2.1.1 Solar Energy

The earth receives solar energy primarily in the form of electromagnetic radiation, which in itself is the product of nuclear fusion reactions, primarily hydrogen fusion to helium, occurring in the sun [4]. The surface temperature of the sun is approximately 6000 K, and the sun emits electromagnetic radiation that is characteristic of a black body at this temperature. Figure 2.2 shows the solar irradiance, which is the power per unit area per unit wavelength as a function of the wavelength of the radiation.

As can be seen from the curve, the peak irradiance occurs approximately at a wavelength of 0.5 μm. The area under the irradiance curve between any two wavelengths is equal to the power radiated per unit area by the sun by the electromagnetic radiation between those two wavelengths. The total area under the curve is the total solar irradiance, that is, the radiation flux from the sun. The value of the total solar irradiance at the top of the earth’s atmosphere, also called the solar constant, is ~1360 W/m² [5]. This incident solar radiation interacts with the constituents of the earth’s atmosphere with fractions of it getting absorbed in the atmosphere and reflected back into space, as well undergoing scattering before reaching the earth’s surface. The earth, in turn, also emits radiation back into space, albeit at a much higher wavelength.³ Overall, the incident solar energy is nearly balanced by the energy emitted by the earth, allowing a stable thermal equilibrium to exist that maintains temperature suitable for life [4]. Figure 2.3 depicts this energy balance for the radiant energy [6].

3. The peak wavelength (λ_max) at which maximum power is emitted by a body is related to its temperature (T) by Wien’s law, which states that λ_maxT ≈ 3 × 10^-3 m·K. The peak wavelength for the earth is 10 μm, well into the infrared region.

4. The incident radiation flux is shown to be 340 W/m² in the figure, rather than the solar constant of 1360 W/m² stated earlier. The lower value is for the radiation reaching the surface of the earth based on the total surface area of earth (4πr²), considering it to be a sphere of radius r. The solar constant is based on the projected area of the earth, which is simply πr².

The majority, nearly 55%, of the solar energy reaching the earth’s surface is in the infrared region, 40% of the energy is in the visible region, and 5% in the ultraviolet region [7]. The earth receives nearly 4 million EJ (~1.1 billion TWh) of solar energy every year, at a rate so large that the amount received in 1 hour exceeds the annual worldwide primary energy consumption [8].

2.1.2 Geothermal Energy

Geothermal energy, as can be inferred from the term, refers to the heat present in the interior of the earth, and is the result of radioactive decay of elements including the heat generated during the formation of earth (primordial heat), as well as ongoing decay of long-lived radioisotopes, such as ⁴⁰K, ²³²Th, ²³⁵U, and ²³⁸U [9]. Figure 2.4 is a schematic of the earth’s interior, showing relative thicknesses of different parts of the earth including the crust, the mantle, and the inner and outer cores.

The inner core of the earth, ~2400 km in diameter, is extremely hot, with a temperature approaching that of the surface of the sun (6000 K). The inner core is surrounded by a ~2400-km-thick outer core of hot molten rock—magma. A mantle layer consisting of ultrabasic and igneous rocks and a thickness of ~2900 km surrounds the outer core. The outermost layer is the crust, which has negligible thickness (<70 km) compared to the earth’s radius of approximately 6400 km [9, 10]. The temperature at the outer core mantle boundary is ~4200 K, while that in the upper mantle near the mantle crust boundary can be as low as 500 K. This radial variation in the temperature results in the establishment of significant temperature gradients within the earth’s interior, giving rise to both conductive and convective heat fluxes toward the earth’s surface. The average heat flux of earth is quite low (~80 mW/m²); however, the total available energy is significantly large. By some estimates, globally, the geothermal energy has the potential to generate 12,000 TWh of electricity annually, again exceeding the worldwide primary energy demand. If the geothermal energy is utilized directly without conversion to electricity, then the resource is even larger, at 600,000 EJ or ~170 million TWh per year [11].

Geothermal energy from the earth’s interior is transmitted to the surface through the mantle at various rates depending on the thermal conductivity of the geological formation and the temperature gradient, which is commonly taken as 30°C/km and may range from 10°C/km to 100°C/km [9, 11]. The interior of the earth is hardly uniform, though, and is characterized by widespread prevalence of geofluids—both aqueous and organic in nature, as well as in liquid and vapor states. These geothermal fluids also facilitate the transmission of the geothermal energy by convection. This heterogeneity leads to different configurations of geothermal energy sources [9, 11, 12]:

• Hydrothermal—hot water or steam at moderate depths (100–4500 m)

• Geopressed—hot water aquifers containing dissolved methane under high pressure at depths of 3–6 km

• Hot dry rock—abnormally hot geologic formations with little or no water

• Magma—molten rock at temperatures of 700°C–1200°C

Of the four types, the hydrothermal source is the only type that can be exploited economically with the prevalent technology. The hydrothermal sources are, as stated earlier, either liquid dominated (characterized by the presence of hot water) or vapor dominated (characterized by the presence of steam, i.e., water vapor). They can also be classified as high-temperature (>180°C), intermediate-temperature (100–180°C), and low-temperature (<100°C) systems. These resources can be harvested for electrical power generation, direct heat utilization, or combined heat and power applications [12].

The enormity of the total energy content of the earth in comparison to the primary energy demand makes geothermal energy practically inexhaustible, earning it its renewable characteristics. However, geothermal energy resources in the interior of earth are heterogeneous in the extreme, and geothermal energy fields are neither contiguous nor uniformly distributed across the earth. It is indeed possible to exhaust a geothermal energy reservoir, if energy is extracted out of it at a higher rate than the rate at which it is replenished by the processes occurring in the core. Proper reservoir management is indispensable for maintaining the presence of fluids in the hydrothermal fields and extraction at sustainable rates for ensuring long-term operation of the geothermal power plant [9].

2.1.3 Biomass Energy

Photosynthesis is the most fundamental process that plants perform to harness solar energy. It is the essential process without which no life will exist on earth. From the energy perspective, it can also be viewed as the process converting solar energy into chemical energy using carbon dioxide and water as the basic raw materials. This chemical energy is stored in the form of biomass and biochemical energy. Historically, as mentioned in Chapter 1, Introduction to Energy Systems, biomass has served as the energy source for all civilizations. Even today, populations in less developed countries continue to rely upon firewood as the primary energy source for cooking and heating applications [11].

Such traditional use of biomass through direct combustion is inefficient and unsustainable, contributes significantly to air pollution and carbon emissions, and affects human health adversely. Biomass-based RESs of the future are aimed at sustainable utilization of biomass through its conversion into chemicals that drive the modern, advanced service technologies. The biomass utilized for these conversions is cultivated and harvested specifically for energy applications. Different types of biomass serve as the energy source for these modern systems, the main ones being [11, 13]:

• Agriculture and forest residues: examples of agricultural residues include corn stover, wheat straw, and rice straw; forest products and residues include hardwoods and softwoods grown in forests as well as the residue left after natural processes and harvesting of products.

• Organic waste streams, including food and yard wastes, manure, and human sewage: examples of such streams include waste edible oils, residue from food processing plants, discarded leftover food, grass clippings from lawn, manure from large dairies and other animal farms, and municipal sewage.

• Crops—either food crops used as fuel or energy crops grown specifically for use as fuel or conversion to chemicals: food crops such as sugar beets and sugarcane, corn and potato, soybean and sunflower, and so on can be easily converted into energy chemicals.⁵ Such diversion of food crops can cause significant disruption in the food supply chain and have disastrous consequences for a large fraction of global human population, and the future trend is to cultivate annual and perennial plant species such as switchgrass, miscanthus, bermudagrass, and microalgae.

5. Diversion of sugarcane in Brazil and corn in the United States to produce ethanol for use as motor vehicle fuel are two of the most prominent examples of repurposing food crops for energy applications.

Biomass is renewable and sustainable, and has the versatility to yield multiple energy carriers—electricity, heat, and chemicals, unlike the other renewable energy sources that are limited mostly to producing electricity. Geothermal and solar energy are exceptions in their ability to provide heat, but do not have the ability to yield chemicals. Biomass has theoretical potential to provide 30,000–80,000 TWh/year, with the low solar energy conversion efficiency (1%) of photosynthesis, and large area requirement limiting its growth [11].

2.1.4 Hydropower Energy

Solar energy is the driver for the water cycle consisting of evaporation, condensation, precipitation, and collection of water in bodies of water on earth, as shown in Figure 2.5.

The flow of water under gravitational flow is harnessed to generate the electricity in the hydropower plants. Hydropower plants can be run-of-river (RoR) plants that utilize the natural flow of water in the river and channels. These plants are subject to significant seasonal and periodic variations depending upon the precipitation and runoff. The uncertainty associated with the RoR plants is avoided in the storage (reservoir) type of hydropower plants, where a dam is constructed to create large reservoirs to store water at an elevation—transforming the solar energy into potential energy in the gravitational force field [12]. The water is released to a lower elevation through turbines to generate electricity, as shown in Figure 2.6.

Pumped storage hydropower plants are a special case of hydropower plants that utilize two reservoirs located at different elevations. These plants are not energy sources but energy storage devices associated with another power plant. Electricity generated by the power plant is used to pump water from the reservoir at the lower elevation to the one at the higher elevation at the times when the generation exceeds the consumer demand. The water stored in the upper reservoir is released through the turbines to augment the electricity generated in the main power plant when demand exceeds its capacity. Energy storage is described in greater detail in Chapter 6, Hybrid Energy Systems.

The inventory of water in the water cycle is estimated to be approximately 0.6 million km³. This volume is evaporated annually due to the solar energy incident on earth and is returned to it through precipitation. The amount precipitating on land is slightly less than 20% of this volume, at 112,000 km³. Slightly more than one-third of this volume ends up as runoff, with the balance getting absorbed by vegetation and land [11]. All of this volume, ~50,000 km³, in theory, can be harnessed to generate hydropower. The total theoretical potential for global hydropower generation, estimated from the runoff volumes and the altitudes at the corresponding locations, ranges from 44,000 to 128,000 TWh/year [11, 14]. Of course, technical limitation makes it impossible to exploit all of the precipitation runoff. Economic considerations further limit the exploitable resources, even when it is technically feasible to construct a hydropower plant. The economic potential for global hydropower production is considerably less than the theoretical potential at 8000–15,000 TWh/year [11, 14].

2.1.5 Wind Energy

Another effect of solar insolation is the heating of the earth’s surface. Earth’s surface being nonuniform, land and water coverage being the most elementary divisions, different regions heat up at different rates, leading to significant variations in temperatures across the globe. Tropical regions have a net gain of heat causing them to have higher temperature, while the polar regions are subject to a net loss and are considerably cooler [11]. This temperature difference, in turn, leads to density and pressure differences across locations, resulting in convective wind currents. Coastal regions experience daily wind cycles as air over the land heats up faster than that over the water, setting off winds from water to land, as shown in Figure 2.7 [15]. The wind direction is reversed in the night as the air mass cools down faster creating higher pressure over the land.

In addition to solar insolation, earth’s rotation contributes to establishment of circulatory patterns in the atmosphere. These patterns are further superimposed by local wind patterns arising out of local natural topographical features that lead to uneven heating. Anthropogenic structures may also interfere and influence these patterns [11].

Wind energy systems harness the kinetic energy of these circulatory patterns. Historically, wind energy was harnessed in direct applications such as sailboats for transportation and windmills for grinding flour or pumping water [16]. Wind power systems, wherein the kinetic energy of wind is first transferred to the kinetic energy of a wind turbine and then converted into electricity, are a relatively recent phenomenon [12]. Wind energy systems (henceforth referring to wind power systems, i.e., where electricity generation is taking place) have seen significant growth over the past two decades, with the global installed capacity approaching 500 GW [16]. These systems produced in excess of 1100 TWh of electricity in 2017 (https://www.eia.gov/energyexplained/wind/history-of-wind-power.php).

Although nearly every location on earth experiences wind patterns, practical technological constraints limit the exploitation of wind energy resources to those locations where the wind power density exceeds 400–500 W/m². The global theoretical wind energy potential, on this basis, is approximately 500,000 TWh/year, exceeding the primary energy requirements. Advances in the wind energy technology can increase this potential even further. However, technical and economic limitations suggest that the actual realizable potential may be closer to 20,000 TWh annually [11].

In addition to the above five forms of energy sources that dominate the current renewable energy landscape, there exists another renewable energy source—marine energy—that can satisfy the primary energy demands for the foreseeable future and beyond. Marine (or ocean) energy actually consists of several different energy forms [11, 12, 17]:

• Tidal current energy—kinetic energy of oceans arising from rotation of the earth in the gravitational field of the sun and the moon

• Tidal barrage energy—potential energy exploiting the change in sea level during high and low tides by creating dams/barrages for the seawater

• Wave energy—which captures the kinetic energy of the wind

• Ocean thermal energy—based on the temperature differences between warmer surface waters and deep cooler waters

• Salinity gradient energy—chemical energy based on the salinity differences between the freshwater that is discharged into oceans and the saline seawater

The theoretical potential of marine energy is in excess of 2 million TWh/year, with the ocean thermal energy exceeding the other types by at least two orders of magnitude [11]. However, all these technologies, except for tidal barrages, are in developmental stages. The current contribution of marine energy to renewable energy is negligible and not expected to rise significantly for the foreseeable future.

As can be gleaned from the above discussion, the renewable energy sources possess theoretical and technical potential to satisfy the entire primary energy demands of the global population. However, these primary sources need to be transformed into appropriate secondary energy sources—the energy currencies—in order to operate the service technologies. These transformations are discussed in the following section.

2.2.1 Transformations of Mechanical Energy Sources

Nearly 99% of the power generated worldwide involves conversion of mechanical energy, specifically kinetic energy, into electrical energy. This conversion is based on Faraday’s law of electromagnetic induction that provides the fundamental explanation of how the movement of a magnet induces electrical current in a conductor placed in its field [18]. Modern electricity generators operate on this principle and consist of an electromagnet rotor surrounded by a stator wound up in conducting wires. The rotor is driven by a turbine that captures the kinetic energy of the motive fluid inducing the current in conductor on the stator. The motive fluid is steam in thermal power stations, for example, those driven by fossil or nuclear energy. Fossil fuel, usually natural gas based combined cycle power plants also utilize a gas turbine driven by the hot combustion gases to generate electricity via a Brayton power cycle. Additional electricity generation is accomplished in a steam turbine operating a Rankine power cycle⁶ with the steam generated through heat transferred from the combustion gases exiting the gas turbine [2].

6. The theoretical basis for such power cycles is well-known and is available in most engineering thermodynamics textbooks.

The two mechanical forms of renewable energy—hydropower energy and wind energy—are converted to electricity using the same principle. As mentioned in the previous section, water stored in the reservoir type of hydropower plants possesses potential energy due to the force of gravity. This potential energy is converted into kinetic energy of the turbine as it is released through the penstock past the turbine located at a lower elevation, as shown in Figure 2.6. The turbine drives the rotor of the generator located in the powerhouse, generating the electricity. Figure 2.8 shows the schematic of a typical hydroelectric turbine. The RoR hydroelectric plants do not have a reservoir of stored water, and the kinetic energy of the flowing water is transferred directly to the turbine.

Wind turbines perform in the same manner as the turbines in the RoR hydropower plants; that is, they capture the kinetic energy of the wind and employ the same principle of electromagnetic induction to generate electricity. The power rating of a wind turbine depends upon the area swept by the blades of the turbine, and most large-scale wind turbines are of the horizontal axis type, that is, the axis of rotation is parallel to the ground and the wind direction. A modern horizontal axis wind turbine (HAWT) in a commercial wind farm installation may have blades that are 50 m long and a power rating of up to 5 MW. The hub of the rotor may be located at a height of 100 meters above the ground to take advantage of the increased wind velocities compared to those near the surface of the earth [16]. Vertical axis wind turbines (VAWTs), where the axis of rotation is perpendicular to the ground, are less common and have smaller power ratings. These VAWTs may find niche applications in smaller residential settings and locations where wind patterns are not consistent or where larger installations are not permitted out of aesthetic or ecological considerations. Figure 2.9 shows a schematic of both horizontal axis and vertical axis turbines. The rotational speed of the blades in a wind turbine is typically 30–60 rpm, and the hub of the turbine contains a gearbox that steps it up to 1200–1500 rpm, which is within the range needed by the generator [19].

Unlike fixed windmills of the past, modern HAWTs contain sophisticated control systems to maintain the orientation of blades perpendicular to the wind to ensure uninterrupted power generation despite any changes in the wind direction. The blades of the turbine have also evolved into longer, sleeker ones that are significantly quieter, despite their size, and have superior aerodynamic performance. Figure 2.10 shows the innovative transformations in blade design over the past four decades [16].

Wind energy installations can be located onshore or offshore, with offshore installations offering the advantages of the availability of a higher-quality wind resource and potential use of larger wind turbines while also avoiding utilizing large land areas that can be repurposed for other applications [12].

Hydropower energy, from reservoir-type plants, can be expected to generate electricity at a consistent rate; however, electricity generation from wind energy installations can be highly variable due to daily and seasonal variations. Possible solutions to improve the reliability and consistency of power generation from variable resources are presented in Chapter 6, Hybrid Energy Systems.

2.2.2 Transformations of Geothermal Energy

Geothermal energy has been utilized for several millennia by human beings, in direct heating applications, such as bathing, washing, and cooking. Direct utilization of geothermal energy in geothermal heat pumps, space heating, and bathing, accounts for 90% of such applications. Direct utilization of geothermal energy also includes industrial and agricultural applications, for example, for food dehydration, milk pasteurization, industrial process heat, and aquaculture pond heating [20]. Geothermal energy usage in direct applications increased by more than 400% over nearly two decades, rising from ~38 TWh in 1997 to ~164 TWh in 2015 [20, 21]. More than half of these applications involve geothermal heat pumps, wherein residential and commercial buildings are heated in winter and cooled in summer by circulating water in a closed loop between the building and subsurface with heat transfer areas provided in each location.

Compared to direct use, converting geothermal energy into electricity is a relatively recent phenomenon that started in the early 20th century. Wells drilled into geothermal reservoirs transport the geothermal fluids to the surface, where thermal energy is extracted from them via a power conversion cycle to generate electricity. Fluids present in the geothermal energy systems are primarily of two types: vapor-dominated systems consisting mainly of steam and liquid-dominated systems consisting of brine [22]. The vapor-dominated systems may typically contain some fraction of noncondensable gases as well. For vapors containing >15% noncondensables, a direct-intake, noncondensing cycle may be used for power generation, wherein the vapor is fed directly to a turbine and exhausted to the atmosphere without attempting to condense the steam [9]. Condensing systems are used for brine-dominated systems, as well as where the vapor consists mostly of dry or saturated steam. In a more general case, the geothermal fluid extracted via a production well consists of brine or a mixture of vapor and brine, and in these cases, single- or double-flash systems are used to generate additional steam from brine. The steam is fed to a turbine to generate the electricity via the Rankine cycle, and the condensate along with the concentrated brine is injected back into the geological formation via an injection well that is distinct from the production well [22]. The fluids present in geothermal energy reservoirs vary greatly with respect to their temperatures, and in some cases, the geothermal fluid may simply be hot water. In such cases, a low-boiling secondary fluid (ammonia or isobutene, for example) is vaporized by heat exchange with the geothermal fluid and used to operate an organic Rankine cycle or Kalina cycle for power generation. These types of plants are called binary-cycle plants. Figure 2.11 shows the conceptual schematics of the three different types of closed-loop geothermal power plants [23].

As mentioned earlier, temperatures of geothermal energy resources vary greatly, and this puts constraints on the type of technology that can be used to harness that energy, as well as the potential applications of the resource. Typically, for any geothermal energy source, a listing of potential applications is developed, arranged in the order of decreasing temperatures and corresponding technology used for those applications. This listing is called the Lindal diagram,⁷ which serves to identify, first the maximum temperature and the corresponding application that is feasible with the geothermal energy resource, and second, the potential to maximize the use of the resource through developing a heat exchange network to cascade through applications requiring progressively lower temperatures [24]. A sample Lindal diagram is shown in Figure 2.12.

7. Named in honor of Baldur Lindal, an engineer from Iceland who first presented such a diagram in 1973.

As can be seen from the above figure, electricity generation via conventional power cycles is the first preference, provided high-temperature steam is available. Lower temperatures would drive the choice to binary power cycles, followed by direct thermal applications. The particular applications shown in Figure 2.12 are of relevance to a geothermal field in an agricultural/fisheries setting. The listing will be quite different for the geothermal field, which is in the proximity of a mine, as shown in reference 24. A situation-specific Lindal diagram needs to be developed to optimize the exploitation of any geothermal field.

2.2.3 Transformations of Solar Energy

The simplest utilization of solar energy is direct low-temperature applications, such as domestic water heating, solar cooking, space heating, and crop drying [21]. Energy management of buildings—heating, cooling, or ventilation—can be accomplished via an active or passive system utilizing solar thermal energy. As significant as these applications are, particularly from the environmental perspective of displacing polluting fuels used for domestic applications (cooking, heating, etc.) in developing economies, it is the conversion of solar energy into electrical energy that is of primary interest for large-scale systems and industrial applications.

Conversion of solar energy into electrical energy is effected in one of two ways: (1) Concentrated solar power (CSP) systems involve focusing solar radiation into a concentrated beam that transfers its thermal energy to a fluid, raising its temperature. This thermal energy conducted by the fluid is typically used to generate steam to drive a power cycle to generate electricity [25]; (2) photovoltaic (PV) systems involve direct conversion of solar energy using the photoelectric effect [26]. PV technology is one of the few exceptions in power generation systems wherein electricity is generated without any intermediate conversion to mechanical energy by using turbines [27].

Technologies for the conversion of solar energy—both CSP and PV systems—into electrical energy are discussed in detail in Chapter 3, Transformations and Chemical Processes in Solar Energy Systems.

2.2.4 Transformations of Biomass Energy

Biomass, particularly from agricultural/forest residues and products and organic waste streams, has been used historically for heating and cooking by direct combustion and continues to be used in this manner in many developing countries. As mentioned earlier, such use is unsustainable, causes environmental pollution, and has adverse health effects on human beings. Furthermore, this traditional burning of biomass hardly allows the energy content of the biomass to be used efficiently and effectively for more complex, value-added applications. Modern transformation technologies enable conversion of biomass to electricity and chemicals in addition to heat, expanding the utility of biomass energy to industrial processes, transportation applications, and consumer services.

Biomass energy is transformed into other forms of energy—electricity, thermal energy, and chemical energy—via a number of different processes. The two major types of conversion processes are thermochemical conversion processes and biochemical conversion processes. Figure 2.13 shows further details of specific processes that are categorized under these types [21]. Thermochemical processing includes direct combustion, pyrolysis, and gasification, while biochemical processing involves digestion or fermentation. Direct combustion of biomass can yield both electricity and heat, while gasification and pyrolysis can also yield other chemicals that can also be used as fuels. Electricity and chemicals can also be obtained through biochemical processing, while extraction processes can yield chemicals, typically biodiesels.

Transformations of biomass energy are described in detail in Chapter 4, Transformations and Chemical Processes in Biomass Energy Systems. Transformation of mechanical energy and other forms of energy are described in Chapter 5, Transformations and Chemical Processes in Mechanical, Geothermal, and Ocean Energy Systems.