Wasteful as we are, technology did progress to be more effective and efficient in our energy use. Mindful of our surroundings, every bit of energy used should be ...
ME5205 Group 7
Using technology to help the environment Manuel Reyes Del Rosario Jr, Modurpalayam Subramanian Selvasurendhiran, Jenn Bing Ong Dept. Mechanical Engineering, National University of Singapore. Climate change is one of the primary concerns of humanity in 21st century. Climate change affects human and environmental health through increased frequency/intensity of heat waves, increased floods/droughts, changes in the vector-borne diseases distribution and risk of disasters and malnutrition, etc. [1]. Large amount of evidence suggests that the increasing concentration of greenhouse gases such as CO2, CH4, CFCs, N2O, and ozone due to anthropogenic emission is trapping the heat radiated from Earth's surface and hence raising the Earth’s surface temperature [2]. The sheer increase of world population, its consumption, and industrial activity, etc. without mindfulness of the environmental health has led to other detrimental consequences such as land, water, and air pollution, acid rain, and stratospheric ozone depletion [3]. The search for a sustainable development has gained much attention over the years. In 2013 alone, US carbon emission is about 5,390 Million Metric Tons with an estimated energy efficiency of only about 39%, suggesting the environmental resources are not fully utilized. Energy waste of 43% during the generation phase alone, equivalent to 3.3 Billion Metric Tons of carbon emission make us take a hard look at ignoring the inefficiencies of our processes and it is here that technology must take part in uplifting the conditions. [4] In Section I of this report, the innovations in some areas of renewable energy to replace fossil fuels in meeting the world energy demand is discussed. Section II introduces the green residential design in saving energy consumption and improving energy effectiveness. The high energy usage nowadays and intermittency in energy supply (especially renewable energy) suggests the need for effective and scalable energy storage. This is covered in Section III and followed by a conclusion. 1. Innovations in Renewable Energy Research Renewable energy sources are those resources that can be repeatedly used to produce energy, e.g. solar, wind, geothermal, biomass, etc [5]. These resources have the potential to replace fossil fuels for sustainable development because they produce minimal or virtually no carbon emission during operation [1]. Harvesting renewable energy in a decentralized manner could meet the rural and small scale energy demands in a reliable, affordable, and environmentally sustainable way, eliminating the need for fuel-transport or transmission lines [6]. 1.1 Wave Power Generation Wave power generation has several advantages, i.e. high energy density compared to solar and wind; widely available; the energy generation is up to 90% of the time and is more predictable compared to wind. Also, waves can travel large distances with little energy loss if they do not encounter head wind or bottom friction and there is a good correlation between wave resources and demand because around 37% of the world population lives at 90km of the coast. But existing Wave Energy Converters (WECs) are bulky and expensive, they need to be over-engineered to withstand extreme weather with unusually high loadings on the devices [11].
1.1.1 Dielectric Elastomer Generator Dielectric Elastomer was first experimentally demonstrated [12] to convert mechanical to electrical energy and the specific energy density of the acrylic elastomer used was shown to be 0.4 J/g. Since then, Dielectric Elastomer Generator (DEG) is increasingly gaining attention because it has attractive attributes of being light, non-toxic and efficient under wide frequency range with few moving parts with high energy density. A number of inexpensive and commercially available materials such as natural rubber, silicone rubber, and acrylic elastomers can be used as dielectric
elastomers. Figure 1 – Principe of DEG [13] With compliant electrode coated on each side of the dielectric elastomer, the elastomer is first stretched mechanically. The surface of the elastomer is then electrically charged. When the elastomer relaxes, the decrease in surface area forces the like charges together and increase in thickness pulls the unlike charges apart and this increases the electrical energy stored in the variable capacitor, i.e. DEG [13]. SRI International and SBM Offshore have independently developed WEC prototypes using DEG SBM Offshore has seen DEG as a breakthrough technology that could offer low structural cost compared to conventional WECs [14]. Figure 2 Lab test of the WEC using dielectric elastomer developed by SBM [14]
1.1.2 Concerns Large scale employment of renewable energy requires careful planning in terms of the power generation, social acceptance, environmental impacts, and risk analysis, etc. For example, large scale wind/wave farm (near-shore/offshore) might have noise and visual impact and harm the immediate biosphere of the region [7]. Many marine animals utilize Earth’s electromagnetic fields for movement, orientation, and foraging and the effects of electromagnetic fields emitted by the power takeoff, transmission networks and supporting infrastructure of wind/wave farm on marine animals is a concern to be addressed [7]. Enhancement of fish populations due to absence of fishing in the said regions and structural foundations functioning as secondary artificial reefs that may attract large number of sessile and motile organisms are some other issues to be considered. 1.2 Solar Power & Biomass Solar photovoltaic system and solar thermal power are employed in large desert area where direct sunlight prevails. The smaller devices that allow convenient household use of solar energy include solar cookers, solar water heater, and solar-dryers [1]. Biomass, an indirect product of solar energy under plant photosynthesis, is another form of renewable resources that is difficult to scale up. More innovative approach is the production of biogas through anaerobic digestion from large amount of animal manure and slurries as well as the wet organic waste streams from households, food industry, and agricultural waste products. This can reduce the emissions of greenhouse gases from these sources and the biogas produced can substitute firewood for cooking and kerosene for lighting/cooking and the organic waste can then be returned to the soil as fertilizers and close the loop from production to utilization by environmental friendly measures [15]. 1.3 Hydrogen The future scenario of hydrogen-economy consists of hydrogen produced through renewable and nonpolluting means; and the H2 so produced will be of appropriate quality for direct usage in devices like fuel cells. Given its wide limits of flammability in air/oxygen and with a very high energy density in terms of weight – it is an ideal fuel, whose emissions are only water. 1.3.1 Alkaline Electrolysis Present day hydrogen production is chiefly from fossil fuels (about 96%) [16] through steam reforming and partial oxidation of hydrocarbons. Other processes include coal gasification and fermentation of bio-masses which are yet polluting. The most promising way of hydrogen generation on a commercial scale is water electrolysis and if the electrical energy had been generated by renewable means, it becomes a zero emission procedure. The other merits of electrolytic generation of hydrogen is that it yields very high purity of the fuel (as high as 99.999% [17]), which can be directly used in low-temperature fuel cells. Also, most of the alkaline electrolysers work at pressures between 25-30 bars, which further reduces a part of work to compress the gas for storage. [18]
1.3.1.1 Working Principle A typical electrolytic cell consists of two electrodes – anode (where oxidation occurs) and cathode (where reduction occurs), a diaphragm which prevents the shorting of electrodes and recombination of hydrogen and oxidation and the electrolyte. Figure 3 (a) shows a typical alkaline electrolytic cell and the typical reaction is: H2O H2 (g) + 0.5 O2 (g)
Figure 3 – (a) Operating Principle (b) I-V Curves of Alkaline Electrolysis Cell [19] Electrolysis is an endothermic and non-spontaneous process, and hence external energy has to be supplied to bring about the decomposition of water. The reversible cell voltage is the lowest voltage required for the process to proceed, but in general – all electrolysers provide thermal energy in addition to the electrical power, hence thermo-neutral voltage is the minimal value required to bring about forward reaction; for standard conditions, this value is 1.481 V [17]. But the cell voltage will increase with respect to the reversible voltage and applied current (as shown in Figure 3 (b)) due to additional resistances which are as follows: Ohmic losses due to resistance of cell elements (wires, electrodes, current collectors, etc.); also caused by gas bubbling on the electrode surfaces and ionic resistivity of electrolyte. Activation overvoltage caused by electrode kinetics and is highly dependent on catalytic properties of electrode materials. Concentration overvoltage because of mass transport processes. 1.3.1.2 Advancements in Alkaline Electrolysis Technology Alkaline electrolysis of water is relatively a well-researched field and besides being a safer method, lifespan of installations may extend up to 15 years [20]. To cater to low cost production, two major strategies have been developed – either to increase the efficiency or to increase current densities. Component-wise, the following are noteworthy: 1. Electrodes a. Nickel is the most widely used electrode, but to overcome deactivation problems iron coating are utilized recently which also give integrity to the electrodes. [21] b. Holes, slots, fins, etc. are provided on surfaces for escape of gas bubbles to lower the Ohmic resistance during production [22]
c. Use of hydrophilic materials helps in bubble management d. Zero-gap configurations i.e. gap of less than 1 mm between electrodes 2. Electrocatalysts a. To reduce the over-potential, alloys of Mo and Pt give enhance activity on electrode surfaces [23] b. Ruthenium dioxide exhibited credible electrocatalytic activity for oxygen electrode. (anode) [24] Nano-rods of Ru were shown to be very good materials for cathode applications, reducing overpotential by 25% and energy consumption by 20%. [25] 3. Electrolyte a. Rather than using alkali electrolytes, usage of ionic liquids gave efficiencies greater than 94.5% [26] but viscosity of these fluids raises concerns. b. Mixing surfactants helps minimize the size of bubbles formed and lowers contact resistances [27] c. Convective electrolyte circulation facilitates easier mass transport and also mechanically breaks bubbles on the electrode surface 4. High-temperature electrolysers to further the electrochemical kinetics and increase electrolyte conductivity 1.3.2 Photoelectrolysis In 1972, [28] discovered that water can be photo-electrochemically decomposed into oxygen and hydrogen using semiconductor electrodes (TiO2) under ultraviolet light. Later in 2001 and 2006, Zou et al. [29] and Maeda et al. [30] identified the photocatalysts that can produce hydrogen under visible light. Due to its simplicity, photocatalytic water splitting is an ideal candidate for large-scale solar hydrogen production [31]. Figure 4 Modular system developed by Prof. Ernest Chua – H2 producing photocatalysis unit [32]
Recently, Prof. Ernest Chua Kian Jon from NUS Mechanical Engineering has developed a rooftop Photocatalysis-Electrolysis-Photovoltaic Tri-hybrid modular system, comprises of three reactor tubes, an innovative hybrid hydrogen filtration system, and two solar panels [32]. The modular system, requires only rainwater and sunlight, is able to produce 120 liters of hydrogen per hour. The hydrogen is stored in fuel cells which could generate power for use. Simple calculation suggests the production rate is enough to power the communal lights of one building level, which could save $1,000 to $1,500 annually in electricity costs. Additionally, the separation process is supported by a
hybrid mechanism, which can achieve almost perfect purity of 99.8 %, a level that is desirable in clean-fuel processing industries. The modular system can be implemented easily on the rooftop of any buildings, providing extra shielding from sunlight while generating electricity. Equipped with battery storage, the modular system can function ceaselessly. Further development aims to set up floating platforms for large-scale hydrogen production. 1.4
Humid Air Turbine
The conventional evaporative cooling (e.g. water cooler) are widely used in dry and hot regions, this direct evaporative cooling system (see Fig. 5) offers a fair drop of temperature (to wet bulb temperature) but the added humidity causes feeling of discomfort [33]. The use of indirect evaporative cooling solves the increased humidity with lower cooling effectiveness (~54%) [33]. Moreover, vapour compression refrigeration systems consume much more electricity and some carry the potential to pollute the environment [33]. Maisotsenko Cycle (popularly known as M-cycle) was introduced by Prof. Valeriy Maisotsenko that uses cross-flow heat exchanger and indirect evaporative cooling to cool the ambience to the dew point temperature. M-cycle air conditioning is proven to be energy efficient (1.5 - 4 times), CFC-free, and water savings compared to the conventional vapor compression cooling system [33]. For years, practical limitations have prevented the Humid Air Turbine (HAT) from being commercialized to generate electricity in an efficient and cost-effective manner, using M-cycle to cool the inlet for compression and Maisotsenko Combustion Turbine Cycle (MCTC) to humidify air with waste heat from boiler significantly reduce the equipment costs while improving the efficiency. The use of humidified air reduces the NOx emission to a very low level in the combustor air. Moreover, M-cycle works at any temperature and pressure, therefore it can help in improving any process that requires fluid cooling and/or saturated air [34]. Figure 5, shows the schematic of direct evaporative cooling (top left), indirect evaporative cooling (top right), Maisotsenko cycle (bottom right), and mechanical design of the system utilizing Maisotsenko cycle (bottom left) [33].
2. Energy Usage Human usage plays a vital role in our step to uplift the environment. Wasteful as we are, technology did progress to be more effective and efficient in our energy use. Mindful of our surroundings, every bit of energy used should be taken account and its full potential utilized. These technologies may not necessarily be a high-tech nature field. Various improvements to known devices through better design, engineering and intelligence geared to energy efficiency and reduction of carbon waste would be the best step in this endeavor. 2.1 Multi-input energy-harvesting Green technology of utilizing ubiquitous, untapped, and unrecognized energy sources can be helpful in relieving stress of hydrocarbon plants. Multi-input energy-harvesting systems by Bandyopadhyay et. al. is an example combining Solar, Thermal and Vibration Energy Harvesting interface in a single chip. [35]
Figure 6, Energy Source Diagram for Multi-input energy-harvesting systems [36] 2.2 Building Insulations Thermal comfort inside the structures consumes a big part of our energy consumption. A/Cs and heaters in general consumes high power in preserving a desired temperature within. However tight our structures might be, our buildings may not necessarily be keeping the internal temperature effectively. Our walls, windows and roofing (building envelop) acts like a big vent for energy to move affected by undesired temperature variations outside. Much like in any system if no proper isolation for a control environment it lessens the efficacy of our control solution. Building Envelope design and material improvements goes in a long way of next generation optimal housing systems and advance materials. Materials of high R-value (m-K/W) is gaining interest in this field. R-value is a measure of thermal resistance of materials that would help insulate our structures from energy leakages thereby conserving air-conditioning and heating power for our personal comfort. Currently, materials only achieve such values shown in
Table 1. It is however in laboratory prototypes to achieve 94.5 to 315 (m-K/W) limited by thermal short circuits from structural spaces and edge loses. With an increase in insulation, energy can be preserved inside our structures and the use of our temperature modifiers need not run 24/7. Table 1. R-values of Common Building Insulation Materials [37]
2.2.1 Phase Change Materials Another concept for building insulation is in the use of Phase Change Materials (PCM), utilizing its characteristic of thermal energy storage specifically through latent heat storage. The concept being, the PCM absorbs the thermal energy coming from the environment at specified temperatures to manage and keep constant the internal building temperature regulating the thermal comfort and energy management indoors. As environmental temperature rises, the PCM insulation gains energy preserving the temperature until eventually it changes into a liquid material storing the heat it has gained attributing it to its latent heat properties thereby retaining the internal temperature. In consequence to that, as temperature falls down the PCM at a certain temperature will release the heat stored keeping the temperature at this level until such time that the material is fully solid. Figure 7, shows a diagram of the process.
Figure 7 PCM Concept of Thermal Storage (http://www.sbec.eu.com/en/concepts/pcm/)
2.3 Smart Devices Smart devices need not only be limited to our handheld mobile devices but to various aspects of energy management and living. These devices managing energy usage can greatly increase energy efficiency reducing wasteful use. The drive comes from the point that if 5% of the grid is more efficient, the energy savings equates to eliminating fuel and greenhouse gas emissions from 53 million cars. [38] 2.3.1 Smart Grid The smart grid concept utilizes power delivery management in such a way that the grid will know when and where a high power demand it is going to supply coupled with source connectivity knowing which power plants need to generate electricity at certain power outputs; thereby maximizing on and off times generating power only when necessary storing excess energy into grid level batteries which can be tapped on demand. Typically this job is done by people, energy professionals predicting daily energy usage from past behaviors. With a smart grid the general process of energy management can be done via a computing device with supervision on how it is working. This management can rely more on efficient power plants rather than the expensive high carbon emitting plants to keep up with the grid peak changes. Seamless integration of renewable sources is also implemented which is a typical grid problem. As these power sources are not generally producing electricity in the same way as traditional plants are, the renewable energy plants would require advanced energy management techniques at the grid operation level. Smart grid’s ability to manage source power would allow seamless integration for the most part. 2.3.2 Smart Appliances Part of this technology are smart appliances, this allows the users to know the effective energy consumption and with it connected to the smart grid can give you efficient energy allows better management of the consumption for the whole energy system. Users can have better energy use control based on their appliance use preference the smart grid-aware appliances can utilize only the necessary power specified at certain times set. Usage control would allow users to be more aware and can act on their part a more efficient household. In conclusion, ABB has this to say. “The “smart grid” will combine established power technologies with advanced analytics, smart devices and automation technologies to create a power system that is more reliable, flexible, secure and efficient, and has a lower impact on the environment.” 2.4 Others There are other ways in making our structures green. Through architecture, material design, and natural sources utilization, we can achieve more efficient and passive structure. Solar illumination design for one can reduce the power use at daytime by not turning on electricity using bulbs. Buildings sourcing part of their power through their own means by solar panels and utilizing natural occurring phenomenon to be tapped as an outsource energy. We live on this day and age wherein everyone is using full utility of their handheld devices. To account such usage can be quite substantial to our dayto-day living. Using handheld energy harvesting applying it to our system can increase battery life of our devices. In turn, our frequent plug-in for charging can be minimized. The days of our devices being plugged constantly can be avoided, with this harvesting technology, our daily movements can give charge and power to our devices. Indoor Lighting in daylight consumes energy that could’ve been avoided if natural light sources are used.
3. Energy Storage Though there are numerous alternatives to generate power through renewable means, most of them are cyclic in nature and to ensure that we meet the demand of day to day activities the presence of energy storage devices is indispensable. For example, solar & wind energy resources aren’t viable above 10% without proper energy storage devices. [39] There are multiple ways to store energy i.e. mechanical, magnetic, chemical and electrochemical [40]; but of these – the electrochemical means of storing energy is the most attractive owing to its reversibility and ease of electrical connections. The major electrochemical contrivances include batteries and fuel cells and we shall discuss a little further on batteries in this article. Further to grid applications, electrification of transport sector will go a long way to curb emissions and reduce global CO2 levels. 78% of people travel less than 40 miles a day (round trip) [41] and small vehicles limited to such applications are comfortable with where the battery technology is today. But batteries need a big boost to extend to a fully electric vehicle with capabilities of present day IC engine vehicles, Table 1 may give a broader perspective on the energy requirements of different classes of vehicles. Table 2: Energy requirements per mile for city & highway conditions for different classes of vehicle (assuming 70% regenerative braking and including 65 kg passengers) [42] Vehicle Type
Mass (kg) No. of Passengers parentheses
City (Wh/mile)
Highway (Wh/mile)
in
Sub-compact
1200 (2)
138
156
Compact Car
1400 (4)
154
171
Mid-size Car
1550 (4)
166
183
Full-size Car
1700 (5)
178
194
Van
2500 (8)
283
361
3.1 Li-Air Batteries Given these demands on road, with the best performing Li-ion battery today yielding around 160 Wh/kg [43], the weight proportion of energy storage systems will amount to more than 65% of the weight and cost of the automobile [44] which needs to be countered. A comparison of gravimetric energy densities of different battery chemistries & gasoline (average tank-to-wheel efficiency 12.6% [45]) is shown in Figure 1. It’s quite clear, to meet the target of long range electric vehicles, Li-air batteries provide credible means of leverage.
Figure 8 Gravimetric energy densities of different battery chemistries & gasoline [43] 3.1.1 Types & Challenges of Li-Air Batteries On the basis of the electrolyte used, the batteries are classified into 4 types as shown in Figure 2: (a) Aqueous, (b) Aprotic, (c) Mixed & (d) Solid-state. The basic reaction in any of the Li-air batteries is given by: 2Li+ + 2e- + O2 -> Li2O2
Figure 9 – Types of Li-Air batteries [46]
Each of the fore mentioned types have their merits and drawbacks; since Lithium reacts violently with water, artificial protection is required for aqueous electrolytes [47], but they prevent electrode clogging which happens with aprotic electrolytes [48]. To overcome the mixed drawbacks of these batteries, solid-state electrolyte was proposed but the issue is lower ionic conductivities of glassceramic electrolytes [49]. Thus, the optimum model is yet to be agreed upon and Table 2 lists the requirements of the various components of Li-air battery. Table 3 Requirements for durable, high-energy automotive Li/air batteries (N = non-aqueous only, A = aqueous only) [50] Component Li Anode Li protection layers
Requirements for durable Li-Air battery Robust & Flexible containment Sufficient conductivity over longer temperature range Negligible electronic conductivity Thin & stable against Li and O2 contaminants Water stable (A)
Air electrode
Continuous gas & electrolyte network High surface area & pore volume (N) Highly selective for OH- & High OH- transport rates (A) Good H2O screening (N) High activity/mass ratio & activity/cost ratio Abundant materials Adequate Li+ conductivity at all temperatures Good stability at high temperatures Low viscosity
Membrane Catalysts Electrolyte
3.1.2 Advancements in Li-air Technology With the necessities listed in Table 2, the trending materials for Li-air batteries are as follows: 1. Air Cathode a. Based on carbon – Ketjen Black exhibited highest specific capacity because of large volume expansions which facilitated extra space to hold reaction products [51] b. Meso-cellular carbon have larger meso-pores which eases gas diffusion inside and electrolyte wetting on the outside and hence supply power for longer periods of time [52] c. To overcome carbon corrosion in the above two materials, nanostructured carbon materials with nitrogen doping has seen improved oxygen kinetics [53] d. Graphene sheets due their higher conductivity and specific surface area provided larger diffusion paths for oxygen and reported capacities as high as 8709 mAh/g [54] 2. Catalyst a. Of various metal oxides investigated, Manganese-di-oxide nanowires proved efficient to enhance capacity as well as improve cyclability [55] b. Use of NiCoO4 with Graphene cathodes have improved the polarization effects that are inherent in Li-air cells [56]
c. Platinum and Gold have been investigated, but alternate materials due to cost constraints are being explored 3. Electrolyte a. On carbonate based electrolytes - propylene carbonate and diethyl carbonate mix exhibited highest specific capacity [57] b. Ethers have higher oxidation potential of 4.5 V vs. Li/Li+ and have lower volatility. Also, they are relatively cheaper and hence are under investigation [58] c. Phosphorous oxynitride (LIPON) solid electrolytes have shown lifetime of upto 1000 cycles but ionic conductivity issues remain unsolved [59] d. Lithium super-ionic conductor has the highest ionic conductivity of all solid electrolytes that have been examined yet, but not very efficient at lower temperatures. [60] Li-Air batteries can serve as viable sources of energy storage for grid applications and automotive sectors as they have one of the highest gravimetric energy densities reported. But challenges such as optimizing the porous structure of cathode, finding inexpensive and abundant catalysts, avoiding degradation due to environmental moisture, development of novel electrolytes without release of CO2 and addressing dendrite formation in Li-metal anode have to be resolved before the technology can be fully commercialized. Nevertheless, they are one of the promising research directions in the domain of energy storage today. 4. Conclusions Technology does not give a panacea solution to solve our environmental quandaries. The technologies we have been producing so far are not sufficient to recover the environmental health. More avenues of untapped potential for efficiency must be explored and development of better energy harvesting techniques should be continued. Energy efficiency and clean energy resources are the smart answers now. Embracing and investing in clean energy sources can provide affordable energy, create a more flexible, secure, and customer-centered grid. This will allow us to reduce the carbon emissions and hence mitigating the global warming.
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