5.3.1 Installed Renewable Power Capacity

According to the International Energy Agency (IEA), the share of renewable energy in the world's total primary energy supply (TPES) was about 14% in 2016, including solid biofuels extensively used in developing countries for conventional residential heating and cooking (IEA 2018b). In modern energy systems, renewable energy sources are used at a commercial scale either for electric generation or industrial heat production. If residential use of conventional solid biofuels (firewood, charcoal, manure, etc.) is excluded, modern renewables account for 10.6% in the world's TPES (REN21 2019, p. 31, fig. 1). The share of renewable sources in electric generation and heat production are shown in Figure 5.1 (REN21 2019). Figure 5.2 shows the installed renewable energy capacity of electric and thermal conversion units in 2018 (REN21 2019, p. 186, Table R1).

Hydropower traditionally has been the biggest source of renewable energy. In modern energy systems hydropower is delivered to end users solely as electric energy.

Use of solar energy has significantly grown since the beginning of the twenty‐first century and almost reached hydropower. In 2018, the world's solar energy conversion capacity was composed of 54% photovoltaic (PV) electric generation facilities and 46% solar thermal units. The largest amount of solar thermal energy is used for water heating by residential scale solar collectors. Commercial scale CSP facilities produce heat for industrial processes or electric generation.

Wind power is the third renewable source but second in electric generation because, like hydropower, wind power is also entirely used for electric generation. Direct conversion of wind force into mechanical work is negligible since sailboats are no longer used for transportation, and mechanical windmills have become antique.

The largest amount of bio‐power is converted into heat. More than two‐thirds of biofuels are used for heat production; including liquid biofuels used in transportation. The rest of bio‐power capacity is used for electric generation.

Although the Earth's crust contains immense geothermal power, only a small portion of it is currently converted to usable forms of energy. About two‐thirds of geothermal capacity produces heat for end‐users. Direct end‐use of geothermal power is either through thermodynamic heat pumps for space heating or hot springs for recreational purposes. At a commercial scale, geothermal energy is delivered to end users in the form of electricity. Geothermal electric generation systems use thermal energy from the Earth to produce steam, and then generate electricity by turbine‐generator units similar to conventional thermoelectric plants.

Among all renewables, bio‐power and industrial scale geothermal power plants emit greenhouse gases. Small‐scale geothermal heat pumps (GHPs) used for space heating have negligible carbon footprint and other gas emissions. Other renewables are carbon‐free and do not emit greenhouse gases during their operation.

Figure 5.3 shows the use of renewable energy sources by geographic regions in 2016 (IRENA 2019). In all regions, hydropower is the leading renewable source for electric generation. Asian countries, in particular China and India, exploit the highest renewable energy potential. Hydroelectric generation in Asia is more than double that of any other region, and solar electric generation is also highest in Asia. Western European countries use wind power more than all other regions. Electric generation from biofuels is also greatest in western Europe. Use of wind, solar, and geothermal energy is insignificant in Eurasia, which includes the Russian Union and former Soviet Union countries in Europe. In the chart, “others” includes Oceania, Central America, and the Caribbean Islands, where the combined renewable electric generation is less than all other regions.

The share of renewable energy in electric generation has considerably increased since the 1980s, and reached 25% in 2017 (BP 2018). The trend of yearly electric generation from major renewable energy sources in the world is shown in Figure 5.4. At the end of the twentieth century, hydropower was generating more than 90% of the renewable electricity and the contribution of other renewable sources was less than 10%. Between 2000 and 2016, wind, sun, and biomass contributed 30% to electricity generated worldwide from all renewable sources.

5.3.2 Capacity Factor

Since renewable sources are not regularly available at all times, the energy output of a renewable conversion unit changes continuously. Such units, operating at variable power, cannot deliver the maximum energy that corresponds to their continuous operation at full capacity. For example, consider a generation unit that has 1‐MW generation capacity that operates only four hours a day and does not produce any energy for the rest of the day; it would produce 4 × 365 = 1460 MWh of energy. On the other hand, if this unit operated at full capacity at all times in a year, it would deliver 8766 MWh of energy. Thus, such a unit is equivalent to a smaller unit with a capacity of 1460/8766 = 0.166 MW capacity.

The capacity factor of an energy conversion unit is the ratio of the electrical energy produced over a certain time interval to the electrical energy that the unit could have produced if it operated continuously at full power during the same period. Suppose that an electric generation facility with P_c kW installed capacity generates W_out kWh output energy over one year. The yearly capacity factor is obtained using the Eq. (5.2), where the factor 8766 is the number of hours in a calendar year, P_c is the rated capacity, and W_out represents the total actual energy output.

(5.2)

Energy output of generation units powered by fossil fuels, and nuclear reactions are more predictable and easier to control. Biofuel fired plants, conventional hydroelectric power plants, and pumped storage hydroelectric units also deliver steady output energy since their input is controllable. The capacity factor of such generation plants depends on the control strategies applied by system operators to balance electric generation and demand on the interconnected network.

On the other hand, wind and solar powered units, as well as small hydropower plants without a reservoir produce variable output energy depending on the uncontrollable input they receive. The capacity factor of such conversion units depends on the frequency distribution of the renewable energy source. Therefore, conversion facilities with the same capacity at separate locations may produce different yearly total energy.

Table 5.1 shows the regional capacity factors of electric generation plants powered by various types of renewable energy sources. Geothermal, biogas, and large hydroelectric power plants are typically used at a higher capacity factor. Solar and wind powered generation units, however, have a lower capacity factor due to the occurrence cycles and frequency distribution of natural phenomena that produce them. Generation stations powered by waste and bioenergy usually have a smaller capacity compared to fossil fuel, nuclear, and conventional hydroelectric plants. They are mostly used to balance the electric generation and demand. This is reflected by the lower capacity factor of such stations.

5.4.1 Assessment of Non‐combustible Resources

Wind and hydro energy are mainly delivered to end users in the form of electricity. Currently, more than two‐thirds of the harnessed solar energy is converted into electric power and one‐third is used for water heating. Yearly total energy potential of a wind, hydro, or solar energy resource can be roughly estimated using the yearly average or mean values. Since wind speed, solar irradiance, and water flow of rivers continuously change in time, a more realistic evaluation should be done based on frequency distributions rather than the long‐term averages. Depending on the type of the renewable source, the maximum, minimum, average, and standard deviation of the variables used in energy calculations are measured and recorded over a monthly or yearly cycle. In practice, 10‐minute or 1‐hour data sampling intervals are adequate for a realistic evaluation.

5.4.2 Assessment of Biomass Resources

Biofuels are evaluated based on their heating value, similar to fossil fuels. The difference between bioenergy sources from fossil fuels is that they are cultivated instead of being extracted. The capacity of a field to grow biofuel resources is estimated similarly to other agricultural products. Land area, water availability, and transportation are the main considerations. Existing farmlands are sometimes used to grow sugar cane, soybean, corn, or other carbohydrate‐rich plants for energy production. Seasonal and yearly production of bioenergy can be estimated at high confidence levels. Forecasting the harvest variations due to climatic changes over several years and even decades is more complicated and depends on projections and assumptions.

Energy production from waste relies on the amount, disposal rate, and composition of the collected waste. Industrial and municipal wastes have different heating values. For example, timber and furniture industries produce considerable amounts of sawdust, wood chips, and pellets, which can be directly used for heating. Municipal waste contains a mixture of combustible and non‐combustible substances. The direct heating value of municipal waste is not suitable for direct energy conversion. Municipal waste must be screened and processed to obtain a liquid or gas fuel. Energy produced from waste depends on the fraction of combustible components and process technologies used to convert waste into synthetic fuel.

The potential of waste to energy in a region is assessed by the type of waste and the rate it is produced. In densely populated areas like European cities, municipal waste has greater potential for district heating. In sparsely populated areas, it is more convenient to produce either biogas or generate electric energy from waste.

5.6.1 Solar Resource Potential

Maximum irradiance at a location mainly depends on the latitude, elevation from sea level, and clarity of the air. Direct normal irradiance (DNI) on the Earth's surface at a certain location is the power per unit area orthogonal to the solar beam, measured in W/m². DNI is equal to the solar radiation that reaches the top of the atmosphere minus atmospheric losses. The length sunlight travels in the atmosphere changes during the day by the solar elevation called zenith angle. In addition, clarity of the sky changes with atmospheric conditions, motion of the clouds, pollution, and precipitation. Consequently, DNI significantly changes in time.

Diffuse horizontal irradiance (DHI) is the radiation received on the Earth's surface from sunlight scattered in the atmosphere by clouds, fog, or haze. DHI is also measured on a horizontal surface in kW/m² and its value depends on the opacity of the air and reflection of the surrounding ground. The radiation emitted from the sun disk (circumsolar radiation) is subtracted from the radiation diffused in the atmosphere. The maps in Figure 5.6 show long‐term average of daily and yearly sums of DNI and global horizontal irradiance (GHI) (Solargis 2017).

GHI is the total radiant power received from the sun on a horizontal surface on Earth. It is the sum of DHI and the component of the DNI perpendicular to the horizontal surface. At an instant when the incident angle of the Sun's ray with the perpendicular to the surface (zenith angle) is z, GHI can be calculated using Eq. (5.4).

(5.4)

Solar energy at a certain location is defined as solar irradiation or simply insolation and measured in kWh/m². Insolation changes throughout the year with the length of daylight time. Yearly total insolation depends on the latitude and clarity of the sky at a particular location. The average of daily irradiations over a year allows a rough estimation of the yearly total solar energy received at a location. Solar irradiation received at a location is obtained by integrating GHI over a certain time interval, typically one day. If DHI and DNI are recorded with a sampling interval of Δt and GHI is calculated for each record, then the integral operation can be simplified with a summation as in the second part of Eq. (5.5).

(5.5)

The available solar potential on Earth is estimated to exceed all global energy needs, even considering the projected energy use until 2100 (UNDP 2000). Development of utility scale solar generation facilities depends on the availability of suitable land and the population density of the region. In addition, air quality and shading considerably affect the energy output. Figure 5.7 shows the range of technically available solar energy potential by geographic regions based on UNDP (2000, Table 5.19). According to the Special Report of the Intergovernmental Panel on Climate Change (IPCC 2011), the total global solar potential is estimated between 1,575 and 49,837 exajoules (EJ) which is well above the current primary energy supply of 576 EJ worldwide reported by International Energy Agency (IEA 2018a).

The estimated amounts reflect the availability of solar energy as a primary resource; thus, they do not consider the limitation and efficiency of solar conversion technologies, nor technical issues regarding the integration of solar electric generation to the interconnected grid. The minimum and maximum limits are based on the assumptions of the land available for solar generation.

5.6.2 End‐use of Solar Energy

End‐uses of solar energy include heating, lighting, and electric generation. Passive solar technologies use natural sunlight without any energy conversion element or system. Active solar technologies convert solar irradiance into heat or electricity for transfer to a point of use.

5.6.3 Strengths and Challenges of Solar Energy

Sunlight is the most environmentally friendly energy source. Solar conversion systems do not release carbon dioxide, any toxic gas, any form of pollutant, or water vapor during their operation. Unlike electromechanical energy conversion systems, PV generation units do not have any moving parts (other than a solar tracker if installed). Therefore, they create no noise and do not need water for cooling. CSP generation systems that use steam turbines are similar to other thermal energy conversion systems in water use, but they are cleaner since no combustion is involved.

Solar thermal and PV systems present minimal risk of accident, and if even an accident occurs, it remains local; unlike nuclear accidents and oil spills, which present life‐threatening and long‐lasting contamination of a large area. Solar energy does not cause any land or water pollution like fossil fuels, or radioactive contamination like nuclear power plants.

Environmental impacts of large‐scale solar energy systems are mainly related to land use. A commercial size PV array changes the vegetation and animal habitat on the land it covers, thus animal migration and biodiversity may be affected of the fragmentation of the land and change of vegetation.

The efficiency of commercially available PV modules ranges between 10% and 20%, which seems lower compared to other energy conversion systems. The significance of efficiency is, however, different for renewables from fossil fuels. A 30–40% efficient fossil fuel powered generation unit burns approximately three units of primary‐source energy to deliver one unit of energy. Solar power plants, on the contrary, do not diminish the available solar energy on site. In addition, on‐site PV generation has no cost to transfer of the energy source to the point of use. Since the fuel is freely available independent of the production rate, efficiency affects the initial cost and the area dedicated to the conversion unit. Lower efficiency PV modules are generally less expensive, therefore, rather than the efficiency of a particular module type, the cost per unit power is more relevant when comparing different technologies. Higher efficiency is particularly desirable at locations where the area is limited, like the roof of a building or the land available for larger commercial solar farms. In some applications, solar trackers may be used to maximize the energy conversion by continuous position control to maintain the collector or module orthogonal to the incident sunlight. The additional cost of a solar tracker versus using fixed solar array is a decision to be made based on a comprehensive economic cost analysis.

Unavailability of sunlight at night and variable irradiance during the day is the biggest challenge of PV generation. The magnitude of daily, monthly, and yearly insolation is predictable at any location. However, the short‐term forecast of fluctuations of irradiance is more complicated and always includes uncertainties.

Integration of PV generation to the interconnected grid supplied by conventional generation systems requires comprehensive analysis and monitoring techniques. In an interconnected network supplied by diverse power plants, sudden changes of the supply or demand can cause electromechanical oscillations that may lead to serious instability problems. Random fluctuations of electric power injected into the distribution system is called penetration. To prevent the risk of instability, utility companies restrict the penetration ratio according to the power variations that can be tolerated by other types of conversion systems. Electric supply cannot depend on solar PV generation unless some form of energy storage provides backup energy. CSP generation systems regulate the short‐term fluctuations by storing thermal energy in the working fluid. Small‐scale off‐grid PV systems are generally supported by a battery backup.

5.7.1 Electric Generation Potential of Wind Resource

Wind energy is basically the kinetic energy of a moving air mass, which can be either used directly as mechanical work or converted into electric power. For example, linear mechanical work caused by the force of the wind moves a sailboat. Windmills transform wind power directly into mechanical power to turn a grinder, pump, or a mechanical apparatus. Wind turbines drive an electric generator to convert the kinetic energy of the wind into electrical energy.

Although in earlier applications wind power was used to directly produce mechanical work, modern conversion systems are mostly based on electric generation. For cost‐effective generation, wind turbines must be exposed to consistent, steady, and sustained wind with a suitable average velocity. Energy output of a turbine depends on the wind regime at the site and characteristics of the selected turbine type. Fast changes of wind velocity, gusts, and turbulence are not desirable for wind powered electric generation.

Electric generation potential of wind resource in a region is generally estimated by using the yearly average wind power density and the land area available for wind farms. Wind power density is the power delivered by a wind turbine per unit area swept by the blades. Realistic assessment of the wind potential, therefore, highly depends on the mean wind speed and frequency distribution at the height at which the wind energy is harnessed, as well as the available energy conversion technologies. Figure 5.8 shows the estimated wind power density at 100‐m above the ground (DTU 2018). The map is obtained by downscaling meteorological data and interpolating wind speed, considering terrain topography and surface roughness.

The maximum electric generation potential of global wind resource is estimated about 6000 EJ, assuming that development of a wind generation facility is feasible at regions where the average wind power density is at least 250–300 W/m² at 50‐m height, and all available area can be used for wind farms (Grubb and Meyer 1993). This estimation considers an overall efficiency of 30% representing wind turbine and transmission line losses.

Wind farms consist of arrays of properly spaced wind turbines. Distance between wind turbines affect the overall efficiency because of the turbulence and wake created by rotating blades. Land area required for commercial wind farms depends on the number of turbines, selected turbine types, rotor diameter, and tower height. For a reasonable generation efficiency, the minimum distance between turbines must be about seven times the rotor diameter, which can be as large as 110–130 m for a 2‐ to 2.5‐MW turbine. A 50‐MW wind power facility utilizing 25 of the 2‐MW turbines would therefore require more than ten square kilometers of land. During construction, each turbine blade is carried by one truck and towers are shipped in separate sections. Figure 5.9 shows a wind farm in the midwestern plains of the US and a truck carrying a wind turbine blade.

A commercial wind turbine site must be easily accessible from highways and located near high‐voltage power lines for connection to the grid. Cities, difficult terrains, forests, and inaccessible mountain areas limit commercial‐scale wind power development. Although lands used to install wind turbines can still be used for farming, a certain area needs to be secured for tower foundations switchgear and other equipment. Public acceptance, environmental concerns, and land use constraints impose additional restrictions to wind power development.

World Energy Council specialists estimated the wind resource available for electric generation by geographic regions as shown Table 5.2. The estimation assumes that 4% of the area with sufficient wind resource can be used for commercial wind farm development. The primary energy equivalent is calculated by dividing the electricity generation potential by a factor of 0.3, which is a typical efficiency of wind turbines, including transmission losses (UNDP 2000, p. 164, Table 5.21).

Kinetic energy available from wind at a certain location depends on many factors including air density, elevation, topography of the land, vegetation, buildings, and large structures.

Utility‐scale wind farms are feasible in areas where the wind speed is consistently in the operational range of available wind turbines. Average wind speed may be used for a rough estimate of the electric generation potential of the wind resource in a region. For a more realistic estimation, the frequency distribution of the wind resource and turbine characteristics should be considered. Methods for assessment of prospective wind turbine sites are described in Chapter 9.

5.7.2 Strengths and Challenges of Wind Energy

Wind is a clean, sustainable, and renewable source of energy. Wind powered generation, like solar and hydroelectric, is free from carbon dioxide or toxic emissions, particulate matter, solid waste, and land or water pollution. Unlike fossil fuel and nuclear power plants, wind powered generation units do not use any water.

Commercial wind farms have a larger footprint for the energy they generate. However, since the turbines are located at a sufficient distance to avoid wake and turbulence, the land under spaced turbine towers can be used for other purposes, including agricultural farms, roads, storage, and recreational or industrial parks.

Unlike solar irradiance, wind may be available at any time of the day. However, wind speed and direction are less predictable than solar irradiance. Because wind is a consequence of uneven solar heating, in some regions wind may complement solar energy in electric generation. Hence, wind‐solar hybrid generation may produce steadier power output during a day or year around.

Wind turbine blades drive an electric generator which directly delivers three‐phase AC power to the external circuit. While advanced electronic controls are still used to regulate voltage and frequency using an electronic converter, utility‐scale units are directly synchronized with the power grid. Wind powered energy conversion does not suffer from low thermodynamic efficiency of fossil fuel fired generation. Depending on the design, the efficiency of a wind turbine can reach as high as the theoretical maximum efficiency of 59%, known as the Betz limit.

Effective use of available wind resources relies on technologic advancements. Composite materials used to make larger, lightweight, and stronger blades combined with advanced airfoil design improve the performance of wind turbines. Strong permanent magnets made from advanced magnetic materials led to the improved design of permanent magnet synchronous generators used in small wind turbines. Since such turbines have become commercially available at affordable prices, residential and small commercial wind generation propagated from rural areas to suburbs and even cities increasing the share of distributed generation in electric supply.

Generators used in large units have evolved from constant‐speed induction machines with soft start to more controllable variable‐speed machines. Advanced technologies to allow control of ramp‐rate, output voltage, and low‐voltage‐ride‐through (LVRT) ability are emerging (Smith 2005). Technologic evolution progressively reduces the adverse effects of the variable wind power generation on the interconnections and increases its reliability.

In addition to turbine and generator technologies, structural improvements allow the use of higher turbine towers. Increased tower height has several benefits. First, longer turbine blades can be used, which result in quadratically increased rotor‐swept area, and consequently quadratic increase of the turbine power output. Larger single units increase the efficiency and unit cost of generated electricity. Turbines installed on higher towers are exposed to more uniform wind at higher velocity, hence generate electric power at a higher capacity factor.

5.7.3 Environmental Impacts of Wind Powered Generation

Although wind power generation has significantly smaller impact on the environment, development of wind farms near populated areas has created considerable public reaction. Public concerns to wind turbines can be combined in three groups; visual impact, interactions with wildlife, and audible noise.

5.8.1 Hydroelectric Potential

Hydropower is not evenly accessible at every region of the world and most of the substantial resources are remotely located. Electric power is the only energy carrier to transmit these resources to populated areas. Moreover, international and intercontinental electric grid connections distribute hydropower resources worldwide.

Hydraulic resource potential is estimated by evaluating the world's annual water balance. About 577 billion (10⁹) cubic meters of water evaporates every year from ocean and land surfaces. Out of this amount, 119 billion cubic meters precipitate on land. About two‐thirds of the precipitation is absorbed by vegetation and soil but most of this water evaporates again. The estimated amount of runoff water, which is theoretically available for energy purposes is about 47 billion cubic meters (UNDP 2000, p. 153). The amount of runoff water by continent can be estimated considering the yearly amount of inland precipitation.

Assuming that the runoff water is evenly distributed across a region, the available potential energy of water should be proportional to the product of runoff volumes and average altitude (W_p = mgh ; g ≃ 9.81). In reality, however, the runoff water is not evenly distributed. Moreover, seasonal variations of precipitation also influence the potential energy.

Technical potential depends on available technologies and feasibility of resources. Development of new technologies to exploit smaller hydroelectric sources increases the technical potential. Estimation of the technical potential varies by countries and is sometimes underestimated or inflated due to incomplete assessments or political reasons. Estimated regional theoretical and technical hydroelectric potentials are shown in Figure 5.10.

Conventional hydroelectric plants are typically designed, engineered, and developed by international consortiums. Less developed countries may not be able to afford the large initial cost of hydroelectric plants because of financial constraints. Moreover, socioeconomic impacts may increase the initial cost and delay construction of large hydroelectric plants.

5.8.2 Strengths and Challenges of Hydroelectric Generation

Hydroelectric power plants are clean and reliable energy sources. Generating electricity by hydropower instead of fossil fuels avoids significant amounts of greenhouse gas emission, air pollution, and release of toxic elements to the environment.

Reservoir backup and pumped storage hydroelectric units add massive energy storage capability to the electric power system. Since the power output can be adjusted in a relatively short time, hydroelectric power plants conveniently adjust the electric generation to the variable demand in the interconnected grid.

Development of a hydroelectric plant is a multipurpose project. Dams regulate water flow in rivers and help flood control. Reservoirs improve the regional climate and vegetation. The area around dams is often used for recreational activities and increases the value of the region.

On the other hand, like any large project, hydroelectric power generation also has adverse environmental and social impacts. Construction of a dam impacts the biodiversity by blocking the fish migration path and changing the food chain. Certain fish species, like salmon, migrate downstream as they grow up and come back to the area where they were born to lay their eggs. Blocking the natural lifecycle of such species may cause a significant decrease in population, and even lead to their extinction.

Construction of a dam causes flooding to a vast area, which generates a wide range of public reactions depending on the population density, education and economic level of the population, existence of historic landmarks, and cultural heritage value of the land. Running river hydroelectric power plants control the river flow for electric generation, which may interfere with the benefits of farmers who need the water for irrigation. Dams constructed on larger rivers also affect transportation and trade along the river. Many dams constructed in the northwest United States were designed with fish ladders and locks to resolve the environmental and socio‐economic issues.

5.9.1 Sources of Geothermal Energy

Thermal energy in the Earth comes from two sources. One of them is extremely hot magma that contains molten or semi‐molten rocks, the other is the radioactive decay of unstable elements. Magma reaching the surface directly through tectonic faults can result in volcanic eruptions, which contain huge uncontrollable thermal energy. The aquifers or ground water heated by magma or radioactive decay can rise to the surface either mixed with steam or gases, and erupt as geysers or simply form hot water springs. Natural discharges of geothermal energy are generally uncontrollable and intermittent. Thermal energy available in the earth can, nevertheless, be extracted in a controllable way by drilling until reaching a feasible heat source, depending on the intended commercial application.

Underground heat sources are transferred to the ground near the surface by either conduction or convection. The temperature gradient in the ground is non‐uniform and highly variable depending on the geological structure of the region. Geothermal energy resources can be grouped in four types (UNDP 2000):

• Hydrothermal resources. Hot water or steam found at moderate depths, typically less than 10 km.
• Geo‐pressured resources. Deep hot‐water aquifers containing dissolved methane under high pressure.
• Hot dry rocks. Extraordinarily hot geologic formations with little or no water.
• Magma. Molten or semi‐molten rocks at much higher temperatures than the average.

Exploration of geothermal sources is similar to oil and gas exploration. Because of the high cost of complete exploration, more promising zones are determined by regional geological and geochemical studies. The structure of the potential area is first thoroughly surveyed by detailed geological, geochemical, and geophysical studies. If commercially valuable resources are located within a reasonable technical certainty, then several test wells are drilled to obtain a more realistic geothermal model of the prospective site. The depth of the exploratory drilling depends on location of the reservoir and the intended use of the resource. Smaller diameter exploratory bores called “slim holes” reduce the initial investment, but their flow rate is significantly lower than standard wells. The diameter of production wells is between 9 and 13 in. (23–33 cm) to provide a fluid flow in the range from tens to hundreds of tons per hour. Test wells normally have the same profile as the production wells. Some of the test wells, therefore, may be large enough to start a small‐scale production that can offset part of the investment for exploration studies. The goal of the test wells is to obtain the physical and chemical characteristics of the reservoir. The temperature and steam/water ratio of the geothermal fluid determines the type of application. Pre‐production wells are operated at different production conditions to assess the probable production for the following 12–20 years. In addition, productive‐size exploratory drilling can produce a quantity of fluid needed for the first operation of the commercial‐size power plant, reducing the overall capital cost of the project. During the exploration phase, the prefeasibility studies are conducted to estimate the cost/benefit ratio of the site. Based on the findings, the project may be terminated or move on to the commercial‐scale development.

Current technologies allow exploitation of hydrothermal resources, which are a small fraction of the geothermal resources available in the Earth. Enhanced geothermal system (EGS) technologies are emerging to reach less accessible resources, lower permeability formations, hot dry rocks, and magma by hydraulic stimulation and fracking (IPCC 2011).

5.9.2 Geothermal Energy Potential

The theoretical global potential of geothermal energy is about 140,000,000 EJ, but 600,000 EJ is classified as useful accessible resource base. The portion of the accessible resource base expected to become economical within 40–50 years is estimated to be about 5000 EJ. Reserves accessible with current and emerging new technologies in a relatively shorter timeframe are only 500 EJ (UNDP 2000). Still, geothermal energy has enormous potential; even the technically and economically accessible reserves (about 434 EJ), exceeds the total annual global primary energy consumption. The regional distribution of geothermal energy potential is shown in Figure 5.7 and Table 5.4.

Like all other renewable resources, availability of geothermal energy also varies by location and the only way to transmit this energy to remote consumers is conversion to electricity. Geothermal power generation has many limitations. First, it suffers from low thermodynamic conversion efficiency, therefore large amounts of geothermal fluids at relatively higher temperatures must be extracted. In addition, use of geothermal energy results in significant air and water pollution. Therefore, rather than the quantity of available energy, conversion technologies and mitigation of environmental impacts determine the exploitation of geothermal energy in power generation.

5.9.3 End‐uses of Geothermal Energy

Geothermal energy is used for direct heat production, electric generation, or combined heat and power (CHP) cogeneration. Direct heat production is more efficient but transmission of heat by steam is limited to short distances from the point of use. Electric generation is more valuable, flexible, and controllable, but less efficient due to the lower thermodynamic efficiency of the conversion of heat into mechanical work. Heat and power cogeneration plants are the most efficient conversion systems since the heat rejected as the part of thermodynamic cycle is used for other purposes.

5.9.3.1 Geothermal Heating

Heating and cooling applications using a geothermal heat pump (GHP) should not be confused with conversion of geothermal energy sources into heat and power. Also known as “earth‐coupled,” and “ground‐source,” such systems operate based on the temperature difference between the air above the surface and ground or water below the surface. Depending on latitude, the temperature below the ground or water surface stays constant year‐around in the range between 45 °F (7 °C) and 75 °F (21 °C). In winter, when the air temperature is lower, a thermodynamic heat pump transfers energy from higher temperatures below the surface to heat the air in a building space. In summer the energy flow is reversed from the higher temperature in a building to cooler ground or water.

Geothermal energy is used in a broad range of direct heating applications depending on the temperature of the geothermal fluid either discharged naturally or extracted by drilling a well. Natural hot springs are among touristic attractions in touristic, therapeutic, and recreational resorts around the world. Space heating in residential districts, commercial buildings, and workshops is common in regions where hot springs are available. Industrial and commercial applications can be grouped as drying, evaporation, distillation, refrigeration, process heating, industrial‐space air conditioning, and other processes such as extraction, washing, baking, etc.

Geothermal heating is achieved by simply direct contact with the pipes that carry geothermal fluid or, in more advanced systems, using a heat exchanger to transfer geothermal steam or water to a fluid that circulates in a secondary loop. Drying, evaporation, and distillation are based on this technique.

Timber and wood preparation, pulp and paper processing, textile industry, leather and fur treatment, synthetic fuel production and enhancement, chemical processes, and mineral production are among common applications of geothermal heating. Agricultural applications include greenhouse and soil heating, mushroom cultivation, fish hatching, food processing, food drying, and canning.

An evaporation process is used to concentrate solutions. One of the common applications is used salt production. Distillation is based on evaporation and controlled condensation of a liquid. Seawater desalination, water purification, heavy water (deuterium) production, and liquor production are traditional distillation applications. Fractional distillation is used to separate components of a liquid mixture that have different boiling temperatures. The hydrocarbon industry uses fractional distillation to separate liquid petroleum gases (LPG) from crude oil and raw natural gas. Geothermal energy can be also used for cooling and refrigeration by using an appropriate thermodynamic system. Figure 5.12 shows heating fluid temperatures needed for various industrial and agricultural operations where geothermal energy can be directly applied.

5.9.3.2 Geothermal Power Generation

Industrial scale geothermal power generation needs higher temperatures and flow rates compared to direct‐use for heat. In practice, two types of reservoirs grouped as steam‐dominated and water‐dominated are exploited for electric generation or electric‐heat cogeneration. Figure 5.13 illustrates the operating principle of four basic systems used for geothermal power generation (Palmerini 1993).

Steam‐dominated reservoirs contain a superheated fluid at high temperature and pressure such that dry and saturated steam extracted at the wellhead can be directly piped to the generation system. In the simplest configuration, dry steam is fed directly to the turbine and exhausted to the atmosphere as shown in Figure 5.14(a). Despite its lower initial cost, such systems have several drawbacks. The steam coming out of the ground generally contains a mixture of gases including carbon dioxide, hydrogen sulfide, and numerous toxic gases. Part of these gases can be condensed and processed to obtain liquid carbon dioxide, sulfur, and other industrial products. The system shown in Figure 5.14(b) uses a gas extractor to separate condensate and non‐condensable gases. More advanced systems use one or more upstream surface heat exchangers/evaporators to separate practically all gas content from steam.

Water‐dominated reservoirs that contain hot water or a mixture of water and steam are more common. In the reservoir the fluid is under high pressure, as it rises in the well, the pressure drops and a mixture of liquid and steam reaches the wellhead. The fluid can be maintained in liquid phase by increasing the pressure by pumps in the well or extracted in a mixed phase. Depending on the reservoir temperature, either pressurized or mixed phase fluid can be used for power generation.

Pressurized fluid evaporated in a flasher/separator the geothermal emits a mixture of steam and gases. In a flashed system shown in Figure 5.14(c) steam and non‐condensate gases extracted from the geothermal fluid enters the turbine and residual water is reinjected into the ground. The reinjected fluid can be as high as 80% of the extracted geothermal fluid. The mixture of steam and gases exiting the turbine is processed in a condenser and gas extractor to separate the condensate and non‐condensate components.

Figure 5.14(d) illustrates the operation of a “binary system” that converts energy from mixed‐phase fluid. A heat exchanger transfers heat to a working fluid, which circulates in a secondary loop through the turbine for thermodynamic conversion. The cooled geothermal fluid exiting the heat exchanger is reinjected into the ground. Such systems utilize a special type of turbine that operates at a lower temperature range than the turbines fed by steam generated in fossil‐fuel fired boilers. The thermodynamic cycle implemented in such systems is named “organic Rankine cycle” (ORC) because it uses an organic compound, such as isobutane, with a lower boiling temperature than water at atmospheric pressure. ORC allows operation of the turbine at the temperature range available from geothermal sources.

The efficiency, which is the ratio of the useful energy output to the thermal energy available from the source strongly depends on the heat‐sink temperature limited by the ambient temperature. Geothermal sources used for power generation can increase the working fluid temperature up to a few hundred degrees Celsius. In the case of low‐temperature sources, especially below 100 °C, the efficiency can be as low as 10–20%. Power‐heat cogeneration systems increase the overall efficiency by recovering the excess heat for direct heating applications.

Depending on the physical characteristics of the heat source, practical geothermal conversion systems may combine multiple stage upstream reboilers, pre‐flash or double flash processes or combinations of flashed and binary cycles (Palmerini 1993).

Geothermal fluid requirement of various power plant types depends on the reservoir temperature, as shown in Table 5.5. The lowest cost plant type is exhausting into the atmosphere, but it requires the greatest amount of geothermal fluid extraction. Single flash power plants are the lower cost, but they require greater amounts of fluid extraction. They are not feasible for reservoir temperature less than 150 °C. Multiple flash and binary cycle facilities need higher initial investment, but they can operate with less geothermal fluid. Direct steam dominant power plants are preferred for reservoir temperature is above 200 °C

From efficiency and fluid consumption standpoint, direct use of geothermal heat is clearly more advantageous than electric generation. Direct heating applications are, however, limited by difficulty of transfer to end‐users. However, electric power is more flexible and controllable.

Cycle	Required GT fluid supply (kg/kWh)
Reservoir temperature (°C)	120	150	200	250
Single flash
Free exhausting (Figure 5.13a)	—	650	150	80
Condensing (Figure 5.13b)	—	150	80	50
Binary cycle (Figure 5.13d)	400	140	70	—

5.9.4 Strengths and Challenges of Geothermal Energy

Geothermal energy resources are considered renewable since the heat tapped from an active reservoir is continuously restored by natural heat production, conduction, and convection from surrounding hotter regions. The extracted geothermal fluids are naturally replenished or returned to the reservoir after heat has been transferred to the conversion process.

Industrial power plants, on the other hand, may cause local declines in pressure and temperature within the reservoir over their economic lifespan. The cooled zones with reduced pressure are eventually reformed from surrounding regions after extraction ends.

Initial and operating cost of geothermal power plants are typically higher than other types of generation facilities because of the special material used to prevent corrosion, as well as periodic maintenance requirements of pipes and equipment. Electric generation in such plants is, however, steady and controllable. In general, geothermal power plants serve as base load supply in interconnected electric systems.

Turbines used in geothermal electric generation operate on the Rankine cycle, which has thermodynamic efficiency limited by the temperature of the heat source above ambient temperature. Power generation units fed from lower temperature reservoirs have overall efficiency from single digits to about 20%. To increase the efficiency, steam leaving the turbine must be cooled either by water or air. Water‐cooled systems evaporate considerable amount of fresh water.

Geothermal energy is not as clean as solar, wind, and hydropower. Environmental impacts of geothermal energy can be divided in two groups. Temporary issues are related to pollution during the exploration and drilling phase. Permanent environmental impacts are associated with maintenance, make‐up drilling, and pollution that result from power plant operation. Geothermal fluids contain sulfur, ammonia, and heavy metals such as mercury. Although geothermal energy conversion processes release carbon dioxide (a mixture of polluting gases including methane, nitrogen oxides, and hydrogen sulfide), they are less than greenhouse gases released by fossil fuel combustion (Goldstein, Hiriart, Bertani et al. 2011) Whereas toxic elements such as mercury and arsenic are also released, they are in trace quantities and generally do not create a significant environmental concern. The operational pollution created by geothermal power plants, however, is still less than fossil fuel fired energy production. On the other hand, numerous geothermal byproducts that have commercial value can be retrieved from geothermal fluid. Some examples are boric acid, sulfur, carbon dioxide, potassium salts, and silica. Processing of exhaust gases has the double benefit of production of valuable chemicals while reducing the toxic emissions.

Residual water from geothermal power plant operation is reinjected into the same reservoir to be reheated by the natural process. The production wells and reinjection bores must be effectively isolated by casing to prevent pollution of fresh ground water. In addition, extraction and reinjection of the thermal fluid may cause seismic activity and vibrations that can trigger earthquakes.

Because geothermal energy is a domestic resource, its use to replace foreign oil and natural gas enhances energy security of nations. Like other renewable resources, geothermal energy is also independent from volatility of energy prices due to economic and political conditions.

5.10.1 Biomass Sources

Bioenergy is produced by processing a wide variety of materials commonly known as biomass, obtained from living organisms such as plants, animals, algae, and bacteria. Such materials are mainly hydrocarbons produced by either photosynthesis or biosynthesis. Biomass resources are abundant in most parts of the world.

Plant‐based biomass includes woody or non‐woody substances, processed waste, or processed fuel. Trees, shrubs, bushes, bamboo, palms are woody biomass. Non‐woody biomass, mainly used for biofuel production, is from energy crops such as sugarcane, soybean, and corn. Other examples are residues, and clippings and roots of industrial plants such as cereal straw, cotton, tobacco, bananas, potatoes, etc. Processed wastes comprise industrial byproducts like sawmill chip and dust, wood bark, plant oil cake, black liquor from pulp and paper mills, and paper products in collected trash. Processed fuels are charcoal, methanol, ethanol, plant oils, biogas, and similar biomass ready to be used as fuel.

Residues from forest industries are valuable byproducts to produce biofuels. However, Biomass feedstock production and regeneration of the forests must be balanced by replanting and sustainable management to allow natural propagation. Agricultural residues can be also used for biofuel production, but some part of these residues must remain at the site for fertilization and soil conditioning.

Ethanol made from sugar cane, soybean, and corn is a potential alternative substitute to petroleum‐based fuels. The fuel‐ethanol industry is extremely water‐ and energy‐intensive. Especially, the cultivation of corn for ethanol production requires vast agricultural land, substantial amounts of ground water, and energy for heavy agricultural equipment. Depending on the climate, production of one unit volume of corn ethanol requires between 3 and 160 units of fresh water from farming to processing (Wu and Chiu 2011). Interactions of biofuel production with ground water and food supply are discussed in more detail in Chapter 12. A cornfield and an ethanol production facility in the Midwest US are depicted in Figure 5.14. In the US, retail gasoline sold at most gas stations contains 10% corn‐based ethanol. Regular vehicle engines can burn gasoline that contains up to 15% (E‐15) or can be modified to use a mixture of 15% gasoline and 85% ethanol (E‐85). The gas pump in Figure 5.14 displays retail prices of various gasoline‐ethanol mixtures in 2017.

Municipal solid waste is a renewable source indirectly associated with biomass since it contains paper, cardboard, vegetable residues, discarded food, etc. Energy potential of municipal solid waste depends on the composition of the refuse materials and recycling policies. Industrialized countries generate 0.9–1.9 kg per capita of municipal solid waste every day. Energy contents range from 4 to 13 MJ/kg and reach as high as 15.9 MJ/kg in Canada and the United States (UNDP 2000).

In many developed countries special power plants burn municipal solid waste to produce steam for electric generation and/or district heating. Municipal waste may also be processed to produce methane and synthetic fuels.

Because disposal of municipal solid waste for landfill in densely populated urban areas is increasingly constrained, energy conversion becomes more profitable. Figure 5.15 shows a waste‐burning CHP plant constructed on the Danube River in a highly populated district of Vienna. The Spittelau waste incineration plant processes 250,000 tons of municipal waste every year to generate 120,000 MW‐hours of electricity and 500,000 MW‐hours of district heating.

Separating and recycling of non‐combustible contents improves the energy output of waste‐to‐energy power plants. Municipal solid waste incineration requires advanced pollution abatement equipment to prevent harmful emission of toxic and polluting gases.

Discarded animal parts from food industry, animal fat, and manure are animal‐based biomass resources. Dung, which is a traditional solid biofuel in rural parts of some poor countries, can be processed to obtain biogas, which is a valuable modern renewable. Whale oil used to be the main fuel for lighting before petroleum. All animal residues can be used to produce liquid or gas biofuels. Biogas extracted from sewage, part of the byproducts of the food industry, and most of municipal wastes are animal‐based biomass. Figure 5.16 shows the share of biomass sources in the primary bioenergy mix (IEA 2009).

5.10.2 Energy Potential of Biomass Resources

Estimation of energy conversion potential of biomass resources is generally based on the size of land available for biomass plantation, the regional distribution of this land and distances to consumption centers, the productivity of the land for biomass production including the technical and economic performance of conversion technologies, and net energy balance.

The growth of biomass supply in industrialized countries is mainly based on the development of cost‐effective biomass production, collection, and conversion systems to create biomass‐derived fuels that can compete with fossil fuels in the energy supply mix with relatively less contribution to the climate change. Given the land area available for agriculture and advanced agricultural methods implemented in developed countries, the biomass production opportunities are generally considered substantial.

In developing countries, biomass production may conflict with land‐use for food production. Moreover, low‐tech use of traditional biomass is inefficient and may cause energy‐related health problems. Exposure to particulate matters and carbon monoxide discharged from incomplete combustion of biomass in closed spaces can cause respiratory infections, problems in pregnancy, and even death. Large amounts of undocumented traditional biomass in poor areas are difficult to estimate.

Several researchers estimated biomass energy potential using integrated models. The projections of primary bioenergy supply in 2050 differ in a wide range from 40–50 to 500 EJ. The variation is mainly due to assumptions and difficulties in accounting traditional applications, which are usually not officially recorded. Regional geographic differences, support policies, and degree of agricultural development are also sources of uncertainties.

According to IEA energy statistics, TPES from bioenergy sources totaled between 48 and 65 EJ in 2008 (IEA 2010). The uncertainty stems from the insufficient data about the consumption of traditional biomass (wood, straw, dung, etc.) in less developed regions using low tech conversion methods. Between 37 and 43 EJ global primary energy from traditional biomass was converted with 10–20% efficiency yielding only 3–6 EJ/yr secondary energy.

Total primary energy of 11.3 EJ supplied from modern bioenergy (processed solid and liquid biomass, biogas) was converted to secondary sources with an average efficiency of 58%. The conversion efficiency was as high as 80% in heating applications, followed by 60% in the use of biofuels for transportation. Efficiency in electric generation is around 30% because of the lower thermodynamic efficiency of heat engines. Overall efficiency of traditional and modern biomass global consumption is from 21% to 27%.

Strapasson and others estimated total global bioenergy production including traditional biomass and residues as 60 EJ/yr in 2015 (Strapasson, Woods, Chum et al. 2017). Based on the integrated models, they project the global bioenergy production in 2050 to be between 70 and 360 EJ/yr, depending on different land‐use prospects, food diet patterns, and climate change mitigation efforts. Data about the global potential of biomass energy is being continuously updated to reach a consensus on a reliable database (Rosillo‐Calle 2016).

5.10.3 Bioenergy Conversion Technologies

Biomass resources are abundant in most parts of the world. The challenge is their sustainable management, efficient conversion, and delivery to the consumers in the form of modern and affordable energy services. Biomass resources can be converted to chemical fuels by several methods.

Feedstock for bioenergy production can be grouped as oil crops, sugar and starch crops, lignocellulosic biomass, biodegradable waste, and photosynthetic microorganisms. Throughout a series of conversion steps, raw biomass is transformed into a final secondary energy source such as heat, electricity, or biofuel. Because of the wide range of biomass feedstock and diversity of end‐uses, many bioenergy process technologies have been developed.

The simplest way to use biomass is direct combustion to obtain heat. Forest wood, bushes, straw, leaves, dung, and other materials are being burned directly in many parts of the world. Advanced bioenergy routes necessitate some sort of screening, pretreatment, upgrading, and conversion steps. For example, firewood sold on the market is selected, cleaned, cut to a convenient size, perhaps treated, and packaged. Charcoal used for direct heating and cooking is basically wood processed to enhance the heating value.

Raw biomass has lower energy density compared to fossil fuels. Moreover, the moisture content may vary considerably. Transportation of biomass is, therefore, more expensive than fossil fuels that contain comparable amount of energy. Biomass must be preprocessed and treated to enhance the fuel quality.

In general, liquid and gas biofuels are cheaper to transport, store, and handle than solid biofuels. The goal of advanced processes is to convert raw biomass into high energy density synthetic fuels that can be substituted to petroleum products and natural gas. Modern bioenergy process chains can be grouped in three main categories identified as thermochemical, physicochemical, and biologic routes.

5.10.4 Strengths and Challenges of Bioenergy

Biomass is a renewable energy resource either naturally grown or produced as agricultural product. Biofuels are in general cleaner than fossil fuels, but more pollutant compared to other renewable energy sources.

Raw biomass needs additional processes to be converted into consumable bioenergy products that can compete in the energy market. Such techniques involve a complex chain of advanced processes. Despite methods developed to enhance the fuel quality, biofuels have lower energy density compared to fossil fuels. This makes transportation and handling more expensive than fossil fuels.

Unlike other renewables, biomass is not a free energy source. The cost of biomass production often represents from 50% to 90% of the total cost of final energy products delivered to consumers.

Biofuels can be easily stored, transported, and delivered to consumers like conventional fuels. In contrast to intermittent wind and solar energy, biofuels can produce controllable, continuous, reliable, and steady energy output, similar to fossil fuels. Liquid biofuels are potential alternatives to petroleum and can be used either alone or blended with oil products. However, energy density of biofuels by volume is lower than all fossil fuels.

Cultivation of biomass crops requires substantial dedicated farmland area. Land requirements depend on energy crop yields, water availability, and the efficiency of biomass conversion into usable fuels. Land use for bioenergy production has been a controversial issue. While unused agricultural land areas in many developed countries could become significant biomass production areas, long‐term projections are alarming (UNDP 2000).

Biomass production requires substantial amounts of water for irrigation. In addition, liquid biofuel and biogas production processes also use water. Water requirements for biomass depend on the climate conditions. It is estimated that in the US, ethanol production consumes 40–140 units of water per unit volume of finished product. Evidently, irrigation water returns to ground even though some part evaporates, and the total water on earth remain the same. However, as biomass production increases, in some regions the limited freshwater resources will have to be shared between agriculture of food products and biomass farming. IPCC studies indicate that even without considering water requirements for biomass production, water shortages are possible for about half the world's population as early as 2025. Thus, the water constraint for extended biomass production will likely be of importance, especially in the long term (UNDP 2000).

Large‐scale biomass production may have negative impacts on soil fertility, biodiversity, and landscape. While use of biofuels instead of fossil fuels eliminate some part of the air pollution, emissions caused by farming equipment and transportation vehicles offset part of this benefit. On the other hand, biomass cultivation can protect land from erosion.

Farming machines and transportation from farmlands to processing plants consume energy, mostly oil products. Energy balance in biofuel production is described by the ratio of energy return on energy invested (EROI). Solid biomass used directly for heat produces 10–30 times greater energy output than the energy used to produce them, hence EROI for solid biofuels ranges from 10 to 30. Energy balance is not that favorable for liquid biofuels; some estimates suggest EROI as low as 1.3 for corn ethanol produced in the US. Nevertheless, ethanol production from sugarcane has higher EROI because of the higher energy content of the initial biomass. Advancements of biofuel process technologies tend to reduce the energy used to produce liquid biofuels.

Biofuel production relies on harvesting methods and process technologies. Cost of energy used for cultivation and processing impact the competitiveness of modern biofuels. Hence, fluctuations of oil prices, market conditions, and political decisions are major factors that affect the resource potential of biofuels in long‐term.

1. Which one of the following statements is not true for renewable energy?
   1. Renewable energy sources are available in limited quantities.
   2. Renewable energy is available at every location in the world in various amounts.
   3. Most renewable sources are delivered to consumers in the form of electricity or heat.
   4. Intermittent nature of wind and solar energy presents challenges in their integration into conventional electric grid.
2. Which renewable source is dominant in electric generation?
   1. Hydropower
   2. Biofuels
   3. Solar power
   4. Wind power
3. Which renewable energy technology is dominant in direct heat production worldwide?
   1. Solar thermal collectors
   2. Photovoltaic cells
   3. Geothermal heat pumps
   4. Biomass combustion
4. Which renewable energy source below is more dominant in the transportation sector?
   1. Biofuels
   2. Biomass
   3. Waste
   4. Hydroelectric
5. Which renewable energy technology has the biggest share in the global thermal capacity?
   1. Modern bio‐heat
   2. Solar water heating
   3. Geothermal heat
   4. Ocean thermal energy
6. Which region below generates the greatest amount of electric energy from hydropower?
   1. North America
   2. South America
   3. Asia
   4. Europe
7. Which region below generates the greatest amount of electric energy from wind power?
   1. North America
   2. South America
   3. Asia
   4. Europe
8. Renewable energy generation from which renewable source below has had the highest increase rate since year 2000?
   1. Biomass
   2. Wind
   3. Solar
   4. Hydro
9. A higher capacity factor indicates that
   1. maximum energy that the installed capacity can generate is larger.
   2. the generation facility has larger installed capacity.
   3. generated energy is closer to the maximum energy the installed capacity can generate in a certain time interval.
   4. the cost of the generated energy per installed capacity is smaller.
10. Which electric generation plants operate at the highest capacity factor worldwide?
    1. Hydroelectric
    2. Geothermal
    3. Wind farms
    4. Bioenergy

Answers: 1‐a, 2‐a, 3‐d, 4‐a, 5‐b, 6‐c, 7‐d, 8‐b, 9‐c, 10‐b.

Research Topics and Problems

Recommended Web Sites

• American Solar Energy Society (ASES): https://www.ases.org
• American Wind Power Association (AWEA): https://www.hydropower.org
• International Geothermal Association (IGA): https://www.geothermal-energy.org
• International Hydropower Association (IHP): https://www.hydropower.org
• International Renewable Energy Agency (IRENA): https://www.irena.org
• IRENA Global Atlas for Renewable Energy: https://irena.masdar.ac.ae/gallery/#gallery
• National Renewable Energy Laboratory (NREL): https://www.nrel.gov
• Renewable Energy Policy Network for the 21st Century (REN21): http://www.ren21.net
• The Intergovernmental Panel on Climate Change (IPCC): https://www.ipcc.ch
• US Energy Mapping System: https://www.eia.gov/state/maps.php
• US Environmental Protection Agency (EPA): https://www.epa.gov/energy
• World Energy Council (WEC): https://www.worldenergy.org