Chapter 5

Transformations and Chemical Processes in Mechanical, Geothermal, and Ocean Energy Systems

The various forms of energy that are the focus of this chapter have been utilized by humankind for centuries, and possibly millennia. Wind energy and hydropower have been harnessed to pump water, grind grains, and drive machinery for a long time. Wind energy has also been exploited for transportation since ancient times—enabling trade across oceans and intrepid explorers to discover new lands. Geothermal energy has also been employed for direct heating, bathing, and cooking applications. The applications of these forms of energy diminished in the past few centuries as humans learned to harness fossil fuel energy and subsequently generate electricity from it. The use of windmills and waterwheels fell by the wayside, replaced by electrical machines powered by other primary energy sources that performed the functions more efficiently and on a larger scale.

Motivation to a transition away from fossil energy has resulted in the development of technologies to convert the energy forms contained in these sources to electricity. These transformations are covered in this chapter, which is organized to discuss first the conversion of mechanical forms of energy—potential and kinetic energy in hydropower and wind power—followed by the conversion of thermal energy from geothermal resources. Hydropower and wind power dominate the current renewable energy landscape, whereas geothermal has its niche applications. The principles of transformations of the mechanical energy sources and technologies based on these principles are well established and are discussed only in brief. Similarly, principles of conversion of thermal energy into electricity are well-known from fossil, solar, and nuclear plants. Geothermal energy conversion is based on the same principles; however, the temperatures available from these sources are much lower than what are possible with other sources requiring innovative approaches. Finally, the transformations of ocean energy are discussed as well, even though it is a very minor contributor to the global primary energy supply at the present time. The transformations of ocean energy are also the most interesting, as it contains all forms of primary energy—mechanical (kinetic and potential), thermal, and chemical.

5.1 Transformations of Mechanical Energy

As mentioned in Chapter 2, Renewable Energy Sources, the final conversion step in the majority of electricity generation devices is the transformation of kinetic energy on the basis of Faraday’s law of electromagnetic induction. Mechanical energy is in the form of kinetic energy or potential energy, and electricity generation technologies are most straightforward when the primary energy source is kinetic energy, as in the moving fluids—wind or water. The challenge in the conversion of kinetic energy is maximizing the energy that can be extracted out of the moving fluid. Development of wind energy or wind power is geared toward this objective. Hydropower is based on the conversion of potential energy available in natural streams and rivers to electricity as discussed below.

5.1.1 Hydropower—Transformation of Potential Energy

Hydropower is one of the most mature, reliable, and predictable technologies that is based primarily on the conversion of potential energy of water stored in a reservoir at a higher elevation to electricity as it is released to a lower elevation. As mentioned in Chapter 2, run-of-river (ROR) hydropower plants do not feature a reservoir but are based on the natural flow of water from higher to lower elevations. Hydropower is one of the least expensive and most efficient (~90%) technologies for power generation [1]. In-stream hydropower plants are essentially ROR plants that take advantage of natural or artificially created falls, barrages, or weirs. Hydropower plant capacities vary greatly, from as low as a few kW to tens of GW. The plant capacity leads to a classification as large or small, based on some power-generation threshold that varies greatly across the globe, which can be as low as 1.5 MW or as high as 50 MW. Further classification into pico- (<5 kW), micro- (5–100 kW), and mini-units (100 kW–1 MW) is also often used. However, these limits are flexible, and there is no consensus over the numeric criteria employed for the classification [1, 2].

This variation in capacities affords hydropower a flexibility to meet the energy needs of large urban areas, as well as localized, decentralized rural needs. Large-scale reservoir hydropower plants are often a part of multipurpose activities such as agriculture irrigation, flood and drought control, water supply, recreation, and so on, for which the reservoir is created.

The power-generation capacity (P) of a hydropower plant is given by equation 5.1:

P=ηρgQH      (5.1)

where ρ is the density of water, g is the acceleration due to gravity, Q is the flow rate, and H is the effective pressure head. η is the hydraulic efficiency of turbine generating the power.

The turbine is the key piece of equipment that is at the heart of a hydropower plant. In general, there are two types of turbines that are used in hydropower plants:

  • Impulse turbines that absorb a part of the kinetic energy of water impinging on the blades of the turbine creating rotatory motion. The turbine is not shrouded in a pressure encasement, and there is no change in the pressure head. Turgo, Pelton, and cross-flow turbines are some of the configurations of the impulse turbine. A schematic of a multijet Pelton turbine is shown in Figure 5.1 [2].

    A representation of multijet pelton turbine.

    Figure 5.1 Multijet Pelton turbine.

    Source: Elbatran, A. H., et al., “Hydro Power and Turbine Systems Reviews,” Jurnal Teknologi, Vol. 74, No. 5, 2015, pp. 83–90.

  • Impulse turbines are most suited for situations where low-flow, high-head conditions exist. Reaction turbines, on the other hand, are preferred for low-head, high-flow situations and harness both the pressure drop and velocity of water. Reaction turbines are typically propeller-type turbines shrouded in a pressure encasement and need to be completely immersed in water. The Kaplan turbine (shown in Figure 2.8) and Francis turbine (shown in Figure 5.2) are examples of reaction turbines.

An illustration of Francis turbine.

Figure 5.2 Schematic of a Francis turbine.

Source: Elbatran, A. H., et al., “Hydro Power and Turbine Systems Reviews,” Jurnal Teknologi, Vol. 74, No. 5, 2015, pp. 83–90.

Considerable research and development activities have led to advanced configurations and refinements of the turbines. Several books and other references are available in the literature, and interested readers are encouraged to consult any of the excellent resources that are focused on turbomachinery principles and operation.

Apart from these classical turbines, devices for harnessing gravitational vortex energy, for example, Vortex Induced Vibration for Aquatic Clean Energy (VIVACE), are also being developed. These devices aim to convert vortex energy into vibratory or oscillatory motion of objects placed in the stream. Vortex shedding occurs when a separation of flow occurs from the surface of a bluff body, such as a cylinder. The flow pattern in the immediate vicinity downstream of the object is strongly dependent upon the Reynolds number (Re) defined on the basis of the dimension of the body. Under certain flow conditions in both laminar and turbulent flow regimes (Re between 40 and 150 or >3.5 × 106), alternating vortices are shed in the wake region setting the body in oscillatory motion [3]. This oscillatory motion is transmitted and converted through appropriate mechanical gears into rotary motion, generating power [4]. The maximum power (PVIVACE) that can be obtained from a cylindrical object in the flowing stream of a fluid based on this technology is given by equation 5.2 [5].

PVIVACE=π2ρCLU2fcylymaxDLsin(ϕ)      (5.2)

where U is the stream velocity, D is the cylinder diameter and L its length, CL is the lift coefficient, fcyl is the frequency of oscillation, ymax is the amplitude of displacement, and f is the phase difference between the lifting force and the cylinder motion. The definition of the lift coefficient is analogous to that of the drag coefficient as shown in equation 5.3 [6]:

CL=FL12ρU2AP      (5.3)

where FL is the lift force and AP is the projected area exposed to the flow. The lift force acts in a direction perpendicular to the stream flow, whereas the drag force acts in the streamwise direction. Unfortunately, unlike for drag coefficient, correlations to predict the lift coefficient are not readily available in the literature and have to be determined through numerical modeling and experimental data for the specific systems.

Other developments in hydropower include improvements in electromechanical components and generators, as well as advanced control strategies to optimize generation under variable flow conditions [7]. Hydropower is the world’s primary source of renewable energy, contributing almost three-quarters of the global renewable supply and nearly one-fifth of all electricity production. Globally, hydropower continues to grow with a proliferation of small hydroelectric projects—more than 80,000 such projects are either in operation or under development worldwide [8].

5.1.2 Wind Power—Transformation of Kinetic Energy

Wind power has experienced an explosive growth over the past three to four decades, with wind turbines evolving from simple, small kilowatt-capacity fans to large, complex megawatt-scale sophisticated machines utilizing advanced geometries and materials for the blades. Wind turbines convert the translational kinetic energy of wind to rotational energy which is then converted into electrical energy. The rate at which kinetic energy is transported by wind, or the power contained in the wind, is given by equation 5.4 [9].

P=12ρAirAU3      (5.4)

where A is the area swept by the rotor and ρAir is the density of the air. It can be seen that the power varies with the cube of wind velocity U.

The wind turbine functions to extract the kinetic energy from wind, reducing its velocity. Clearly, not all the energy can be captured with the turbine, as it would reduce the wind velocity to zero past the turbine, which would mean no flow. Theoretical analysis conducted in the early 20th century revealed that maximum capture efficiency of such a device is 59.3%, a number that is commonly termed as the Betz limit, Lanchester–Betz limit, or Lanchester–Betz–Joukowsky limit1 [1012].

1. In honor of A. Betz, F. W. Lanchester, and N. E. Joukowsky who, working independently in Germany, Britain, and Russia, respectively, arrived at the result.

An object placed in a moving stream of fluid experiences drag as well as lift. The earliest wind turbines operated on the transfer of energy through drag with the blade surface oriented perpendicular to the flow direction. This technique is inefficient as the relative wind velocity is simply the difference between actual wind velocity and the blade velocity, and the maximum possible efficiency is only 16%. Subsequent developments involved turbines designed to operate with lift force by changing the angle of attack, which is the angle between the relative wind velocity and the chord line (the reference line going through the farthest points on the leading and trailing edges, which are the tip and base of the blade). The relative wind speed is obtained by vector subtraction, and the maximum possible efficiency is ~50% [9].

Modern wind turbines are designed with cut-off wind velocities that specify lower and upper limits of operation: turbines start operation above the cut-in velocity which is ~3–4 m/s, while they also cut out of operation at higher velocities of ~20–25 m/s to prevent damage to the rotor and structural components. Power harvested by the wind turbine increases as the wind velocity above the cut-in speed increases until the rated design capacity is reached, which is usually around wind velocities of 11–15 m/s. Above these velocities, the control system manipulates the blade angle and other parameters to limit the output to prevent overloading the generator until the cut-off velocity is reached. Figure 5.3 shows the typical power curve for a modern variable-speed wind turbine [13].

A graph presents the characteristics of a modern wind turbine.

Figure 5.3 Characteristic power curve for modern wind turbine.

Source: Wiser, R., et al., “Wind Energy,” IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation, edited by O. Edenhofer, et al., Chapter 7, pp. 535–607, Cambridge University Press, Cambridge, UK, 2011.

As mentioned in Chapter 2, wind turbines can be either horizontal axis (HAWT) or vertical axis (VAWT) as shown in Figure 2.9. HAWTs have come to dominate the large-scale generation, and most turbines are three-blade (50% more energy and less noisy than two-blade), upwind rotor (rotor in front of supporting column) designs. The blades are mounted on a hub and main shaft connected to a generator through gears. As can be inferred from equation 5.4, increasing the power generation requires increasing the swept area, which in turn requires longer blades. Advances in this area have seen the maximum power of a wind turbine rising from ~750 kW in the late 1990s to ~1.5 MW in 2009. The largest wind turbine on the verge of deployment in this decade will be General Electric’s 13-MW turbine featuring blades that are more than 100 m in length. These will be installed in an off-shore wind farm in the North Sea on towers that will stand 250 m tall.

Such dramatic increase in the power-generation capacity is attributable mainly to two factors: advanced materials and improvements in blade designs. The blades of state-of-the-art turbines are substantially larger than those used just a couple of decades ago; however, they are also significantly lighter, as much as 90% lighter than what a 1980s blade would weigh if scaled up to today’s rated power [14]. In other words, the mass of the blade has not increased proportionally with volume. Most blades are made of fiberglass composite—fiberglass reinforced with polyester or epoxy material. E-glass, which is alumino-borosilicate glass with <1% by weight of alkali oxides, is the predominant blade material. S-glass, which is similar to E-glass but with MgO instead of CaO, has superior tensile strength, and even though more costly than E-glass, may be more attractive for larger rotors. Other materials being developed include carbon fibers and carbon/wood hybrids [14]. Energy harvested by the turbines is also being increased by refining the blade design. The aerodynamic performance is enhanced through improved airfoils, aeroelastic tailoring passively reducing loads through coupling blade bending and twist, vortex generator add-ons, and other improvements [15, 16]. Detailed discussion of these developments is beyond the scope of this book, and myriad sources are available in the technical literature and open sources.

5.2 Transformations of Geothermal Energy

As mentioned in Chapter 2, geothermal energy is the practically inexhaustible quantity of heat present in the interior of the earth. This heat is being continually transported, primarily by conduction, to the surface of the earth. Unfortunately, most of this energy is available at depths too great to be exploited economically. In other words, the temperature gradient from the surface to the interior of the earth is too small at most locations to encounter the temperatures necessary for electricity generation at reasonable depths. Geothermal fields around the globe are the regions where the temperature gradient is sufficiently high to permit accessing high temperatures within relatively smaller depths. Furthermore, the subsurface geology is favorable enough at these locations to contain a geothermal fluid or allow facile circulation of an external fluid that functions as a carrier for the convective transport of heat. The geothermal fluids are essentially precipitation (rainwater) recharges accumulated in aquifers that are the integral parts of most geothermal fields. The water may reach the surface in some cases, appearing as hot springs, geysers, mud pits, or fumaroles. In many other cases, the aquifer may be confined by impermeable rock layers that prevent it from reaching the surface, creating regions of water/steam mixtures under pressure [17]. The heterogeneity of subsurface environment results in many different types of geothermal reservoirs as described below.

5.2.1 Classification and Characteristics of Geothermal Resources

Various geothermal resources can be classified on the basis of several different criteria. Based on the geological formation and the nature of fluids, they may be categorized as convective (hydrothermal) systems, conductive systems, and deep aquifers. Both hydrothermal and deep aquifer systems contain in situ fluids, whereas conductive systems are essentially formed of dry rocks and magma. Deep aquifers involve fluids circulating in porous formations at depths exceeding 3 km. The geothermal resources where the recharge water can escape back to the surface, either as vapor or liquid, are called hydrothermal resources [17]. Deep aquifer resources can be classified as hydrostatic pressured or geo-pressured (where the pressure is more than that of the hydrostatic head). The different types of geothermal resources are illustrated in Figure 5.4 [18].

An illustration of different types of geothermal resources.

Figure 5.4 Types of geothermal resources.

Source: Goldstein, B., et al., “Geothermal Energy,” IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation, edited by O. Edenhofer, et al., Chapter 4, pp. 401–436, Cambridge University Press, Cambridge, UK, 2011.

The reservoirs can also be categorized on the basis of the enthalpy potential or temperature of the reservoir. Reservoirs having temperatures lower than 160°C are generally classified as low-enthalpy reservoirs, whereas those having temperatures in excess of 190°C are classified as high-enthalpy reservoirs. Reservoirs having intermediate temperatures are termed medium-enthalpy resources [19]. It should be noted that several different temperature limits and ranges have been reported in the literature for this classification, with maximum temperature for the low-enthalpy reservoirs as low as 70°C, and minimum temperatures as high as 225°C for categorization as a high-enthalpy reservoir. An alternative scheme involves using exergy, useful work potential at the system conditions, rather than enthalpy or temperature for classification. Specific exergy Ψ is related to specific enthalpy h and specific entropy s by the relation [20].

Ψ=(h-h0)-T0(s-s0)      (5.5)

where subscript 0 denotes the dead state where no useful work is possible. T0 is the temperature of the dead state, and generally taken to be that of the surroundings that function as the heat sink. In order to compare different geothermal resources that have different dead state temperatures, a specific exergy rate (SER) is defined by taking the dead state to be that of the triple point of water (0.6112 kPa, 0.01°C) and normalizing the exergy at any condition with the maximum exergy possible (1192.6 kJ/kg) with this dead state. Since the specific enthalpy and specific entropy at the dead state are taken to be equal to zero, the SER is obtained by the following equation [20]:

SER=h273.15s1192.6      (5.6)

The specific enthalpy and entropy values are available from thermodynamic steam tables, allowing the calculation of the SER. Since SER is a normalized matrix, its values range from 0 to 1. Resources with SER smaller than 0.05 are low-exergy resources, whereas those with SER greater than 0.56 are considered high-exergy resources. The intermediate SER values will result in the resource being classified as a medium-exergy resource. It should be noted that although SER provides useful information, it does not by itself convey information about available temperatures and pressures, which is essential for determining the power conversion technology.

Hydrothermal reservoirs are generally classified as water-dominated or vapor-dominated. Water-dominated reservoirs are capable of providing hot water at temperatures up to 100°C or pressurized water systems that contain small quantities of steam. Vapor-dominated reservoirs primarily contain dry saturated or superheated steam at higher pressures [17].

Waters in the geothermal reservoirs are at elevated temperatures and pressures and in contact with the subsurface formations. Their interaction with the minerals present in the subsurface environment results in the presence of several chemical species that are characteristics of the formation. Geothermal fluids contain high concentrations of anions (halides, bicarbonate, sulfate), cations (alkali and alkaline earth, manganese, iron, etc.), and neutral species (silica, ammonia, noble gases, arsenic, etc.) [17]. The most common type of fluid is near-neutral-pH salt (NaCl) brine containing dissolved gases, such as CO2, H2S, SO2, H2, NH3, and so on. The dissolved gases include noble gases as well. Other species present in the waters may include isotopes of various elements such as B, Sr, S, and many others [21]. The presence of these dissolved species has significant bearing on the operation of power plants in several ways:

  • The sulfate-rich, chloride-rich, or bicarbonate-rich waters will have their characteristic corrosive properties attacking the piping and other equipment in the power plant.

  • The presence of noncondensable gases in the steam generated from the fluid will reduce the efficiency of power generation and affect the operation of the heat exchange equipment.

  • Contaminants such as As, B, Hg, F, Sb, and so on, which are present in high concentrations, have the potential to cause severe harm. Discharges from the power plant need to be managed carefully to prevent any adverse impacts on human health and the environment.

The geothermal fluid present in any reservoir is unique to that location, and careful consideration must be given to the hydrogeochemistry of the reservoir before developing it for energy applications.

5.2.2 Energy Conversion Technologies

Direct utilization of geothermal energy has been practiced worldwide since ancient times, and such usage continues at the present time for applications such as geothermal heat pumps, space heating, swimming and bathing, greenhouse heating, agriculture drying, and so on. Geothermal heat pumps dominate such applications with unit capacities ranging from 5 to 150 kW, and along with space heating and bathing/swimming account for more than 80% of direct utilization of geothermal energy [22]. Figure 5.5 shows a schematic of a geothermal heat pump for a large commercial installation [23].

An illustration of closed loop systems is shown.

Figure 5.5 Geothermal heat pump system.

Source: U.S. Department of Energy, “Geothermal Heat Pumps,” https://www.energy.gov/energysaver/heat-and-cool/heat-pump-systems/geothermal-heat-pumps.

The system essentially consists of vertical piping that extends into the ground to a depth ranging typically from 100 to 400 ft and forms a recirculation loop. The subsurface temperature is relatively constant throughout the year, higher in winter and lower in summer than the atmospheric temperature. This piping arrangement in the heat pump provides an opportunity to transfer heat to the structure in winter and function as a heat sink during summer. Heat pump systems for smaller residential installations function in a similar manner; however, they are located at shallow depths due to the smaller heat exchange area needed in the underground piping system. The schematic of a space heating system is shown in Figure 5.6 [18].

An outline of the geothermal space heating system is shown.

Figure 5.6 Geothermal space heating system.

Source: Goldstein, B., et al., “Geothermal Energy,” IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation, edited by O. Edenhofer, et al., Chapter 4, pp. 401–436, Cambridge University Press, Cambridge, UK, 2011.

Hot geothermal fluid is pumped from the geothermal reservoir and loses its heat as it circulates through a radiator located inside the house or the structure. The cooled fluid is returned to the reservoir. Figure 5.6 illustrates an open-loop arrangement. A closed-loop system involves a heat exchange between the geothermal fluid and a secondary fluid (water) that recirculates through the radiator.

Compared to the direct use, power production using geothermal energy is a relatively recent development starting in the early 20th century. Advances in the technology since then have resulted in various power plant configurations that can be broadly classified into five categories: (1) dry steam, (2) single flash, (3) double flash, (4) binary (organic Rankine–Kalina cycle), and (5) advanced systems that feature various combinations of single–double flash systems, as well as combinations of geothermal plants with other primary sources including fossil and other renewable sources [24]. Alternately, these systems can also be classified on the basis of motive fluid that drives the turbine in the power conversion system (PCS), leading to a simpler classification of steam-driven plants and organic-driven plants. High-enthalpy reservoirs typically will have steam-driven plants, whereas the low-enthalpy reservoirs are exploited with organic-driven plants. Further classification can be made for the steam-driven plants on the basis of condensation and the number of flashing stages.

5.2.2.1 Steam-Driven Plants

As mentioned in Section 5.2.1, hydrothermal reservoirs can be water-dominated or vapor-dominated, and the vapor-dominated systems contain dry steam at high pressures. If significant quantities of noncondensable gases (>15% by weight) are present in this steam, then a direct intake noncondensing power cycle is used for power generation, where steam from the geothermal well is simply passed through a turbine and exhausted to the atmosphere without passing it through a condenser post turbine. This simplifies the design of the power plant by obviating the need for a condenser as well as for an injection well to return the condensate to the reservoir. Vapor-dominated reservoirs, where the noncondensable fraction is much lower, operate with a condensing steam cycle, where high-pressure steam is piped directly to the turbine, and the turbine exhaust is condensed by passing it through a condenser. The condensate is injected back into the ground.

Liquid-dominated medium- to high-enthalpy reservoirs yield high-temperature pressurized water rather than steam. Single- and double-flash power plants operate by generating steam from this water by reducing its pressure in flash drum(s) once it is pumped to the surface. The steam is fed to the turbine to generate the power. Turbine exhaust is passed through the condenser. A fraction of the resulting condensate and the concentrated brine generated from the flash operation is injected back into the reservoir. A double-flash plant will have two flash stages, producing high-pressure and low-pressure steam. The high-pressure steam from the first flash stage is fed to a high-pressure turbine. The exhaust of the high-pressure turbine is combined with the low-pressure steam from the second flash stage and fed to the low-pressure turbine for additional power generation. The exhaust steam is condensed and returned back to the reservoir along with the brine from the flash stages. A double-flash plant can produce 25% more power than a single-flash unit. Figure 5.7 shows a schematic of a flash power plant.

An outline of the steam flash power plant is shown.

Figure 5.7 Geothermal condensing steam flash power plant.

Source: Goldstein, B., et al., “Geothermal Energy,” IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation, edited by O. Edenhofer, et al., Chapter 4, pp. 401–436, Cambridge University Press, Cambridge, UK, 2011.

5.2.2.2 Binary Cycle Plants

Power generation using steam as the motive fluid is not possible with hydrothermal low-enthalpy reservoirs yielding geothermal fluids at temperatures lower than 90°C. Binary cycle or organic Rankine cycle (ORC) plants are used under these conditions for power generation, where the turbine is driven by a secondary organic fluid. Vapor of this low-boiling secondary fluid is generated in a vaporizer (boiler) by heat exchange with the geothermal fluid. The geothermal fluid does not experience any phase change and is returned to the reservoir. Figure 5.8 shows a schematic of a binary cycle geothermal power plant [18].

A schematic of the binary cycle power plant is shown. It is similar to steam flash power plant, except there is a heat exchanger and feed pump at the beginning of the setup.

Figure 5.8 Geothermal binary cycle power plant.

Source: Goldstein, B., et al., “Geothermal Energy,” IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation, edited by O. Edenhofer, et al., Chapter 4, pp. 401–436, Cambridge University Press, Cambridge, UK, 2011.

The geothermal fluid does not undergo any phase change and contacts only the piping to and from the heat exchanger. As a result, all dissolved species remain confined in the geothermal fluid, eliminating the need for any environmental controls. Furthermore, the quality of the secondary fluid can be controlled rigorously such that the turbomachinery (turbine and pumps) is contacted only by a clean fluid, unlike in a flash plant where the steam can contain several undesired contaminants [25]. Commonly used secondary fluids include propane, normal and isobutane and pentane, and ammonia. Propane and ammonia are particularly attractive due to their low boiling points. Their vapor pressures at 300 K are ~1 MPa, compared to the vapor pressure of 3 kPa for water. Several studies have also looked at fluorocarbons for use as secondary fluids, with promising results. A Rankine power cycle based on the secondary fluid is typically used for power generation, though other cycles, such as the Kalina cycle, have also been examined [24].

Exploitation of geothermal sources requires not only high temperatures but also the presence of geothermal fluids that have sufficient permeability through the subsurface such that adequate, reliable withdrawal is possible for power generation. In several locations, high temperatures are available at convenient depths; however, the formation is either dry (lacks sufficient quantity of the geothermal fluid) or does not have sufficient porosity to provide consistent supply of the geothermal fluid. Such fields can be exploited through enhanced geothermal systems (EGSs) wherein water is injected into the subsurface and the formation is modified/fractured to increase the porosity and permeability of the strata. The key challenge for EGSs is to ensure sufficient fluid volumes can be maintained in the system to sustain long-term production at acceptable rates, while minimizing water losses and risk from induced seismicity [18].

Although geothermal plants do not produce carbon dioxide, they do emit some carbon dioxide which is the major dissolved gas constituent in the geothermal fluids [18]. Other noncondensable gases present in the geothermal fluids include nitrogen, hydrogen sulfide, methane, argon, and oxygen. These dissolved gases are released into the vapor phase in the flash operations and accompany the steam through the PCS. Unlike steam, these do not condense at the temperatures present in the condenser and are released into the atmosphere, except for the hydrogen sulfide [26]. Hydrogen sulfide emissions are averted typically by caustic scrubbing, followed by oxidation and catalytic conversion to sulfur. The hydrogen sulfide present in the geothermal fluids has the potential to generate additional revenue through conversion to sulfuric acid. However, the quantity generated will typically not be large enough for the conversion to be economical. The emission of noncondensables does contribute to greenhouse gas emissions, albeit small, and these emissions can be eliminated by injecting the gas back into the reservoir along with the spent brine. However, this results in parasitic power consumption, reducing the overall energy efficiency of the plant. It should be noted that binary cycle plants do not have such emissions, as the geothermal fluid merely passes through a heat exchanger before being injected back into the formation.

Geothermal plants also produce solid waste streams associated with their activities. These include chemical sediment in tubes and vessels that may have heavy metals, deposit in cooling towers possibly contaminated with mercury, waste materials from treatment systems, and so on. In general, the quantities are manageable and have potential to yield valuable byproducts such as lithium, zinc, and silica, improving the economics of the power plant [27].

5.3 Transformations of Ocean Energy

The potential of ocean energy to satisfy the global energy demand has been known for a long time. However, its contribution to the energy mix is minimal at the current time. The global total installed capacity of all forms of ocean energy technologies is ~535 MW, an overwhelming majority of it (~520 MW) based on tidal range or tidal barrage that utilizes the changes in height of ocean level accompanying the cycle of tides. Tidal stream/current, wave energy, ocean thermal energy conversion (OTEC), and salinity gradient energy (SGE) systems account for the remaining capacity as shown in Figure 5.9 [28].

A pie chart presents the contribution of four different factors toward ocean energy technologies.

Figure 5.9 Installed capacity (in megawatt) of various ocean energy technologies.

Source: Redrawn from IRENA, Innovation Outlook: Ocean Energy Technologies, International Renewable Energy Agency, Abu Dhabi, 2020.

Unlike other renewable energy sources, ocean energy is characterized by the variety of primary energy types contained within it: tidal range/barrage harnesses the potential energy associated with the tides, wave energy and tidal current technologies involve conversion of kinetic energy, OTEC utilizes thermal energy, and SGE systems are based on the conversion of chemical energy. Energy transformations in each of these technologies are discussed below.

5.3.1 Potential Energy Transformations

Tidal range power generation is based on the rise and fall of the sea level with the tides, driven by the gravitational forces between the earth and the celestial bodies, primarily the moon and, to a lesser extent, the sun. Alignment of the sun and moon on new and full moons results in high tides (spring tides), whereas the misalignment of the three bodies on waxing and waning half-moons results in low tides (neap tides). Consequently, the rise and fall in the sea levels vary from 4 to 12 m along the coastlines [29]. Tidal range power plants involve impounding the water and then allowing it to flow through a turbine to generate electricity [30]. The potential energy stored in the impounded water is given by [2931]:

PE=0HρgAh·dh=12ρgAH2      (5.7)

where H is the tide water level over the base datum line, h is the level at any instant, and A is area of the impoundment or the barrage basin. ρ is the density of the ocean water, and g is the acceleration due to gravity (9.81 m/s2). The average power generated can be estimated by dividing this energy by the time taken to empty the impounded water, which is 6 hours and 12.5 minutes—the time taken by the tide to flow out. Assuming the density of the ocean water to be 1025 kg/m3, the average power from this plant will be [29]:

P¯=225AH2      (5.8)

It should be noted that the instantaneous power at any time will be different and can be obtained from the definition of power:

P=d(PE)dt=ρgAhdhdt      (5.9)

A tidal range power plant has the following four basic components [30]:

  1. Embankments to form the impoundment for the tidal water; these need to be designed to minimize the disturbance to the natural impoundment. To maximize the stored volume, the embankments should enclose as large an area as possible.

  2. Openings in the embankments fitted with control gates, or sluice gates, to transfer flows at a particular time and with minimal obstruction.

  3. Turbines for conversion of the potential energy to electricity. These are located in water passages across the embankment.

  4. Locks along the structure for the passage of vessels across the impoundment.

The impoundments get filled when the tide flows in from the ocean and are emptied as the tide flows out. Three different operating modes are possible for power generation to occur during this movement of the ocean water [31].

  1. Ebb generation mode involves filling the impoundment by opening the sluice gates as the tide flows in. The gates are closed when the desired level is reached. The stored water is held until the ocean level outside the impoundment is sufficiently reduced for the turbines to operate. At this point, the impounded water is released through the turbines to generate electricity until the height differential makes the turbine operation impractical.

  2. Flood generation mode operates in exactly the opposite manner to the ebb generation mode. Here the sluice gates are kept closed as the tide flows in until a sufficient height differential between the water levels outside and inside the impoundment is developed. The gates are opened at this time, allowing the tide flow into the impoundment through the turbines, generating electricity.

  3. Bidirectional generation mode involves operating the system during both filling and emptying of the impoundment, that is, operating it in both flood and ebb generation mode.

As there are two tide cycles every day, the single operation flood and ebb generation modes can produce electricity during two time periods. Double operation bidirectional plants, on the other hand, are able to generate electricity four times a day, as they operate during both incoming and outgoing tides. Flood generation mode is typically less efficient than the ebb generation mode due to the fact that the impoundment or basin cannot be emptied (or filled) completely. Furthermore, the storage capacity of the impoundment is generally much greater in the upper sections or heights, meaning that the lower sections fill up fast, reducing the height differential for the power generation. Bidirectional generation mode enables increased power generation; however, it requires more advanced and expensive bidirectional turbines. It should also be noted that the output of the bidirectional plant is not double, but only 15% more than that of a single operation plant. It should also be noted that although the total time needed for the inflow or outflow of tides is 6 hours and 12.5 minutes as mentioned earlier, power generation is not possible during the entire period. A sufficient height differential between the impoundment and ocean water levels must be built up before the turbines can start the operation. The actual period of operation during any leg of the tidal cycle is only ~3.5 hours [29].

Several tidal barrage plants are active around the world, with a total global capacity of ~520 MW. About 95% of this capacity is accounted for by just two plants: La Rance tidal barrage plant in France and Lake Sihwa tidal barrage plant in South Korea [28]. The La Rance plant is a bidirectional plant having twenty-four 10-MW turbines for a total capacity of 240 MW, whereas the Lake Sihwa plant is rated at 254 MW with operation in flood generation mode [31]. Turbines used in both the plants are ~10 MW Kaplan bulb turbines.

5.3.2 Kinetic Energy Transformations

Wave energy and tidal stream/current energy technologies are the major contributors to ocean energy after the tidal barrage systems. Although both these technologies involve harnessing the energy of moving ocean streams for power generation, the natural phenomena that result in these moving streams are different.

5.3.2.1 Tidal Current Energy

Tidal currents, as the name indicates, have their genesis in the regular tidal cycles under the influence of the moon and the sun. These currents are highly predictable and quantifiable with respect to their timing and the magnitude. Furthermore, as they are governed by the gravitational forces between the earth and the celestial objects, climatic phenomena have no effect on them [32]. Tidal current technologies differ from tidal barrage systems, in that no storage of water is involved to create a potential difference. Rather, the movement of water in the open ocean is exploited without creation of any barrier [28]. The resource potential of the tidal current can be expressed through its power density (PD—power per unit area), which is given by [33]:

PD=12ρU3      (5.10)

where U is the velocity of the current.

The apparent limiting total power generated can be obtained by multiplying the power density by the area swept by the turbine, which is π4D2 if D is the diameter of the turbine. However, a correction factor is needed to account for the area occupied by the turbine, which is generally taken to be a rectangle having a length of 10D in direction parallel to the flow and width 1.5D in the direction perpendicular to the flow. The area occupied can thus be visualized as the area influenced by a turbine, and when deploying a battery of turbines, no other turbine should be located within this area. Therefore, the average power density intercepted by the turbine (API) is actually only ~5% of the apparent power density obtained from equation 5.10 [33].

API=PDAsweptAoccupied=PD(π/4)D215D2=0.0524·PD      (5.11)

The power-generation capacity of the tidal current installation can then be obtained by multiplying this average intercepted power density with the area of the installation perpendicular to the direction of the flow.

The turbine is the key piece of equipment in the tidal current systems, and several different designs of turbines, as well as other devices that can capture the kinetic energy of moving fluid, have been developed and deployed for power generation. These include the following [34]:

  • Horizontal-axis turbines that are similar in principle and operation to wind turbines.

  • Vertical-axis turbines that feature turbines with blades rotating about a vertical rather than horizontal axis.

  • Ducted turbine or enclosed tips that are essentially horizontal-axis turbines encased in a nozzle conduit for reduced turbulence.

  • Oscillating hydrofoil that experiences a lift due to the tidal current and then transfers that lift into a rotational motion of fluid.

  • Archimedes screw that rotates in the tidal current and in the process pumps the water through helical channels of the screw to a higher elevation.

  • Tidal kite that intensifies the flow to a small turbine to which it is tied.

These six configurations are shown in Figure 5.10 [28].

Six different turbine configurations are shown.

Figure 5.10 Tidal current turbine configurations: (a) horizontal axis, (b) vertical axis, (c) ducted turbine, (d) oscillating hydrofoil, (e) Archimedes screw, and (f) tidal kite.

Source: Redrawn from IRENA, Innovation Outlook: Ocean Energy Technologies, International Renewable Energy Agency, Abu Dhabi, 2020.

The MayGen project off the coast of Scotland, United Kingdom, is currently the largest tidal current power-generating station, having a total installed capacity of 6 MW. It utilizes a 15-m tall horizontal-axis turbine with a blade diameter of 16 m and rated power of 1.5 MW.

Tidal current technologies are deployed near shore where they take advantage of the daily tidal cycles. Significant currents also exist in the open ocean, driven by wind and thermohaline circulations. The Gulf Stream in the Atlantic, Kuroshio off the coast of Japan in the Pacific, and Agulhas/Mozambique current off the African coast in the Indian Ocean are some of the prominent examples. Several other currents are also present over the rest of the globe [32, 35]. These currents are slower than the tidal currents, but still sufficiently fast to have power-generation potential. Unlike tidal currents, these are unidirectional, which may simplify the design requirements for the turbines. However, the infrastructure requirements are likely to be far more challenging.

5.3.2.2 Wave Energy

Tidal currents, by their nature, are periodic, coinciding with the tidal cycle. Waves offer another alternative for power generation on a more continuous basis. Waves contain tremendous amounts of energy and can travel over long distances. Wave energy consists of both potential energy (displacement from the mean ocean level) and kinetic energy (through motion of the water particles). The source of this energy is the kinetic energy of wind, some of which gets transferred to water. This interchange results in waves that are between 1 and 3 m high at the coasts but several meters high in the deep sea. Most energetic waves are found on the globe between 30° and 60° latitudes [35]. The global potential for wave energy is ~29,500 TWh/year, exceeding the global energy demand [28]. Waves, while exhibiting seasonal and short-term variations, can be predicted with reasonable accuracy, and thus be a reliable primary energy source with nearly 90% temporal availability [29]. The energy stored in a wave per unit area of the ocean surface is described by the following equation [36]:

Ewave=116ρgHmo2      (5.12)

where Ewave is the energy per unit area (J/m2) and Hmo is the significant wave height in m. The wave energy flux Ef depends upon the periodicity of the wave. Using the standard g and ρ values, the wave energy flux can be expressed as [36]:

Ef=0.491Hmo2TM      (5.13)

where Ef is in kW/m of crest length and TM is the wave period in seconds.

Wave energy conversion (WEC) devices are broadly classified into three types [28]:

  1. Oscillating water columns (OWC) that transform the wave motion into an up-down motion of a water column in a confined conduit that is open to the atmosphere. The opening is fitted with an air turbine. The up-down motion of the water column compresses the air column, which drives the air turbine.

  2. Overtopping devices (OD) use floating structures that capture water spilled by the waves over the top to create a water reservoir and drive a hydro-turbine.

  3. Oscillating bodies (OB) converters use different mechanical linkages.

Schematics of the three types of devices are shown in Figure 5.11 [37].

An outline of three different wave energy converters are shown.

Figure 5.11 Wave energy converters: (a) oscillating water column, (b) overtopping devices, and (c) oscillating bodies.

Source: Ahmad, R., K. McKee, and L. Howard, “Advancements of Wave Energy Converters Based on Power Take Off (PTO) Systems: A Review,” Ocean Engineering, Vol. 204, 2020, #107248 (25 pages).

Several wave energy developmental projects are in operation around the world, the largest of which is in Portugal with a rated power capacity of 0.35 MW. The capacities of most of the other installations are in tens of kilowatts [28].

5.3.3 Thermal Energy Transformations

OTEC systems rely upon the temperature difference between the warm surface waters and the cold water at the depth of the ocean. The surface temperatures may reach as high as 25°C in the tropics from the absorption of solar radiation, whereas the temperatures at a depth of ~1 km may range from 5 to 10°C. A temperature difference of ~20°C is generally required for OTEC systems to operate, and based on the temperature distributions across the globe, optimistic estimates of 30,000–90,0000 TWh/year have been suggested for electricity generation through OTEC systems. This theoretical potential considerably exceeds the global electricity demand; however, the technical potential is likely less than one-fifth of the theoretical potential [35, 37, 38].

OTEC systems are primarily of three kinds: open cycle, closed cycle, and hybrid cycle. The open-cycle systems use steam as the working fluid for the PCS, whereas the closed-cycle systems use some other working fluid to drive the turbines in the PCS. Hybrid cycles combine the features of both open-cycle and closed-cycle systems and typically will have two working fluids: steam and another fluid. In all the systems, warm surface waters are used to generate vapor to drive a turbine for power generation, and the cold water from the depth of the ocean is used as the heat sink in the power-generation cycle.

The open-cycle system is conceptually simple and relies upon the ocean water itself to provide the working fluid for the turbine—the steam. The warm water is pumped into an evaporator, which operates at subatmospheric conditions. Steam that is produced drives a turbine generating power and is then condensed using the cold ocean or seawater from the deep. The residual concentrated solution from the evaporator is returned to the ocean. Condensed water is almost pure and is not recycled back to the evaporator. The power plant thus also functions as a desalination unit, yielding a valuable by-product. The conceptual schematic of the open-cycle OTEC plant is shown in Figure 5.12 [39].

An illustration of an open-cycle OTEC plant is shown.

Figure 5.12 Conceptual schematic of an open-cycle OTEC plant.

Source: Zhang, W., et al., “Review of the Applied Mechanical Problems in Ocean Thermal Energy Conversion,” Renewable and Sustainable Energy Reviews, Vol. 93, 2018, pp. 231–244.

Although the open-cycle system is attractive as it does not require a separate working fluid, the low operating temperatures and pressures of the PCS translate into very low conversion efficiencies. Closed-cycle systems utilize a low-boiling fluid that can provide much higher operating pressures to drive the turbines, and thus increase the power conversion efficiency. The conceptual schematic of a closed-cycle system is shown in Figure 5.13 [39].

An illustration of a closed-cycle OTEC plant is shown.

Figure 5.13 Conceptual schematic of a closed-cycle OTEC plant.

Source: Zhang, W., et al., “Review of the Applied Mechanical Problems in Ocean Thermal Energy Conversion,” Renewable and Sustainable Energy Reviews, Vol. 93, 2018, pp. 231–244.

The working fluid in the closed-cycle systems is a low-boiling compound such as ammonia, propane, or a chlorofluorocarbon. The temperature of the surface water is sufficient to create a high-pressure vapor of this fluid, which then drives the turbine generating the power, typically in a Rankine cycle operation. The benefit of using a closed cycle over an open cycle can be readily understood from the differences in the vapor pressures of the working fluids: at 25°C, the vapor pressures of ammonia and fluorocarbon R152a are in excess of 600 kPa, and that of propane is 936 kPa, whereas the vapor pressure of water is only ~3 kPa. Low-temperature deep ocean water is used to condense the vapor, and the condensed liquid is circulated back to the evaporator as seen in Figure 5.13. Despite the high pressures, the power conversion efficiency is quite low, that is, 3%–4%, and improvements in the efficiency are sought through incorporation of vapor–vapor or liquid–vapor ejectors, use of more sophisticated Uehara or Kalina power cycles, or using injection or absorption power cycles. These modifications can improve the power conversion efficiency by additional 1%–2% [40].

Hybrid cycles share features of both open and closed cycles. The power generation is through the use of a secondary working fluid, as in the case of a closed-cycle system. This working fluid is vaporized using the steam generated from the warm surface water in an evaporator, as in the case of the open-cycle system. The schematic of a hybrid cycle is shown in Figure 5.14 [39].

An illustration of an hybrid-cycle OTEC plant is shown.

Figure 5.14 Conceptual schematic of a hybrid-cycle OTEC plant.

Source: Zhang, W., et al., “Review of the Applied Mechanical Problems in Ocean Thermal Energy Conversion,” Renewable and Sustainable Energy Reviews, Vol. 93, 2018, pp. 231–244.

The efficiency of the electricity generation can be increased by combining the OTEC systems with other energy sources: integration of solar energy collectors is an attractive proposition to raise the temperature of the ocean water; it may also be possible to combine geothermal sources, if such a source is available in the vicinity [40].

Currently, only small-scale OTEC plants are operational in Japan and the United States. The maximum capacity of these plants is 100 kW. South Korea has planned to deliver a 1-MW OTEC project to Kiribati, which will be a significant contributor to its total electricity generation capacity of 6 MW and result in displacement of fossil energy by renewable energy [28].

Although most of the OTEC systems utilize power cycles for electricity generation, the temperature difference between the surface and deep waters can also be utilized to generate electricity directly on the basis of the Seebeck effect. The Seebeck effect (or the thermoelectric) effect is the phenomena wherein a temperature differential results in the establishment of a potential gap (∆V) across a conductor or a semiconductor [41]. The magnitude of this potential gap is dependent upon the temperature differential and the Seebeck coefficient (S). Mathematically,

V=STH-TC      (5.14)

where TH and TC are the temperatures of the hot and cold points, respectively. Seebeck coefficients for metals are typically between −100 and +100 μV/K, for semiconductors 300 and 900 μV/K, and between −2000 and +2000 μV/K for alloys. The composition of the alloy has a significant impact on the coefficient; Pb15Ge37Se58 has a value of −1990 μV/K, whereas another alloy of the same element Pb0.3Ge39Se58 has a value of +1670 μV/K [29].

A thermoelectric generation system consists of three components: a heat source, the thermoelectric module, and a heat sink. In the ocean thermoelectric generation (OTEG) systems, the warm surface waters function as the heat source, and the deep waters function as the sink. The conceptual schematic of an OTEG configuration is shown in Figure 5.15 [42].

An illustration of an OTEG plant is shown.

Figure 5.15 Conceptual schematic of an OTEG system.

Source: Bohn, M. S., D. K. Benson, and T. S. Jayadev, “Thermoelectric Ocean Thermal Energy Conversion,” Journal of Solar Energy Engineering, Vol. 102, 1980, pp. 119–127.

The energy required for the pumping of the deep cold water can be obtained from the energy of ocean waves, essentially using a device similar to that shown in Figure 5.11. Figure 5.16 shows an alternate arrangement when the oscillatory action of waves is used for the operation of the OTEG device [43].

A comparison of OTEG system with a graph is shown.

Figure 5.16 OTEG system based on wave motion.

Source: Liu, L., “Feasibility of Large-Scale Power Plants based on Thermoelectric Effects,” New Journal of Physics, Vol. 16, 2014, #123019 (13 pages).

The performance of material used for thermoelectric applications is indicated by a dimensionless figure of merit (ZT), which is defined according to the following equation [44]:

ZT=S2kσT      (5.15)

where k and σ are thermal and electrical conductivities, respectively. The figure of merit for Sb–Bi–Se–Te alloys, promising materials for OTEG applications, is shown in Figure 5.17 [42].

A graph presents the relationship between the figure of unit and temperature.

Figure 5.17 ZT for Sb–Bi–Se–Te alloys.

Source: Bohn, M. S., D. K. Benson, and T. S. Jayadev, “Thermoelectric Ocean Thermal Energy Conversion,” Journal of Solar Energy Engineering, Vol. 102, 1980, pp. 119–127.

The maximum thermal efficiency (ηmax) of a thermoelectric generator operating between hot and cold temperatures (TH and TC), respectively, is given by the following equation [44]:

ηmax=THTCTH1+ZT¯11+ZT¯+TC/TH      (5.16)

where

ZT is the figure of merit at the average temperature, (TH + TC)/2.

It can be seen that the first term on the right-hand side is simply the efficiency of the Carnot cycle operating between the two temperatures, and thus the maximum efficiency of the OTEG system is always less than that of the Carnot cycle. The Carnot cycle efficiency increases with increasing difference between the two temperatures. Unfortunately, this difference is quite small for OTEG systems, which can be expected to have very low power conversion efficiencies. OTEG ηmax can approach the Carnot cycle efficiency when ZT approaches infinity, which is possible in theory. However, in practice, it has been found to be <3, severely limiting the efficiency of the OTEG systems. OTEG efficiency can be improved by increasing the surface temperature through incorporation of solar heating or utilizing other heat sources (geothermal/fossil, etc.), if available. Currently, OTEG systems are limited to providing power in the range of 1–10 kW to subsurface instrumentation [29].

5.3.4 Chemical Energy Transformations

The chemical energy transformations for the generation of electricity in ocean energy systems are based on the salinity difference between freshwater and ocean water [35]. The chemical potentials of the two streams are different due to the difference in their salinity, and consequently the mixing of the two streams at any river mouth results in a release of Gibbs energy—the Gibbs energy of mixing. SGE systems involve harnessing this released Gibbs energy to produce electricity rather than allowing it to be dissipated as heat. The driving force for electricity generation from the mixing of freshwater and ocean water is given by [45]:

Driving Force = (Go+Gr)Gb=ΔGmix      (5.17)

where Go, Gr, and Gb are Gibbs energies of the ocean water, freshwater, and brackish water after mixing, respectively.

The mixing process can be visualized as an isothermal and isobaric process, with only the concentration differences contributing to the Gibbs energy change ∆Gmix. The molar Gibbs energy change (∆ gmix) for the mixing process can then be written in terms of mole fractions of the solute species as follows [46]:

ΔgmixRT = ixi,bln(γi,bxi,b)nonbixi,oln(γi,oxi,o)nrnbixi,rln(γi,rxi,r)      (5.18)

where γi and xi represent the activity coefficient and mole fraction of the solute i; n represents the total moles of the solute in any stream; subscripts b, o, and r represent the brackish water after mixing, ocean water, and freshwater, respectively; and R and T are the gas constant and temperature, respectively.

Equation 5.18 can be simplified by assuming that the activity coefficients for the solutes in a dilute solution are equal to 1 and the molar ratios can be approximated by volume ratios to obtain the specific Gibbs energy change per unit volume of the mixed brackish solution (∆ gv,mix).

Δgv,mixjRT = Cbln(Cb) (1ϕ) Coln(Co)  ϕCrln(Cr)      (5.19)

where C is the total concentration of solutes in any stream and ø is the volume fraction of the freshwater in the total volume. The van’t Hoff factor j can be taken to be 2 if the salinity is attributed to NaCl, a strong electrolyte that dissociates completely in a solution.

The amount of energy dissipated in this simple mixing process is enormous, amounting to as much as 2 kJ/L. The mixing of two solutions of unequal concentrations can also be analyzed from the perspective of osmotic pressure of ocean water, which can range from 20 to 25 atm. The potential for electricity generation can be readily recognized by visualizing this loss to be equivalent to the release of potential energy from a 700-ft-high waterfall [47, 48]. In other words, wherever a river empties into a sea or an ocean, there is a potential to tap energy equivalent to what can be obtained in a hydroelectric plant operating with a height differential of ~700 ft. The power-generation potential at any location can be calculated simply by multiplying this Gibbs energy change by the volumetric flow rate. Estimates of the theoretical global potential for SGE have varied greatly from a high value of 27,500 TWh/year (exceeding the global electricity demand) to a more realistic 625 TWh/year [36, 45].

Harnessing electricity from this natural phenomenon of a river flowing into an ocean involves controlling the mixing of the two waters by using a semipermeable membrane that inhibits the transport of the solute molecules across it. Under these conditions, the riverine freshwater will flow across the membrane effectively pumping the mixed water to a height of ~700 ft. In the simplest configuration of a power generation system, this water can be released through a waterwheel generating electricity as shown in Figure 5.18 [49].

An illustration of the osmotic salination energy converter.

Figure 5.18 Conceptual schematic of osmotic salination energy converter.

Source: Norman, R. S., “Water Salination: A Source of Energy,” Science, Vol. 186, No. 4161, 1974, pp. 350–352.

The practical implementation of SGE systems is primarily through pressure-retarded osmosis (PRO) and reverse electrodialysis (RED) [35, 45]. Other concepts under development for electricity generation include capacitive mixing (CAPMIX) and capacitive reverse electrodialysis (CRED) [50].

5.3.4.1 Pressure-Retarded Osmosis

Osmosis is the process of transport of water across a semipermeable membrane from a solution containing a lower concentration of dissolved species to a solution containing a higher concentration of the dissolved species. In the salinity gradient systems, ocean water containing a high concentration of salt thus draws water from the river or freshwater stream across the membrane. The freshwater stream is referred to as the feed stream and the ocean water, the draw stream. The mixed stream is the brackish water exiting the membrane module on the permeate side. However, for the transport to occur, the pressure on the permeate side, that is, the pressure of the draw stream, must be lower than the osmotic pressure π, calculated using the van’t Hoff equation [51]:

π=jCRT      (5.20)

where C is the concentration of the osmotically active solute, R is the gas constant, T is the temperature, and j is the van’t Hoff factor, which is the ratio of the actual concentration of the osmotically active particles to the nominal concentration of the solute species. It is related to the degree of dissociation of the solute (α) and the dissociation stoichiometry (υ—the number of osmotically active particles per molecule of the solute) by the following relationship:

j=1+α(υ1)      (5.21)

For a strong electrolyte such as NaCl, which dissociates completely in water, α = 1 and υ = 2, yielding a value of 2 for the van’t Hoff factor.

In a batch osmosis process, the transport of water across the membrane results in an increase in the hydrostatic pressure on the permeate or the draw side. The driving force for the transport, the difference between the osmotic pressure and the hydrostatic pressure, decreases with time, reducing the flow until the driving force becomes zero. In a PRO process, the permeate side is maintained at a constant pressure such that the hydrostatic pressure differential between the feed and draw stream is less than their osmotic pressure differential. This creates a constant driving force for the flow of the feedwater across the membrane. This additional flow is exhausted through a turbine to generate power as shown in Figure 5.19.

An illustration of PRO system.

Figure 5.19 Conceptual schematic of a PRO system.

Source: Redrawn from Helfer, F., C. Lemckert, and Y. G. Anissimov, “Osmotic Power with Pressure Retarded Osmosis: Theory, Performance and Trends–A Review,” Journal of Membrane Science, Vol. 453, 2014, pp. 337–358.

Typically, the pressure on the draw side is maintained at half the osmotic pressure, and only one-third of the brackish water is used for power generation. The remaining two-thirds of the brackish water is passed through a pressure exchanger to pressurize the incoming draw water to maintain a pressure balance.

The semipermeable membrane is the key component of the process. Ideally, the membrane should have high hydrophilicity for a greater flux of water, high selectivity or solute rejection, and mechanical strength to withstand the high pressure differential. Initially, reverse osmosis (RO) membranes were investigated for the PRO applications; however, these membranes were found to be susceptible to concentration polarization—deposition or accumulation of salt on the membrane that reduces the driving force and hence the flux across the membrane. Subsequently, cellulose acetate and cellulose triacetate membranes were used in the PRO applications. Further developments have resulted in the use of asymmetric thin film composite (TFC) membranes consisting of a dense active thin film deposited on a microporous support. The first TFC membrane consisted of an active polyamide (PA) layer on a polysulfone (PSf) substrate backed by nonwoven polyethylene terephthalate fabric [52]. Continued research on membrane material has resulted in the use of materials such as polyethersulfone resin (PES), polyacrylonitrile (PAN), polyamide imine (PAI), polyetherimide (PEI), and polyvinyl pyrrolidone (PVP) for the substrate layer in addition to PSf. PES and PAN have been found to be particularly suitable for PRO due to their low cost and excellent mechanical, chemical, and thermal stability. The active layer in most of the membranes is made of PA; however, cellulose acetate is also used in the active layer, and the recent developments have seen applications of other materials such as polybenzimidazole (PBI) [53]. The membranes are typically used in a hollow fiber module rather than a flat plate geometry due to the higher surface area, packing density, and mechanical strength characteristics of the hollow fiber module configuration. Such modules can have a water flux up to 50 L/m2 h and power densities of 10–40 W/m2 [53]. Recent trends have investigated inorganic ceramic or metallic membranes for their robustness and inherent properties such as high mechanical, chemical, and thermal stability; antifouling; and hydrophilicity.

5.3.4.2 Reverse Electrodialysis

RED is another alternative that utilizes semipermeable membranes to exploit the chemical potential difference between the river and ocean waters. The seawater and river water are allowed to mix after flowing through compartments separated by a series of alternating anion and cation exchange membranes (AEM and CEM) as shown in Figure 5.20. The chemical potential difference between the two streams generates a potential across each membrane, and the overall potential is the sum of the individual potential differences.

An illustration of RED system.

Figure 5.20 Conceptual schematic of an RED system.

Source: Lewis, A., et al., “Ocean Energy,” IPCC Special Report on Renewable Energy Sources and Climate Mitigation, edited by O. Edenhofer, et al., Chapter 6, pp. 497–533, Cambridge University Press, Cambridge, UK, 2011.

The electrolytes in the anode and cathode compartments at either end contain a reversible Fe2+/Fe3+ couple, typically a ferrocyanide/ferricyanide reaction couple ([Fe(CN)6]4−/[Fe(CN)6]3−), in a salt (NaCl) solution to convert the ionic transport into electric current in the external circuit.

The potential E of any individual cell in the arrangement shown earlier can be obtained by writing the Nernst equation for the cell visualizing it as consisting of a central ocean water compartment separated from two half-cells containing river water on each side [54]. The schematic of the cell is as shown in Figure 5.21.

A diagram represents one cell from the RED system.

Figure 5.21 Individual cell in RED system.

E=E1/2,  CEM+E1/2,  AEM=(αCEMz++αAEMz)RTFln(CoCr)      (5.22)

where E1/2 is the half-cell potential, α is the permselectivity of the membrane, and z is the ion valence. F is the Faraday constant. If the ion-selective membranes are perfectly selective for the respective ions, then the potential of a single cell is ~308 mV,2 if the salt content is assumed to be solely due to NaCl, where each ion has a unit charge. In reality, both the streams contain other salts as well, in particular divalent cations such as Ca2+, and Mg2+, as well as divalent anions such as sulfate, which results in a lower potential [54]. Continuous operation of RED devices involves cocurrent flows of ocean water and river water through adjacent cells and recirculation of the electrolyte solution containing the redox couple through the cathodic and anodic compartments.

2. The potential clearly depends upon the salt concentrations in the two streams. This number is based on salt concentrations of 600 and 1.5 mM in ocean water and river water, respectively, at 25°C.

The ion exchange membranes are the key components of the RED system. AEMs are positively charged barriers that permit only the anions to move across, whereas CEMs are negatively charged barriers that permit the cations to move through. Both AEMs and CEMs can be homogeneous or heterogeneous, depending on the uniformity of the charge distribution in them. These are typically styrene–divinylbenzene–based polymers that are sulfonated to obtain CEMs or subjected to chloromethylation followed by amination to obtain AEMs [55]. CEMs and AEMs are both available commercially from vendors like Asahi Glass, Dupont (Nafion membranes), and so on.

Electrodes are equally important components of the RED systems, with dimensionally stable anodes (DSAs) made of Ti-supported metal oxide (e.g., Ru/Ir) or noble metal (e.g., Pt) and graphite/carbon cathodes preferred for their low electrical resistances and good thermal/chemical stabilities. The ferro-/ferri-cyanide system is also preferred over the Fe2+/Fe3+ system (chloride salts), as the Fe2+ ion is highly susceptible to oxidation, which creates practical difficulties.

5.3.4.3 Capacitive Mixing

CAPMIX systems rely on cyclic charging and discharging of electrodes when exposed to solutions of different salinity. This charging is accomplished by means of ion intercalation, redox reactions, or through the use of ion-selective membranes [48]. The operation principle of CAPMIX systems is shown in Figure 5.22 [56].

An illustration of CAPMIX operation system is shown.

Figure 5.22 CAPMIX system operation: (a) operational cycle and (b) network/energy output.

Source: Morais, W. G., et al., “Electrochemical Systems for Renewable Energy Conversion from Salinity and Proton Gradients,” Journal of the Brazilian Chemical Society, Vol. 29, No. 5, 2018, pp. 934–947.

In the first step of the cycle, the electrodes/accumulators are exposed to the concentrated solution resulting in the transfer of ions from the solution to the accumulator. The second step involves exchanging the concentrated solution with the dilute solution when the external circuit is open. The accumulated charges are discharged into the solution. The third step involves closing the electric circuit resulting in the discharge of the accumulated charge into the external circuit. The cycle is completed by exchanging the dilute solution with the concentrated solution in the fourth step under open circuit conditions. The net energy gain is given by the area enclosed by the cycle as shown in Figure 5.22(b). Mathematically,

W=ΔEdq      (5.23)

Activated carbon has been commonly used as a capacitive accumulator/electrode for both positive and negative electrodes. Recent developments have investigated polymeric materials such as poly(acrylic acid), poly(methacrylic acid), and ethylene diamine to use as soft electrodes and the use of ion exchange membranes to improve the performances of activated carbon electrodes [57, 58].

Mixing entropy batteries (MEBs) function in a similar manner as the CAPMIX process as described earlier. However, they utilize electrodes consisting of MnO2 and silver wire for positive and negative electrodes, respectively. Furthermore, the charging and discharging processes take place in dilute and concentrated solutions, respectively, reverse of the process described earlier.

5.3.4.4 Capacitive Reverse Electrodialysis

CRED can be visualized as a hybrid of CAPMIX and RED processes. RED processes operate continuously, with electrodes merely functioning as the sites for charge transfer reactions. CRED processes employ capacitive electrodes and function in a cyclic manner [59]. The cyclic CRED operation is shown in Figure 5.23 [50].

An illustration of CRED operation is shown.

Figure 5.23 CRED operation.

Source: Vermaas, D. A., et al., “Clean Energy Generation Using Capacitive Electrodes in Reverse Electrodialysis,” Energy and Environmental Science, Vol. 6, 2013, pp. 643–651.

The energy/work output from the CRED system is similar to the cyclic CAPMIX process. However, it can be seen that the CRED process employs only two electrodes for multiple chambers, unlike the CAPMIX process, where each compartment uses two electrodes/accumulators. Furthermore, the use of multiple compartments provides greater potential while minimizing the membranes needed. The power density obtained in CRED is higher than that of CAPMIX alternatives. The major advantage of CRED over the conventional RED process is the avoidance of a redox couple for the reactions at the two electrodes. As mentioned earlier, typically the ferro-/ferri-cyanide couple is used in the RED process. The redox reactions have associated overpotentials for faster kinetics, which represents an energy loss. A more significant benefit is realized from environmental considerations—the inevitable leakage of complex cyanide ions, howsoever small it may be, results in an environmental hazard while also affecting the ionic balances in the device compartments. These difficulties are averted in the CRED process [50].

SGE conversion systems are extremely attractive from the viewpoint of conceptual simplicity. PRO and RED systems have seen greater developments than the other alternatives. The actual realizable technical potential for these systems, while not at the same scale as the other ocean energy systems, is still large enough to make a significant contribution to clean, renewable energy, pointing to a need for further research and development in this arena.

It should be noted that although all these technologies are considered climate-friendly and do offer benefits of zero or extremely low carbon emissions, they are not without some environmental impacts. Reservoir hydropower systems, for example, dramatically alter natural flows, disrupting life cycles of various organisms by changing the landscape. Flooding of large natural swathes of land and vegetation growing on it leads to anaerobic conditions and generation of methane, a gas that has an order of magnitude larger global warming potential. The impact of wind power installations on birds has also been well-documented. Similarly, ocean energy systems, while considered environmentally benign or “blue” energy alternatives, are not without their share of impact on the environment. The OTEC system involves large flows of nutrient- and microorganism-rich deep waters, and their transport to the surface can have a severe impact on the ecosystem of surface waters. As the deepwater functions as the heat sink, warming it by even a few degrees may result in the release of carbon dioxide contained in it [38]. Submarine cables needed for electricity transmission will create electromagnetic fields, and the impact of such fields on marine organisms is not known. Tidal barrage systems are located generally at locations that feature a complex, biologically rich habitat. Estuary sites (where an SGE system has to be situated) are even more complex in this regard. Creation of impoundments and controlling the flows through it can have a disastrous consequence on the marine life, on both macro- and microorganisms. The economic impact of such changes will be felt by fishing and other industries that are developed in the particular locations. It is also possible that such systems may have beneficial impacts on some species, providing them with niche environments to thrive. Careful analysis of all potential impacts must be conducted by responsible agencies and authorities before implementing any kind of renewable energy project [60, 61]. Further discussion of potential environmental impacts of various renewable technologies is presented in Chapter 7, Techno-Economic Analysis of Renewable Energy Systems.

5.4 Summary

Wind power and hydropower are the two most dominant primary sources for the generation of renewable electricity. Energy systems involving these two are based on the conversion of mechanical energy—kinetic and potential energy, respectively—and the technologies for such conversions are mature and well established. Globally, the number of small-scale hydropower installations continues to grow, though they may account for only ~10% of the total hydropower generated. Apart from traditional technologies, some interesting alternative techniques are also under development to harness the potential energy of flowing streams.

Wind power has grown significantly, driven by, among other things, the astonishing growth in the size and capacity of wind turbines. A single wind turbine today is capable of generating power in excess of 10 MW. Improved materials and blade designs are behind this capacity expansion.

Geothermal energy is overall a relatively minor contributor to the renewable energy portfolio. However, it has a large untapped potential, and innovative thermodynamic cycles are being investigated for power conversion using this energy.

Ocean energy is unique among all energy resources in possessing multiple types of primary energy—mechanical, thermal, and chemical. Technologies for the conversion of ocean energy show the greatest variety and present maximum opportunities for development.

Finally, it should be realized that all these technologies do have some environmental impacts, which may be understood better in the case of mature technologies. A careful comprehensive assessment of these impacts must be conducted, particularly for technologies under development, before any projects based on such technologies are implemented in practice.

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Problems

5.1 Compare the reservoir and ROR hydropower plants on the basis of factors such as capacity, cost, site requirements, human and environmental impacts, and other relevant factors.

5.2 Consult the literature to obtain data on the increase in the maximum capacity of wind turbines over time. Present the results as a graph of capacity versus time.

5.3 Obtain the SER as a function of steam quality for low-, medium-, and high-enthalpy geothermal reservoirs. Choose appropriate representative temperatures. Thermodynamic properties of water can be obtained from steam tables or other sources.

5.4 What are the advantages and disadvantages of open- and closed-cycle geothermal steam power plants?

5.5 What are the challenges associated with binary cycle plants as compared to steam plants for power generation from geothermal resources? What are the benefits offered by the binary cycle plants?

5.6 Consider an arbitrary tidal barrage power plant. Plot the power-generation profile as a function of time over the course of a day. Account for the idle periods during which the available head is insufficient. How will the profile change with the generation mode? Make reasonable assumptions and justify them.

5.7 What are the operational challenges faced by tidal current power plants?

5.8 Consult the literature for data on the Seebeck coefficient. What are the potentials generated by some of these materials in an OTEG system? Calculate the figures of merit for the materials and present the results in the form of a plot. What are the maximum thermal efficiencies of the thermoelectric generators?

5.9 Equation 5.19 can be applied to systems involving the discharge of a saline stream into freshwater. Calculate the specific Gibbs energy change for such a situation as a function of concentration and volumetric ratios of the two streams. Assume that the salinity is due to NaCl. How will the values change if the salt is an equimolar mixture of MgCl2 and CaCl2?

5.10 What would be the potential of an individual RED cell operating with the streams described in question 5.9?

5.11 Compare the operation of RED and CRED power-generation systems.

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