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ENERGY SCIENCE, ENGINEERING AND TECHNOLOGY

ENERGY MODERN ENERGY STORAGE, CONVERSION, AND TRANSMISSION IN THE 21ST CENTURY

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ENERGY SCIENCE, ENGINEERING AND TECHNOLOGY

ENERGY MODERN ENERGY STORAGE, CONVERSION, AND TRANSMISSION IN THE 21ST CENTURY

LARS ROSE EDITOR

New York

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Copyright © 2013 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book.

Library of Congress Cataloging-in-Publication Data Energy : modern energy storage, conversion, and transmission in the 21st century / editor, Lars Rose. pages cm Includes bibliographical references and index. ISBN:  (eBook)

1. Energy storage. 2. Energy conversion. 3. Energy transfer. 4. Electric power. 5. Power resources. I. Rose, Lars. TJ165.E495 2013 621.31--dc23 2013009429

Published by Nova Science Publishers, Inc. † New York

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To Bertram Moniz, a world class materials engineer, prolific author, friend, and mentor and to Heidrun Spohr, a world class chemist, with whom all good things may happen

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CONTENTS Preface

ix

Chapter 1

Modern Energy – Do We Have the Answers? Lars Rose

Chapter 2

Carbon Nanotubes and Energy Esteban E. Ureña-Benavides and Virginia A. Davis

Chapter 3

A Review of Energy Saving Potential and Strategies for Electric Lighting in Future Low Energy Office Buildings Marie-Claude Dubois

1 13

47

Chapter 4

Photovoltaic, Thermal and PV/T Solar Collectors: An Overview Marc A. Rosen

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Chapter 5

Grid Integration Issues for Wind Power K. John Holmes, Lawrence T. Papay and Elizabeth A. Santori

107

Chapter 6

Offshore Wind Energy Alexia Aubault and Dominique G. Roddier

121

Chapter 7

Building Integrated Wind Energy Conversion Systems for Future Cities Wen Tong Chong, Sin Chew Poh and Ahmad Fazlizan

145

Chapter 8

Energy and Exergy Performance Analysis of Heat Storage Systems Onyemaechi Valentine Ekechukwu, Howard O. Njoku and Samuel O. Onyegegbu

Chapter 9

Sustainable EU Monitoring: Efficiency of Consumption of Energy Resources Mirjana Golušin

193

Case Study: Economic Index of Energy Security - Monetization of All Costs Approach Mirjana Golušin, Holger Schlör and Jürgen-Friedrich Hake

223

Chapter 10

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viii

Contents

Chapter 11

Recent Advances in Algal Biofuels Lakshimi Mangamoori, Razif Harun, Ravicandra Pothumarti, Parcha Lakshmi and Michael K. Danquah

Chapter 12

Physicochemical Investigation of Microalgal Cell Disruption Technologies with Energy Consumption Analysis Razif Harun and Michael K. Danquah

Chapter 13

Microalgae Dewatering: Technology Advancement Using Electrocoagulation Nyomi Uduman, Michael K. Danquah and Andrew F. A. Hoadley

Index

243

263

275

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PREFACE The amount of energy we use per capita increases, as does the total population. As a result, we require an increasing amount of energy. This energy can be supplied by various means, some of which have been found to come along with heavy environmental footprints. These circumstances represent a huge economic opportunity to utilize novel methods of converting, transmitting, storing, and consuming energy more efficiently. As a result, energy related research, from policy and fundamental science to engineering and technology implementation and business model developments, is pushed by a large global community. This professionally peer reviewed work presents but a small part of this vast global research. The selected, invited authors significantly contribute to the furthering of energy related research in the 21st century. In chapter 1, a very high elevation overview over the issues related to energy conversion, transmission, consumption, and storage is presented by the editor, without going into scientific details. This chapter sets the stage for the in-depth scientific, engineering, and policy analyses performed in the other chapters. One of the most often dissed issues in modern energy-related research is the use of nano technology and specifically the use carbon nanotubes. Esteban Ureña-Benavides and Virginia Davis present the current status of carbon nanotubes research in all aspects of energy related technology in chapter 2. The uses and applications of nanotubes, while still limited due to the only very recent discovery of these tubes and the fact that we cannot produce them at transmission-line length, are outlined. It is shown that there are a multitude of energy-related applications, all of which can benefit significantly from utilizing carbon nanotubes. Some of the intermittent sources of clean energy conversion, solar collectors, are described in chapter 3. Marie-Claude Dubois reviews the energy saving potential and strategies for electric lighting in low energy office buildings, with a strong correlation of international efforts to the Swedish approach to modern building technologies. A Canadian perspective on the global photovoltaic, thermal, and combined photovoltaic/thermal solar collectors is given by Marc Rosen in chapter 4. It shows how specific solar collector systems can be tailored to a building, and summarizes many different uses of solar collectors in a modern building. The chapter also shows that a strong focus in research is given to photovoltaic – thermal combined solar generation. In chapter 5, John Holmes, Lawrence Papay, and Elizabeth Santori introduce the complexities surrounding grid integration issues for wind power. While the expansion of all alternative sources of energy is desirable form an energy security point of view, the

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intermittent nature of the power conversion of wind and solar energy makes the integration of these new and often small power stations into an existing, old, and static power grid very challenging. Alexia Aubault and Dominique Roddier summarize the state-of-the-art in offshore wind energy conversion in chapter 6. It can be seen that many different types of technologies already exist and that the global market in offshore energy conversion is expanding. Some technology infancy issues, such as component corrosion and grid connections, are currently addressed through pilot projects that are already installed at sea. Building integrated wind energy conversion systems are analyzed based on computer modeling, and correlated to existing structures around the globe in chapter 7 by Wen Tong Chong, Sin Chew Poh, and Ahmad Fazlizan. The analysis shows how simple modifications to a building can convert a standard office tower into a modern clean energy converter, without impacting the occupants. Onyemaechi Ekechukwu and co-workers Howard Njoku and Samuel Onyegegbu outline the fundamental research performed to increase efficiency and applicability of heat storage systems through computational modeling in chapter 8. This research aids in developing novel methods to store energy for a later use with fewer losses and a faster energy conversion rate than current energy storage systems. The complexities of energy policies are explained on the example of the European Union in chapter 9 by Mirjana Golušin. The chapter explores the policies behind monitoring the emissions and energy conversion methods, and correlates the efficiency of consumption of energy resources to the policies used in the various member states of the European Union. The policy foundations outlined in chapter 9 are applied to a practical example by Mirjana Golušin, Holger Schlör, and Jürgen-Friedrich Hake in the case study described in chapter 10, focusing on the economic index of energy security and the monetization of all costs approach. Without a structured approach to energy policy, any changes in technology may not be used to their full potential, and this chapter outlines one of the many research studies performed to improve the governmental ability to handle energy related issues efficiently. Finally, we are depending mostly on fossil fuels when it comes to supplying today’s economy with energy. However, many alternatives are being researched. Research of biofuels derived from algae, performed by Australian researcher Michael Danquah and associates shows that, while potential biofuels are abundantly available in nature, harvesting biofuels useful to us in a cost-effective way requires significant further research. However, it can also be seen that, in a sufficiently focused effort, we can change the source of our fuels in the future, if we decide to do so. Danquah shows in chapter 11, together with Lakshimi Mangamoori, Razif Harun, Ravicandra Pothumarti, and Parcha Lakshmi, recent advances in research, development, and commercialization furthering the extraction of biofuels from algae. In chapter 12, Razif Harun and Michael Danquah show the technologies researched to destroy the cell walls of algae in order to extract their contents, and in chapter 13, Nyomi Uduman, Andrew Hoadley, and Michael Danquah show how electrocoagulation can be used for the technologically critical step of dewatering algae in order to be able to use them in fuel production. Please note that the information presented in this book does not necessarily reflect the views of the authors’, editors’, or contributors' host institutes.

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In: Energy Editor: Lars Rose

ISBN: 978-1-61942-526-2 © 2013 Nova Science Publishers, Inc.

Chapter 1

MODERN ENERGY – DO WE HAVE THE ANSWERS? Lars Rose Materials Engineering Department, University of British Columbia, Frank Forward Building, Vancouver, B.C., Canada

ABSTRACT Since managing the control of fire, humans have come a long way in harnessing power to their own needs. Yet, despite the abundance of modern ideas, we have not fundamentally changed the generation of electricity and heat, or their distribution through the electric grids for many decades. This chapter shall give a very brief and high-level overview over novel ideas which have generated interest in the scientific world and which could be used on an everyday basis if the right policies and incentives are put into place.

INTRODUCTION Humans have a long history of trying to harness electricity, a feat that has become common over the past centuries. Electric power is, for example, commonly used to drive electric motors. Generation of electricity by means of rotating magnets has been used since approximately 1873. While some progress has been made to the materials and methods used in converting electric energy, thereby making the process more efficient, the underlying physical method to do so has remained the same since the discovery of electromagnetism in 1831, almost two centuries ago. Two methods of intentional electric charge conduction along predefined vectors have been discovered, Tesla’s alternating current (AC) method [1], and Edison’s direct current method [1,2]. The majority of today’s infrastructure for long distance 

University of British Columbia, Materials Engineering Department, 309-6350 Stores Road, Vancouver, B.C., Canada V6T 1Z4, suburp (at) interchange.ubc.ca. DuPont De Nemours (Nederland) B.V., Baanhoekweg 22, Postbus 145, ES&S 20, Dordrecht 3300AC, NL, Dossiernummer 54013445.

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transmission of electric power is based on AC technology. The advantages and disadvantages of this technology will be briefly discussed later in this chapter. Modern engineering materials developments include high temperature superconductors and carbon nanotubes, which could theoretically transmit electric power with significantly fewer losses. Also, energy conversion in stationary power converters has remained very similar over the past century. From coal fired steam engines to compressed natural gas fired gas turbines, the principle of energy conversion has remained the same. And while alternative sources of electric power generation exist, their overall percentage contribution to the modern electricity grid remains sadly negligibly small. In mobile power generation, power storage systems such as batteries have come a long way, mainly due to intensive research and development, but applications of these energy storage systems remain mostly limited to small handheld devices. Early automotive technology favored electric storage systems, since oil was scarce and expensive, and gear shifts were mechanically difficult to construct compared with simple electromotors. The expansion of oil exploration at the beginning of the 20th century, however, with a resulting drop in oil prices shifted automotive engine technology towards internal combustion engines used to this day. This chapter aims to give a very brief introduction to technologies that enable energy conversion, transmission, and storage. It thus also serves as an overview to the technological challenges, alternative methods of generating fuels, green energy technologies, and the political frameworks described in the other chapters of this book.

1. ENERGY CONVERSION Traditional methods of energy conversion involve the burning of fossil fuels and other organic matter. Originally, this was (and mostly still is) the main source of thermal energy in large parts of the world. The thermal energy produced during the exothermal reaction of the oxidation of organic matter can also be converted into electricity by the means of gas and steam turbines. Large scale (i.e. GW sized) power plants burn chiefly oil, natural gas, or coal, and convert the chemical energy liberated in the furnaces into electricity. The by-products of this energy conversion include carbon oxides (CO, CO2), as well as other oxides such as NOx and SOx, and a compliment of metal oxides of all metals (including radioactive isotopes [3,4]) present in small quantities in the source fuels, collectively summarized as fly ash. These often toxic and sometimes radioactive byproducts of coal burning may cause damage to the environment, as the particulate matter in the air dissipates throughout the atmosphere [5,6]. Large coal ash deposits can lead to environmental problems in the vicinity of the storage sites [7,8]. Additionally, some of the gases emitted during the process contribute to the greenhouse effect, of which the impact on humankind is discussed worldwide [9,10] and the mining of these resources can take a heavy toll on the environment [11,12]. Furthermore, the use of compressed natural gas regularly leads to industrial accidents [13], despite the existence of alternative, cleaner, and safer methods of energy conversion. Another widely used method of energy conversion involves the controlled fission of radioactive materials in nuclear power plants. The reaction heat generated during the fission process heats water in the same way that the exothermal reaction of burning fuel heats water

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to drive gas and steam turbines. During this process, no greenhouse gases are produced as byproducts, although the building and maintenance of the infrastructure constitutes a net contribution to greenhouse gas emissions on the same scale as for any other similarly-sized power station. However, all materials that come in close proximity to the fissibles become hazardous radioactive waste, and currently not a single solution exists that addresses the safe disposal of this waste. Germany engaged in an experiment to permanently store radioactive waste in an abandoned salt mine in the 1960s, with almost no cost to the industry [14]. As the salt mine is slowly being penetrated by ground water barely fifty years later, an engineering project is underway to recover the mostly unknown waste from the mine, a process that will cost several dozen billions of Euros, and may take several decades [15,16]. No long-term plan exists to satisfactorily deal with the radioactive waste to be extracted from this mine, much less from the still active nuclear power plants all over the globe. However, since the power plants are not sufficiently insured against major accidents, the electricity converted by nuclear power plants can be delivered at a low price level and is consequently much in demand globally [17,18]. Alternative methods to convert energy exist, but their overall contribution to the global energy conversion remains small. Government incentives have been successful in the past to facilitate a wider spread of the various methods, but when competing against low oil, coal, and natural gas prices and insufficiently insured atomic energy, these methods have not had a significant impact on the market yet. However, significant research is underway and energy conversion methods with less detrimental effects on health and environment are being developed globally. Some of these outstanding efforts are elaborated in the other chapters of this book. The most widely used source of alternative energy is the conversion of kinetic or potential energy of water into electrical energy in hydroelectric installations. While hydroelectric installations have no on-site emissions, the construction of dams is controversial as they interrupt established ecologic systems. However, of the alternative methods available to convert energy, hydroelectric installations convert the largest amount of energy. Solar electric and solar thermal systems are another promising and clean method to convert energy. These installations require a high initial investment in energy and a set of not always benign materials of construction. However, opposed to fossil fuel energy plants that also require a large input in energy during their construction, not to mention the installation of the necessary distribution grid, alternative energy conversion methods such as solar energy require no continuous input of (fossil or nuclear) fuels and, after a few short years, generate more energy than was originally invested in them, for free, until the end of their useful life of potentially decades. Wind power converters are also becoming more prevalent. However, these converters are sometimes frowned upon for the impact they have on natural vistas and many, especially older, models generate noise that often requires them to be placed in remote areas. Nevertheless, wind energy conversion is a significant global business, and in the United States, Texas is today the largest producer not only of petroleum, but also of wind power [19,20]. Geothermal energy conversion extracts the heat of Earth’s core to electricity by heating steam that drives gas and steam turbines. While large scale generators are usually placed in the vicinity of geological faults, at least the heating and cooling load of most buildings could be covered today by ground source heat pumps almost anywhere on the planet. The buildings of the National Research Council in Vancouver, Canada, for example,

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extract the energy required for heating in winter and part of the energy required for cooling in summer from such ground source heat pumps [21,22,23]. These systems typically involve a closed looped system that transports a liquid several dozen meters deep through the earth, extracting the latent heat from these shallow depths [24]. Other research projects are involved in energy conversion from organic matter that does not involve fossil fuels [25]. One readily available source of stored energy is human refuse, which, if converted into usable energy, also reduces the volume of landfills. The energy stored in waste can readily be converted into electricity in waste incineration plants, with the additional benefit of removing slow degrading waste products from ending up in the environment. The exhaust gases require modern scrubbing to remove aerial pollutants but beyond such particulates, carbon dioxide is also emitted in the process. Incineration facilities are used extensively in densely populated Europe, but other parts of the world appear reluctant to use them. Some controversy exists about the amount of dioxin produced in the facilities, as compared to the storing the same volume of refuse in landfills [26,27,28,29]. Furthermore, institutes such as the Chalmers University in Gothenburg Sweden, for example, cover their entire requirement for heat through the burning of wood chips [30,31,32]. Other less used, though promising, alternatives to fossil fuels include algae produced in the oceans and other water systems. Here, plants and organisms that may be blamed by some for depleting parts of the ocean of oxygen that is vital to most other life, can be harvested to be used in energy conversion instead [33]. Other methods involving the oceans in energy conversion are tidal reactors and wave generators that use the energy of the tides or waves to generate electricity. While tidal reactors require large installations in biologically sensitive areas, wave generators can be placed as buoys in the ocean without a large impact on their environment [34]. However, these methods are still in the research phase and are not widely applied yet. Once electrical energy has been converted from other types of energy as described above, this energy has to be transmitted from the conversion source to the end user. The next section of this chapter addresses current developments in the field of energy transmission.

2. ENERGY TRANSMISSION Unless energy is converted on-site where it is consumed by an end user via, for example, clean energy technologies such as solar cells, it is produced off-site in a location far from the end user, or close to where the energy source is largest, for example solar radiation in a desert. This energy then has to be distributed to the end users. Depending on the length, location, landscape, and availability of older distribution networks, this transmission can be performed via AC (alternating current) or DC (direct current) transmission lines. In order to reduce losses during transmission, high voltages are used. Some of the advantages of HVDC (High voltage direct current) transmission over (HV)AC electricity distribution networks shall be mentioned briefly in this chapter. When using transmission lines effectively, energy conversion can occur far away from the load centers, where conditions are better for, for example, large wind farms, solar cell farms, or nuclear or fossil fuel power plants. HVDC enables transmission of large amounts of power over long distances with lower capital cost and with significantly fewer losses than AC

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transmission. For distances longer than approximately 600 km overland, the HVDC technology offers capital cost savings over AC transmission hardware. Also, long (i.e. >200 km) undersea cables and overland endpoint-to-endpoint long-haul bulk power transmission without intermediate stations can generate high capacitances, which can significantly increase the losses, especially in AC systems. The current required to charge and discharge the capacitance of the cable system causes significant I2R power losses in AC systems in addition to dielectric losses. The current density of an existing power grid can easily be adapted in HVDC systems, whereas additional wires can be expensive and difficult to install into existing AC systems. At similar charge densities, significant economic savings in copper can be realized. Also, several separate AC grids that are not in phase can be easily connected via an HVDC line. Furthermore, the amount of wiring and pylons per line can be reduced, since the wiring can be placed underground due to a reduced necessity for insulation, since the HVDC system only requires two conductor lines, and since the charge capacity per area is increased. HVDC can carry more power per conductor cable cross sectional area since the constant voltage in a DC line at any given power rating is lower than the peak voltage in an AC line. This voltage also determines the requirement for environmental insulation, which is consequently thinner for HVDC lines. Additionally, HVDC lines do not exhibit the same strong electromagnetic fields as observed in the surroundings of AC overland lines. In AC grids, cascading surges can disturb entire grids, and propagate through the grids. Modern fly wheels can be used to store and dispense energy, as needed, within fractions of a second and any surges in the grid can thus be smoothed out within the capacity of the fly wheel facility, preventing a buildup of irregularities within the grid. However, the quantity and distribution of failsafes such as fly wheels can be much lower in HVDC grids. Even more, a predominantly AC power grid can be stabilized with HVDC hardware without the necessity to increase maximum prospective short circuit currents. Significant rapid changes in load that would cause portions of an AC network to become unsynchronized would not have a similarly damaging effect on an HVDC link. In fact, HVDC links can act as stabilizers or, to a limited degree, surge propagation protection in AC networks. Nevertheless, harmonics cannot be completely eliminated in HVDC systems, either. The magnitude and vector of power flow through HVDC grids can be directly controlled and altered instantly if needed to support unstable AC networks. However, the static inverter hardware is typically costly and only has a limited overload capacity. The energetic losses in the static inverter hardware can be higher for short transmission distances for HVDC lines compared with AC grids. Consequently, the higher cost of the DC inverter hardware may not necessarily be offset by reductions in line construction cost and lower energetic line losses. Also, multi-terminal systems can become complex and expanding simple existing HVDC hardware to multi-terminal systems requires higher initial capital investments than the expansion of AC stations. Additionally, the proper control of power flow in a multi-terminal HVDC system requires instant, uninterrupted communication between all the terminals, and power flow cannot be passively observed, but must be actively controlled and regulated. Electrical energy has to be consumed once produced, or it may be lost if it cannot be stored. Current energy storage technology can store no significant fraction of the energy converted in power stations. The following section of this chapter summarizes modern

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methods to store energy for later use. These energy storage technologies become increasingly more important for our daily lives due to the increased use of mobile applications.

3. ENERGY STORAGE While storage of electrical energy, for example in the form of batteries or capacitors, has been developed over two centuries ago, these mobile storage systems have remained astonishingly inefficient over the years. However, other methods to store energy exist. Pumping water up a mountain, compressing gas in a defined container, hydrolyzing water into hydrogen for later consumption by oxidation either in a fuel cell or via direct combustion, the acceleration of fly wheels, or the storage of energy from oil in polymers are just some examples of how energy is stored. This section aims to discuss the advantages and disadvantages of some of these methods and the advances in research and development pushing these storage methods towards becoming more effective systems. Batteries are devices that store electricity via typically reversible chemical reactions. Different types of batteries involve different chemicals, in solid, liquid, and sometimes gaseous states of matter, which can store electricity chemically. Early devices included the Voltaic pile, a set of dissimilar metals in electric contact arranged into a series of cells in 1800 [35], and the Daniell cell, the early predecessor of today’s batteries, developed in 1836 [36]. Many of the early automobiles developed used electric engines powered by the energy stored in batteries, since petrol was expensive and rare in those days [37]. Most commonly used electrical energy storage devices nowadays include lead acid batteries, wherein lead is reversibly oxidized to lead oxide inside an acidic electrolyte solution. While being heavy with a low charge capacity, and while they contain toxic lead, these devices are robust, rechargeable, and can operate well at low temperatures. As a consequence, they have been the battery type of choice in the automotive sector for many decades. Other often used battery types based on nickel include rechargeable Ni metal hydride (Ni-MH) batteries and the Ni-Cd single use batteries that are being phased out in the industrialized part of the world due to the toxic cadmium content [38]. These battery types still have a low charge capacity to mass ratio, and are consequently disadvantageous for mobile devices compared to modern batteries with a higher specific energy capacity [39]. For stationary storage, Ni-Cd batteries have low specific energy storage capacity (40 Wh/kg compared with 80 Wh/kg for Ni-MH, and 35 Wh/kg for lead acid batteries), and Ni-MH batteries lose their recharge capacity rapidly with repeated recharge cycles. A modern approach to battery storage is represented by Li ion batteries. Due to the low mass, lithium is a good material for ion charge transfer in electrochemical storage devices, resulting in a high charge capacity to mass ratio. While representing a significant advancement to other battery types, Li ion batteries may run the risk of thermal runaway and modern batteries are thus outfitted with safety devices such as thermal interrupts to prevent overcurrent, cell separators that shut the cell down if the temperature increases beyond a determined safe limit, and tear-away tabs and vents to relief internal pressure. The charge capacity of lithium ion batteries is rapidly and irreversibly lost at elevated temperatures due to microstructural changes in the construction material of the electrochemical cell comprising the Li ion battery [40,41].

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The challenge in battery research is to construct devices that are non-toxic, have a low mass and a high volumetric and gravimetric current density, a fast charge-discharge cycle, and can deliver high currents at high voltages without becoming limited by surface polarization or diffusion limits. Several promising candidate electrochemical energy storage cells include lithium-sulphur batteries [42], sodium sulphur batteries [43], vanadium redox batteries, and zinc cerium batteries [44]. However, further research is necessary to continue to improve all battery technologies in order to be able to meet the increasing global demand for mobile energy storage systems. Rather than chemically storing electricity, charges can also be stored physically in supercapacitors. In these devices, metal electrodes, on which charges can accumulate, are separated by thin non-conductors. Research here is aimed at increasing current density, while reducing the device volume and mass, thus reducing charge/discharge cycle times, and lengthening the duration during which the devices can hold charges. Modern supercapacitors are also becoming more efficient, producing less waste heat during charge cycles [45,46]. Electric energy can also be converted into rotational motion via fly wheels. While this technology is successfully used to perform load balancing in electric networks, it can also be used to store energy. However, due to the large mass required for fly wheels to be efficient, and the induced moments of inertia of rotating heavy masses, they are most often used in stationary installations. However, these devices represent a very interesting method to store unused energy from vehicle brakes which is currently dissipated as thermal energy. Pressurized gases constitute another physical method to store energy, although the energy storage capacity is low. Proposals to store energy on a large scale in caves or in fabric sacs underwater, however, are somewhat optimistic options to increase the available storage volume [47]. Usable prototypes of this technology for industrial scale energy storage are still missing, but are being researched. More commonly, energy storage with an industrially suitable energy capacity is realized by water storage facilities at elevated heights. During night, when most power plants are idling and underutilized, water can be pumped to higher elevations. During peak energy use times, in the morning and late afternoon, the stored water can be allowed to rush down to the lower levels again, covering the increased energy demand. While most mobile devices today would be unthinkable without the advances in storage technology over the past century, the overall realized energy storage capacity is still very low, and most of the energy in our distribution grids has to be consumed on site. All energy storage methods require significant further research and development in order to increase the currently available energy storage capacity. Furthermore, if the electrification of vehicle technology is to progress as prescribed in the policies of most countries [48], energy storage is one of the most crucial areas of energy research and development.

CONCLUSION While novel methods of energy conversion, transmission, and storage are available today, we still mostly use technologies that have changed little since their inception a long time ago. As a consequence, for example, we still have differences in voltage in the electrical networks worldwide and the networks are very susceptible to breakdowns. Additionally, we continue to derive most of our energy from fossil fuels, while cleaner and healthier alternative methods

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are available and sufficiently mature to use. Clearly, there is a huge margin for improvement through good global policies and the continuous research and development of new clean energy technologies. This includes more efficient ways to convert, transmit, store, and consume electricity, with a resulting net benefit to all energy consumers. This consequently represents significant global business opportunities in the energy market. This book highlights some of the multi-faceted aspects of these technology improvements and the implementations of policies to support their developments. We clearly have discovered some of the answers to address energy conversion, transmission, and storage in an efficient way already, but we still need to apply them on a larger scale than we currently do, if we truly intend to obtain solutions that are affordable to all and less polluting and if we intend to reduce potentially negative side-effects of dealing with energy on a global scale.

REFERENCES [1]

Hadzigeorgiou, Y., Klassen, S., Klassen, C.F. (2012). Encouraging a "romantic understanding" of science: The effect of the Nikola Tesla story, Science And Education, 21(8) 1111-1138. [2] Cheney, M., Uth, R., Glenn, J., Tesla (2001). Master of Lightning, Metro Books, Barnes and Noble, October 2001. [3] Turhan, Ş., Parmaksiz, A., Köse, A., Yüksel, A., Arikan, I.H., Yücela, B. (2010). Radiological characteristics of pulverized fly ashes produced in Turkish coal-burning thermal power plants, Fuel 89 (12), 3892–3900. [4] Papp, Z. Dezsö, Z., Daroczy, S. (2002). Significant radioactive contamination of soil around a coal-fired thermal power plant, Journal of Environmental Radioactivity 59 (2), 191–205. [5] Santiago, R. (2012). Imminent and substantial endangerment to human health and the environment from use of coal ash as fill material at construction sites in Puerto Rico: A case study, Procedia - Social and Behavioral Sciences 37, 389-396. [6] Hvistendahl, M. (2011). Coal Ash Is More Radioactive than Nuclear Waste, Scientific American, Nature America, Inc., 13 Dec. 2007. [7] Bian, Z., Inyang, H.I., Daniels, J.L., Otto, F., Struthers, S. (2010). Environmental issues from coal mining and their solutions, Mining Science and Technology 20 (2), 215-223. [8] Ruhl, L., Vengosh, A., Dwyer, G.S., Hsu-Kim, H., Deonarine, A. Bergin, M., Kravchenko, J. (2009). Survey of the Potential Environmental and Health Impacts in the Immediate Aftermath of the Coal Ash Spill in Kingston, Tennessee, Environmental Science and Technology 43, 6326–6333. [9] Rose, L. (2011). Review of cornerstone parameters influencing future energy policy, in: Global Environmental Policies: Impact, Management and Effects; Environmental Science, Engineering and Technology Series, eds.: R. Cancilla, M. Gargano Nova Publishers, Hauppauge, New York. [10] Haines, A., Kovatsa, R.S., Campbell-Lendrum, D., Corvalan, C. (2006). Climate change and human health: Impacts, vulnerability and public health, Public Health 120 (7), 585-596.

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[11] Silva, C.A., Ribeiro, C.A.O., Katsumiti, A., Araújo, M.L.P., Zandoná, E.M., Costa Silva, G.P., Maschio, J., Roched, H., Silva de Assis, H.C. (2009). Evaluation of waterborne exposure to oil spill 5 years after an accident in Southern Brazil, Ecotoxicology and Environmental Safety 72 (2), 400-409. [12] Owens, E.H., Henshaw, T. (2002). The OSSA II Pipeline Oil Spill: The Distribution of Oil, Cleanup Criteria, and Cleanup Operations, Spill Science and Technology Bulletin, 7 (3-4), 119-134. [13] Greenberg, M.I. (2006). Liquefied natural gas explosion, Disaster Medicine, Chapter 152, 781-783. [14] Bossy, H. (2006). The Closure of the Asse Research Mine, Joint Convention on the Safety of Spent Fuel Management and on the Safety of Radioactive Waste Management, Second Review Meeting, May 17th, Federal Ministry of Education and Research, Bonn, Germany, http://www.bmu.info/ files/ pdfs/allgemein/application/pdf/ jc_asse.pdf [15] Blowers, A., Lowry, D. (1997). Nuclear conflict in Germany: The wider context, Environmental Politics, 6 (3), 148-155. [16] Hofmann, H., Bauer, A., Warr, L.N. (2004). Behavior of smectite in strong salt brines under conditions relevant to the disposal of low- to medium-grade nuclear waste, Clays and Clay Minerals, 52 (1), 14-24. [17] Tyran, J.R., Zweifel, P. (1993).Environmental risk internalization through capital markets (ERICAM): The case of nuclear power, International Review of Law and Economics, 13 (4), 431-444. [18] Trebilcock, M., Winter, R.A. (1997). The economics of nuclear accident law, International Review of Law and Economics, 17 (2), 215-243. [19] Oliveira, W.S.D., Fernandes, A.J. (2012). Global wind energy market, Industry and Economic Impacts, Energy and Environment Research, 2(1) 79-97. [20] Langniss, O., Wiser, R. (2003). The renewables portfolio standard in Texas: an early assessment, Energy Policy 31(6) 527–535. [21] CANMET Energy Technology (2005). Ground-Source Heat Pump Project Analysis Chapter, RETScreen® International, Natural Resources Canada. [22] Briller, D.L. (2011). Using Leed® to Facilitate The EISA Goal of Zero Fossil Fuel Use in New Federal Buildings, Strategic Planning for Energy and the Environment 30(4), 770. [23] Astill, T. (2010). Vancouver testing and validation center gets fuel cell technology to market faster, Fuel Cells Bulletin 2010, 12-15. [24] Hotte, N. (2012). Ground source heat pumps, Editor: Franceschetti, D.R., Salem Press: Applied Science, May 2012, ISBN: 978-1-58765-781-8. [25] Zerbe, J.I. (1991): Liquid fuels from wood – ethanol, methanol, diesel, World Resource Review, 3[4], 406-414. [26] Buekens, A., Huang, H. (1998). Comparative evaluation of techniques for controlling the formation and emission of chlorinated dioxins/furans in municipal waste incineration, Journal of Hazardous Materials 62(1), 1–33. [27] Buekens, A., Huang, H. (1998). Comparative evaluation of techniques for controlling the formation and emission of chlorinated dioxins/furans in municipal waste incineration, Journal of Hazardous Materials 62, 1-33.

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[28] Unilabs Environmental (2001). Characterisation and estimation of dioxin and furan emissions from waste incineration facilities, Study for the Australian Government, Department of sustainability, environment, water, population, and communities. [29] Sakai, S.I., Hayakawa, K., Takatsuki, H., Kawakami, H. (2001). Dioxin-like PCBs Released from waste incineration and their deposition flux, Environmental Science and Technology (2001) 35, 3601–3607. [30] Sikkema, R., Steiner, M., Junginger, M., Hiegl, W., Hansen, M.T., Faaij, A. (2011). The European wood pellet markets: current status and prospects for 2020, Biofuels, Bioproducts and Biorefining 5(3), 250–278. [31] Röser, D., Asikainen, A., Stupak, I., Pasanen, K. (2008). Sustainable Use of Forest Biomass for Energy, Managing Forest Ecosystems, Forest Energy Resources and Potentials 12, 9-28. [32] Svedberg, U., Samuelsson, J., Melin, S. (2008). Hazardous Off-Gassing of Carbon Monoxide and Oxygen Depletion during Ocean Transportation of Wood Pellets, The Annals of Occupational Hygiene 52(4), 259-266. [33] Worm, B., Barbier, E.B., Beaumont, N., Duffy, J.E., Folke, C., Halpern, B.S., Jackson, J.B.C., Lotze, H.K., Micheli, F., Palumbi, S.R., Sala, E., Selkoe, K.A., Stachowicz, J.J., Watson, R., (2006). Impacts of biodiversity loss on ocean ecosystem services. Science 314 (5800) 787-790. [34] McArthur, S., Brekken, T.K.A. (2010). Ocean wave power data generation for grid integration studies, Power and Energy Society General Meeting, IEEE, 25-29 July 2010, Minneapolis, MN., USA. [35] Mauro, A. (1969). The role of the voltaic pile in the Galvani-Volta controversy concerning animal vs. metallic electricity, H. Schuman Publishing, New York. [36] Bird, G. (1838). Report of the seventh meeting of the British society for the advancement of science London, 6, 45. [37] Rose, L. (2012). Electric automobile technology, Editor: Franceschetti, D.R., Salem Press: Applied Science, May 2012, ISBN: 978-1-58765-781-8. [38] E. Callender, E. (2003). Heavy Metals in the Environment—Historical Trends, US Geological Survey, Westerly, RI, USA, Treatise on Geochemistry Volume 9: Environmental Geochemistry, 67–105. [39] Logan, D., Neil, C., Taylor, A. (1994). Modeling Renewable Energy Resources in Integrated Resource Planning, NREL Report No. TP-462-6436. [40] Vetter, J., Novák, P., Wagner, M.R., Veit, C., Moeller, K.C., Besenhard, J.O., Winter, M., Wohlfahrt-Mehrens, M., Vogler, C., Hammouche, A. (2005). Ageing mechanisms in lithium-ion batteries, Journal of Power Sources 147, 269–281. [41] Shim, J., Kostecki, R., Richardson, T., Song, X. (2002). Electrochemical analysis for cycle performance and capacity fading of a lithium-ion battery cycled at elevatedtemperature, Journal of Power Sources 112, 222–230. [42] Ji, X., Lee, K.T., Nazar, L.F. (2009). A highly ordered nanostructured carbon–sulphur cathode for lithium–sulphur batteries, Nature Materials 8, 500-506. [43] Jones, I.W. (1977). Recent advances in the development of sodium-sulphurbatteries for load levelling and motive power applications, Electrochimica Acta 22, 681–688. [44] Leung, P.K., Leon, C.P.D., Walsh, F.C. (2012). The influence of operational parameters on the performance of an undivided zinc–cerium flow battery, Electrochimica Acta 80, 7–14.

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[45] Conway, B.E., Electrochemical supercapacitors: scientific fundamentals and technological applications (1999). [46] Das, J., Tang, M.B., Kim, K.B., Theissmann, R., Baier, F., Wang, W.H., Eckert, J. (2005). “Work-Hardenable” Ductile Bulk Metallic Glass, Physical Review Letters 94, 205501. [47] Pimm, A., Garvey, S. (2009). Analysis of flexible fabric structures for large-scale subsea compressed air energy storage, ournal of Physics: Conference Series 181[1], 012049. [48] Rose, L., Hussain, M., Ahmed, S., Malek, K., Costanzo, R., Kjeang, E. (2013). A comparative life cycle assessment of diesel and compressed natural gas powered refuse collection vehicles in a Canadian city, Energy Policy 52, 453-461.

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Chapter 2

CARBON NANOTUBES AND ENERGY Esteban E. Ureña-Benavides and Virginia A. Davis Department of Chemical Engineering, Auburn University, AL, US

ABSTRACT Numerous national and international organizations have recognized that the energy challenge is one of the most pressing problems facing mankind. Both population growth and increasing technological sophistication are increasing global need inexpensive, clean energy sources. Carbon nanotubes will be an integral part of addressing all aspects of the energy challenge: conversion, transmission, storage, and consumption. For example, the use of carbon nanotubes in traditional fuel cell and biofuel cell electrodes can increase efficiency and power output. Carbon nanotubes have also been shown to enhance the efficiency of thin-film and hybrid solar cells. In terms of transmission, electrical cables comprised of carbon nanotubes could revolutionize the power grid. With current materials there is approximately 5 % power loss for every 160 km (100 miles) of transmission. Carbon nanotube quantum wires could greatly reduce these losses as well as increase charge carrying capacity. These improvements in transmission efficiency could increase the economic feasibility of long distance transport from optimally placed solar energy farms to metropolitan areas. Carbon nanotubes have also shown significant potential in energy storage devices including supercapacitors, and batteries. Finally, the low density, high strength, and thermal stability of carbon nanotubes enable their use in strong light-weight composites. Since the majority of vehicle fuel consumption is related to vehicle mass, the increased use of carbon nanotubes in transportation will significantly reduce energy consumption. While other emerging materials have the potential to address one or more aspects of the energy challenge, carbon nanotubes are unique in their versatility and the fact that they are already being produced in large quantities by major industrial corporations.

Keywords: Carbon nanotubes, energy, conversion, transmission, composites, batteries, fuel cells, photovoltaic, supercapacitors



Auburn University, 212 Ross Hall, Auburn, AL, (334) 844-2060, [email protected]

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INTRODUCTION A broad range of factors such as the advancement of technology, world population growth, the development of emerging economies, and increasing lifespans are creating an increasing demand for energy availability. [1-5] It has been estimated that by 2035 the world’s use of primary energy will have increased by approximately 53 % compared to 2008. [6] The challenge of meeting the world’s energy needs in an economical way, with minimal impact on the planet’s climate, has often been termed the energy challenge or the Terawatt challenge. [4] While energy is not the only global problem, it has often been termed the most important. Access to clean affordable energy is integral to solving other challenges such as access to clean water, food, medicine, or education. [4,7] In addition, the worldwide dependence on fossil resources has generated significant concerns related to the economic, social, environmental, and public health risks of traditional fuels. [1-3] International Energy Agency statistics for 2009 reveal that 80.9 % of the world’s primary energy supply comes from oil, coal, and natural gas. [8] In recent years, various organizations worldwide have recognized the significant need for access to newer, inexpensive and clean energy resources. [2] Solving the energy challenge cannot be achieved by focusing solely on efficient energy conversion. Adequate transmission and storage, as well as a reduction of consumption, are necessary to fulfill future world energy requirements. Renewable energies such as water, wind, and solar depend on the availability of areas with adequate conditions for electricity generation, which may not be located near large population centers. [9,10] As a consequence, efficient transmission channels are necessary for the success of renewable energy technologies. A commonality among these types of energy sources is that they can only be produced intermittently. [9,10] To address this problem, various studies have suggested the combined use of alternative energy sources with effective energy storage devices. [9,10] A complementary route to tackle energy availability is by reducing consumption. According to the United States Energy Information Administration, transportation accounted for 28 % of the total United States energy use in 2010. [11] Since the majority of fuel consumption is related to vehicle mass, producing light-weight and mechanically strong composites that can perform equal to, or better than, traditional materials is a promising method to reduce transportation related energy consumption. If carbon nanotubes (CNT) can enable applications that provide low cost energy conversion, storage, and transmission, they will become a key material for supporting economic growth, sustainable development, and national security. [7] The exceptional properties of CNT could enable them to make a key contribution in each one of these aspects of the energy challenge. The outstanding electrical conductivity of CNT makes them attractive for the development of electrodes for energy conversion (e.g. biofuel and solar cells) and storage devices (e.g. supercapacitors and batteries), as well as materials for photovoltaics. [12-16] They are also promising for the manufacture of efficient electrical cables for long distance electricity transmission. [17] Their ability to adsorb and release hydrogen in a controlled manner have attracted attention for hydrogen storage applications. [18,19] Moreover, CNT are among the strongest materials known to man. At the same time, they have high thermal stability and low density, which make them ideal for developing light-

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weight materials to reduce energy consumption. [20] It has also been suggested that they can be used to store mechanical energy. [21] The specific properties of CNT greatly depend on their structural characteristics. They are all formed by one or more graphene sheets rolled into concentric cylinders with nanoscale diameters. CNT which consist of a single rolled graphene sheet are known as single-walled carbon nanotubes (SWNT). If they consist of two concentric sheets, they are known as double-walled carbon nanotubes (DWNT), and if they consist of three or more sheets, they are known as multi-walled carbon nanotubes (MWNT). The number of concentric sheets affects the tube diameter. SWNT are approximately 1 nm in diameter. In DWNT and MWNT, the spacing between concentric sheets is ~3.4 Å and the inner diameter typically ranges between 3 and 20 nm. The outer diameter of a MWNT depends on the inner diameter and the number of concentric layers which can be up to approximately 100 sheets. [12] Another class of carbon nanomaterials is vapor grown carbon fibers (VGCF) which are hollow core carbon fibers with diameters that are often below 200 nm. They are composed of several graphene layers stacked parallel or at an angle to the fiber axis. Their cylindrical shape is not as smooth as carbon nanotubes. They contain a greater number of defects in their molecular structure, resulting in lower cost and easier dispersion, but also in poorer physical properties. MWNTs’ electronic properties are generally similar to graphite and they are thus considered semimetals. SWNT can be categorized as metallic (M) or semiconducting (S) depending on their chirality or (n,m) index. The direction of sheet rolling is defined by the chiral vector Ch = na1 + ma2; where a1 and a2 are the unit vectors of the graphene lattice (see Figure 1). The magnitude Ch is the circumference of the nanotube with diameter

dt 

a



n 2  nm  m 2

where a is 0.246 nm, while the chiral angle  is given by



3m     m  2n 

  tan 1 

All the (n,0) nanotubes are called zigzag and have  = 0; the armchair structure is obtained when n = m and consequently  = 30; while all other (n,m) combinations are known as chiral nanotubes. [12,22,23] The M or S character depends on the structure based on the following conditions:[12,22] If n – m = 3z If n – m = 3z  1 If n = m

the nanotubes are considered metallic the nanotubes are semiconductors the nanotubes are ballistic metallic conductors.

Here, z can be any integer. Consequently, for all applications there is particular interest in assembling nanotubes of a single chirality or at least a single electronic classification. It is still not possible to obtain nanotubes of a specific (n,m) index. However, there have been

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several recent advances in both synthesis and separation techniques to yield mostly metallic or semiconducting specimens. [23] The ongoing research and development focused on obtaining specific chiralities or ranges of chiralities can be considered analogous to the 20th century polymer industry’s ongoing efforts to reduce molecular weight polydispersity.

Figure 1. Schematic representation of an unrolled carbon nanotube. Reprinted from Composites Science and Technology, Vol. 61, E.T. Thostenson et al., Advances in the science and technology of carbon nanotubes and their composites: a review, pp. 1899–1912, Copyright (2012), with permission from Elsevier. [24]

Several global chemical companies already produce commercial quantities of CNT, and increasing process efficiencies and economies of scale will continue to reduce cost. Many nanotubes are produced using chemical vapor deposition (CVD) which is considered a potentially low cost and scalable technique for both SWNT and MWNT. Large scale production of high quality nanotubes that are easily purified has been more successful for MWNT than for SWNT. However, uniformity of SWNT production is rapidly improving. Recently, there have also been a number of efforts to develop scalable laser vaporization methods to produce SWNT with high quality and low cost; some estimates predict a future cost of ~$3.20/kg using this technique. [12]

1. CONVERSION Carbon nanotubes have been explored in nearly every technology for converting the world’s energy resources to forms that are used by mankind. For example, in the oil industry, carbon nanotube composites have been explored for use as O-rings in deep drilling well components. [25,26] In the case of wind energy, carbon nanotube composites (see Section 4

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of this chapter) are being evaluated for wind turbine blades. [27-29] In addition to high strength and low density, CNT materials have the additional advantages of being corrosion resistant and providing lightning strike protection. [30] In biomass, CNT are being explored for use as catalysts for the conversion of biomass to fuel. [31-33] In addition, carbonization of biomass is being explored as a route for producing CNT and other allotropes of carbon. [34,35] Various applications in hydroelectric and nuclear energy are also being explored. [3638] However, the majority of research and applications development to date has focused on solar energy (photovoltaics), and chemical energy (fuel cells). In addition, there is an emerging interest in thermocells.

1.1. Photovoltaic Devices Energy conversion through solar cell technologies has so far been very limited; there is a need for both technological improvements in efficiency and significant cost reduction. These two issues are interrelated since the cost of electricity generation is directly related to the installed area, which in turn depends on the overall efficiency of the devices. In 2009, less than 0.8 % of the world primary energy supply was fulfilled through photovoltaic devices. [8] However, ongoing research is motivated by the fact that the sun is a virtually inextinguishable energy source. It supplies more energy to the earth in a single hour than is consumed by the global population in one entire year. [39] In fact, the sun is the only source that has the potential to satisfy the predicted 10 – 20 TW global energy need in 2050. [39] However, the standard home rooftop solar power system only has 2 kW peak power. At that power level, over 500,000 rooftop solar power systems would need to be installed everyday between now and 2050 in order to meet the projected 3 TW need for the United States (US) alone. [39] However, the land required for solar farms is actually relatively small. The amount of land area each of the US states would need to fulfill its total electric energy needs has been estimated using 2007 United States National Renewable Energy Laboratory (NREL) data. The demand of 31 states could be fulfilled by installing less than 1 % of their total land area with solar panels; for 19 other states the percentage ranges from 1 to 5 %. Only the state of New Jersey would require 5 to 9 % of its area (Figure 2). [40,41] By another estimate, the land required for US solar energy farms operating at 10 % efficiency would be comparable to that devoted to numbered highways. [39] However, since the sun is an intermittent power source and optimal locations may be geographically distant from population centers, effective use of solar energy farms requires not only advances in solar technology but also in energy transmission and storage (see Sections 2 and 3). CNT have been introduced in photovoltaics with the goal of improving charge generation, separation, transfer, and collection. They have tunable semiconductor properties depending on nanotube diameter, structure and surface chemistry, unique high mobility and conductivity, and can be assembled into flexible and transparent thin films. [12,42] Consequently, CNT have been used as both p-type and n-type semiconductors for inorganic solar cells, as electron donors and acceptors in organic solar cells, and as transparent electrodes. [12,42] Moreover, the excellent mechanical performance and chemical stability of CNT make them a promising material for developing long lasting solar cells.

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Figure 2. Solar electric footprint of the United States in 2005. The green squares are scaled to represent the area of installed photovoltaic devices required to provide 100 % of the state’s energy demand. The inset shows a color code indicating the respective percentage of the state total area. Reprinted from Energy Policy, 36, Denholm, P. and Margolis, R., Land-use requirements and the per-capita solar footprint for photovoltaic generation in the United States, Copyright 2008, with permission from Elsevier. [41]

1.1.1. Inorganic Solar Cells In any type of solar cell, a photon with the right energy must be absorbed by the active layer to form an electron – hole pair. In the case of inorganic solar cells, the charges are separated and transported through the semiconductors to be collected by the electrodes. An electric field is induced in the p-n heterojunction, which directs the electrons to the n-type semiconductor while the holes are transported through the p-type semiconductor. A solar cell was fabricated by Wei et al. (2007) where a percolated film of double wall carbon nanotubes (DWNT) was deposited on an n-type silicon. [43] The fabrication of the device was performed by floating a thin film of DWNT on water with a few drops of ethanol and transferring it onto a silicon substrate. The substrate was previously patterned by photolithography to remove the silicon oxide layer and expose the n-type silicon. A power efficiency of 1.38 % was reported. The p-type DWNT film served for both photogeneration and charge transfer. A similar work in 2008 by Li et al., reported a solar cell which was constructed by spraying a SWNT/dimethylformamide (DMF) suspension on top of n-type silicon. [44] The power efficiency obtained was 0.95 %; however, a post-treatment with SOCl2 resulted in a 45 % higher efficiency. The treatment had strong electron withdrawing effects and shifted the Fermi level, thereby increasing the conductivity of the SWNT network.

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Higher efficiencies, between 5 and 7 %, were reported by Jia et al. (2008) using DWNT deposited on n-type silicon with a fabrication process analogous to Wei et al. (2007). [43,45] The authors explained the significantly higher efficiency as being due to an effective removal of the native silicon oxide layer via etching in dilute hydrofluoric acid previous to photolithography. They stated that the CNT film aided charge separation, transport, and collection by acting as both a p-type semiconductor and thin film transparent electrode. Moreover, separation of metallic from semiconductor nanotubes was not necessary. Further improvements to the latter design were reported in 2011 by Jia et al. [46] Their pre-prepared CNT/silicon composite was immersed in nitric acid to grow an oxide film and form a metalinsulator-semiconductor (MIS) device which had a stable efficiency of 10.1 %. Addition of a polydimethylsiloxane (PDMS) antireflection coating to the MIS solar cell yielded an improved efficiency of 10.9 %. Figure 3 shows the fabrication procedure for a MIS solar cell with a PDMS coating. Carbon nanotubes can also be n-doped by adequate functionalization. [47-49] Li et al. (2010) reported the first solar cell built with n-type SWNT and p-type silicon. [49] The SWNT were doped by submerging overnight in a polyethyleneimine (PEI) solution. The resulting PEI-SWNT were dispersed in DMF and sprayed onto a p-type silicon substrate. The measured short current density was exceptionally low which yielded efficiencies of roughly 0.006 %. Much more research is still required for adequate use of carbon nanotubes for solar cell applications, yet the authors suggest that a pure carbon solar cell might be possible after significant improvements.

Figure 3. a) Schematic representation of the fabrication procedure for a MIS solar cell with a PDMS coating. b) Scanning electron microscopy (SEM) picture of the deposited CNT film. Reprinted with permission from Jia, Y. et al. Appl. Phys. Lett. 98, 133115 (2011). Copyright 2011, American Institute of Physics. [46]

1.1.2. Organic Photovoltaics The large cost of common inorganic photoactive materials as well as the sophisticated fabrication techniques required for assembling solar cells have driven a significant volume of research to developing organic photovoltaics (OPV). [50,51] Photoactive semiconductor polymers are relatively low cost and can be processed by easy to scale solution based systems. The widespread industrial availability of various printing and deposition technologies makes it possible to use roll-to-roll techniques to significantly lower the cost of OPV devices. Moreover, the ability to make light weight and flexible solar cells make OPV

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attractive for portable electronic devices. However, the efficiencies of OPV are significantly lower than inorganic solar cells. To the authors’ knowledge, at the time of writing, the highest ever confirmed efficiency for a ‘one sun’ silicon solar cell is 25 % (near the thermodynamic limit of 31 % for a standard single-junction device); while for an OPV the record is 9 %. [12,52,53] If multi-junction devices are considered, then current ‘one sun’ inorganic solar cell efficiencies can go up to 34 %. [52] When an electron is excited in an OPV, the electron – hole pair relaxes and stays bound forming an exciton, which has to diffuse to an interface where the chemical potential energy drop is large enough to separate the charges. [51,54] Once the charges dissociate, they have to be transported to the respective electrodes before they recombine. Polymer semiconductors typically possess low charge mobilities and exciton diffusion lengths (3 – 10 nm), giving rise to the low efficiencies generally encountered. [51,54,55] Nanostructuring the photoactive layer of OPV is thus fundamental to overcome the difficulties encountered with polymeric materials. Grain sizes of less than twice the exciton diffusion length are required, while the overall thickness of the device must not be so large as to cause charge recombination before reaching the electrodes. [51,54] It has been suggested that the charge mobilities of the polymers should be between 10-4 and 10-2 cm2/Vs for a total thickness ranging from 200 to 500 nm. [51] Some of the most common semiconductor polymers employed for OPV are the electron donors regioregular poly(3-hexylthiophene) (RR-P3HT) and poly(2-methoxy-5-(3’,7’dimethyloctyloxy)-1,4-phenylene-vinylene) (MDMO-PPV) and the fullerene derivative electron acceptor (6,6)-phenyl-C61-butyric acid methyl ester (PCBM). There have been various attempts to improve the efficiency of solar cells by adding CNT in the photoactive layer of OPV. In 2003, Kymakis et al. assembled an OPV using poly(3-octylthiophene) (P3OT) and SWNT as the active layer. [56] They prepared the cells by drop casting or spin coating a SWNT/P3OT/chloroform solution on an indium tin oxide (ITO) electrode and then coating with aluminum. The results demonstrated that the SWNT behaved as an electron acceptor material and yielded a power efficiency of 0.06 % with 1 mass % of SWNT in the active layer. Nogueira et al. (2007) prepared SWNT covalently modified with thiophene groups, and used them as an electron acceptor material to build a P3OT/SWNT solar cell. [57] The presence of thiophene groups on the SWNT improved the dispersibility in the P3OT matrix, and increased the efficiency to 0.184 %. A similar attempt to use modified SWNT in a P3OT matrix was undertaken by Bhattacharyya et al. (2004); they used the dye N-(1pyrenyl)maleimide (PM), which interacts with the nanotubes via –stacking interactions. [58] The authors suggested that the dye participated in electron excitation and improved electron transfer to the SWNT and hole transfer to the P3OT. The efficiency of the devices increased from 7.5x10-4 % without dye to 0.036 % with the dye. In 2007, Berson et al. introduced CNT in a mixture of P3HT and PCBM. [59] They used SWNT produced by the HiPCO process with 85 % and 95 % purities as well as high purity MWNT. The maximum efficiency found with the 85 % pure SWNT was only 0.2 %, while with the 95 % pure SWNT, the maximum efficiency was 1.3 %. Using MWNT gave a slightly higher efficiency of 2.0 %. Saddhu et al. (2011) prepared thiophene functionalized MWNT and introduced them in the bulk heterojunction of P3HT/PCBM solar cells. [60] A maximum efficiency of 2.5 % with a MWNT concentration of 0.3 % was obtained; this compared to an efficiency of 2.3 % when only P3HT and PCBM were used. In the same year,

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Derbal-Habak et al. employed esterified SWNT to improve the efficiency of P3HT/PCBM OPV. [61] An efficiency increase from 1.89 % to 2.85 % was observed; the authors attributed this to partial crystallization of the P3HT. In order to further improve the crystallinity of the photoactive layer, the films were thermally annealed at 110 C. Interestingly, the efficiency of the cells without SWNT increased to 4.0 %, while the cells containing nanotubes had a maximum efficiency of 3.7 %. After several unsuccessful attempts to induce a significant improvement in OPV by introduction of CNT, Alley et al. (2012) reported a study where SWNT were decorated with C60 molecules linked by amination. [62] The modified nanotubes were blended with a photoactive layer of P3HT/PCBM to form solar cells. Characterization of the OPV resulted in reduced efficiencies upon addition of the SWNT. The authors discussed that SWNT, which often have lengths larger than 1 m, can short the solar cells that are typically 100 to 200 nm thick, as represented in Figure 4. They reduced the length of the nanotubes to ~ 60 nm by fluorination methods but the obtained efficiencies were still low. The authors interpreted their results and various previous studies by stating that successful incorporation of carbon nanotubes in OPV could only be successful if a large-scale source of pure semiconducting nanotubes was available.

Figure 4. Diagram of an OPV on the right. On the left, a schematic of carbon nanotubes introduced into the active layer showing a) long CNT shorting the device, b) high concentration of short CNT and c) low concentration of short CNT. Reprinted from Synthetic Metals, 162, N.J. Alley et al., Effect of carbon nanotube-fullerene hybrid additive on P3HT:PCBM bulk-heterojunction organic photovoltaics, 95-101, Copyright 2012, with permission from Elsevier. [62]

1.1.3. Transparent Electrodes Solar cells require the use of conductive transparent electrodes that allow the light to reach the active layer, but at the same time collect the charges generated and transport them to the outer circuit. The most common materials used are transparent and conductive oxides (TCOs). Indium tin oxide (ITO) is the most prominent due to its low sheet resistivity of 18 /sq, and high transmittance of 95 % at 550 nm. [12,41] The unit /sq is used for sheet resistivity to avoid confusion with bulk resistivity; it is obtained by dividing the resistivity by the dimensionless quantity of width/length. However, indium is an expensive element that has had considerable price fluctuations over recent years. For this reason, ITO is not an ideal candidate to meet the increasing demand for photovoltaic devices. [12,41,63] Another common material used for transparent electrodes is poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS). [12,41] Ultrathin films of predominantly metallic carbon nanotubes with thicknesses ranging from 30 to 150 nm are another potential

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alternative to ITO. [64-66] In 2004, Wu et al. demonstrated the ability to form CNT films with sheet resistance and transmittance comparable to ITO for visible light, and superior transmittance for infrared (IR) wavelengths between 2 and 5 m. [66] The simple fabrication procedure, consisted of vacuum filtration of a dilute SWNT suspension followed by washing of the surfactant and dissolution of the filtration membrane. The films were used to construct an optical field-effect transistor, and opened the opportunity for other applications, including photovoltaics. Rowell et al. (2006) followed a similar vacuum filtration method to fabricate organic solar cells on a flexible polyethylene terephthalate (PET) support using SWNT as the anode to replace ITO. [65] The efficiency obtained was 2.5 %, which is just slightly lower than the 3.0 % efficiency measured for a control solar cell containing ITO on glass. Although less efficient, the SWNT based solar cell film provides additional benefits such as lower cost and higher flexibility. Figure 5 shows the current density (I) versus voltage (V) curves from which the efficiencies were obtained; the inset shows the significant flexibility of the SWNT based solar cells. The stability of the devices upon flexing was tested and revealed a complete retention of the efficiency after bending the SWNT based cell to a radii of curvature of ~ 5 mm, and only a 20 – 25 % loss after bending to a ~ 1 mm radii. As a comparison, a PET backed cell with ITO as the anode completely failed after bending to ~ 5 mm radii of curvature.

Figure 5. Current density versus voltage curves of solar cells using ITO and SWNT electrodes. The insets show a schematic of the solar cell design and a flexible solar cell with a SWNT electrode. Reprinted with permission from Rowell, M.W. et al. Appl. Phys. Lett. 88, 233506 (2006). Copyright 2006, American Institute of Physics. [65]

SWNT ultrathin films have also been prepared through spray coating techniques. [67] An organic solar cell was built with a 30 nm thick SWNT film as the anode resulting in the structure SWNT/PEDOT:PSS/P3HT:PCBM/Al. When compared to a reference cell with the layers ITO/PEDOT:PSS/ P3HT:PCBM/Al, the efficiency of the SWNT cell (1.5 %) was also just slightly lower than the reference cell (2.0 %). Another design was attempted substituting both ITO and PEDOT:PSS by SWNT, however the efficiency was only 0.47 %. Even though

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spray coated SWNT electrodes result in only a small loss of efficiency, the spraying method provides difficulties with agglomeration of carbon nanotubes, giving rise to rough surfaces. [68] In an attempt to improve the efficiencies of spray coated SWNT electrodes, a team from NREL employed an ultrasonic spraying method to deposit uniform films of SWNT over large area substrates to substitute for ITO in an OPV. [68] The SWNT were dispersed as individuals in water with carboxymethyl cellulose (CMC) and mild sonication and then sprayed on a 6 x 6 inch substrate. Uniform SWNT films with root mean square (rms) roughness of only 3 nm were obtained over an area of 10x10 m after spraying and washing with nitric acid to remove the CMC and p-dope the nanotubes. The efficiency of the solar cells with P3HT and PCBM as the active layer and SWNT as the electrode was 3.1 %. When ITO was used instead of SWNT, the efficiency was 3.6 %. Interestingly, the open circuit voltages, series resistances and fill factors of both devices were virtually equal; however the shunt resistance of the ITO cell was three times larger than the SWNT based photovoltaic. The low shunt resistance is an indication of device shorting due to possible intercalation of the nanotubes into the active layer. [68] Ultra-thin, self-standing nanotube sheets can be dry drawn from forests of MWNT as demonstrated by Ray Baughman’s group in 2005. [69] This method has numerous potential applications due to the ability to make mechanically strong sheets with favorable transparency and electrical properties. In one study, MWNT sheets were used to replace ITO in solar photovoltaics;[70] an efficiency of 1.1 % was obtained when the MWNT sheet was used compared to 1.3 % for the ITO based solar cell. Interestingly, the MWNT film had a sheet resistance of ~600 /sq, approximately 30 times higher than ITO. The authors suggested that an improvement in charge collection of the MWNT sheet was responsible for maintaining a similar efficiency despite the significantly larger sheet resistance. In 2007, Ulbricht et al. demonstrated that the dry drawn MWNT sheets could be applied as an additional layer in ITO based OPVs. [71] Three types of OPV were assembled. Type 1 was the reference with only ITO as the electrode, type 2 corresponded to the CNT sheet replacing ITO, and type 3 was a hybrid ITO/CNT electrode. The respective reported efficiencies were 1.30, 1.32, and 1.74 %; the short circuit current density of the hybrid electrode solar cell was twice as large as for the reference cell. These results demonstrated that the 3-D network of MWNT indeed improved the collection of charges from the photoactive layer and effectively transferred them to the ITO anode. Another example in which CNT were used as an additional layer in ITO based solar cells was given by Su et al. in 2010. [72] For this design, the SWNT film was grown from a silicon oxide substrate and transferred to ITO. To transfer the film, the SiO2 substrate with the grown SWNT film was dipped into a 6:1 buffer oxide solution. After immersion, the SWNT were peeled off and allowed to float on the solution as a free-standing film. The film was then transferred to deionized water, and picked up with the ITO substrate. An efficiency of 3.61 % was obtained compared to 3.41 % when only ITO was used as the anode. The authors also attributed the increase in efficiency to an improved charge collection caused by the SWNT film. [72] The examples discussed so far focus on transparent nanotube electrodes for OPV; however, CNT can also be used to substitute the expensive platinum counter electrode in dye sensitized solar cells (DSSC). [13,73,74] Zhang et al. (2011) used SWNT films to develop a

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fiber shaped DSSC with an efficiency of 1.65 %, which was lower than the 2.79 % obtained with the platinum electrode. [74] When a silver wire was added to the CNT electrode, the efficiency improved to 2.60 %. The dry spun MWNT sheets from Baughman’s group [69] were also used to replace platinum in DSSC, but in the typical 2-D conformation (as opposed to fiber shaped DSSC). [73] When the MWNT were used instead of platinum an efficiency of 6.6 % was obtained, compared to 8.8 % for the reference cell. However, the addition of graphene to the MWNT sheet increased the efficiency of the cell to 7.5 %. The presence of graphene was important for the reduction of I3- at the counter electrode. Dong et al. (2011) used vertically aligned SWNT as the counter electrode of flat DSSC. [13] The SWNT were grown directly on Si/Fe/Al2O3 substrates; the interface between the SWNT and the substrate was then etched enabling transfer of the CNT film by a simple contact printing process. The efficiency using vertically aligned SWNT was 5.5 %, virtually the same as the reference cell with platinum as the counter electrode (5.6 %). The recent research suggest that carbon nanotubes might, in the future, be a feasible alternative to the expensive and difficult to process ITO and platinum electrodes. The opportunities for carbon nanotubes will expand as the demand for flexible photovoltaics increases and the cost of carbon nanotubes decreases. Moreover, additional research in the coming years is expected to further improve the efficiencies of OPV and DSSCs that incorporate transparent conductive CNT films as electrodes.

1.2. Fuel Cells Fuel cells convert chemical energy from a fuel to electrical energy through an oxidation reaction at the anode and a reduction reaction at the cathode. The high electrical conductivity and surface area of carbon nanotubes make them attractive electrode materials for both traditional chemical fuel cells and biofuel cells. For example, the incorporation of CNT into proton exchange membrane (PEM) fuel cells could reduce the amount of platinum (or other precious metal) catalyst required, thereby reducing the overall cost. [75] The CNT can be incorporated into the cells’ conducting surface by several methods including direct growth or by casting a film from a dispersion containing a polymer binder or electrophoretic deposition. An example of incorporating carbon nanotubes into a PEM fuel cell is shown in Figure 6. In this work, the electrophoretic deposition of SWNT onto carbon paper increased the electrochemically accessible surface area per gram of platinum by 10 %. [75] Additional increases may be possible by optimizing the microstructure of the deposited SWNT or using SWNT significantly enriched with metallic chiralities that were not available at the time of research. The influence of microstructure is illustrated by Jyothirmayee Aravind et al. (2011) who used graphene-MWNT composites for catalyst supports. [76] They found that incorporating MWNT into the structure nearly doubled the power density achieved by their graphene design alone. They hypothesized that this result was due to bridging defects in the electron transfer structure and increasing the basal spacing between graphene sheets. [76]

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Figure 6. Schematic of SWNT/Pt Films for use in a PEM fuel cell. Reprinted with permission from Girishkumar, G. et. al. Langmuir. 2005, 21 8487-8494. Copyright 2005 American Chemical Society.

In the case of direct methanol fuel cells, broader commercialization has been limited by not only the cost of platinum but also by the reactions’ kinetic limitations. Yuan and Xin (2012) explored the effectiveness of carbon nanotubes for reducing these limitations. Instead of using a carbon nanotube electrode, they explored the effects of doping a commercial carbon black supported PtRu electrocatalyst ink with single-walled carbon nanotubes. [77] The authors concluded that SWNT induced significant microstructural changes to the ink. The catalyst activity increased by five times at a low SWNT loading of 1:150 SWNT:PtRu/CB by mass. However, increasing the amount of carbon nanotubes to 1:100 induced tube aggregation; this resulted in a lower effective surface area and lower activity. [77] Venkateswara Rao et al. (2012) investigated nitrogen doped vertically aligned carbon nanotubes coated on a rotating disk electrode in an anion-exchange membrane fuel cell (alkaline fuel cell). The use of an alumina membrane as the substrate enabled synthesis of the nanotubes without the addition of a separate catalyst material, thereby reducing impurities in the final system. These electrodes showed good stability and a higher tolerance to ethanol than a commercial platinum carbon electrode. However, the maximum power density was 37 mW/cm2 for the CNT-N electrode compared to 62 mW/cm2 for the platinum carbon material under anionic fuel cell conditions. [78] Much of the recent interest in carbon nanotube fuel cells has focused on biofuel cells, fuel cells that use biochemical pathways to produce electrical current. Galvani’s 18th century experiments on electrically induced motion of frogs’ legs demonstrated that electrical action can generate biological reactions. [79,80] Since that time, understanding of how biological processes generate power has continually evolved. Research into using carbon nanotubes to enhance biofuel cell performance has been motivated by their electrical conductivity, high specific surface area, the ability to achieve a range of 2- and 3- dimensional morphologies, and the ability to achieve either direct or mediated electron transfer with enzymes. Enzymatic biofuel cells are generally being explored for small, low power consumption devices including implantable devices such as pacemakers and insulin pumps. Enzymatic

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biofuel cells mimic the natural Krebs’ cycle in which a cascade of enzyme catalyzed oxidation-reduction reactions produces a flux of electrons from the oxidation of sugars and alcohols. The majority of enzymes in the Krebs cycle are dependent on the oxidation of nicotinamide adenine dinucleotide (NADH) to NAD+. For example, Figure 7 shows this pathway for a glucose/O2 enzymatic fuel cell. [81] Limitations of enzymatic biofuel cells have included slow reactions kinetics for NADH oxidation, large overpotentials, and corrosion of bare electrodes. Electrocataylsts such as polymethylene blue and polymethylene green can be incorporated into the anode to facilitate the reaction. Several studies have shown that carbon nanotubes can also improve biofuel cell performance. However, the specific results have been dependent on the types of carbon nanotubes and electrocatalyst used, the method of nanotube incorporation, the electrode design, and even the method of electrocatalyst incorporation.

Figure 7. Schematic of a glucose/O2 enzymatic biofuel cells using the bioelectrocatalysts polymethylene blue and laccase. [81] Reprinted with permission from Yan Y et al., Advanced Materials. 2006, 18, 2639-2643.

For example, Narváez Villarrubia et al. (2011) investigated the effects of adding methylene green before or after fabrication of the electrode. [82] In the first case, SWNT bundles dispersed in isopropanol (IPA) were filtered to form solid papers known as buckypapers. Polymethylene green was then deposited on the surface using electrodeposition, resulting in a material with a resistivity of 14.7 mΩ/mg. In the second case, methylene green was added to the SWNT/IPA dispersion and polymerized after the dry bucky-paper was formed. This resulted in a resistivity of 47.2 mΩ/mg; the higher value was likely due to incorporation of PMG throughout the structure instead of just on surfaces accessible after drying. In both cases, the papers were incorporated into a cavity electrode using Teflon-ized carbon powder (XC50) as the adherent material. The performance of the resulting two types of electrodes was characterized using oxidation and regeneration of NADH and for L-malate oxidation by malate dehydrogenase (MDH). NADH/NAD+ is the cofactor needed for oxidation of L-malate by MDH in the Krebs cycle. Both electrodes were found to have high

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surface area and porosity which facilitated mass transfer. Hydrodynamic polarization curves showed that incorporation of methylene green directly into the dispersion increased the current at a given potential and NADH concentration. For example, at 0.2 V and 10 mM NADH, the paper with PMG throughout the electrode had a current of 95 µA compared to 25 µA for the electrode where PMG was deposited after the solid SWNT paper was formed. This result suggests that further optimizing the morphology and contact with the electrocatalyst may enable improvements in electrode performance. Microbial fuel cells use whole microbes such as those found in domestic wastewater and biomass waste. It has been estimated that using microbial fuel cells in conjunction with wastewater treatment for a town with a population of 150,000 could generate 0.5 to 2.3 MW of power. [79] Although they have the potential for greater power generation, microbial biofuel cells face even more challenges than those using enzymes. Specifically, fuel cell functionality requires a habitat of living microbes whose transport is uninhibited. One approach for making electrodes for microbial fuel cells is to use vertically aligned nanotube forests. For example, Mink et al. (2012) used an oxidized forest of MWNT; oxidation was performed to not only remove impurities but also to improve cell adhesion. [83] The anode was tested using mixed bacterial culture and a solution. The fuel cell was inoculated with the mixed bacteria. After a stable biofilm was achieved, an acetate nutrient medium was used. Significant colonization was seen in the MWNT anode and at a maximum power density of 19.6 mW/m2 (396 mW/m3) the current was 197 mA/m2 (3947 mA/m3). [83] In contrast, Xie et al. (2012) drew inspiration from natural aquatic sponges; they dipped pieces of industrial polyurethane sponges in aqueous SWNT-sodium dodecyl sulfate dispersions and allowed them to dry. [84] This created an electroactive coating throughout the interior of the sponges’ 3-dimensional pore network. This met the goal of having a high surface area 3-dimensional network of large diameter (> 10 microns) pores that provided a habitat for the microbes and would not easily be clogged. A 200 nm thick coating resulted in a conductance of 1 S/cm, and the surface area was estimated as 104 m2/m3. In 1 g/l glucose media the sponge achieved a current density of 2.13 mA/cm2 (10.63 mA/cm3), which was significantly higher than had been achieved by textile based CNT electrodes. [85] With filtered domestic wastewater, the CNT sponge achieved a current density of 0.52 mA/cm2 (2.58 mA/cm3); the lower value with wastewater was expected based on previous research. Microbial fuel cells using the CNT sponge electrode had a maximum areal power density of 1.24 W/m2 and a maximum volumetric power density of 182 W/m3. [84] Although the authors did not comment on it, further improvements may be achievable through using metallic chirality enriched carbon nanotubes, and by increasing the conductivity and effectiveness of the material used to stabilize the initial SWNT ink.

1.3. Thermocells Several industrial processes produce large amounts of low-grade waste heat which is typically lost to the environment in cooling towers or in waste streams. Thermocells can convert this thermal energy, typically at a temperature below 100C, to electricity based on the Seebeck effect. Other applications include incorporating thermocells into vehicles to recover the heat energy lost; or in solar cells, where a lot of the sun’s energy is also lost in the form of heat. [86] Unfortunately, the efficiencies are very low and the cost of the electrode

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materials is high. [87,88,89] Thermocells have typically been developed based on p-type and n-type semiconductors connected by metal junctions; an electric current is generated when the two metal junctions are maintained at different temperatures. The Seebeck effect results from two dissimilar electrodes being maintained at different temperatures and inducing differences in the electrochemical potential of a redox reaction. [88-90] These cells have the potential advantages of simple designs, low expected maintenance, low cost, and flexible devices that could be wrapped around small pipes. [89] Thermo-electrochemical cells have been demonstrated using MWNT electrodes in contact with an aqueous Fe(CN)63-/Fe(CN)64- electrolyte. [88] The electrodes were fabricated from either bucky papers or vertical forests of MWNT directly grown on the surface of stainless steel electrodes. Various different configurations were used for the bucky paper thermocells including coin cells, MWNT scroll electrodes, flow electrolyte cells, and cells wrapped around pipes. Efficiencies as high as 1.4 % of the Carnot efficiency were obtained; this was three times higher than previous thermo-electrochemical cells. The coin configuration was tested with a vertical forest of MWNT, which resulted in a 30 % higher efficiency than the bucky paper electrodes. [88] The efficiencies of the thermocells depend on various parameters including the type of nanomaterial, cell orientation, electrode size, electrode spacing, electrolyte concentration, temperature, etc. [89] Kang et al. (2012) performed a detailed study comparing the effect of those parameters. They found differences for the nanomaterials studied: purified SWNT (PSWNT), unpurified SWNT, purified MWNT (P-MWNT), reduced graphene oxide (RGO) and P-SWNT/RGO composite. The highest specific power (~6.8 W/kg) was obtained for PSWNT, followed by P-MWNT (~6.1 W/kg), P-SWNT/RGO composite (~5.3 W/kg), unpurified SWNT (~5.2 W/kg) and finally RGO (3.9 W/kg). [89] The low specific power of RGO was attributed to degradation of the graphene structure during oxidation and inefficient recovery of the reduced state after reduction.

2. TRANSMISSION Efficient transport of electrical energy is important for energy savings at all length scales including within devices, within equipment and over long distances for the power grid. Cables for electrical transmission typically have several form factors, each with one or more conducting and insulating elements. [91] In direct current (DC), and less than 1 kHz alternating current (AC) power distribution, the conducting wire is clad with insulation to prevent corrosion and shorting. In power cords and other applications where the cables need to withstand flexure, they are constructed in a multistrand configuration while permanent installations with limited flexure typically use solid conductors. At frequencies between 1 kHz and 1 GHz, wires are usually arranged in twisted pairs to reduce radiative transmission losses. Above 1 GHz, coaxial configurations are used. The most common material used as a conductor in cables is copper. However, other materials including aluminum, steel, silver, nickel, and even silver plated nylon composites are used in some applications. [91] Carbon nanotubes are an attractive alternative to currently used conductors for several reasons. In general, carbon nanotubes are of interest for electrical power transmission due to their high electrical conductivity, high current density, potential quantum conduction, high

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aspect ratio, corrosion resistance, and ability to be spun into long flexible cables. In addition, a light-weight conductor may have additional advantages with regard to weight savings in aerospace and other applications. Richard Smalley, 1996 Chemistry Nobel Laureate, devoted the last few years of his life communicating the importance of solving the energy challenge. He envisioned large scale electrical power transmission using carbon nanotube quantum wire cables that would carry 10 times the current as copper wire, weigh 1/6th as much, and not undergo significant energy loss over long distances. Currently, electrical transmission and distribution losses approximate 6 %, but vary significantly with transport distance. Some energy concepts such as transmission from solar energy farms require transportation over long distances to large population centers. For example, the DESERTEC project plans to construct solar power plants in the Sahara and transport energy back to Europe. [92] Significant energy savings could therefore be achieved by the development of an electrical cable that could carry the significant load and eliminate distribution losses. Many researchers continue to dedicate themselves to the achievement of Richard Smalley’s bold vision. [93] The ultimate realization of this vision requires the ability to produce long cables consisting solely of armchair quantum wires. [94-96] The idea of the quantum wire is based on the fact that armchair SWNTs are one dimensional ballistic conductors with a conductivity of approximately 108 S/m. Several researchers have demonstrated ballistic or quasi-ballistic 1D electrical transport for armchair single walled carbon nanotubes with a mean free path of electron scattering that exceeded a few micrometers. For example, Maruyama et al. (2011) deposited SWNT fibers on a tungsten tip using dielectrophoresis and characterized the electrical properties using a vertical conductance measurement approach where the fiber was immersed in a mercury electrode. The experimental results could be fit to a model based on the metallic SWNT in the bundle acting as the dominant conductors. The results showed quasi-ballistic conductance with a mean free path on the order of microns at room temperature. [97] This compares to a typical room temperature mean free path of 40 nm for copper wire. [7] While researchers are progressively growing longer nanotubes, the longest SWNT synthesized to date is less than 20 cm in length. [8,32] While the ultimate length limit is not yet known, long CNT wires will have to be formed from continuous bulk materials comprised of numerous CNT. In bulk CNT materials such as films, ribbons, fibers, or cables, the electrical conductivity is dependent on both the intrinsic properties of the nanotubes and the properties of the bulk network. Intrinsic properties include nanotube type, chirality distribution, presence of defects, and presence of impurities/dopants. For example, the barrier to electrical transport between different chiralities of SWNT can have a significant effect. The network properties depend on microstructural alignment and defects which can be influenced by the intrinsic nanotube length and diameter distribution as well as processing conditions. [91,97] To date, conductivity results for carbon nanotube fibers have spanned several orders of magnitude from 10-4 to 10-7 Ωm, depending on the nanotube type, nanotube purity, presence of impurities or dopants, and spinning methodology. [17] Therefore, significant advances toward carbon nanotube electrical transmission wires/cables hinge on ongoing advancements in SWNT synthesis, purification, chirality selectivity (via synthesis or separation), and cable production technology through fiber spinning, mechanical drawing of nanotube forests, or direct harvesting from synthesis reactors. [17,101-103] One particularly interesting work is Zhao et al.’s (2011) achievement of a 1.5 x 10-7 Ωm resistivity in cables of aligned iodine doped double-walled carbon nanotubes (DWNT). [17]

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This conductivity compares to 1.7 x 10-8 Ω m for oxygen free copper and 2.8 x 10-8 Ω m for oxygen free aluminum at 20 °C. However, given that the average DWNT cable density was 0.33 g/cm3, the specific conductivity was 1.96 S m2/kg which is higher than that of copper or aluminum. The high specific conductivity could be significant for applications where mass reduction is important. For example, there are over 130 miles (210 kilometers) of electrical cable on a Boeing 747-8 freighter. [98] Based on a density of 8.9 g/cm3 and a diameter on the order of 1 mm, this equates to a mass of over 1000 kg (2000 lbs.). While this is less than 1 % of the maximum takeoff mass, a 1 tonne reduction would allow for additional fuel savings or the ability to carry additional passengers or cargo. In addition, the current carrying capacity, or amount of current that can be transported through a unit of cross sectional area without failure, of the iodine doped DWNT cables was also significant. It was on the order of 104 to 105 A/cm2 compared to 106 A/cm2 for nanoscale copper. [17] While numerous challenges remain to replacing large portions of the power grid with carbon nanotubes, other applications may be realized in the near term. Lab scale demonstrations of carbon nanotubes in coaxial, USB, and Ethernet cables show they are viable replacements for metallic conductors. [91] The flexural tolerance of carbon nanotube cables is particularly noteworthy for applications that require spooling and unspooling. In one study, the CNT wires showed no change in resistance after 200,000 bend cycles while the resistivity of a commercial aerospace wire increased rapidly after 8,000 cycles. [91] For other applications, corrosion resistance is an important consideration. Although insulation is meant to prevent corrosion, unintended exposure and oxidation at contact points can result in cable deterioration or failure. Immersion of a non-insulated CNT wire and a 16 gauge stranded copper wire in 1 M hydrochloric acid showed that the CNT wire had a stable resistivity for over 80 days, while the copper wire underwent dramatic deterioration within eight days. [91] Therefore, while significant progress is still needed before the conductivity of bulk carbon nanotube cables is competitive with copper, other attributes of CNT cables may result in nearer term commercializations.

3. STORAGE Many renewable energies such as solar and wind are intermittent. For example the peak sun hours during summer time generally do not coincide with peak consumption hours;[12] during winter time, the availability of the sun is even smaller. Thus, it is critical to develop technologies that can efficiently store energy from renewable sources. Furthermore, the ongoing development of light weight devices with high energy density and power will be required for ongoing advances in hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV). Additionally, mechanical flexibility without loss of properties is desired for portable electronic devices. Micron thick CNT films possess large electrical conductivity and surface area, excellent mechanical performance, are light-weight and can be processed as flexible films; all of these properties make them a good candidate for developing light, strong, flexible, and efficient batteries and capacitors. [41] Several types of energy storage employing carbon nanotubes have been, or are being, explored. For example, storage of mechanical energy by ultralong and strong CNT has been suggested. [21] Research into carbon nanotubes’ potential for hydrogen storage began in 1997

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but has been controversial. [18,99,100] Hydrogen is considered a clean and sustainable alternative to fossil fuels; however it faces several limitations. [19] A key concern is safely storing large amounts of hydrogen that can be easily released on demand in a controlled manner. [19,101,102] In addition, controversy exists regarding the efficiency of carbon nanotubes to store hydrogen, and the reported capacities have rarely exceeded ~2 mass %. [18,99] During the last decade, improvements in carbon nanotube purity and characterization techniques have resulted in a general consensus that some results were due to catalyst impurities and carbon nanotubes were unlikely to ever meet US Department of Energy (DOE) requirements for ~6 mass % on board storage. Research has generally shifted to other types of high surface area materials such as metal organic frameworks. [7,18] However, a recent study demonstrated the formation of stable C-H bonds in platinum-sputtered SWNT by a hydrogen spillover mechanism; even though the storage capacity was low (1.2 mass %), the authors suggested that improvements are possible by optimizing the diffusion kinetics of atomic hydrogen from platinum to SWNT. [99] In addition, it has been suggested that carbon nanotubes can still be used to enhance the absorption and release kinetics of other hydrogen storage materials. [18] Therefore, while the early promise of carbon nanotubes for hydrogen storage will likely go unfulfilled, they may yet contribute to hydrogen storage technologies. The most promising energy storage applications for carbon nanotubes are supercapacitors and batteries. In electrochemical capacitors (EC), also known as supercapacitors, energy is stored as electric charges on the surface of the electrodes by either an electric double layer or pseudocapacitance induced by fast reversible redox or Faradaic reactions at the surface. [12,15,103] Since only the surface stores energy, the densities obtained (~ 5 Wh/kgcell) are smaller than in batteries. Since the chemical processes involved during charge and discharge are much faster, ECs have higher power (~ 10 kW/kgcell). [12,15,103] Also, given that the energy storage mechanism is limited to the surface, the structural framework of the ECs is not significantly affected by consecutive charge/discharge cycles giving rise to longer life times than batteries. [15,103] In lithium ion batteries, chemical energy is stored in the bulk of active materials. Lithium (Li) metal oxides (e.g. LiCoO2, LiNiO2, or LiMnO2) or Li phosphate compounds such as LiFePO4 are often used as positive electrodes; graphite is commonly used as the negative electrode. This storage mechanism results in large gravimetric energy densities (~150 Wh/kgcell), but also in slow power delivery (~1 kW/kgcell); the Li ions must diffuse slowly through the bulk of the materials. [12,15,103] Batteries also have a relatively low life time, since the constant diffusion of the large Li ions during consecutive charge/discharge cycles negatively modifies the structure of the active materials. Nanostructuring electrodes has advantages for both batteries and supercapacitors. A large surface area increases the energy storage capability of supercapacitors. In the case of batteries, nanostructured electrodes reduce the lithium diffusion length. This increases the specific power density and enables maintaining a stable network for a longer time. [12,15] Moreover, the addition of transition metal oxides, like ruthenium and manganese oxides, and conductive polymers, like polypyrrole (PPy) and polyaniline (PANi), can improve the energy storage capacity by inducing pseudocapacitance. The following sections review literature focusing on the use of carbon nanotubes in batteries and supercapacitors.

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3.1. Supercapacitors Various methods have been used to fabricate CNT electrodes, one of which consists of coating a prefabricated electrode with uniformly dispersed nanotubes. This method was used by Yan et al. to coat carbon paste electrodes (CPE) with either pristine MWNT or MWNT that had been decorated with ruthenium oxide nanoparticles. [104] Suspensions of the modified and unmodified MWNT were dripped on the CPE and allowed to dry at room temperature. When MWNT decorated with 40 % RuO2 particles were used a large energy density was obtained. The capacitance was approximately 400 F/g at 200 mV/s, compared to ~7 and 60 F/g for plain CPE and unmodified MWNT electrodes, respectively. The large energy storage capacity was attributed to the enhanced internal and external surface area of the MWNT framework, which allows storing more charges, as well as pseudocapacitance involving the RuO2 nanoparticles. A similar method was used by Lin and Xu to deposit PPy-coated MWNT on glassy carbon electrodes (GCE) from an ionic liquid suspension. [105] Using the ionic liquid enabled a relatively good dispersion of MWNT. The GCE was first coated with PPy only, which resulted in a capacitance of 207 F/g measured at 100 mV/s in 1 M KCl. This value was attributed mostly to induced pseudocapacitance from the PPy. When the GCE was coated with both MWNT and PPy, a specific capacitance of 790 F/g was obtained at the same conditions. After 1000 cycles, only 9 % of the capacitance was lost. Other methods involve dispersing CNT and PPy in aqueous media followed by filtration of the suspensions and preparation of the electrodes from the solid residue. In order to obtain better dispersions of MWNT in aqueous media, Fu et al. (2011) functionalized MWNT by carbonizing a glucose/toluenesulfonic mixture in the presence of the nanotubes. [106] The non-covalently bound sulfate groups arising from modification significantly improved the dispersibility of the MWNT in aqueous suspension, improving the coverage of PPy on the MWNT. The electrodes were prepared by grinding and pressing the filtered MWNT/PPy specimens with carbon black and polytetrafluoroethylene. The capacitors prepared from the sulfate modified MWNT and PPy showed a 58 % larger capacitance (135 F/g) than those prepared with non-modified MWNT and PPy; the measurements used a current density of 0.25 A/g. Moreover, a reduction in capacitance of only 3 % was measured after 1000 cycles. Lu et al. (2012) prepared CNT/PPy/graphene flexible electrodes by filtering the composite from an aqueous suspension and recovering the bucky paper film from the filter. [107] The capacitance of the films containing 52 mass% CNT/PPy was 211 F/g at 0.2 A/g, compared to 64 F/g for plain graphene sheets. The flexible films maintained 95 % of the capacitance after 5000 cycles. The Baughman group at the University of Texas at Dallas (UTD) has shown numerous applications for MWNT aerogel sheets drawn from forests produced by chemical vapor deposition. [69] The ability to continuously produce the sheets, process them into different forms, and incorporate different materials gives this approach excellent flexibility. In one work, they showed that vapor phase polymerization could be used to coat the densified membranes with PEDOT. Both the composite membranes and yarns produced from the membranes showed excellent mechanical properties in addition to the electrochemical properties needed for supercapacitor applications. Figure 8 shows the fabrication process and resulting structures. [14] The process started with stacks of carbon nanotube aerogel sheets which had been densified by contact with, and evaporation of, an oxidant containing a butanol solution. Subsequent vapor phase polymerization enabled composite nanomembranes. An

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82 % oxidant solution enabled an 85 % PEDOT nanomembrane with thickness of 66 nm in one layer, and a sheet resistance of 312 Ω/sq. The volumetric capacitances of the composite nanomembrane were 123 and 40 F/cm3 when going from 0.01 to 100 V/s, and the time constant was 16 ms; the latter was measured through electrochemical impedance spectroscopy. The corresponding power and energy densities were calculated to be ~7910 W/cm3 (~4391kW/kg) and ~70mWh/cm3 (~37 Wh/kg) respectively. In addition to this electrochemical behavior, the membranes also had other interesting physical properties. The optical transmittance was 56 % at 550 nm, which is higher than that for ITO but comparable to graphene films. The membranes’ mechanical properties were a specific tensile strength of 51 ± 3 MPa/(g/cm3) and a specific modulus of 12.6 ± 5 GPa/(g/cm3) These membranes could be twisted into yarns with or without the incorporation of guest materials such as carbon black. The yarns showed less capacitance than the membranes; measured without a metal current collector, the yarns had power and energy densities of ~538 W/cm3 and ~47 mWh/cm3, respectively. Incorporation of carbon black into the yarn improved the power and energy density to 836 W/cm3 and ~55 mWh/cm3.

Figure 8. Hybrid nanomembranes prepared by vapor phase polymerization. a) Schematic of a PEDOT coated CNT sheet. b) SEM image of a hybrid membrane. c) AFM image and height profile of a hybrid nanomembrane on a Si wafer, showing an average thickness of 66 nm. d) Photo of a free standing, optically transparent hybrid nanomembrane in ethanol. e) Photo of a helically twisted hybrid nanomembrane backed on PET. Reprinted with permission from Lee, J.A. et al. ACS Nano. 2012, 6, 327–334. Copyright 2012 American Chemical Society. [14]

Researchers at the Massachusetts Institute of Technology (MIT) have done significant work on using layer-by-layer assembly (LBL) to fabricate MWNT electrodes. [15,16,108111] In one study by Lee et al. (2009), two suspensions of MWNT functionalized with negative carboxylate (MWNT-COOH) and positive amine (MWNT-NH2) groups were

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employed to assemble the layers as shown in Figure 9. [110] The MWNT were deposited by repeated, alternate substrate dipping in both suspensions followed by washing steps between each immersion. The prepared films were flexible and uniform, with a thickness and morphology that can be controlled by changing the pH of the suspensions and the number of deposited bilayers. All-MWNT electrodes prepared by this method showed capacitances of 159 F/g (132 F/cm3) at 50 mV/s in 1.0 M H2SO4. In a later work, Lee et al. (2010) reported the redox deposition of MnO2 nanoparticles on the MWNT films previously prepared by LBL assembly. [111] The MnO2 nanoparticles were used to provide pseudocapacitance, while the carbon nanotubes provided a high surface area network with a large electrical conductivity. The nanoparticles uniformly coated the MWNT inside the films, yielding electrodes with a specific capacitance of 290 F/g (246 F/cm3) at a scan rate of 10 mV/s (~ 1.8 A/cm3) in neutral K2SO4. A good cycling stability was observed with ~11.6 % loss after 1000 cycles. In 2011, Byon et al. incorporated chemically reduced graphene (CRG) sheets and MWNT-NH2 in an all-carbon LBL electrode.108 The LBL assembly was performed by alternatively dipping a substrate in suspensions of MWNT-NH2 and graphene oxide (GO). After assembly, the GO sheets were reduced with hydrazine vapors, which induced crosslinking of the graphene sheets and the MWNT-NH2 through amide bond formation. The resulting electrode (MWNs/CRG) films showed a high capacitance of 157 F/g in 0.5 M H2SO4 at a 50 mV/s sweep rate, compared to 63 F/g for the MWNT/GO films. The authors suggested that pseudocapacitance was provided by redox reactions between the protons and oxygen containing groups at the surface; this was supported by a 50 % lower capacitance in 1.0 M KCl. When the neutral electrolyte was used, the capacitance only arose from an electric double layer.

Figure 9. Schematic of layer-by-layer assembled MWNT-COOH and MWNT-NH2. Reprinted with permission from Lee, S.W. et al. J. Am. Chem. Soc. 2009, 131, 671–679. Copyright 2009 American Chemical Society. [110]

3.2. Batteries Lahiri et al. (2010, 2011) have demonstrated MWNT forests directly grown on a copper current collector as a new anode structure for lithium ion batteries. [112,113] During lithiation, the Li+ ions pass through the forest, reach individual MWNT, and attach to their

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sidewalls. During delithiation, the majority of ions return to the opposite electrode. This ionic movement is countered by the movement of electrons through the circuit. Efficient electron transport between the CNT and the current collectors is therefore critical. As in other studies using carbon nanotubes, the high specific surface area of the CNT enabled better Li-ion transport and battery performance was enhanced by the CNT electrical conductivity. In addition, the authors highlighted several advantages to their approach. First, the fabrication method was a simple two-step process consisting of catalyst deposition through sputtering followed by CNT growth using chemical vapor deposition. Second, their approach avoided polymeric binders, thereby enabling higher temperature applications and increasing the battery’s specific capacity. Third, unlike Si and SnO2, CNT are not believed to have expansion and contraction issues that can cause structural degradation during long cycles. Finally, the interface-enabled Ohmic contact and strong bonding between the CNT and the substrates could be controlled during manufacturing. The electrodes showed an irreversible gravimetric charge capacity loss of 42 % from 2500 mAhg-1 between the first and second cycles but stable performance after that. At a high charge/discharge rate of 1,116 mAg-1, the reversible capacity was 767 mAg-1, which is over two times greater than that of graphite. In addition, there was no capacity degradation over 50 cycles and the Coulombic efficiency was above 99 %. The carbon nanotube aerogel sheets produced by the Baughman group enable the incorporation of other materials to enhance performance while retaining structural integrity and flexibility. In one study, guest materials that cannot typically be spun into fibers were placed on carbon nanotube webs and twisted to form a fiber in a process referred to as biscrolling. [114] The twist insertion resulted in Archimedean, dual-Archimedean or Fermat scrolls, similar to the spirals commonly found in nature. For example, flexible battery cathodes were produced by incorporating LiFePO4 into a MWNT yarn. LiFePO4 is an inexpensive high performance Li battery cathode material with a theoretical storage capacity of 170 mAh/g; charge collection has been a challenge for this material due to its low electrical conductivity. However, a biscrolled yarn of 98.5 mass % LiFePO4 incorporated into a MWNT host showed an unexpectedly high gravimetric conductivity of 8 S cm2/g, indicating that the presence of LiFePO4 did not decrease the intrinsic conductivity of pure twist-spun MWNT yarn. [114] Furthermore, the yarn remained weavable in spite of the high amount of powder incorporated. Based on the total electrode weight, reversible charge storage capabilities of 115 mAh/g were obtained at a C/3 rate (the discharge rate needed to release 1/3 of theoretical storage capacity in 1 hour). Energy storage densities of 379 Wh/kg and 135 Wh/kg were calculated for power densities of 180 W/kg and 4590 W/kg, respectively. [114] Electrodes produced from the LBL method have also been demonstrated in lithium ion batteries by researchers at MIT. A battery was fabricated using the LBL all-MWNT films, prepared as in Figure 9, as the positive electrode and lithium titanium oxide as the negative electrode. [16] A gravimetric energy density of ~ 200 Wh/kgelectrode was obtained with an outstanding power density of 100 kW/kgelectrode and negligible charge capacity loss after 2500 cycles. The results correspond to 5 times more energy density compared to traditional EC, and a 10 times larger power density than lithium batteries. The mechanism for energy storage involves Faradaic reactions between the oxygen containing groups at the surface of the MWNT and the lithium; while the graphitic layers in the MWNT provide a conductive framework for electrons.

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Hyder et al. employed LBL assembly to fabricate PANi/MWNT electrodes for batteries and supercapacitors. [109] Negatively charged carboxylated MWNT were used to form the bilayers with positively charged HCl doped PANi fibers. An interconnected network with nanosized pores that provide good ionic and electrical conductivity was obtained. The electrochemical properties were tested in lithium cells with 1 M LiPF6 in a 3:7 mixture of ethylene carbonate and dimethyl carbonate. A specific capacitance of 418 F/g (238 F/cm3) was reported for the PANi/MWNT electrodes at 1 mV/s. Also, a high energy density of 386 Wh/kgelectrode (220 Wh/Lelectrode) was reported at a high power of 175 kW/kgelectrode (100 kW/Lelectrode). A Ragone plot of the LBL PANi/MWNT and all-MWNT electrodes are shown in Figure 10. Additionally, a gravimetric charge capacity loss of only 4 % was observed after 1000 cycles. The large energy density was attributed to pseudocapacitance arising from Faradaic reactions between the PF6- anions and PANi fibers, along with redox reactions between the Li+ cations and the oxygen containing groups at the surface of the MWNT.

Figure 10. Ragone plot based on electrode mass of LBL all-MWNT and PANi/MWNT electrodes. The electrode densities are 0.57 and 0.83 g/cm3. Reprinted with permission from Hyder, M.N. et al. ACS Nano. 2011, 5, 8552–8561. Copyright 2011 American Chemical Society. [109]

4. CONSUMPTION The positive impact of improvements in energy conversion, transmission, and storage will be negated if energy consumption continues to grow unchecked. A significant factor in reducing energy consumption is societal perception of the importance of reducing energy consumption. Attari et al. found in a 2010 survey that most Americans had little understanding of the comparative energy use inherent in different behaviors. [115] In particular, most of the respondents focused on achieving energy conservation through energy curtailment measures such as turning off lights and driving less instead of expert

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recommended improvements in energy efficiency. This discrepancy between consumer perception and actual energy savings was attributed to several factors. First, energy curtailment measures such as turning off lights are easy to imagine and implement. However, improving energy efficiency requires the consumer to research the different means to improve efficiency for a given activity. For example, with regard to driving, consumers typically do not know whether reducing speed from 70 mph to 60 mph, driving a car that has a purported gas mileage of 30 mpg instead of 20 mpg or simply getting a tune up twice per year would have the most impact on energy consumption. Second, in many cases improving energy efficiency may require the consumer to spend money. This may be a small investment in the case of light bulbs, but a significant investment in the case of an appliance or hybrid automobile. Attari et al. concluded from their study that increasing public understanding about energy effectiveness could result in significant gains in conservation. [115] The potential for carbon nanotubes to improve energy conservation is greatest in the transportation industries which include 1) automotive, 2) air (aircraft and aerospace), and 3) mass transportation (buses, trains, and ships). Transportation accounts for 25 % of global energy use and the majority of oil consumption. [116] Since global mobility or the distance people travel over a certain timeframe is expected to triple by 2050,[116] addressing energy conservation in the transportation industries is of particular importance. In general, approximately 75 % of vehicle fuel consumption is related to vehicle mass. [116] Other factors include engine efficiency, air/water/rolling resistance, and velocity. Carbon nanotubes’ high specific strength (strength/density) can potentially result in significant reductions in fuel usage in both the automotive and aerospace industries. A comparison between the specific strength and Young’s modulus of carbon nanotube composites and other materials is shown in Table 1. Several studies have noted, however, that the potential for energy reduction during use is not the only factor that should be considered. Instead, a total life cycle analysis should be performed that includes the energy consumption in synthesis, processing, use, and disposal. This type of analysis has significant degrees of uncertainty for new materials such as carbon nanotubes. Some studies have found that the intense energy consumption during synthesis negates savings during use. [117,118] However, the authors recognized that their analyses are subject to numerous uncertainties. First, the production scale and efficiency of carbon nanotube synthesis is still in a high rate of flux, and it is uncertain what improvements can reasonably be expected. Second, the analyses hinge on what percentage of an individual carbon nanotube’s mechanical properties can be achieved in a bulk material or a composite. This is an active area of research that requires overcoming two key challenges: dispersion and interfacial interaction. [119-122] For single-walled carbon nanotubes in particular, van der Waals attractions make it difficult to achieve dispersion of individual nanotubes. Since the diameter of a bundle is larger than that of an individual, the effective aspect ratio is decreased. Second, since mechanical reinforcement requires achieving load transfer, the nanotubes must not only be dispersed but have an interface that enables load transfer. Ongoing progress toward overcoming these challenges should significantly reduce the amount of carbon nanotubes required to achieve a given property and may make the overall life cycle analysis more favorable.

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Esteban E. Ureña-Benavides and Virginia A. Davis Table 1. Mechanical properties of materials [123] Specific Tensile Strength (MPa/(kg/m3)) 0.079 0.243 0.173 0.938 1.540 2.991 5.517

Steel Titanium Aluminum CFRPa 50 % SWNT HDPEb 60 % SWNT HDPE 70 % SWNT HDPE a b

Young's Modulus (GPa) 200 110 73 181 58 98 162

CFRP – Carbon fiber reinforced polymer, HDPE – High density polyethylene.

It should be noted that the efforts to reduce vehicle mass through materials development have been ongoing since the 1920’s when the aerospace industry started using aluminum alloys. Over the last ninety years, use of light-weight metal alloys, composites, and plastics has continued to grow. The recent commercialization of the Airbus A380 (2005) and Boeing 787 Dreamliner (2011) highlight innovative materials usage as well as the time needed for development. Both planes use significant amounts of carbon fiber reinforced polymers. The idea of using carbon fiber dates back many decades but the rigorous standards required by the aerospace industry resulted in a prolonged period of time between concept and commercialization. In the case of the Boeing 787, 80 % of the structure by volume (50 % by mass) is constructed from composite material. Energy savings resulted from not only mass reduction but also higher cabin pressurization that enabled better control of ventilation and climate control systems. [116] The use of composites is also beneficial for reducing corrosion and fatigue, improved aerodynamics, and a reduction in the number of fasteners. CNT composites have the promise of even higher performance and mass reductions. The potential for CNT in airplanes includes CNT composites and/or metal alloys for structural components, wiring, fuel cells, sensors, and lubricant fluids. In addition to mass reduction, CNT and CNT composites are also of interest for transportation due to their potential for electromagnetic interference (EMI) shielding [124] and lightning strike protection. [125] As was the case for carbon fiber, commercialization of CNT in the aerospace industry will require a high degree of confidence in both the supply chain and performance property reliability. However, studies have already highlighted the potential mass and energy savings that may someday be achieved. O’Donnell et al. (2004) compared the potential mass savings for a CNT composite using (10,10) tubes in high density polyethylene or epoxy. [124] The mechanical properties were calculated using the method of mixtures where

X comp  V f X f  1  V f  X m 

1



Xcomp, Xf, Xm are the mechanical properties of the composite, fiber, and polymer matrix, respectively. Vf is the volume fraction of the composite, and α is the stress index, α = 1 corresponds to a unidirectional composite in which the stress is aligned parallel to the fiber axis, and α = -1 corresponds to a fiber where the stress is aligned perpendicular to the fiber

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axis. The authors used α = 0.01 to get a first order approximation of a bi-directional composite with a higher modulus fiber in a lower modulus matrix. For the scenario of substituting the airframe of an E145, 747-400, 757-200, or A320 aircraft with 70 % composite, the increase in specific strength would enable a 10 to 18 % mass savings, depending on the assumptions of aircraft type, fuel load, and payload. This equates to an average fuel savings ranging from 9 % to 14 %. It should be noted that the above assumption of 70 % composite is probably not achievable by adding carbon nanotubes directly to the resin. As the nanotube fraction in a melt or uncured resin is increased, two factors retard further increases in concentration. First, the viscosity increases significantly due to their high aspect ratio. [122] Second, the nanotubes are forced into closer and closer contact as their volume fraction increases; this will eventually result in aggregation. [120] Therefore, achieving such a high mass ratio composite would likely require impregnating a high density carbon nanotube array with polymer. Successful infiltration requires that the spacing between the nanotubes be greater than the polymer radius of gyration and that there is sufficient wetting and interaction to enable complete infiltration and load transfer. Boncel et al. (2011) explored these thermodynamic and kinetic requirements using a range of fluids for impregnation of highly aligned SWNT and MWNT arrays. [126] The authors further demonstrated that even polymer melts such as polystyrene could successfully infiltrate multiple layers of SWNT and MWNT arrays enabling sandwich composites. In addition to sandwich composites, nanocomposite foams,[124,127] functionally graded beams,[128] films,[129-136] and fibers [131,133,137-139] are all being explored and may find application in the transportation industries. There is also rapidly growing interest in hierarchical materials that use carbon nanotubes in conjunction with traditional fiber reinforcements. Mechanical properties of hierarchical CNT/epoxy composites with glass and carbon primary fibers show little change in the fiber dominated in-plane properties, but significant improvement in toughness and interlaminar shear strength. [140] The hierarchical materials approach has already been successfully commercialized in a number of applications particularly sporting goods,[141] and may provide the best combination of cost and performance in many transportation applications. [140]

CONCLUSION Carbon nanotubes’ ability to form strong, light-weight, highly conductive, and even flexible materials make them highly desirable for a broad range of applications. The use of carbon nanotubes has been demonstrated in fuel cells, thermo-electrochemical cells, solar cells, quantum wires, capacitors, batteries, and light-weight materials to reduce transportation related energy consumption. In addition to application specific research, optimization of the synthesis and purification of carbon nanotubes will be fundamental to reducing the costs associated with the large scale production of high quality materials. Efficient control of carbon nanotube chirality either by separation or direct synthesis will be necessary. Low cost and efficient methods for tuning the length and aspect ratio of carbon nanotubes are also important. Some applications, like organic photovoltaics, require short nanotubes, but others, like light-weight composites and

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transmission cables, will benefit from high aspect ratio nanotubes. Fortunately, noteworthy improvements have already been achieved in this area. Optimization of the various processes and parameters is needed to continue to improve the efficiencies of carbon nanotube based devices and materials for energy applications. The carbon nanotube industry is very young compared to the metallurgical, semiconductor, and polymer industries. However, even at this early stage, carbon nanotubes have proven their potential to help address all aspects of the energy challenge: conversion, transmission, storage, and consumption.

ACKNOWLEDGMENTS The authors would like to thank the late Dr. Richard E. Smalley for his vision and inspiration. The authors also thank the many researchers who continue to explore how carbon nanotubes and other emerging materials can be used to address the energy challenge. The authors gratefully acknowledge reviewing by Dr. Lars Rose of DuPont De Nemours. The authors acknowledge funding from the National Science Foundation under Grant Numbers NSF 1131633 and 1116080. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

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[66] Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Science 2004, 305, pp 1273-1276. [67] van de Lagemaat, J.; Barnes, T. M.; Rumbles, G.; Shaheen, S. E.; Coutts, T. J.; Weeks, C.; Levitsky, I.; Peltola, J.; Glatkowski, P. Appl. Phys. Lett. 2006, 88, pp 233503233503-3. [68] Tenent, R. C.; Barnes, T. M.; Bergeson, J. D.; Ferguson, A. J.; To, B.; Gedvilas, L. M.; Heben, M. J.; Blackburn, J. L. Adv. Mater. 2009, 21, pp 3210-3216. [69] Zhang, M.; Fang, S.; Zakhidov, A. A.; Lee, S. B.; Aliev, A. E.; Williams, C. D.; Atkinson, K. R.; Baughman, R. H. Science 2005, 309, pp 1215-1219. [70] Ulbricht, R.; Jiang, X.; Lee, S.; Inoue, K.; Zhang, M.; Fang, S.; Baughman, R.; Zakhidov, A. Phys. Stat. Sol.(b) 2006, 243, pp 3528-3532. [71] Ulbricht, R.; Lee, S. B.; Jiang, X.; Inoue, K.; Zhang, M.; Fang, S.; Baughman, R. H.; Zakhidov, A. A. Sol. Energy Mater. Sol. Cells 2007, 91, pp 416-419. [72] Su, C.-Y.; Lu, A.-Y.; Chen, Y.-L.; Wei, C.-Y.; Weng, C.-H.; Wang, P.-C.; Chen, F.-R.; Leou, K.-C.; Tsai, C.-H. J. Phys. Chem. C 2010, 114, pp 11588-11594. [73] Velten, J.; Attila, J. M.; Li, D.; Officer, D.; Wallace, G.; Baughman, R.; Zakhidov, A. Nanotechnology 2012, 23, pp 085201. [74] Zhang, S.; Ji, C.; Bian, Z.; Liu, R.; Xia, X.; Yun, D.; Zhang, L.; Huang, C.; Cao, A. Nano Lett. 2011, 11, pp 3383-3387. [75] Girishkumar, G.; Rettker, M.; Underhile, R.; Binz, D.; Vinodgopal, K.; McGinn, P.; Kamat, P. Langmuir 2005, 21, pp 8487-8494. [76] Jyothirmayee Aravind, S. S.; Imran Jafri, R.; Rajalakshmi, N.; Ramaprabhu, S. J. Mater. Chem. 2011, 21, pp 18199-18204. [77] Yuan, X.; Xin, L. IEEE Transactions on Nanotechnology 2012, 11, pp 148-151. [78] Venkateswara Rao, C.; Ishikawa, Y. The J. Phys. Chem. C 2012, 116, pp 4340-4346. [79] Davis, F.; Higson, S. P. J. Biosens. Bioelectron. 2007, 22, pp 1224-1235. [80] Bullen, R. A.; Arnot, T. C.; Lakeman, J. B.; Walsh, F. C. Biosens. Bioelectron. 2006, 21, pp 2015-2045. [81] Yan, Y.; Zheng, W.; Su, L.; Mao, L. Adv. Mater. 2006, 18, pp 2639-2643. [82] Narvaéz Villarrubia, C. W.; Rincoín, R. A.; Radhakrishnan, V. K.; Davis, V. A.; Atanassov, P. ACS Appl. Mater. Interfaces 2011, 3, pp 2402-2409. [83] Mink, J. E.; Rojas, J. P.; Logan, B. E.; Hussain, M. M. Nano Lett. 2012, 12, pp 791795. [84] Xie, X.; Ye, M.; Hu, L.; Liu, N.; McDonough, J. R.; Chen, W.; Alshareef, H. N.; Criddle, C. S.; Cui, Y. Energy and Environmental Science 2012, 5, pp 5265-5270. [85] Xie, X.; Hu, L.; Pasta, M.; Wells, G. F.; Kong, D.; Criddle, C. S.; Cui, Y. Nano Lett. 2010, 11, pp 291-296. [86] Tritt, T. M.; Harald, B.; Cheng, L. MRS Bulletin 2008, 33, pp 366-368. [87] Vining, C. B. Nat Mater 2009, 8, pp 83-85. [88] Hu, R.; Cola, B. A.; Haram, N.; Barisci, J. N.; Lee, S.; Stoughton, S.; Wallace, G.; Too, C.; Thomas, M.; Gestos, A.; Cruz, M. E. d.; Ferraris, J. P.; Zakhidov, A. A.; Baughman, R. H. Nano Lett. 2010, 10, pp 838-846. [89] Kang, T. J.; Fang, S.; Kozlov, M. E.; Haines, C. S.; Li, N.; Kim, Y. H.; Chen, Y.; Baughman, R. H. Adv. Funct. Mater. 2012, 22, pp 477-489.

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[116] Marquis, F. JOM Journal of the Minerals, Metals and Materials Society 2012, 64, pp 367-373. [117] Dhingra, R.; Naidu, S.; Upreti, G.; Sawhney, R. Sustainability 2010, 2, pp 3323-3338. [118] Khanna, V.; Bakshi, B. R. Environ. Sci. Technol. 2009, 43, pp 2078-2084. [119] Radhakrishnan, V. K.; Zagarola, S. W.; Davis, E. W.; Davis, V. A. Poly. Eng. Sci. 2011, 51, pp 460-473. [120] Vaia, R. A.; Wagner, H. D. Mater. Today 2004, 7, pp 32-37. [121] Vaia, R. A.; Maguire, J. F. Chem. Mater. 2007, 19, pp 2736-2751. [122] Kayatin, M. J.; Davis, V. A. Macromolecules 2009, 42, pp 6624-6632. [123] O' Donnell, S.; Sprong, K.; Hatli, B. AIAA 4th Aviation Technology, Integration and Operations (ATIO) Forum, McLean, VA, Sep. 20-22, 2004 2004. [124] Chen, L.; Ozisik, R.; Schadler, L. S. Polymer 2010, 51, pp 2368-2375. [125] Gou, J.; Tang, Y.; Liang, F.; Zhao, Z.; Firsich, D.; Fielding, J. Composites Part B: Engineering 2010, 41, pp 192-198. [126] Boncel, S.; Koziol, K. K. K.; Walczak, K. Z.; Windle, A. H.; Shaffer, M. S. P. Mater. Lett. 2011, 65, pp 2299-2303. [127] Zeng, C.; Hossieny, N.; Zhang, C.; Wang, B. Polymer 2010, 51, pp 655-664. [128] Ke, L.-L.; Yang, J.; Kitipornchai, S. Compos. Struct. 2010, 92, pp 676-683. [129] Feng, Q.-P.; Shen, X.-J.; Yang, J.-P.; Fu, S.-Y.; Mai, Y.-W.; Friedrich, K. Polymer 2011, 52, pp 6037-6045. [130] Sreekumar, T. V.; Liu, T.; Kumar, S.; Ericson, L. M.; Hauge, R. H.; Smalley, R. E. Chem. Mater. 2003, 15, pp 175-178. [131] Liu, T.; Kumar, S. Nano Lett. 2003, 3, pp 647-650. [132] Zhang, M.; Fang, S. L.; Zakhidov, A. A.; Lee, S. B.; Aliev, A. E.; Williams, C. D.; Atkinson, K. R.; Baughman, R. H. Science 2005, 309, pp 1215-1219. [133] Coleman, J. N.; Khan, U.; Blau, W. J.; Gun'ko, Y. K. Carbon 2006, 44, pp 1624-1652. [134] Coleman, J. N.; Khan, U.; Gun'ko, Y. K. Adv. Mater. 2006, 18, pp 689-706. [135] Shim, B. S.; Zhu, J.; Jan, E.; Critchley, K.; Ho, S.; Podsiadlo, P.; Sun, K.; Kotov, N. A. ACS Nano 2009, 3, pp 1711-1722. [136] Olek, M.; Ostrander, J.; Jurga, S.; Mohwald, H.; Kotov, N.; Kempa, K.; Giersig, M. Nano Lett. 2004, 4, pp 1889-1895. [137] Perrot, C.; Piccione, P. M.; Zakri, C.; Gaillard, P.; Poulin, P. J. Appl. Polym. Sci. 2009, 114, pp 3515-3523. [138] Ericson, L.; Fan, H.; Peng, H.; Davis, V.; Zhou, W.; Sulpizio, J.; Wang, Y.; Booker, R.; Vavro, J.; Guthy, C.; Parra-Vasquez, A.; Kim, M.; Ramesh, S.; Saini, R.; Kittrell, C.; Lavin, G.; Schmidt, H.; Adams, W.; Billups, W.; Pasquali, M.; Hwang, W.; Hauge, R.; Fischer, J.; Smalley, R. Science 2004, 305, pp 1447-1450. [139] Davis, V. A.; Parra-Vasquez, A. N. G.; Green, M. J.; Rai, P. K.; Behabtu, N.; Prieto, V.; Booker, R. D.; Schmidt, J.; Kesselman, E.; Zhou, W.; Fan, H.; Adams, W. W.; Hauge, R. H.; Fischer, J. E.; Cohen, Y.; Talmon, Y.; Smalley, R. E.; Pasquali, M. Nat. Nano 2009, 4, pp 830-834. [140] Qian, H.; Greenhalgh, E. S.; Shaffer, M. S. P.; Bismarck, A. J. Mater. Chem. 2010, 20, pp 4751-4762. [141] Endo, M.; Strano, M.; Ajayan, P.; Springer Berlin / Heidelberg, 2008, p 13-61.

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Chapter 3

A REVIEW OF ENERGY SAVING POTENTIAL AND STRATEGIES FOR ELECTRIC LIGHTING IN FUTURE LOW ENERGY OFFICE BUILDINGS Marie-Claude Dubois Division of Energy and Building Design, Department of Architecture and the Built Environment, Faculty of Engineering (LTH), Lund University, Sweden

ABSTRACT This chapter presents key energy use figures and explores the energy saving potential for electric lighting in office buildings based on a review of relevant literature, with special emphasis on a North European context. The review outlines that an energy intensity of around 10 kWh/m2yr is a realistic target for electric lighting in future low energy office buildings, based on theoretical calculations, measurements in full-scale rooms and simulations with validated lighting programs. This target would yield a significant reduction in energy intensity of at least 50 % compared to the actual average electricity use for lighting in offices. Strategies for reducing energy use for electric lighting are also discussed, which include: improvements in lamp, ballast and luminaire technology, use of task/ambient lighting, improvement in maintenance factor, application efficacy and spectral quality of light sources, reduction of maintained illuminance levels and total switch-on time, use of manual dimming, and switch-off occupancy sensors. Strategies based on daylight utilization are also presented and the relevant aspects such as effects of latitude, window characteristics, properties of shading devices, reflectance of inner surfaces, ceiling and partition height are discussed.

Keywords: Electric lighting, daylighting, office buildings, energy-efficiency, lamp, ballast, luminaire, task and ambient lighting, control systems, daylight utilization, occupancy sensors 

[email protected], Tel.:+46 46 222 7629.

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Marie-Claude Dubois

INTRODUCTION Lighting is one of the key end-uses to consider in minimizing global energy use. It ranks among the end-uses dominating global power demand and was the first service ever offered by electric utilities. According to a recent report of the International Energy Agency [1], gridbased electric lighting consumes 19 % of total global electricity production, which is slightly more electricity than used by the nations of OECD Europe for all purposes. The same report reveals that energy consumed to supply lighting generates greenhouse gas emissions of an equally impressive scale: 1,900 Mt of CO2 per year, equivalent to 70 % of the emissions from the world’s light passenger vehicles1 [1]. In industrialized countries, lighting typically accounts for 5 to 15 % of electric energy consumption, while in developing countries, this value may be as high as 86 %, according to Mills [2]. In a well-developed, industrial country like Sweden, for instance, lighting energy use sums up to 14 TWh annually [3], which is roughly 10 % of total energy consumption within the building sector or roughly 10 % of total electricity use in the country. According to many stakeholders and researchers [4], the lighting service sector offers considerable potential for energy savings, which is the main topic of this chapter. In buildings where high visual performance is expected, like commercial and office buildings, lighting is a key issue from health, productivity, and energy use points of view. According to Santamouris and Descalaki [5], office buildings, in particular, are classified among the building types presenting the highest energy consumption with a total annual energy use varying in the range of 100–1000 kWh/m2yr, depending on the geographic location, use and type of office equipment, operational schedules, type of envelope, use of HVAC systems, type of lighting, etc.. In Northern Europe, for example, office energy intensity lies in the range of 269–350 kWh/m2yr, and for offices all over Europe, the energy intensity is approximately 306 kWh/m2yr, with the mean electric index at 150 kWh/m2yr and the mean fuel index at 158 kWh/m2yr [6]. Taking Sweden as an example again, office buildings have an average energy intensity of 210 kWh/m2yr in this country, with a high electricity use by square meter (93 kWh/m2yr, excluding heating), as revealed by a recent inventory for 123 office buildings of different age [7] [8]. Given the high energy consumption of such building types, the recent recast of the Energy Performance of Buildings Directive (EPBD) [9], is putting high pressure on governments and the building industry, to reduce this high energy consumption to a drastic near-zero level2 within the next 8 years. The challenge is certainly considerable but on the other hand, many studies [10] [11] have demonstrated that modern office buildings have a very high energy savings potential. Electric lighting is one of the energy posts in office buildings where energy savings are possible at a reasonable cost in new buildings as well as in retrofit projects. One recent study [12] indicated that investments in energy-efficient lighting figure among some of the most cost-effective ways to reduce CO2 emissions and many studies show that electricity use for lighting could be reduced by 50 % using existing, well-proven technology [13] [14] [15] [16]. 1

Hydrocarbon fuel-based lighting, used both in vehicles and in areas beyond the range of electricity grids, amplifies these consumption figures and lighting’s secondary effects on public health and the environment. 2 The near-zero energy level is not fully defined yet in many countries. Work is going on as part of International Energy Agency Task 40, to arrive at such definition.

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The following sections explore the potential and strategies for energy savings in office lighting, including control systems, mainly applicable to the context of Northern Europe, with some specific information from Sweden.

ACTUAL ENERGY USE AND ENERGY SAVING POTENTIAL IN OFFICE LIGHTING Lighting is one of the most important environmental attributes of a workplace [17]. Not only does it influence an individual’s perception of work-related tasks, it also affects its general emotional/motivational state and health [18]. The lighting service is generally responsible for 20-45 % of electricity demand in commercial buildings [11], but this figure varies a lot from one building to another and can sometimes be as high as 40 % of the gross energy consumption in some types of buildings [13]. The most significant environmental impact (80-90 %) of lighting is generated during the operation of the lighting system while the cost of an electric lighting installation typically represents only 15 % of total costs and electricity used during operation represents around 70 % of total costs3 [19]. Using the Swedish example again, lighting typically accounts for 25-30 % of electricity use in nonresidential premises [3] [8] [19]. The recent inventory [11] of 123 office buildings mentioned earlier showed that electric lighting and atmospheric ventilation via fans represent the most significant portion (37.5 %) of total electricity use for the majority of office buildings inventoried. The same inventory also indicated an energy intensity for electric lighting ranging between 7 and 53 kWh/m2yr, with an average value of 23 kWh/m2yr, representing around 21 % of total electricity use (108 kWh/m2yr according to the 2006 inventory). More recently, the Swedish Energy Agency published slightly lower numbers (21 kWh/m2yr) based on an extensive review of the inventory [8]. The reported average installed lighting power density (LPD) of 10.5 W/m2 also varied according to room type: 13.1 W/m2 for individual office rooms, 12.4 W/m2 for landscape offices, and 8.6 W/m2 for common rooms (including corridors). According to previous research [20] [21], average LPDs of 7–11 W/m2 are achievable using efficient lamp circuits (based on T8 i.e. 26 mm fluorescent tubes and standard electronic high frequency ballasts) for general office lighting of 300–500 lx. Note that in 1990, office electric lighting was 30 kWh/m2yr in Sweden; a reduction of approximately 9 kWh/m2yr has thus occurred over the 20 years interceding the analyses alluded to herein [7], which is a result of increased efficiency. According to Borg [22], a modern advanced lighting installation should only use 11 kWh/m2yr and if occupancy and daylight sensors are integrated in the installation, the annual energy consumption for lights may come down to as low as 5 kWh/m2yr. Bülow-Hübe [14] investigated the daylight availability and electricity use for lights in offices located in Gothenburg, Sweden, through simulations using the validated programs Rayfront/ RADIANCE and DAYSIM. The study included single-cell and open-plan offices with three different façades (30, 60, and 100 % window-to-wall ratios). Assuming an installed LPD of 3

These figures are valid for industrial countries like Sweden.

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12 W/m2, the annual electricity use for lighting was calculated to be 28 kWh/m2yr if the lights were switched on for 9 h/day, 5 days/week. Assuming manual light switching, with a mix of active and passive users, the electricity use dropped to 20–23 kWh/m2yr and to 11–18 kWh/m2yr using a perfectly commissioned photosensor dimming system, and a mix of passive and active users. This study thus demonstrated by simulation that it is possible to cut down electricity use for office lighting by about 50 % (from 23 to 11 kWh/m2yr) using existing technology, in reference to a case with manual on/off switch near the door. An active user considers interior daylight levels when setting the lighting and blinds as opposed to a passive user who keeps blinds lowered and lighting switched on during occupied hours. Both behavioral patterns have been observed in field studies. More recently, other researchers [15] arrived at similar conclusions. They performed a simulation study using DAYSIM, where daylight autonomy and electric light consumption in perimeter office rooms were analyzed in the climates of Sweden and Canada. The following parameters were varied: glazing-to-wall ratios (GWR), climate, orientation, inner surfaces’ reflectances, glazing visual transmittance, Venetian blind management and electric lighting dimming and switching systems. They concluded that the choice of electric lighting dimming and switching systems had a more significant impact on electricity use than the GWR, orientation, and the other variables examined. They also showed that perfectly commissioned photoelectric dimming combined with occupancy switch-off allows reducing electricity use by at least 50 % compared to all cases with manual switches near the door. A simple occupancy switch-off system yielded intermediate savings, i.e. around 25 % compared to the case with manual switch near the door. Interestingly, electricity savings were not significantly lower on the North than on the South façade. The authors stated that the installed LPD is an important design decision, especially if there is no possibility for photoelectric dimming. In this case, it is possible to achieve an annual electricity use below 10 kWh/m2yr with automatic switch-off combined with LPD 8 W/m2 regardless of GWR. For all cases with LPD > 10 W/m2, photoelectric dimming combined with absence detector is required to obtain a lighting electricity use below 10 kWh/m2yr, which is a proposed realistic target for future low energy office lighting [23]. This number is also in line with results obtained by Bourgeois, Reinhart and Macdonald [24], who compared energy use in a single perimeter office room located in Quebec City (Canada) and Rome (Italy) by using advanced dynamic computer simulations with ESPr/SHOCC/Lightswitch2002. In this research, a constant lighting output resulted in a high annual electricity use of 38 kWh/m2yr for lights in both cities, considering a relatively high LPD of 15 W/m2. The use of manual control (i.e. being able to manually switch off the lights when daylight is sufficient) reduced annual lighting use significantly to 8.1 kWh/m2yr in Rome and 8.6 kWh/m2yr in Quebec, which represented less than 23 % of the initial estimate in lighting use. If automated lighting control was added to manual control, lighting energy use was further reduced to 0.8 kWh/m2yr in Rome and 2.0 kWh/m2yr in Quebec, a spectacular reduction which can be explained by the generally high daylight availability in both locations. In an older study, Santamouris et al. [25] reported the findings of a large monitoring campaign in 186 office buildings in Greece, where the specific energy consumption of the buildings for heating, cooling, and lighting purposes, as well as the electric consumption of office equipment were monitored. The data for electric lighting showed an average energy use ranging from 15 to 25 kWh/m2yr, depending on the type of building. Around 50 % of the

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buildings presented a lighting consumption lower than 11 kWh/m2yr while for the majority of buildings (86 %), the consumption was less than 20 kWh/m2yr. These numbers are generally higher, but the study dates back to 1994. The recent European standard EN-15193 [26] introduced LENI (Lighting Energy Numeric Indicators), prescribing installed LPD for small individual office rooms of 10 W/m2 with a preferable target of approximately 8 W/m2 (considering ‘normal’ illuminance levels for offices). Taking into consideration a reference annual time of use (2500 h) and various lighting control strategies, the calculated annual energy use ranges from 7-20 kWh/m2yr, which shows the large potential for energy savings through control strategies (up to 65 % reduction). For large office rooms (>12 m2), this standard recommends an installed LPD under 12 W/m2 with a preferable target of under 10 W/m2, which results in annual energy use in the range of 17-30 kWh/m2yr, depending on the selected lighting control strategy (see Table 1)4. A combination of occupancy sensors and daylight dimming provides the lowest energy intensity values. To sum up, these various sources suggest that it is possible to achieve energy savings of the order of 45–65 % depending on room type and control strategy, and this, with existing technology. Loe [20] also presented detailed calculations showing that 50 % savings are possible when an installation has a task and a planned building lighting approach and is controlled to provide illumination only when needed5; he also demonstrated mathematically that greater energy savings can be achievable. Table 1. Guidelines for installed LPD (W/m2), reduction factors and LENI (kWh/m2yr) LPD (W/m2) Type of room Individual office rooms (10m2) Obligatory 10 Preferable 8 Large office rooms (12m2) Obligatory 12 Preferable 10 Corridor Obligatory 8 Preferable 6

LENI (kWh/m2yr )

Reduction factor Manual control

Absence/ presence control

Daylight control

Manual control

+ Absence/ + Daylight presence control control

0.8 0.8

0.75 0.75

0.56 0.56

20 16

15 12

8 7

1 1

0.90 0.90

0.77 0.77

30 25

27 23

21 17

1 1

0.75 0.75

0.57 0.57

20 15

15 11

9 6

Also worth noting here is that besides direct electricity savings due to the reduced use of lights, indirect energy savings can also be obtained as a result of the reduced heat production and cooling needs [24] [27] [28].

4

Note, however, that landscape offices normally have a higher occupancy ratio (persons/m2) which means that the energy use per employee may be the same or even lower than in individual office rooms. 5 In his calculations, approximately half the savings were due to the task/ambient lighting approach and about half due to the controls applied to the task/ambient lighting systems.

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However, in cold climates, there may be an increase in energy use for heating, but since electricity is generally a more valuable energy carrier, it is often preferable to save electricity and spend more on heating in most normal cases. Besides, it is wiser to heat the building with the heating system which is placed and designed for optimal thermal comfort than use excess heat from lights as space heating since lights are placed at ceiling height, away from the body and feet of the occupants. The next sections examine in further detail the implementation strategies to reach the proposed low energy intensities in office lighting.

STRATEGIES TO REDUCE ENERGY USE FOR ELECTRIC LIGHTING Strategies to reduce energy use for electric lighting in offices include: 1) Strategies directly related to the electric lighting installations:           

Improvement in lamp technology; Improvement in ballast technology; Improvement in luminaire technology; Use of task/ambient lighting; Improvement in maintenance factor; Improvement in application efficacy; Reduction of maintained illuminance levels; Improvement in spectral quality of the light sources; Reduction of switch-on time; Use of manual dimming; Use of occupancy sensors.

2) Strategies related to daylight utilization. In this case, the following parameters need to be considered carefully:      

Effect of latitude and orientation; Effect of window characteristics; Effect of shading devices; Effect of reflectance of inner surfaces; Effect of ceiling height; Effect of partition height.

The next sections discuss each strategy in detail. An overview of the related energy saving strategies and reported savings is presented in Table 2.

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Table 2. Overview of energy saving strategies and relative energy saving potential Energy saving strategy

Relative saving potential 10 % (T12 to T8) 40 %a (T12 to T5)

References [29] [3, 29] [29]

1

Improvement in lamp technology

2

Improvement in ballast technology

4-8 %

3 4 5

Improvement in luminaire technology Use of task/ambient lighting Improvement in maintenance factor

6

Improvement in application efficacy

40 %b [3] 22-25 % [20, 30] 5 %c [19] Depends on application and context

7 8 9 10 11 12

Reduction of maintained illuminance levels Improvement in spectral quality of the light source Reduction of total switch-on time Use of manual dimming Use of switch-off occupancy sensors Use of daylight dimming

20 % (500 to 400 lx)

[31]

35 %

[32]

6 %d 7-25 % 20-40 % 25-60 %e

[7] [33-35] [33, 36-38] [14, 15, 39, 40]

a

This number also includes improvements due to high-frequency ballast. However, this number also includes dimming (occupancy and daylight) and improved high-frequency ballasts. c Approximately 5 % light output would be lost each year without a proper maintenance program. d By reducing average existing total switch-on time to only 2600 h/year. e This number is highly dependent on the climate, shading strategy, and the base line for comparison (if compared to a case where lights are switched on for 100 % of the time, the savings appear to be much higher than a more realistic case with, for example, manual on/off switch at the door). b

2.1. Energy Saving Strategies Related to Electric Lighting Installations 2.1.1. Improvement in Lamp Technology Although T5 fluorescent lamps have existed for 15 years, recent statistics (for Sweden) [8] indicate that many existing lighting installations still use T8 or even older T12 lamps, which have a much lower luminous efficacy (lm/W). Replacing T12 with T8 lamps can save up to 10 % of the energy consumption, while giving 10 % more light [29]. Newer T5 (16 mm tube diameter) lamps have even higher efficacies (90–104 lm/W), achieving a 40 % reduction in energy use (compared to T12 lamps of 60 lm/W with magnetic ballasts), but these lamps need different fittings [3] [29]. Unfortunately, the replacement rate of lighting systems is low with approximately 3 % replacements per year (in Sweden), which implies that it takes roughly 33 years to replace old lighting installations with new, energy-efficient ones. Since 1995, a period during which several leaps forward were made in energy-efficiency developments, not even 40 % of lighting installations have been changed and it will take another 20 years before the potential for energy savings is fully exploited [3].

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Recent statistics for Sweden show that fluorescent lamps with conventional ballasts represent nearly half (46 %) of the installed electric lighting power density in offices [11]. Also, traditional incandescent lamps represent 12 % of total installed lighting power density [8]. A theoretical calculation [7] has shown that changing all fluorescent tubes to T5 tubes and all incandescent light bulbs to compact fluorescent lamps (CFL) in Sweden would reduce the energy intensity for electric lighting in offices by 5.5 and 1.4 kWh/m2yr respectively. Note that, in the extensive Greek study mentioned earlier [25], it was shown that the replacement of the existing lamps with fluorescent lamps with an efficacy of 80 lm/W would reduce the total energy requirements for lighting by up to 35 %; the use of very high efficacy lamps (117 lm/W) would give reductions of up to 55 %. Incandescent lamps which are changed for a CFL are directly economical and provide up to 15 times increase in durability for these types of lamps. Changing a conventional fluorescent tube with a T5 tube can allow saving electricity use by up to 80 % (including savings from the high-frequency ballast, better luminaire and occupancy combined with daylight dimming) and at the same time obtain flicker-free light. And while all CFL systems contain the toxic material mercury in gaseous form, newer lamps, especially the T5 lamps, contain less mercury than older lamps and have a longer lamp life, which means that fewer lamps need to be disposed of in time [8]. According to Borg [22], today’s most energy-efficient practice scenarios use modern technologies available on the market, which means extensive use of occupancy and daylight sensors, T5 or metal halide light sources, and efficient luminaires. Current predictions reveal, however, that light emitting diodes (LEDs), which do not contain any mercury, will provide the majority of light sources by 2035 [41]. The light efficacy of LEDs is increasing very quickly; it has nearly doubled every other year over the past decades. The highest values are from the LEDs with a high color temperature that is blue-white light; the improvement rate is about 5-10 lm/W per year [42]. In 2009, white LEDs with a light efficacy of 100 lm/W were available [43]. These solid state LED devices are also non-toxic, and can easily be scaled and shaped. However, it is expected that, as a result of pricing and current technological limitations, more traditional light sources will have a major role to play for some time yet, which means that it will be developments in the design and control of lighting installations that are likely to provide substantial energy saving opportunities in the immediate future [44].

2.1.2. Improvement in Ballast Technology In the past, ballasts were relatively simple wire-wound devices and consumed an appreciable amount of energy – typically 10–20 % of the lamp wattage [20]. In order to improve efficiency, high-frequency (HF) ballasts have been developed over the course of the last decades, which have fewer losses. HF ballasts use less than half the energy required by the conventional wire-wound types [29]. They can be used with both T8 fluorescent lamps and the newer, smaller T5 lamps. T5 lamps are always operated with HF-ballasts so an upgrade of lamp technology from T12 to T5 automatically entails an upgrade in ballast technology. Inefficient ballasts are being steadily phased out across the European Union following the adoption of the Ballast Directive 2000/55/EC [45]. Since November 21, 2005, only low loss magnetic ballasts with typical efficiencies of 85 % (depending on the lamp power) and high frequency electronic control gear with efficiency values of more than 92 % are allowed [27] [45]. Recent statistics [8] for Sweden indicate that fluorescent tubes with HF-ballasts, with T8

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and T5-tubes, represent so far only 27 % of the total installed lighting power density in offices in spite of the fact that HF ballasts have existed for over 20 years. HF lighting has many advantages: an improved lighting quality, flicker-free lighting, and reduction in power demand, compatibility with occupation sensing and daylight control, better controllability, and longer life [41]. The effect of light flicker on health, comfort and productivity has been a subject of considerable research. For example, Küller and Laike [46] found that, when a light source was powered by the conventional ballasts, individuals with high critical flicker fusion frequency (CFF) responded with a pronounced attenuation of electroencephalography (EEG) waves, and an increase in speed and decrease in accuracy of performance. These results may be understood in terms of heightened arousal in the central nervous system in response to the pronounced light modulation caused by the conventional ballasts. Likewise, experiments achieved by Veitch and Newsham [47] with 292 office employees working one day under nine different light conditions indicated that people who worked under HF ballasts showed better visual performance, did better on reading and writing tasks and rated the tasks as being less difficult. The most likely explanation of these findings, which were consistent with previous research at the National Research Council (NRC) and elsewhere, is that high frequency of operation (20 kHz) of electronic ballasts, being undetectable, causes no ‘sensory noise’. By contrast, magnetic (conventional) ballasts, which operate at 100 Hz (in Europe, 120 Hz in North America) cycle at a rate that the nervous system can detect, although observers do not report flicker.

2.1.3. Improvement in Luminaire Technology Lighting equipment essentially consists of a lamp, controls and control gear if needed, and a luminaire, each contributing to the overall efficiency [29]. New lighting fixtures reflect light in such a way that more light can be used where needed and less light gets lost in the light fixture itself [20]. While the introduction of T5 lamps in 1995 allowed a 40 % reduction in energy use compared to T12 lamps, the combination of new reflector material in lighting fixtures with dimming (daylight and occupancy) allows achieving another 40 % energy reduction. These improvements combined indicate that modern lighting installations may use less than one fifth (20 %) of the energy used by older installations [3]. The efficiency of the luminaire is defined by light output ratio (LOR):

LOR = lum/lamp

(1)

where lum is the initial luminous flux released by the luminaire and lamp is the initial luminous flux released by the lamp. The LOR describes the efficiency of the luminaire in emitting lamp flux in lumens into the interior space. Its value is determined by the optical layout, the quality of the optical materials (reflectors, diffusers, filters, etc.), the ambient temperature of the lamp and the requirements for preventing glare. The use of new materials, such as coated reflectors and holographic diffusers, allows LOR values of 75 % and higher to be obtained [27].

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2.1.4. Use of Task/Ambient Lighting Loe [20] [44] suggested an alternative approach to lighting design which consists of separating the elements of task lighting and building or amenity lighting and to control them both independently, but in an integrated way. According to the author, this is not a new approach as it was used in the early part of the 20th century when lighting was extremely expensive, both in terms of the electricity it consumed and the cost of equipment, particularly lamps. Rogers [50] also suggested using task/ambient electric lighting system where daylight could provide an ambient level of light adequate for circulation and general tasks, and electric task lighting could provide higher localized illumination. This approach is already used in Denmark, where relatively low general illuminance levels (50–100–200 lx) required in the office are often provided by a combination of electric light and daylight, and illumination on the task (500 lx) is achieved with individual task lamps. Worth noting that the Danish system is based on previous research, which has shown that more uniform (monotonous) lighting normally demands higher illuminance levels and that users are normally more satisfied with control over their own task lamp [30]. In addition to better integration with daylight, a task/ambient system often yields lower LPDs, resulting in a greater base level of energy savings, because illuminance is only provided when and where needed [50]. In experiments achieved by Veitch and Newsham [47] in the nineties, nine light conditions including three levels of LPDs (9, 14, 25 W/m2) and three levels of designers’ lighting quality (DLQ) were evaluated by temporary office workers. It was shown that lighting systems incorporating both task and ambient lighting (9W/m2, measured LPD including task lighting) were rated as providing better quality lighting than systems without task lighting (14 W/m2). Furthermore, in recent experiments in Denmark [30], installations combining low level general daylighting/lighting levels with task lighting achieved total LPDs of 5.4 W/m2, including the task lamp and respecting the Danish code DS700 (500 lx on task, 200 lx in immediate surroundings, 100 lx in remote surroundings and 50 lx for general lighting). This installation resulted in 25 % reduction in electricity use compared to a standard energy-efficient installation. Loe [20] also presented calculations showing energy savings of around 22 % (compared to fixed general lighting solutions) by simply using a combination of general lighting level (200 lx) combined with task lighting. However, some researchers [51] expressed reserves about the use of task lighting: because of the increased risk of visual fatigue, desktop lamps should not be used for prolonged periods of time, and never as the sole light source. This was also an important result of the Danish study reported earlier: users complained that the general light level, and especially the amount of daylight, was too low [30]. Recent measurements [21] of electric lighting use performed in individual office rooms in Lund, Sweden, in the months of November and December are in line with the Danish results. At high latitude, the ambient lighting generated by daylighting alone is not sufficient. It is necessary to supplement the electric task lighting with an electrically driven ambient lighting system. Note, however, that the authors of this study also mentioned that this situation occurs mostly at the beginning and end of the working day in the winter or under strongly overcast skies and that there are many hours during the day when daylight is a sufficient complement to electric task lighting, even in November and December. In this particular experiment, the electricity use of the task/ambient lighting system was extremely low due to the low power of the LED task lamp (6 W).

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Furthermore, new research supporting the idea that ambient or ‘surrounding’ light is very important needs to be mentioned here. Govén et al. [52] showed that the level of background luminance, and particularly the luminance of the walls, has an influence on visual, emotional, and even biological aspects. They also recommended wall luminance of around 100 cd/m2 for future lighting applications (in an office context with 500 lx on task). Regarding the energy use by task lamp, Borg [22] claimed that no more than 20 W should be allowed for one task lamp to reach the recommended 500 lx in the task area, including the electricity consumption of the ballast. Technologies using 6 W and less have been demonstrated with integrated LEDs in Swedish tests and are now being offered by several manufacturers, see [21]. In the Danish study [30], the recommended energy intensity of task lamps was 1–2 W/m2 and for the general lighting, it was 5–6 W/m2, which resulted in a total annual energy use of 9.6 kWh/m2yr for electric lighting using daylight-responsive dimming and assuming normal work hours i.e. from 8:00 to 17:00, five days a week.

2.1.5. Improvement in Maintenance Factor The maintenance factor (MF) is the ratio of the average illuminance on the working plane after a certain period of use of a lighting installation to the initial average illuminance obtained under the same conditions for the installation. This can be equivalently formulated as: MF = TAfin/TA

(2)

where TAfin is the maintained luminous flux on the task area. The maintenance factor takes into account lamp burnouts, lamp lumen maintenance, luminaire dirt depreciation and room surface reflectance maintenance. The rate of reduction of illuminance is influenced by the equipment choice, routine cleaning of the lamp, luminaire, and room surfaces, and the environmental and operating conditions. In offices, schools, and shops, which do not normally become very dirty, the lighting output can be reduced by up to 5 % per year [19]. This reduction in light output depends on the fact that lighting fixtures, light sources, walls and ceiling, become dirtier and also some lamps get older or burn [19]. A high maintenance factor (cleaning) together with an effective maintenance program promotes energy efficient design and limits the installed lighting power requirements [27]. Hanselaer et al [27] proposed a realistic threshold value of MF  0.75 for future energy efficient light installations. A high degree of installation maintenance involves cleaning of the luminaires every year, and of room surfaces every three years, as well as bulk lamp replacement every 10,000 h [20].

2.1.6. Improvement in Application Efficacy An important principle of energy efficient lighting design is to make the most of any light sources available by directing the light to where it is needed [44]. Traditionally, luminaire efficacy has been rated with the luminaire efficacy rating (LER), which is a measure of the total luminous flux emitted by the entire lamp -luminaire system per unit power consumed. The LER is either a calculated quantity obtained from measurements or an estimate of the efficiencies of each luminaire component [48]. These components’ efficiencies and the luminous efficacy of the lamp(s) are then combined into an efficacy rating for the luminaire.

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Work at the Lighting Research Center [49] has shown that LER as currently defined is a good predictor of energy effectiveness for linear fluorescent lamp luminaires, but it is not a good predictor for all CFL luminaires, despite some claims to that effect. While LER can characterize the total luminous flux emitted from the luminaire, it does not consider light distribution. In other words, it ignores the efficacy of delivering light in a particular direction and thus to a particular location. For example, in architectural lighting, some specifications recognize that the most effective lamp and luminaire combination for a given application is often not the one with the highest lamp luminous efficacy. The widespread use of tungsten halogen sources in display and downlighting applications, despite the low lamp luminous efficacy of these technologies relative to others, suggests that lamp luminous efficacy is only partially related to the effectiveness of a lighting installation [32]. In order to bridge this gap, Rea and Bullough [48] [32] introduced the term application efficacy that is, first, based upon the lamp and luminaire combination rather than, as usually considered, solely on lamp luminous efficacy. Application efficacy is defined [48] as the average luminous flux within a specific solid angle per unit power, which is measured in lumens per steradian (lm/sr) or intensity in candelas, per watt (thus cd/W), expressed as: AE = LE/

(3)

where LE is the luminous efficacy (luminous flux/power, lm/W) and  is the solid angle measured in steradians. This new measure is concerned specifically with delivering light where it is needed in the most energy efficient manner. Rea and Bullough [48] suggested to use application efficacy as a new measure of efficacy for lighting applications based upon the lamp and luminaire rather than, as usually is considered, solely on lamp efficacy. The application efficacy is related to the concept of utilance factor U, which is defined as [27] [20]: U=TA/lum

(4)

where TA is the initial luminous flux reaching the task area and lum is the luminous flux released from the luminaire. The utilance U relates the luminous flux from the luminaires to the luminous flux on the target area. It depends on: 1) the arrangement of the luminaires in the room in relation to the position of the task area; 2) the luminous intensity distribution of the luminaires and the spacing to height ratio; 3) the reflectance of the surroundings, which determines the indirect contribution. The LOR and utilance U are combined in what is called the utilization factor UF defined as: UF = TA/lamp

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The value of the utilance is even more important than the LOR in reaching energy efficiency targets and target utilance values for efficient interior lighting have been recently proposed [27].

2.1.7. Reduction of Maintained Illuminance Levels Recommended maintained illuminance levels are normally prescribed over the task area on the reference surface, which may be horizontal, vertical, or inclined. In the USA and Canada, for example, an illuminance of 500 lx on the work plane is recommended for office work [59] [60]. A design illuminance of 500 lx is, in fact, commonplace throughout much of the developed world [61]. In the UK, the working plane illuminances recommended for offices are in the range of 300–500 lx, the lower limit being recommended for mainly computer-based work and the upper limit for mainly paper-based work [62]. In Sweden, an illuminance of 500 lx is also recommended on the task area for individual office rooms while 300 lx are normally accepted as general lighting level for landscape offices [19]. It is acknowledged that there is a large range of lighting conditions over which the human eye performs satisfactorily, and that there is a large range of variation among individuals as to what comprises satisfactory visual conditions [61] [63]. Many studies have indicated that office workers generally prefer illuminance levels that are lower than recommended by the standards [33] [31] [64] [65] [66] [67], particularly if they work with a computer most of the time [63]. For example, a Canadian field study [40] recorded that illuminances larger than, or equal to, 150 lux were classified as appreciable daylight. Moreover, the Illuminating Engineering Society of North America recommends 50 to 100 lux, provided directly onto the individual task area, as the general range of illuminance required for working with CRT screens in laboratory areas [68]. In fact, in a survey of work spaces of a computer hardware and software distribution company, where each of the offices contained at least two computers, measurements showed that most employees felt comfortable with a lighting level of around 100 lux (as opposed to the standard regulations of workplaces demanding 300 lux to 500 lux at desk level) [69]. In line with these recommendations, a French field study [63] involving worker interviews in three office buildings, distinguished between two distinct groups: a small group spending more than 70 % of their time working on the computer, for which light levels were low (100-300 lux), and a bigger group spending less than 70 % of their time working on the computer for which light levels were higher (300-600 lx). Similar results were also found in another French study [70]. However, one extensive study under office conditions has shown that people prefer artificial lighting in addition to the normal daylighting present in an office environment: an average 800 lx on top of the prevailing daylight contribution [71]. Also, several other studies [72] [73] have indicated a preference for very high illuminance levels (including daylight) ranging from 0 to 3000 lx. However, Fotios and Cheal [74] recently demonstrated that the preferred illuminance is significantly influenced by the range of illuminances available to the research participant (the stimulus range), and that occupants tend to select the middle point of the range available. They concluded that studies with different stimulus ranges will lead to different estimates of preferred illuminance, with studies involving a large range resulting in a higher preferred illuminance selected. This may explain why such high preferred illuminance values have been found in some studies.

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Fotios and Cheal [74] proposed to give occupants a restricted range of illuminances to choose from, this range being chosen so that the expected preferred illuminance will be less than the standard 500 lx, meaning they will be satisfied with their environment despite an illuminance less than 500 lx, and energy consumption will be reduced. Boyce et al. [31] also claimed that lighting practice that uses 500 lx as the target for maintained illuminance is excessive. By using 400 lx as a design criterion, a 20 % decrease in energy consumption could be gained together with a likely increase in the percentage of office workers who are within 100 lx of their preferred illuminance. Tregenza et al. [75] claimed that a universally preferred illuminance does not exist, since, as indicated by their research, in both seasons the range of illuminance deemed acceptable is greater than the range considered as unacceptable. Boyce [76] also noted a lack of association between illuminances and their subjectively viewed suitability when subjects were carrying out realistic tasks, i.e. tasks for which visibility requirements were satisfied at relatively low levels of illuminance. Loe [44] suggested recommending a band of adjustable task illuminance for particular situations rather than a minimum level, which would probably be more appropriate and yield higher energy savings, as some individuals would probably select lower illuminance levels than the recommended levels.

2.1.8. Improvement in Spectral Quality of Light Sources According to Rea and Bullough [53], photometry is simply the measurement of light according to an agreed-upon set of definitions. Significant among the many important definitions in photometry are two classes of spectral weighing functions sanctioned by international agreement: the photopic luminous efficiency functions based upon the spectral sensitivity of human cones, and the scotopic luminous efficiency function based upon the spectral sensitivity of rods. Above approximately 3 cd/m2 (photopic condition), the cones are the primary photoreceptors in human vision, and below 0.001 cd/m2 (scotopic conditions), the rods are the primary visual photoreceptors [54]. This difference is significant for lighting practice because rods and cones have different spectral sensitivities (cones peak at 555 nm and rods at 507 nm). According to Rea and Bullough [53], photometry does not reflect the responses of the human visual system at many levels. Unlike mass and time, which exist independently of the human condition, light is defined in terms of a particular human response to the spectrum of electromagnetic radiation. The decision to weight the electromagnetic radiation in terms of human visual response has practical significance because light sources developed for commerce are usually developed to meet human visual needs. Interestingly, the establishment of a system of photometry nearly 100 years ago has created a situation where light is now both the stimulus to, and the response from, the human visual system. This inherent circularity has led to a continuous conceptual difficulty [53]. At a recent conference on energy efficient lighting, Rea [32] cleverly linked these observations to the issue of energy saving, since it is obvious that better matching the lighting system’s spectral qualities and the user’s visual response can provide an optimal, energyefficient lighting solution. Daylighting research seems to support this idea: it has been shown [55] that people tolerate much lower illuminance levels of daylight than artificial light, particularly in diminishing daylight conditions at the end of the day. This may be explained by the fact that daylight, the light source under which humans have evolved, provides a continuous spectrum of light for which the human visual function is optimally adapted.

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In the world of electric lighting, Rea, Bullough and Akashi [56] have shown, for instance, that they could achieve energy savings (of the order of 37 % according to [32]) in outdoor lighting applications by using metal halide (MH) lamps instead of the more common HPS (High-pressure sodium) since MH spectra are better tuned to the spectral sensitivity of the human retina at mesopic6 light levels. Note that the illuminance ratio between an MH and an HPS light source has been measured to be about 0.7 for equivalent brightness perception in the high end of the mesopic luminance range (>0.1 cd/m2) [57] [58]. The same logic can be applied to indoor lighting situations. Rea [32] showed, for instance, that at the same brightness level, 6500 K T8 fluorescent lamps use 35 % less energy than 3000 K T8 fluorescent lamps.

2.1.9. Reduction of Switch-on Time The total number of units of electricity consumed by the lighting installation is also obviously affected by the length of time the lighting is switched on. The time of use with the electrical lighting switched on is affected by the amount of daylight that is present in the space and whether the room is occupied, but also whether there are suitable lighting controls. These may be manual or automatic, or a combination between the two to ensure optimum lighting conditions without lighting being left on unnecessarily, which is unfortunately very often the case and yields tremendous energy waste worldwide. The European standard EN 15193 [26] recommends a total utilization time for electric lighting in offices of 2500 h (2250 daytime hours + 250 nighttime hours). In the Swedish context, for example, recent calculations [7] have shown that reducing time of use of electric lighting in offices to 2600 h/yr would reduce energy intensity for electric lighting by 1.3 kWh/m2yr, thus going from the actual 21 kWh/m2yr to 19.7 kWh/m2yr. An annual time budget of 2500 h corresponds to about 48 h per week and thus 9.6 h per day (5 days/week) of total switch on time, which is feasible even taking into consideration flextime. Office towers fully lit at night can be seen all over the world, particularly in North America. These edifices are a clear sign of energy waste, which is no longer acceptable today. 2.1.10. Use of Manual Dimming Several studies have generated promising results showing that electrical energy use can be substantially reduced by using lighting control systems such as manual dimming and occupancy sensors. For manual dimming, the electric lighting energy savings obtained range between 7–25 % [33] [34] [35] [36]. Moore et al. [77] reported on a survey of user attitudes toward control systems and the luminous conditions they produce in 14 similar UK office buildings. They observed that controllable systems were typically operated at 50 % of maximum output but they did not specify the exact corresponding electricity savings. In a French field study [63] where 41 office workers were interviewed and light levels were measured, results suggested that manual dimming could produce more energy savings (69 %) than those reported earlier by Maniccia et al. [35] and Jennings et al. [36] but the authors did not provide explicit energy saving figures. The same study showed that ‘automatic dimming with a manual choice of illuminance level’ was one of the main characteristics of an ideal lighting system. In other words, despite automatic electric light dimming, people wanted to have manual control to switch lighting on or off according to their needs. 6

Mesopic vision corresponds to luminances in which both the rods and cones contribute to vision.

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Providing building occupants with control over their own environment is generally considered desirable [78]. According to Vischer [79], more control is positive in several ways: it helps people cope with environmental demands, and it encourages people to find new ways of solving problems, so that users increase their learning and knowledge about their building and workspace. The author also claims that strain occurs in the workplace, partly because users cannot control those environmental features that are uncomfortable.

2.1.11. Use of Occupancy Sensors Previous research has shown that the use of automated systems for controlling the electric lighting can yield considerable energy savings [38] [80] - automated control systems save energy compared to manual switching. For occupancy sensors, the reported lighting electricity savings range from 20 to 40 % [33] [36] [37] [38]. According to Guo et al. [80], automated control systems save energy compared to manual switching, but differences between observed savings and industry estimated savings that result from the application of these systems are often observed [80] [81]. Studies comparing energy use after installation of occupancy sensors with manual switching as the baseline show energy savings of approximately 25 % in private offices with a sensor time delay setting of 20 min, which is the lower bound for manufacturer claims [80]. Also, some studies have stressed the fact that poor acceptance of automatic systems had led to the deactivation of the systems [82] [83] [84] [85]. Furthermore, a recognized annoyance of occupancy sensors is when the lighting is automatically switched off because the occupancy sensor no longer detects any movement [21] [63] [83] [84] [86] [87]. If automated systems are used, a manual override function should be provided to avoid serious complaints and possible action by occupants leading to the disabling or even destruction of automated control systems [88]. An extensive simulation study [15] performed with the program DAYSIM showed, on one hand, that automatic on/off (presence/absence) detectors yield more energy use than a simple manual switch at the door in individual office rooms located in Sweden and Canada. This is due to the fact that some occupants always choose to switch-off lights and work only with daylight, which is not allowed by the automatic switch on/off system. The automatic switch on/off is therefore performing less efficiently than a simple manual switch at the door. On the other hand, the same study showed that the use of an automatic switch-off system or so-called ‘absence detector’ with a delay time of 5 minutes yields electricity savings of about 25 % compared to a manual switch at the door, and this for any glazing-to-wall ratio and any orientation. The interest of this system lies in the fact that it switches lights off when occupants leave the office but does not switch them on again. Thus, if there is enough daylight in the room upon their return, they normally leave the lights off for a longer period, which saves energy. This solution has thus been identified as a very promising one in future low energy use office rooms, especially if the initial installed LPD is low and if simplicity and robustness are desired characteristics of the lighting installations.

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2.2. Energy Saving Strategies Related to Daylight Utilization Most commercial spaces have enough daylight next to windows to eliminate the need for electric lighting [89]7. The exploitation of daylight, commonly referred to as ‘daylight utilization’, is recognized as an effective means to reduce the artificial lighting requirements of nondomestic buildings. Daylight utilization may allow energy savings compared to electric lighting due to its intrinsic higher luminous efficacy. For a given quantity of illumination, light from clear blue skies delivers the least amount of heat gain [90]. Generally, experts agree that a well-lit building should have a daylight control system, so that residents are able to switch or dim electric lights when sufficient daylight is present [91]. Research has shown that daylight-linked lighting control systems, such as automatic on/off and continuous dimming, have the potential to reduce the electrical energy consumption in office buildings by as much as 30–65 % [38] [39]. Bodart and De Herde [92] examined previous literature on the subject and concluded that it is difficult to evaluate the energy savings coming from light dimming as a function of daylight availability. For office buildings with classical windows (no specific daylight system), they found previous research indicating electricity savings for lights ranging from 20 % to 77 %. They achieved a study by computer simulations with ADELINE and TRNSYS for the Belgian climate and showed that daylight harvesting allows reductions of electric lighting consumption of 50–80 %, which would yield primary energy savings for the building of up to 40 %, considering a glazing type normally used in offices. The savings obtained depended on the glazing visual transmittance, façade configuration, orientation of opening, room width as well as reflectance of interior walls. In Canada, Athienitis and Tzempelikos [28] developed a simulation methodology and carried out simulations for a typical office room (5 m × 5 m × 3 m) located in Montreal with a Vision Control window measuring 2 m × 4 m on the façade facing 10° east of south with no obstructions. Comparing the results with an office space where lights are switched on 100 % during all working hours, they obtained electricity savings for lights (with the window system and controllable highly reflective venetian blinds plus light dimming) reaching 76 % on overcast days and 92 % on clear days. In parallel to this, research has also shown that, in spite of few promising laboratory test results and computer predictions, most daylight-linked systems do not provide the anticipated energy savings when installed in real buildings [39] [21]. Post-occupancy studies carried out in real buildings have shown that the actual energy performance of daylit buildings is invariably markedly worse than that predicted at the design stage [90]. One significant reason for this is the inability of the standard predictive methods to account for realistic conditions and the wants and needs of the occupants. Dimming electric lights based on available daylight is also expensive with significant equipment (dimming ballasts) and commissioning costs. Papamichael et al. [89] claimed that only a small fraction of side-lit dimming applications operate satisfactorily today. While useful in low daylight areas, dimming is not really necessary in areas with high levels of daylight, where dimming is only useful during the early morning and late afternoon [89]. Moreover, dimming ballasts are less efficient than nondimming ballasts and consume 10–20 % power even at the lowest possible light output [89]. While it is true that occupants may be more distracted by the dramatic changes in light levels caused by switching (as opposed to a smooth dimming function), switching will generally 7

This does not apply to buildings located in the far north of Scandinavia with very little daylight during winter.

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only occur twice a day, during early morning and late afternoon or early evening hours. Moore et al. [77] also claimed that there is little merit in equipping locally dimmable systems with photoelectric controls, given the general dislike of photoelectric control reported in the literature. In general, the literature reveals a number of reports of switching behavior or electric light use being related to daylight availability [34] [38] [71] [94] [95] [96]. However, a number of studies have indicated no relationship between daylight availability and electric lighting use [64] [77] [97] [98] [99] [100] and even higher levels of electric light with higher external illuminances [71]. Begemann et al. [71] proposed two possible explanations to this phenomenon. First, occupants could be attempting to balance the brightness of window areas with those of the interior. Secondly, the phenomenon could be caused by occupant adaptation to exterior conditions prior to entering the building, and too short adaptation periods to interior conditions prior to switching. Interestingly, Escuyer and Fontoynont [63] found that people tended to choose a lower level of artificial light when more daylight entered the office, because they felt a clear need to ‘benefit from daylight’. In their field study, many of the 41 office workers interviewed added less than 280 lx of artificial light on the desktop, even in workplaces where daylight illuminances were very low (0-100 lx), and this because they wanted to ‘feel’ the daylight and experience a sense of connection to outdoor conditions. Despite all these arguments, we have to consider that daylight utilization is just one of the many arguments for admitting daylight in buildings. In recent years, research has identified the benefits of daylight and sunlight in buildings for the health and well-being of occupants, including its necessity for the regulation of circadian rhythms [101] [102]. A number of studies have stressed the importance of daylight and windows and have demonstrated their numerous positive effects on building occupants. Most importantly for office environments, daylight is generally preferred to electric lighting [103] and ‘having plenty of daylight’, that is to say principally being located near the windows with a clear view outside, is one important desirable characteristic of an office [63]. In addition, daylight remains a predominant factor in how a space is revealed and perceived by its users [104]. Daylight presence adds a significantly positive contribution to lighting quality [105] and makes an interior space look more attractive [106]. Glare studies have shown that glare is tolerated much more from a daylight source than from its artificial equivalent [107]. Significantly less incidents of eyestrain are reported by people whose workstations received large proportions of natural light [108]. Furthermore, Sutter et al. [109] showed that high luminance contrasts were more tolerated when the window occupied a large portion of the visual field. However, daylight, because of its variability and intensity, poses additional challenges and needs to be carefully considered to realize its potential to provide healthy and comfortable office environments. Boyce [110] argued that daylight offers no guarantees of a better work experience. Tzempelikos and Athienitis [111] warned that large fenestration areas often result in excessive solar gains and highly varying heating and cooling loads. Daylighting could thus lead to a net increase in energy consumption if the additional cooling load due to daylight (i.e. including the solar component) exceeds the energy saved due to reduced electric lighting, or if the net heat gains and losses through the fenestration do not compensate for the lighting energy saved [90]. Some reduction in the variability can be achieved by installing solar cells on the fenestration, providing shading, but this may just lead to additional daytime hours with lights switched on or additional heating loads in the winter. In fact, an all too common scenario in most overglazed buildings is where the blinds are down

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to control glare and the lights are on [111]. Cell placement is ideal on opaque, well insulated parts of the building envelope. In many cases in North America, office lighting is switched on anyway, no matter what the external lighting condition. A full consideration of the potential for daylight to save energy should, at some point, also account for the thermal effects of daylight [90]. Fortunately, many recent studies indicate that daylight utilization can provide electric lighting savings with ‘reasonable’ fenestration areas (around 30–40 % of facade area) including the use of shading devices when needed [14] [92] [111]. The next sections examine the effect of some design parameters on daylight utilization potential.

2.2.1. Effect of Latitude and Orientation The potential for daylighting during winter is limited in the Nordic countries, due to the high latitudes and restricted daylight availability during winter. Reinhart [40] studied the influence of various design variables on the daylight availability and electric lighting requirements in open plan office spaces using DAYSIM. Five climatic centers that represent the ambient daylight conditions of 186 North American Metropolitan Areas were identified. For these five climatic centers, 1000 office settings were investigated with varying external shading situations, glazing types, facade orientations, ceiling designs, and partition arrangements. The daylight performance of the offices was expressed in terms of their daylight autonomy distributions and energy savings for an ideally dimmed lighting system. The results indicated that energy savings were falling with rising latitude and total annual solar radiation. An analysis of the monthly energy savings for the five sites studied in the United States and Canada showed that most differences appear in the winter months due to shorter day lengths in the North. In particular, the Vancouver region was characterized by dark overcast winter skies. Concerning orientation, Bodart and De Herde [92] found that north oriented room consumption was always higher than for other orientations, but they did not use solar shading devices in their research, so the savings obtained for the south, east, and west orientations may not be realistic given the fact that shading devices would probably be used in reality for these orientations. They also noted that the influence of orientation was minor and even nonexistent. They explained this by the fact that daylight availability was so important that the diffuse and the external reflected lighting portions that added to the internal reflected daylight were sufficient to reach the minimum lighting requirement. Furthermore, a user assessment survey by Osterhaus [88] in real daylit office spaces achieved in 1992–1994 in nine office buildings in the USA and Germany, where 250 questionnaires were distributed to individual office workers, indicated that north-facing windows (all survey sites were located in the northern hemisphere), have a lower impact than those facing other directions. Respondents in offices with northern orientation reported presence of daylight glare somewhat less frequently. However, there was no evidence that windows facing other directions created higher levels of glare. While it can reasonably be expected that north-facing windows create fewer concerns for glare discomfort, the author was surprised that east and west-facing windows showed no higher levels than south-facing windows. The author mentioned, however, that the beautiful view over certain landscape elements may have mitigated the effect of daylight glare for some orientations.

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2.2.2. Effect of Window Characteristics The size, shape, position, orientation, and quantity of windows influence the daylight indoors, as does the framing and transmittance of glazing [51]. Gratia and De Herde [112] provided some recommendations regarding windows: 1) Generally, the higher the window is, the better the lowest part of the room is lit and the deeper the naturally lit zone is. 2) The surface area of the window plays an important role. In his simulations, Reinhart [40] found that changing from a high (75 %) to a low (35 %) transmittance, solar-protective glazing reduced lighting energy savings by about 20 percentage points for the peripheral office. Reinhart outlined, however, that care has to be taken that such energy savings on the electric lighting side are not compromised by additional cooling or heating loads. Therefore, the author suggested that an ‘adequate’ blind control strategy had to be chosen. In Belgium, Bodart and De Herde [92] performed a simulation study where they observed that the electricity consumption did not vary linearly with the glazing transmittance. They observed that when an illuminance level of 500 lx was reached, any daylight availability had no more influence on the artificial light consumption. They concluded that a high visible transmittance is beneficial for the lighting energy consumption, but that beyond a certain value, the benefits stabilize. They showed that in an office room, an increase in the ratio Swindow/Sfloor from 16 to 32 % reduced the electric lighting consumption by 12 % for glazing with 20 % visible transmittance and by 36 % for glazing with 81 % visible transmittance. However, these numbers did not include the additional energy use for heating and cooling caused by the larger window. In general, highly glazed façades, often with poor shading, have become very common. This, together with the extra heat gains from the electric lighting made necessary by deep floor plans, and the wider use of false ceilings, increase the risk of overheating [112]. Poirazis et al. [10] showed that office buildings in Sweden with fully glazed facades are likely to have a higher energy use for heating and cooling than buildings with conventional façades (e.g. 30 % window to external wall area). In their simulation study, the total energy use (heating, cooling, electricity for pumps, fans, and office equipment) of the 30 % glazed building ranged from 123 to 136 kWh/m2yr and that of the 100 % glazed building ranged from 143 to 176 kWh/m2yr. In the ‘best’ case, the additional energy use of the 100 % glazed building was 15 % higher compared to the energy use of the 30 % glazed building. The authors also outlined that one of the main arguments for using increased glazed areas in buildings is the provision of better indoor environment due to daylight. However, increased window area does not necessarily directly correlate with a reduction in energy use for building illumination. Glare problems that can be caused by the large amount of daylight entering a highly glazed working space often reduce the quality of visual comfort and shading devices are used more frequently in highly glazed buildings, often maintaining the same levels of daylight used in a building with a conventional façade. Tzempelikos and Athienitis [111] showed that, for a 30 % window-to-wall ratio (WWR) and a south orientation, daylight provides peripheral offices in Montreal with 500 lx on the work plane 76 % of the working time in a year. They showed that increasing the WWR above 30 % did not result in a significant increase in useful daylight in the room (9 % additional daylight for 80 % WWR). Therefore, the 30 % WWR was identified by the authors as the daylighting saturation region for south-facing facades in

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Montreal. Recent results by Dubois and Flodberg [15] and Bülow-Hübe [14] are generally in line with these reported findings.

2.2.3. Effect of Shading Devices According to Küller [51], the reduction in thickness of the exterior walls and the increased use of glazing in the façades of modern architecture has made the design of good daylighting more difficult. Various kinds of shading devices may be necessary not only to avoid overheating but also to control the interior lighting and avoid glare from windows. For computer tasks, where the normal line of sight is more horizontal than for reading or handwriting tasks, glare from windows is usually a considerable concern and needs to be carefully controlled [88]. Shading devices generally reduce the amount of daylight available in a space. Christoffersen et al. [113] noted that for Venetian blinds, the best utilization of daylight is achieved with horizontal slats, because this evens out the big differences in luminances between the window zone and the rear wall zone. They explained that if the slats are tilted downwards (+45°), light comes mainly from the ground and thus reduces illuminance levels by 75–90 % in the case of white Venetian blinds and by slightly less in the case of Venetian blinds with reflective slats. They explained that it is thus essential to raise the slats when not needed to provide natural light in the space. They also showed that for light shelves, the only situation in which the light shelf increases the illuminance level is when it is specularly reflecting and hits direct sunlight from a relative azimuth angle so small that most of the reflected light hits the ceiling. Under overcast sky, the light shelf reduces the illuminance level on the working plane because it directly cuts off part of the view to the sky. Galasiu et al. [39] achieved a field experiment in Ottawa (Canada, latitude 45.24°N), where the performance of two commercial photocontrolled lighting systems, continuous dimming and automatic on/off, was evaluated as a function of various configurations of manual and photocontrolled automatic Venetian blinds. They showed that under conditions with a clear sky and without blinds, both lighting control systems reduced the lighting energy consumption on average by 50–60 % when compared to lights permanently switched on from 6 AM to 6 PM. These savings, however, dropped by 5–45 % for the dimming system, and by 5–80 % for the automatic on/off system with the introduction of various static window blind configurations. The savings in lighting energy were more significant when the lighting control systems were used with photocontrolled blinds. This was due to the capability of the blinds to adjust their position automatically in direct response to the variable daylight levels. According to Reinhart [40], one error source for overoptimistic energy savings predictions in office buildings is the treatment of blinds. It is often assumed that blinds are retracted all year round (maximum daylight availability), while the lighting is always activated during office hours. In the open-plan office study, the simulation results revealed that the daylight availability in peripheral offices allowed for electric lighting energy savings between 25 % and 60 % for an ideally commissioned, dimmed lighting system. Reinhart observed, however, that electric lighting energy savings for a dimmed lighting system in an open plan office decisively depended on the underlying blind control strategy. A number of researchers have attempted to investigate whether occupants of office buildings use the shading devices according to predictable patterns and if these patterns are dependent on factors such as window orientation, time of day, sky condition, season, latitude,

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and workstation position. Galasiu and Veitch [103] presented an exhaustive review of this research, which allows to conclude the following:      

Some research indicates no relation between exterior environmental conditions and use of shading devices [114]. Sunlight entering the space (especially more than 2 m away from the outer wall) triggers the use of shading devices [85] [115] [116] [117] [118]. Solar radiation levels above 250–300 W/m2 normally induce a significant percentage of blind utilization [116] [119] [120]. Below 50–60 W/m2, occupants most certainly do not use shading devices [116] [121]. Most people operate the blinds based on perceptions formed over long periods of time, rather than primarily in response to current conditions [116] [121]. Once closed, the blind will usually remain closed the entire day [114] [122].

More recent research [123] [124] suggests that when maintaining sky luminance under 2500 cd/m2, only a minority of occupants would want to lower the window blinds. In another study [125] about daylighting of the New York Times Headquarters building, a threshold value of 2000 cd/m2 was used, based on the assumptions that the primary task involved a liquid crystal display (LCD) type monitor with an average luminance of 200 cd/m2. The window was within the occupant’s peripheral field of view so that a maximum luminance ratio of 10:1 between window and task was just acceptable, and that the average background luminance was 50–100 cd/m2. It was also based on subjective survey results that found that there was a 50 % probability that blinds would be lowered when the average window luminance was 2100 cd/m2 [124].

2.2.4. Effect of Inner Surface Reflectance The use of brighter colors for inner walls is necessary to maximize the reflection of natural light in the space and even the reflection of electric lights on walls. According to the European Standard EN 12464 [126], proposed ranges of useful reflectances for the major interior surfaces are:    

ceiling 0.6–0.9 walls 0.3–0.8 working planes 0.2–0.6 floor 0.1–0.5

Loe [44] noted examples for which the correct horizontal task illuminance has been provided, but the occupants were dissatisfied because the room appeared gloomy. Often, the problem was caused by low reflectance wall finishes in combination with luminaires which provided little light on vertical surfaces. Consequently, the room did not appear ‘light’ and was deemed to be under-lit and therefore unsatisfactory. In Reinhart’s open-plan office study [40], the simulations showed that reducing partition reflectance seriously reduced the amount of daylight at second row offices (for landscape office layouts) and should be avoided if daylighting is desired. The author also pointed out that increasing the ceiling reflectivity has a

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positive effect on energy savings and leads to a more uniform distribution of daylight throughout the space. Gratia and De Herde [112] presented general guidelines for the design of low energy office buildings in Belgium, based on a series of parametric simulations using the programs TAS, OPTI, and ADELINE 2.0 (Superlite). The simulations were carried out for a rectangular, 3041 m2 office building with five floors and 60 % WWR. Results of their simulations indicated average horizontal task illuminance values of 7552, 7617, and 7127 lx with ceiling reflectances of 70, 80, and 0 % respectively (wall and floor reflectances of 45 % and 15 % respectively), for a situation with a clear sky on March 15th. They recommended high reflectance values (70–80 %) for the ceiling, especially when a light shelf is used, because the light shelf redirects daylight toward the ceiling. For walls, they obtained average horizontal task illuminance values of 7552, 8149 and 7105 lx with wall reflectances of 45, 80 and 0 % respectively (ceiling 70 % and floor 15 % in this case). Therefore, they recommended keeping wall reflectances above 50 % to maximize reflection of daylight in the space. For the floors, they obtained average horizontal task illuminance values of 7552, 9080 and 7314 lx with floor reflectances of 15, 80 and 0 % respectively (wall 45 % and ceiling 70 %). This surface and the surface of desks in the office play a major role in light distribution due to geometrical considerations (exposure to skylight) and therefore, the authors recommended selecting floor and desk reflectances above 50 %. However, the authors noted that the floors are often relatively dark in order to facilitate maintenance and reduce visibility of optical flaws and stains. Consequently, a compromise has to be made in order to simultaneously meet the requirements of visual comfort and maintenance. Finally, they mentioned that light colors of desks also allow a reduction of contrast between the paper and the desk surface, which contributes to visual comfort. For the furniture, a reflectance value between 25 and 45 % has been recommended [127]. Osterhaus [88] also advised to select light monitor screen borders and lighter desktop surfaces to keep luminance ratios low. Too dark desk and/or furniture surfaces may give rise to high contrasts and unacceptable luminance ratios in the direct field of view [128]. On the other hand, too bright surfaces can yield disturbing reflections and glare.

2.2.5. Effect of Ceiling Height Few studies have been found about the effect of ceiling height. In his open-plan office study, Reinhart [40] found that the ceiling was a crucial design element for daylighting as the majority of daylight that penetrates into a building beyond the 1st work station is reflected from the ceiling at least once. The author found that second row offices receive considerably less daylight even though a reduced partition height and increased ceiling reflectances can double electric lighting energy savings up to 40 %. It was also noted that reducing the ceiling height from 9 ft (2.74 m) to 8 ft (2.44 m) cuts energy savings for electric lighting in half, which is significant. However, with a reduced height, heating becomes much more efficient during winter, as less unoccupied head volume has to be heated. However, this is mainly an issue in homes. On the other hand, ventilation becomes less efficient with a reduced height, because of the presence of heated, used air in head height. During the day, there is little heating load in offices, most of the heating load occurs during night. The main issues for offices are ventilation, lighting, and cooling.

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2.2.6. Effect of Partition Height In Reinhart’s open-plan office study [40], it was also shown that lowering partition heights from 64 in. (162.6 cm) to 48 in. (121.9 cm) nearly doubled energy savings for the automated and manually controlled blind scenario. Another benefit of reduced partition heights between peripheral and second row offices was that the latter get a partial view outside. On the other hand, lower partitions reduce the acoustical separation between two work spaces. A smart design option might be to group workplaces that require intense communication between co-workers in peripheral and second row offices and reserve inner spaces with higher partitions for more noise sensitive tasks, as proposed by the author. Another solution might be to use transparent or semi-transparent partitions.

CONCLUSION Key figures for energy consumption and energy saving potential for office lighting were presented based on a review of relevant literature, with a special emphasis on a North European context. The chapter reveals that the replacement of older lighting installations (T12 fluorescent lamps) with modern energy-efficient T5 lamps with high-frequency (HF) ballasts could provide up to 40 % energy savings. An additional 40 % energy savings could be obtained by using a combination of more energy-efficient luminaires, task/ambient lighting, occupancy switch-off, and daylight dimming. The overall potential energy savings that can be achieved by combining all the positive effects exceed 80 % compared to older T12 fixed lighting installations. The chapter shows that measurements in full-scale rooms, theoretical calculations, and simulations with validated lighting programs indicate that an energy intensity of around 10 kWh/m2yr is a realistic target for electric lighting in future medium energy use office buildings as well as in office building retrofits. It is interesting to compare the proposed energy intensity target (10 kWh/m2yr) with the baseline building energy use described in the U. S. Green Building Council LEED 2009 Certification System for New Construction and Major Renovations [129]. In this rating system, up to 19 points can be earned by demonstrating that the building reduces energy use compared to a baseline building performance rating according to the method in Appendix G of ANSI/ASHRAE/IESNA Standard 90.1-2007, which prescribes ‘Whole Building (LPD) allowance’ of 1 W/ft2 (equivalent to 10.76 W/m2) for electric lighting. Assuming ‘normal’ office hours of 2250-2500 hours per year (9-10 hours per day during 250 days), such building would use 24.2-26.9 kWh/m2yr, which is slightly higher than the average Swedish building. In the LEED 2009 system, a 12 % improvement in overall whole building energy use compared to this baseline building gives only 1 point, while a 48 % improvement provides 19 points, which is taken into consideration in the overall certification (Certified 40–49 points, Silver 50–59 points, Gold 60–79 points, Platinum 80 points and above). Furthermore, it is interesting to consider that this target, which assumes typical illuminance levels for office rooms, would yield a significant reduction in energy intensity of at least 50 % compared to the actual average electric lighting use (21 kWh/m2yr in Sweden, for example). Note, however, that this figure may vary according to room type (i.e. individual office rooms versus landscape rooms and common rooms). This chapter also indicates that

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lower energy intensities are achievable by accepting lower installed illuminance levels (500 to 400 lx) and task/ambient lighting using very energy-efficient task lamps (