Green Energy - An Introduction

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Green energy is at the heart of all ecological strategies because it affects ... source. New renewable energy sources (solar energy, wind energy, modern.
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12 Green Energy - An Introduction SAMEER SAADOON AL-JUBOORI*

ABSTRACT

Green energy is at the heart of all ecological strategies because it affects companies in three vital areas: environmental, economic, and social. Conventional energy sources based on oil, coal, and natural gas have proven to be highly effective drivers of economic progress, but at the same time damaging to the environment and to human health. The potential of renewable energy sources is enormous as they can in principle meet many times the world’s energy demand. Renewable energy sources such as biomass, wind, solar, hydropower, and geothermal can provide sustainable energy services, based on the use of routinely available, indigenous resources. Renewable energy sources currently supply somewhere between 15 percent and 20 percent of world’s total energy demand. The supply is dominated by traditional biomass, mostly fuel wood used for cooking and heating, especially in developing countries in Africa, Asia and Latin America. A major contribution is also obtained from the use of large hydropower; with nearly 20 percent of the global electricity supply being provided by this source. New renewable energy sources (solar energy, wind energy, modern bio-energy, geothermal energy, and small hydropower) are currently contributing about two percent. A number of scenario studies have investigated the potential contribution of renewables to global energy supplies, indicating that in the second half of the 21st century their contribution might range from the present figure of nearly 20 percent to more than 50 percent with the right policies in place. Key words: Green energy, Renewable energy, Sustainable, Conventional energy, Energy scenarios Head of Electronic and Contol Engineering Dept. and Head of Climate change group at Kirkur Technical college, Kirkuk, Iraq. *Corresponding author: E-mail: [email protected].

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1. INTRODUCTION

Renewable Energy RE is any form of energy from solar, geophysical or biological sources that is replenished by natural processes at a rate that equals or exceeds its rate of use. RE is obtained from the continuing or repetitive flows of energy occurring in the natural environment and includes resources such as biomass, solar energy, geothermal heat, hydropower, tide and waves, ocean thermal energy and wind energy. However, it is possible to utilize biomass at a greater rate than it can grow or to draw heat from a geothermal field at a faster rate than heat flows can replenish it. On the other hand, the rate of utilization of direct solar energy has no bearing on the rate at which it reaches the Earth. Fossil fuels (coal, oil, natural gas) do not fall under this definition, as they are not replenished within a time frame that is short relative to their rate of utilization. Renewable energy sources are often considered alternative sources because, in general, most industrialized countries do not rely on them as their main energy source. Instead, they tend to rely on non-renewable sources such as fossil fuels or nuclear power. Because the energy crisis in the United States during the 1970s, dwindling supplies of fossil fuels and hazards associated with nuclear power, usage of renewable energy sources such as solar energy, hydroelectric, wind, biomass, and geothermal has grown. Renewable energy comes from the sun (considered an “unlimited” supply) or other sources that can theoretically be renewed at least as quickly as they are consumed. If used at a sustainable rate, these sources will be available for consumption for thousands of years or longer. Unfortunately, some potentially renewable energy sources, such as biomass and geothermal, are actually being depleted in some areas because the usage rate exceeds the renewal rate. Fig. 1, shows paths of energy from source to service[1-4]. 2. WHY RENEWABLE ENERGY?

Today we primarily use fossil fuels to heat and power our homes and fuel our cars. It’s convenient to use coal, oil, and natural gas for meeting our energy needs, but we have a limited supply of these fuels on the Earth. We’re using them much more rapidly than they are being created. Eventually, they will run out. And because of safety concerns and waste disposal problems, the United States will retire much of its nuclear capacity by 2020. In the meantime, the nation’s energy needs are expected to grow by 33 percent during the next 20 years. Renewable energy can help fill the gap. Even if we had an unlimited supply of fossil fuels, using renewable energy is better for the environment. We often call renewable energy technologies clean or green because they produce few if any pollutants.

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Fig. 1: Paths of energy from source to service[4]

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Burning fossil fuels, however, sends greenhouse gases into the atmosphere, trapping the sun’s heat and contributing to global warming. Climate scientists generally agree that the Earth’s average temperature has risen in the past century. If this trend continues, sea levels will rise, and scientists predict that floods, heat waves, droughts, and other extreme weather conditions could occur more often. Other pollutants are released into the air, soil, and water when fossil fuels are burned. These pollutants take a dramatic toll on the environment and on humans. Air pollution contributes to diseases like asthma. Acid rain from sulfur dioxide and nitrogen oxides harms plants and fish. Nitrogen oxides also contribute to smog. Renewable energy will also help us develop energy independence and security. Replacing some of our petroleum with fuels made from plant matter, for example, could save money and strengthen our energy security[5,6]. 3. SOLAR ENERGY

Solar energy is the ultimate energy source driving the earth. Though only one billionth of the energy that leaves the sun actually reaches the earth’s surface, this is more than enough to meet the world’s energy requirements. In fact, all other sources of energy, renewable and non-renewable, are actually stored forms of solar energy. The process of directly converting solar energy to heat or electricity is considered a renewable energy source. Solar energy represents an essentially unlimited supply of energy as the sun will long outlast human civilization on earth. The difficulties lie in harnessing the energy. Solar energy has been used for centuries to heat homes and water, and modern technology (photovoltaic cells) has provided a way to produce electricity from sunlight. There are two basic forms of radiant solar energy use: passive and active. Passive solar energy systems are static, and do not require the input of energy in the form of moving parts or pumping fluids to utilize the sun’s energy. Buildings can be designed to capture and collect the sun’s energy directly. Materials are selected for their special characteristics: glass allows the sun to enter the building to provide light and heat; water and stone materials have high heat capacities. They can absorb large amounts of solar energy during the day, which can then be used during the night. A southern exposure greenhouse with glass windows and a concrete floor is an example of a passive solar heating system. Active solar energy systems require the input of some energy to drive mechanical devices (e.g., solar panels), which collect the energy and pump fluids used to store and distribute the energy. Solar panels are generally mounted on a south or west-facing roof. A solar panel usually consists of a glass-faced, sealed, insulated box with a black matte interior finish. Inside

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are coils full of a heat collecting liquid medium (usually water, sometimes augmented by antifreeze). The sun heats the water in the coils, which is pumped to coils in a heat transfer tank containing water. The water in the tank is heated and then either stored or pumped through the building to heat rooms or supply hot water to taps in the building. Photovoltaic cells generate electricity from sunlight. Hundreds of cells are linked together to provide the required flow of current. The electricity can be used directly or stored in storage batteries. Because photovoltaic cells have no moving parts, they are clean, quiet, and durable. Early photovoltaic cells were extremely expensive, making the cost of solar electric panels prohibitive. The recent development of inexpensive semiconductor materials has helped greatly lower the cost to the point where solar electric panels can compete much better cost wise with traditionally-produced electricity. Though solar energy itself is free, large costs can be associated with the equipment. The building costs for a house heated by passive solar energy may initially be more expensive. The glass, stone materials, and excellent insulation necessary for the system to work properly tend to be more costly than conventional building materials. A long-term comparison of utility bills, though, generally reveals noticeable savings. The solar panels used in active solar energy can be expensive to purchase, install and maintain. Leaks can occur in the extensive network of pipes required, thereby causing additional expense. The biggest drawback of any solar energy system is that it requires a consistent supply of sunlight to work. Most parts of the world have less than ideal conditions for a solar-only home because of their latitude or climate. Therefore, it is usually necessary for solar houses to have conventional backup systems (e.g. a gas furnace or hot-water heater). This double-system requirement further adds to its cost[2],[4],[5]&[6].

3.1. Photovoltaic Systems A photovoltaic system is composed of the PV module, as well as the balance of system (BOS) components, which include an inverter, storage devices, charge controller, system structure, and the energy network. The system must be reliable, cost effective, attractive and match with the electric grid in the future. At the component level, BOS components for grid-connected applications are not yet sufficiently developed to match the lifetime of PV modules. Additionally, BOS component and installation costs need to be reduced. Moreover, devices for storing large amounts of electricity (over 1 MWh or 3,600 MJ) will be adapted to large PV systems in the new energy network. As new module technologies emerge in the future, some of the ideas relating to BOS may need to be revised. Furthermore, the quality of the system

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needs to be assured and adequately maintained according to defined standards, guidelines and procedures. To ensure system quality, assessing performance is important, including on-line analysis (e.g., early fault detection) and off-line analysis of PV systems. The knowledge gathered can help to validate software for predicting the energy yield of future module and system technology designs. To increasingly penetrate the energy network, PV systems must use technology that is compatible with the electric grid and energy supply and demand. System designs and operation technologies must also be developed in response to demand patterns by developing technology to forecast the power generation volume and to optimize the storage function. Moreover, inverters must improve the quality of grid electricity by controlling reactive power or filtering harmonics with communication in a new energy network that uses a mixture of inexpensive and effective communications systems and technologies, as well as smart meters. Photovoltaic applications include PV power systems classified into two major types: those not connected to the traditional power grid (i.e., off-grid applications) and those that are connected (i.e., grid-connected applications). In addition, there is a much smaller, but stable, market segment for consumer applications. Off-grid PV systems have a significant opportunity for economic application in the un-electrified areas of developing countries. Of the total capacity installed in these countries during 2009, only about 1.2% was installed in off-grid systems that now make up 4.2% of the cumulative installed PV capacity of the IEA PVPS countries. Off-grid centralized PV mini-grid systems have become a reliable alternative for village electrification over the last few years. In a PV minigrid system, energy allocation is possible. For a village located in an isolated area and with houses not separated by too great a distance, the power may flow in the mini-grid without considerable losses[2],[4],[5]&[6]. 4. HYDROELECTRIC ENERGY

Hydroelectric power is generated by using the energy of flowing water to power generating turbines for producing electricity. Most hydroelectric power is generated by dams across large-flow rivers. A dam built across river creates a reservoir behind it. The height of the water behind the dam is greater than that below the dam, representing stored potential energy. When water flows down through the penstock of the dam, driving the turbines, some of this potential energy is converted into electricity. Hydroelectric power, like other alternative sources, is clean and relatively cheap over the long term even with initial construction costs and upkeep.

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But because the river’s normal flow rate is reduced by the dam, sediments normally carried downstream by the water are instead deposited in the reservoir. Eventually, the sediment can clog the penstocks and render the dam useless for power generation. Large-scale dams can have a significant impact on the regional environment. When the river is initially dammed, farmlands are sometimes flooded and entire populations of people and wildlife are displaced by the rising waters behind the dam. In some cases, the reservoir can flood hundreds or thousands of square kilometres. The decreased flow downstream from the dam can also negatively impact human and wildlife populations living downstream. In addition, the dam can act as a barrier to fish that must travel upstream to spawn. Aquatic organisms are frequently caught and killed in the penstock and the out-take pipes. Because of the large surface area of the reservoir, the local climate can change due to the large amount of evaporation occurring. The total worldwide technical potential for hydropower generation is 14,576 TWh/yr (52.47 EJ/yr) with a corresponding installed capacity of 3,721 GW, roughly four times the current installed capacity. Worldwide total installed hydropower capacity in 2009 was 926 GW, producing annual generation of 3,551 TWh/y (12.8 EJ/y), and representing a global average capacity factor of 44%. Of the total technical potential for hydropower, undeveloped capacity ranges from about 47% in Europe and North America to 92% in Africa, which indicates large opportunities for continued hydropower development worldwide, with the largest growth potential in Africa, Asia and Latin America. Additionally, possible renovation, modernization and upgrading of old power stations are often less costly than developing a new power plant, have relatively smaller environment and social impacts, and require less time for implementation. Significant potential also exists to rework existing infrastructure that currently lacks generating units (e.g., existing barrages, weirs, dams, canal fall structures, water supply schemes) by adding new hydropower facilities. Only 25% of the existing 45,000 large dams are used for hydropower, while the other 75% are used exclusively for other purposes (e.g., irrigation, flood control, navigation and urban water supply schemes). Climate change is expected to increase overall average precipitation and runoff, but regional patterns will vary: the impacts on hydropower generation are likely to be small on a global basis, but significant regional changes in river flow volumes and timing may pose challenges for planning. Hydropower can provide important services to electric power systems. Storage hydropower plants can often be operated flexibly, and therefore are valuable to electric power systems. Specifically, with its rapid response load-following and balancing capabilities, peaking capacity and power quality attributes, hydropower can play an important role in ensuring

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reliable electricity service. In an integrated system, reservoir and pumped storage hydropower can be used to reduce the frequency of start-ups and shutdowns of thermal plants; to maintain a balance between supply and demand under changing demand or supply patterns and thereby reduce the load following burden of thermal plants; and to increase the amount of time that thermal units are operated at their maximum thermal efficiency, thereby reducing carbon emissions. In addition, storage and pumped storage hydropower can help reduce the challenges of integrating variable renewable resources such as wind, solar photovoltaics, and wave power[2],[4].

4.1. Environmental and Social Impacts Although hydroelectricity is generally considered a clean energy source, it is not totally devoid of greenhouse gas emissions (GHG) and it can often have significant adverse socio-economic impacts. There are arguments now that large-scale dams actually do not reduce overall GHG emissions when compared to fossil fuel power plant. To build a dam significant amounts of land need to be flooded often in densely inhabited rural area, involving large displacements of usually poor, indigenous peoples. Mitigating such social impacts represents a significant cost to the project, which if it is even taken into consideration, often not done in the past, can make the project economically and socially unviable. Environmental concerns are also quite significant, as past experience has shown. This includes reduction in biodiversity and fish populations, sedimentation that can greatly reduce dam efficiency and destroy the river habitat, poor water quality, and the spread of water-related diseases. In fact, in the U.S. several large power production dams are being decommissioned due to their negative environmental impacts. Properly addressing these issues would result in an enormous escalation of the overall costs for producing hydropower making it far less competitive than is usually stated. As many countries move toward an open electricity market this fact will come into play when decisions regarding investments in new energy sources are being made. If the large hydro industry is to survive it needs to come to grips with its poor record of both cost estimation and project implementation[1]. 5. WIND POWER

Wind is the result of the sun’s uneven heating of the atmosphere. Warm air expands and rises, and cool air contracts and sinks. This movement of the air is called wind. Wind has been used as an energy source for millennia. It has been used to pump water, to power ships, and to mill grains. Areas with constant and strong winds can be used by wind turbines to generate electricity. Wind energy does not produce air pollution, can be virtually limitless, and is relatively inexpensive to produce. There is an

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initial cost of manufacturing the wind turbine and the costs associated with upkeep and repairs, but the wind itself is free. The major drawbacks of wind-powered generators are they require lots of open land and a fairly constant wind supply. Windmills are also noisy, and some people consider them aesthetically unappealing and label them as visual pollution. Migrating birds and insects can become entangled and killed by the turning blades. However, the land used for windmill farms can be simultaneously used for other purposes such as ranching, farming and recreation[1],[2]&[4].

5.1. Technology and Applications Modern, commercial grid-connected wind turbines have evolved from small, simple machines to large, highly sophisticated devices. Scientific and engineering expertise and advances, as well as improved computational tools, design standards, manufacturing methods, and O&M procedures, have all supported these technology developments. As a result, typical wind turbine nameplate capacity ratings have increased dramatically since the 1980s (from roughly 75 kW to 1.5 MW and larger), while the cost of wind energy has substantially declined. Onshore wind energy technology is already being manufactured and deployed on a commercial basis. Nonetheless, additional R&D advances are anticipated, and are expected to further reduce the cost of wind energy while enhancing system and component performance and reliability. Offshore wind energy technology is still developing, with greater opportunities for additional advancement. Specifically, modern large wind turbines typically employ rotors that start extracting energy from the wind at speeds of roughly 3 to 4 m/s. A wind turbine increases power production with wind speed until it reaches its rated power level, often corresponding to a wind speed of 11 to 15 m/s. At still-higher wind speeds, control systems limit power output to prevent overloading the wind turbine, either through stall control, pitching the blades, or a combination of both. Most turbines then stop producing energy at wind speeds of approximately 20 to 25 m/s to limit loads on the rotor and prevent damage to the turbine’s structural components. Wind turbine design has centered on maximizing energy capture over the range of wind speeds experienced by wind turbines, while seeking to minimize the cost of wind energy. Increased generator capacity leads to greater energy capture when the turbine is operating at rated power. Larger rotor diameters for a given generator capacity, meanwhile, as well as aerodynamic design improvements, yield greater energy capture at lower wind speeds, reducing the wind speed at which rated power is achieved. Variable speed operation allows energy extraction at peak efficiency over a wider range of wind speeds. Finally, because the average wind speed at a

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given location varies with the height above ground level, taller towers typically lead to increased energy capture. To minimize cost, wind turbine design is also motivated by a desire to reduce materials usage while continuing to increase turbine size, increase component and system reliability, and improve wind power plant operations. A system-level design and analysis approach is necessary to optimize wind turbine technology, power plant installation and O&M procedures for individual turbines and entire wind power plants. Moreover, optimizing turbine and power plant design for specific site conditions has become common as wind turbines, wind power plants and the wind energy market have all increased in size; site-specific conditions that can impact turbine and plant design include geographic and temporal variations in wind speed, site topography and access, interactions among individual wind turbines due to wake effects, and integration into the larger electricity system. Wind turbine and power plant design also impacts and is impacted by noise, visual, environmental and public acceptance issues[1].

5.2. Regional and National Status and Trends The countries with the highest total installed wind power capacity by the end of 2009 were the USA (35 GW), China (26 GW), Germany (26 GW), Spain (19 GW) and India (11 GW). After its initial start in the USA in the 1980s, wind energy growth centered on countries in the EU and India during the 1990s and the early 2000s. In the late 2000s, however, the USA and then China became the locations for the greatest annual capacity additions as shown in Fig. 2[1][4].

Fig. 2: Top-10 countries in cumulative wind power capacity

6. BIOMASS ENERGY

Biomass is the term used for all organic material originating from plants including algae, trees and crops and is essentially the collection and storage of the sun’s energy through photosynthesis. Biomass energy, or bioenergy,

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is the conversion of biomass into useful forms of energy such as heat, electricity and liquid fuels. Biomass for bioenergy comes either directly from the land, as dedicated energy crops, or from residues generated in the processing of crops for food or other products such as pulp and paper from the wood industry. Another important contribution is from post consumer residue streams such as construction and demolition wood, pellets used in transportation, and the clean fraction of municipal solid waste (MSW). The biomass to bioenergy system can be considered as the management of flow of solar generated materials, food, and fiber in our society. These interrelationships are shown in Fig. 3, which presents the various resource types and applications, showing the flow of their harvest and residues to bioenergy applications. Not all biomass is directly used to produce energy but rather it can be converted into intermediate energy carriers called biofuels. This includes charcoal (higher energy density solid fuel), ethanol (liquid fuel), or producergas (from gasification of biomass)[1][6].

Fig. 3: Biomass and bioenergy flow chart[1]

The use of biomass as a fuel source has serious environmental effects. When harvested trees are not replanted, soil erosion can occur. The loss of photosynthetic activity results in increased amounts of carbon dioxide in the atmosphere and can contribute to global warming. The burning of biomass also produces carbon dioxide and deprives the soil of nutrients it normally would have received from the decomposition of the organic matter.

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Burning releases particulate matter (such as ash) into the air which can cause respiratory health problems. In 2008, biomass provided about 10% (50.3 EJ/yr) of the global primary energy supply. Major biomass uses fall into two broad categories: • Low-efficiency traditional biomass such as wood, straws, dung and other manures are used for cooking, lighting and space heating, generally by the poorer populations in developing countries. This biomass is mostly combusted, creating serious negative impacts on health and living conditions. Increasingly, charcoal is becoming secondary energy carrier in rural areas with opportunities to create productive chains. • High-efficiency modern bioenergy uses more convenient solids, liquids and gases as secondary energy carriers to generate heat, electricity, combined heat and power (CHP), and transport fuels for various sectors. Liquid biofuels include ethanol and biodiesel for global road transport and some industrial uses. Biomass derived gases, primarily methane, from anaerobic digestion of agricultural residues and municipal solid waste (MSW) treatment are used to generate electricity, heat or both. The most important contribution to these energy services is based on solids, such as chips, pellets, recovered wood previously used and others. Heating includes space and hot water heating such as in district heating systems. The estimated total primary biomass supply for modern bioenergy is 11.3 EJ/yr and the secondary energy delivered to end use consumers is roughly 6.6 EJ/yr. Additionally, the industry sector, such as the pulp and paper, forestry, and food industries, consumes approximately 7.7 EJ of biomass annually, primarily as a source for industrial process steam. Global bioenergy use has steadily grown worldwide in absolute terms in the last 40 years, with large differences among countries. In 2006, China led all countries and used 9 EJ of biomass for energy, followed by India (6 EJ), the USA (2.3 EJ) and Brazil (2 EJ). Bioenergy provides a relatively small but growing share of Total Primary Energy Supply: TPES (1 to 4% in 2006) in the largest industrialized countries (grouped as the G8 countries: the USA, Canada, Germany, France, Japan, Italy, the UK and Russia). The use of solid biomass for electricity production is particularly important in pulp and paper plants and in sugar mills. Bioenergy’s share in total energy consumption is generally increasing in the G8 countries through the use of modern biomass forms (e.g., co-combustion or co-firing for electricity generation, space heating with pellets) especially in Germany, Italy and the UK. By contrast, in 2006, bioenergy provided 5 to 27% of TPES in the largest developing countries (China, India, Mexico, Brazil and South Africa), mainly through the use of traditional forms, and more than 80% of TPES in the

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poorest countries. The bioenergy share in India, China and Mexico is decreasing, mostly as traditional biomass is substituted by kerosene and liquefied petroleum gas within large cities. However, consumption in absolute terms continues to grow. This trend is also true for most African countries, where demand has been driven by a steady increase in wood fuels, particularly in the use of charcoal in booming urban areas. Three principal categories are more or less comprehensively considered in assessments of biomass resource potentials: • Primary residues from conventional food and fiber production in agriculture and forestry, such as cereal straw and logging residues; • Secondary and tertiary residues in the form of organic food/forest industry by products and retail/post consumer waste; and • Plants produced for energy supply, including conventional food/fodder/ industrial crops, surplus round wood forestry products, and new agricultural, forestry or aquatic plants. Given that resource potential assessments quantify the availability of residue flows in the food and forest sectors, the definition of how these sectors develop is central for the outcome[1].

6.1. Biomass Energy Conversion Technologies and Applications There are a variety of technologies for generating modern energy carriers electricity, gas, and liquid fuels from biomass, which can be used at the household (~10 kW), community (~100 kW), or industrial (~MW) scale. The different technologies tend to be classed in terms of either the conversion process they use or the end product produced[1]. 6.1.1. Combustion Direct combustion remains the most common technique for deriving energy from biomass for both heat and electricity production. In colder climates domestic biomass fired heating systems are widespread and recent developments have led to the application of improved heating systems which are automated, have catalytic gas cleaning and make use of standardized fuel (such as pellets). The efficiency benefit compared to open fireplaces is considerable with advanced domestic heaters obtaining efficiencies of over 70 percent with greatly reduced atmospheric emissions. The application of biomass fired district heating is common in the Scandinavian countries, Austria, Germany and various Eastern European countries[1]. 6.1.2. Gasification Combustible gas can be produced from biomass through a high temperature thermochemical process. The term gasification commonly refers to this high temperature thermochemical conversion with the product gas called

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producer-gas, and involves burning biomass without sufficient air for full combustion, but with enough air to convert the solid biomass into a gaseous fuel. Producer-gas consists primarily of carbon monoxide, hydrogen, carbon dioxide and nitrogen, and has a heating value of 4 to 6 MJ/Nm3, or 10–15 percent of the heating value of natural gas. The intended use of the gas and the characteristics of the particular biomass (size, texture, moisture content, etc.) determine the design and operating characteristics of the gasifier and associated equipment. After appropriate treatment, the resulting gases can be burned directly for cooking or heat supply, or can be used in secondary conversion devices such as internal combustion engines or gas turbines for producing electricity or shaft work. The systems used can scale from small to medium (5–100 kW), suitable for the cooking or lighting needs of a single family or community, up to large grid connected power generation facilities consuming several hundred of kilograms of woody biomass per hour and producing 10-100 MW of electricity[1]. 6.1.3. Anaerobic digestion Combustible gas can also be produced from biomass through the low temperature biological processes called anaerobic (without air) digestion. Biogas is the common name for the gas produced either in specifically designed anaerobic digesters or in landfills by capturing the naturally produced methane. Biogas is typically about 60 percent methane and 40 percent carbon dioxide with a heating value of about 55 percent that of natural gas. Almost any biomass except lignin (a major component of wood) can be converted to biogas animal and human wastes, sewage sludge, crop residues, carbon laden industrial processing byproducts, and landfill material have all been widely used. Anaerobic digesters generally consist of an inlet, where the organic residues and other wastes are fed into the digester tank; a tank, in which the biomass is typically heated to increase its decomposition rate and partially convert by bacteria into biogas; and an outlet where the biomass of the bacteria that carried out the process and non-digested material remains as sludge and can be removed. The biogas produced can be burned to provide energy for cooking and space heating or to generate electricity. Digestion has a low overall electrical efficiency (roughly 10–15 percent, strongly dependent on the feedstock) and is particularly suited for wet biomass materials. Direct non-energy benefits are especially significant in this process. The effluent sludge from the digester is a concentrated nitrogen fertilizer and the pathogens in the waste are reduced or eliminated by the warm temperatures in the digester tank[1]. 6.1.4. Liquid biofuels Biofuels are produced in processes that convert biomass into more useful intermediate forms of energy. There is particular interest in converting

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solid biomass into liquids, which have the potential to replace petroleumbased fuels used in the transportation sector. However, adapting liquid biofuels to our present day fuel infrastructure and engine technology has proven to be nontrivial. Only oil producing plants, such as soybeans, palm oil trees and oilseeds like rapeseed can produce compounds similar to hydrocarbon petroleum products, and have been used to replace small amounts of diesel. This “biodiesel” has been marketed in Europe and to a lesser extent in the U.S., but it requires substantial subsidies to compete with diesel. Other alternative biofuels to petroleum-based fuels are alcohols produced from biomass, which can replace gasoline or kerosene. The most widely produced today is ethanol from the fermentation of biomass. In industrialized countries ethanol is most commonly produced from food crops like corn, while in the developing world it is produced from sugarcane. Its most prevalent use is as a gasoline fuel additive to boost octane levels or to reduce dependence on imported fossil fuels. In the U.S. and Europe the ethanol production is still far from competitive when compared to gasoline and diesel prices, and the overall energy balance of such systems has not been very favorable. The Brazilian Proalcool ethanol program, initiated in 1975, has been successful due to the high productivity of sugarcane, although subsidies are still required. Two other potential transportation biofuels are methanol and hydrogen. They are both produced via biomass gasification and may be used in future fuel cells. While ethanol production from maize and sugarcane, both agricultural crops, has become widespread and occasionally successful it can suffer from commodity price fluctuation relative to the fuels market. Consequently, the production of ethanol from lignocellulosic biomass (such as wood, straw and grasses) is being given serious attention. In particular, it is thought that enzymatic hydrolysis of lignocellulosic biomass will open the way to low cost and efficient production of ethanol. While the development of various hydrolysis techniques has gained attention in recent years, particularly in Sweden and the United States, cheap and efficient hydrolysis processes are still under development and some fundamental issues need to be resolved. Once such technical barriers are surmounted and ethanol production can be combined with efficient electricity production from unconverted wood fractions (like the lignin), ethanol costs could come close to current gasoline prices and overall system efficiencies could go up to about 70 percent (low heating value). Though the technology to make this an economically viable option still does not exist, promising technologies are in the works and there are currently a number of pilot and demonstration projects starting up[1].

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7. GEOTHERMAL ENERGY

Geothermal energy uses heat from the earth’s internal geologic processes in order to produce electricity or provide heating. One source of geothermal energy is steam. Groundwater percolates down though cracks in the subsurface rocks until it reaches rocks heated by underlying magma, and the heat converts the water to steam. Sometimes this steam makes its way back to the surface in the form of a geyser or hot spring. Wells can be dug to tap the steam reservoir and bring it to the surface, to drive generating turbines and produce electricity. Hot water can be circulated to heat buildings. Regions near tectonic plate boundaries have the best potential for geothermal activity. The western portion of the United States is most conducive for geothermal energy sources, and over half of the electricity used by the city of San Francisco comes from the Geysers, a natural geothermal field in Northern California. California produces about 50 percent of the world’s electricity that comes from geothermal sources. Entire cities in Iceland, which is located in a volcanically active region near a mid ocean ridge, are heated by geothermal energy. The Rift Valley region of East Africa also has geothermal power plants. Geothermal energy may not always be renewable in a particular region if the steam is withdrawn at a rate faster than it can be replenished, or if the heating source cools off. The energy produced by the Geysers region of California is already in decline because the heavy use is causing the underground heat source to cool. Geothermal energy recovery can be less environmentally invasive than engaging in recovery methods for non-renewable energy sources. Although it is relatively environmentally friendly, it is not practical for all situations. Only limited geographic regions are capable of producing geothermal energy that is economically viable. Therefore, it will probably never become a major source of energy. Global geothermal technical potential is comparable to global primary energy supply in 2008. For electricity generation, the technical potential of geothermal energy is estimated to be between 118 EJ/yr (to 3 km depth) and 1,109 EJ/yr (to 10 km depth). For direct thermal uses, the technical potential is estimated to range from 10 to 312 EJ/yr. The heat extracted to achieve these technical potentials can be fully or partially replenished over the long term by the continental terrestrial heat flow of 315 EJ/yr at an average flux of 65 mW/m2. Thus, technical potential is not likely to be a barrier to geothermal deployment (electricity and direct uses) on a global basis. Whether or not the geothermal technical potential will be a limiting factor on a regional basis depends on the availability of EGS technology.

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There are different geothermal technologies with distinct levels of maturity. Geothermal energy is currently extracted using wells or other means that produce hot fluids from: (a) Hydrothermal reservoirs with naturally high permeability; and (b) EGS-type reservoirs with artificial fluid pathways. The technology for electricity generation from hydrothermal reservoirs is mature and reliable, and has been operating for more than 100 years. Technologies for direct heating using geothermal heat pumps (GHP) for district heating and for other applications are also mature. Technologies for EGS are in the demonstration stage. Direct use provides heating and cooling for buildings including district heating, fish ponds, greenhouses, bathing, wellness and swimming pools, water purification/ desalination and industrial, and process heat for agricultural products and mineral drying. Geothermal resources have been commercially used for more than a century. Geothermal energy is currently used for base load electric generation in 24 countries, with an estimated 67.2 TWh/yr (0.24 EJ/yr) of supply provided in 2008 at a global average capacity factor of 74.5%; newer geothermal installations often achieve capacity factors above 90%. Geothermal energy serves more than 10% of the electricity demand in 6 countries and is used directly for heating and cooling in 78 countries, generating 121.7 TWh/yr (0.44 EJ/yr) of thermal energy in 2008, with GHP applications having the widest market penetration. Another source estimates global geothermal energy supply at 0.41 EJ/yr in 2008. Environmental and social impacts from geothermal use are site and technology specific and largely manageable. Overall, geothermal technologies are environmentally advantageous because there is no combustion process emitting carbon dioxide (CO2), with the only direct emissions coming from the underground fluids in the reservoir. Historically, direct CO2 emissions have been high in some instances with the full range spanning from close to 0 to 740 g CO2/kWhe depending on technology design and composition of the geothermal fluid in the underground reservoir. Direct CO2 emissions for direct use applications are negligible and EGS power plants are likely to be designed with zero direct emissions. Life cycle assessment (LCA) studies estimate that full lifecycle CO 2 equivalent emissions for geothermal energy technologies are less than 50 g CO2eq/ kWhe for flash steam geothermal power plants, less than 80 g CO2eq/kWhe for projected EGS power plants, and between 14 and 202 g CO2eq/kWhth for district heating systems and GHP. Local hazards arising from natural

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phenomena, such as micro-earthquakes, may be influenced by the operation of geothermal fields. Induced seismic events have not been large enough to lead to human injury or relevant property damage, but proper management of this issue will be an important step to facilitating significant expansion of future EGS projects. Several prospects exist for technology improvement and innovation in geothermal systems. Technical advancements can reduce the cost of producing geothermal energy and lead to higher energy recovery, longer field and plant lifetimes, and better reliability. In exploration, research and development (R&D) is required for hidden geothermal systems (i.e., with no surface manifestations such as hot springs and fumaroles) and for EGS prospects. Special research in drilling and well construction technology is needed to reduce the cost and increase the useful life of geothermal production facilities. EGS require innovative methods to attain sustained, commercial production rates while reducing the risk of seismic hazard. Integration of new power plants into existing power systems does not present a major challenge, but in some cases can require extending the transmission network. Geothermal-electric projects have relatively high upfront investment costs but often have relatively low Levelized costs of electricity (LCOE). Investment costs typically vary between USD2005 1,800 and 5,200 per kW, but geothermal plants have low recurring ‘fuel costs’. The LCOE of power plants using hydrothermal resources are often competitive in today’s electricity markets, with a typical range from US cents2005 4.9 to 9.2 per kWh considering only the range in investment costs provided above and medium values for other input parameters; the range in LCOE across a broader array of input parameters is US cents2005 3.1 to 17 per kWh. These costs are expected to decrease by about 7% by 2020. There are no actual LCOE data for EGS power plants, as EGS plants remain in the demonstration phase, but estimates of EGS costs are higher than those for hydrothermal reservoirs. The cost of geothermal energy from EGS plants is also expected to decrease by 2020 and beyond, assuming improvements in drilling technologies and success in developing well-stimulation technology. Fig. 4 shows schematic diagram of a geothermal binary-cycle power plant. Current levelized costs of heat (LCOH) from direct uses of geothermal heat are generally competitive with market energy prices. Investment costs range from USD2005 50 per kWth (for uncovered pond heating) to USD2005 3,940 per kWth (for building heating). Low LCOHs for these technologies are possible because the inherent losses in heat-to electricity conversion are avoided when geothermal energy is used for thermal applications. Future geothermal deployment could meet more than 3% of global electricity demand and about 5% of the global demand for heat by 2050.

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Fig. 4: Schematic diagram of a geothermal binary-cycle power plant[1]

Evidence suggests that geothermal supply could meet the upper range of projections derived from a review of about 120 energy and GHG reduction scenarios. With its natural thermal storage capacity, geothermal energy is especially suitable for supplying base-load power. By 2015, geothermal deployment is roughly estimated to generate 122 TWhe/yr (0.44 EJ/yr) for electricity and 224 TWhth/yr (0.8 EJ/yr) for heat applications. In the long term (by 2050), deployment projections based on extrapolations of longterm historical growth trends suggest that geothermal could produce 1,180 TWhe/yr (~4.3 EJ/yr) for electricity and 2,100 TWhth/yr (7.6 EJ/yr) for heat, with a few countries obtaining most of their primary energy needs (heating, cooling and electricity) from geothermal energy. Scenario analysis suggests that carbon policy is likely to be one of the main driving factors for future geothermal development, and under the most favorable climate policy scenario (