Ramakumar. R. Barnett. A M. Kazmerski. L L. Benner. J. P. Coutts. T.J. " Power Systems and Generation The electrical Engineering Handbook Ed. Richard C. dorf Boca Raton: CRc Press llc. 2000
Ramakumar, R., Barnett, A.M., Kazmerski, L.L., Benner, J.P., Coutts, T.J. “Power Systems and Generation” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000
60 Power Systems and generation R. Ramakumar 60.1 Distributed power generation Allen m. Barnett Geothermal. Tidal Energy. Fuel Cells. Solar-Thermal-Elect Conversion. Biomass Energy. Thermoelectrics Lawrence L Kazmerski Thermionics. Integrated System Concepts.System Impacts ational Renewable energy 0.2 Photovoltaic Solar Cells Solar Cell Operation and Characteristics.Solar Cell Types and hn p. benner Their Optimization.Crystalline Silicon. Ill-V Semiconductors ational Renewable energy Thin-Film Solar Cells. Dye-Sensitized Cells. Module Technologies. Photovoltaic Power Systems 60.3 Thermophotovoltaics Timothy J. Coutts Background. Design Considerations of a TPV System. Optical National Renewable energy Control of Sub-bandgap Energies.Development of PV cells Status of System Development . Systems and Applications 60.1 Distributed Power generation Distributed generation(DG)refers to small(a few watts up to 1 MW) power plants at or near the loads, operating in a stand-alone mode or connected to a grid at the distribution or subtransmission level, and geographically scattered throughout the service area. Typically they harness unconventional energy resources such as insolation, wind,biomass, tides and waves, and geothermal. Small plants powered by site-specific conventional energy resources such as low-head and small hydro and natural gas are also included in this general group Interest in DG has been growing steadily since the dramatic oil embargo of 1973. In addition to the obvious advantages realized by the development of renewable energy sources, DG is ideally suited to power small remote loads, located far from the grid. An entire family of small power sources has been developed and employed for space, underwater, and biomedical applications. Another niche for these systems is in energizing remote rural areas of developing countries. It is estimated that there are more than one million remote villages in the world with no grid connection and minimally sustained by locally available energy sources. Integrated renewable energy systems(IRES), a special subset of DG, are ideally suited for these situations. General Features DG will have one or more of the following features: Stand-alone or interface at the distribution or subtransmission level Located near the loads e 2000 by CRC Press LLC
© 2000 by CRC Press LLC 60 Power Systems and Generation 60.1 Distributed Power Generation Photovoltaics • Wind-Electric Conversion • Hydro • Geothermal • Tidal Energy • Fuel Cells • Solar-Thermal-Electric Conversion • Biomass Energy • Thermoelectrics • Thermionics • Integrated System Concepts • System Impacts 60.2 Photovoltaic Solar Cells Solar Cell Operation and Characteristics • Solar Cell Types and Their Optimization • Crystalline Silicon • III-V Semiconductors • Thin-Film Solar Cells • Dye-Sensitized Cells • Module Technologies • Photovoltaic Power Systems 60.3 Thermophotovoltaics Background • Design Considerations of a TPV System • Optical Control of Sub-bandgap Energies • Development of PV Cells • Status of System Development • Systems and Applications 60.1 Distributed Power Generation Distributed generation (DG) refers to small (a few watts up to 1 MW) power plants at or near the loads, operating in a stand-alone mode or connected to a grid at the distribution or subtransmission level, and geographically scattered throughout the service area. Typically they harness unconventional energy resources such as insolation, wind, biomass, tides and waves, and geothermal. Small plants powered by site-specific conventional energy resources such as low-head and small hydro and natural gas are also included in this general group. Interest in DG has been growing steadily since the dramatic oil embargo of 1973. In addition to the obvious advantages realized by the development of renewable energy sources, DG is ideally suited to power small remote loads, located far from the grid. An entire family of small power sources has been developed and employed for space, underwater, and biomedical applications. Another niche for these systems is in energizing remote rural areas of developing countries. It is estimated that there are more than one million remote villages in the world with no grid connection and minimally sustained by locally available energy sources. Integrated renewable energy systems (IRES), a special subset of DG, are ideally suited for these situations. General Features DG will have one or more of the following features: • Small size • Intermittent input resource • Stand-alone or interface at the distribution or subtransmission level • Extremely site-specific inputs • Located near the loads R. Ramakumar Oklahoma State University Allen M. Barnett AstroPower, Inc. Lawrence L. Kazmerski National Renewable Energy Laboratory John P. Benner National Renewable Energy Laboratory Timothy J. Coutts National Renewable Energy Laboratory
Availability of energy storage and reconversion for later use Potential and Future Globally, the potential for DG is vast. Even extremely site-specific resources such as tides, geothermal, and small hydro are available in significant quantities. Assessments of the future for various DG technologies vary, depending on the enthusiasm of the estimator. However, in almost all cases, the limitations are economic rather than technical Concerns over the unrestricted use of depletable energy resources and the ensuing environmental problems such as the greenhouse effect and global warming are providing the impetus necessary for the continued development of technologies for DG. Motivation Among the powerful motivations for the entry of dg are: Less capital investment and less capital at risk in the case of smaller installations Easier to site smaller plants under the ever-increasing restrictions Likely to result in improved reliability and availability Location near load centers decreases delivery costs and lowers transmission and distribution losses In terms of the cost of power delivered, DG is becoming competitive with large central-station plants, especially with the advent of open access and competition in the electric utility industry DG Technologies Many technologies have been proposed and employed for DG Power ratings of DG systems vary from milliwatts to megawatts, depending on the application. a listing of the technologies is given belot Wind-electric conversio Tidal and wave energy conversion Solar-thermal-electric conversion Biomass utilization Thermoelectrics Thermionics Small cogeneration plants powered by natural gas and supplying electrical and thermal energies The technology involved in the last item above is mature and very similar to that of conventional thermal power plants and therefore will not be considered in this section. PV refers to the direct conversion of insolation(incident solar radiation)to electricity. A PV cell (also known a solar cell) is simply a large-area semiconductor pn junction diode with the junction positioned very close to the top surface. Typically, a metallic grid structure on the top and a sheet structure in the bottom collect the minority carriers crossing the junction and serve as terminals. The minority carriers are generated by the incident photons with energies greater than or equal to the energy gap of the semiconductor material. Since the output of an individual cell is rather low(1 or 2 W at a fraction of a volt), several( 30 to 60)cells are combined to form a module. Typical module ratings range from 40 to 50 w at 15 to 17V PV modules are progressively put together to form panels, arrays(strings or trackers), groups, segments(subfields), and ulti mately a Pv plant consisting of several segments. Plants rated at several Mw have been built and operated fully e 2000 by CRC Press LLC
© 2000 by CRC Press LLC • Remoteness from conventional grid supply • Availability of energy storage and reconversion for later use Potential and Future Globally, the potential for DG is vast. Even extremely site-specific resources such as tides, geothermal, and small hydro are available in significant quantities. Assessments of the future for various DG technologies vary, depending on the enthusiasm of the estimator. However, in almost all cases, the limitations are economic rather than technical. Concerns over the unrestricted use of depletable energy resources and the ensuing environmental problems such as the greenhouse effect and global warming are providing the impetus necessary for the continued development of technologies for DG. Motivation Among the powerful motivations for the entry of DG are: • Less capital investment and less capital at risk in the case of smaller installations • Easier to site smaller plants under the ever-increasing restrictions • Likely to result in improved reliability and availability • Location near load centers decreases delivery costs and lowers transmission and distribution losses • In terms of the cost of power delivered, DG is becoming competitive with large central-station plants, especially with the advent of open access and competition in the electric utility industry DG Technologies Many technologies have been proposed and employed for DG. Power ratings of DG systems vary from milliwatts to megawatts, depending on the application. A listing of the technologies is given below. • Photovoltaics (PV) • Wind-electric conversion systems • Mini and micro hydro • Geothermal plants • Tidal and wave energy conversion • Fuel cells • Solar-thermal-electric conversion • Biomass utilization • Thermoelectrics • Thermionics • Small cogeneration plants powered by natural gas and supplying electrical and thermal energies The technology involved in the last item above is mature and very similar to that of conventional thermal power plants and therefore will not be considered in this section. Photovoltaics PV refers to the direct conversion of insolation (incident solar radiation) to electricity. A PV cell (also known as a solar cell) is simply a large-area semiconductor pn junction diode with the junction positioned very close to the top surface. Typically, a metallic grid structure on the top and a sheet structure in the bottom collect the minority carriers crossing the junction and serve as terminals. The minority carriers are generated by the incident photons with energies greater than or equal to the energy gap of the semiconductor material. Since the output of an individual cell is rather low (1 or 2 W at a fraction of a volt), several (30 to 60) cells are combined to form a module. Typical module ratings range from 40 to 50 W at 15 to 17 V. PV modules are progressively put together to form panels, arrays (strings or trackers), groups, segments (subfields), and ultimately a PV plant consisting of several segments. Plants rated at several MW have been built and operated successfully
POINT (SOURC RECTANGLE FILL-FACTOR E VOLTAGE, V FIGURE 60.1 Typical current-voltage characteristic of an illuminated solar cell. ntages of PV include demonstrated low operation and maintenance costs, no moving parts, silent and simple operation, almost unlimited lifetime if properly cared for, no recurring fuel costs, modularity, and minimal environmental effects. The disadvantages are its cost, need for large collector areas due to the diluteness of insolation, and the diurnal and seasonal variability of the output. PV systems can be flat-plate or concentrating type. While flat-plate systems utilize the global(direct and diffuse)radiation, concentrator systems harness only the direct or beam radiation. As such, concentrating systems must track(one axis or two axis)the sun. Flat-plate systems may or may not be mounted on trackers. 1990, efficiencies of flat-plate crystalline and thin-film cells had reached 23 and 15%, respectively Efficiencies as high as 34% were recorded for concentrator cells. Single-crystal and amorphous PV module efficiencies of 12 and 5% were achieved by the early 90s. For an average module efficiency of 10% and an insolation of 1 kW/m- on a clear afternoon, 10 m of collector area is required for each kw of output. o The output of a PV system is dc and inversion is required for supplying ac loads or for utility-interactive PV system is determined by external factors such as cloud cover, time of day, season of the t, the input to a ation. While the required fuel input to a conventional power plant depends on its outp or near their maximum ouput etry of the collector. Therefore, PV systems are operated, as li sar,geographic location orientation, an in their outputs due to moving cloulds pv plants have inertialess generation and are subject to rapid changes The current-voltage(Iv) characteristic of an illuminated solar cell is shown in Figure 60. 1. It is given as I=I where I, and I, are the dark and source currents, respectively, k is the Boltzmann constant(1.38 X 10-2 J/K), T is the temperature in K, and e is the electronic charge. Under ideal conditions(identical cells), for a PV module with a series-parallel arrangement of cells, the IV characteristic will be similar, except that the current ale should be multiplied by the number of parallel branches and the voltage scale by the number of cells in series in the module. The source current varies linearly with insolation. The dark current increases as the cell e 2000 by CRC Press LLC
© 2000 by CRC Press LLC Advantages of PV include demonstrated low operation and maintenance costs, no moving parts, silent and simple operation, almost unlimited lifetime if properly cared for, no recurring fuel costs, modularity, and minimal environmental effects. The disadvantages are its cost, need for large collector areas due to the diluteness of insolation, and the diurnal and seasonal variability of the output. PV systems can be flat-plate or concentrating type. While flat-plate systems utilize the global (direct and diffuse) radiation, concentrator systems harness only the direct or beam radiation. As such, concentrating systems must track (one axis or two axis) the sun. Flat-plate systems may or may not be mounted on trackers. By 1990, efficiencies of flat-plate crystalline and thin-film cells had reached 23 and 15%, respectively. Efficiencies as high as 34% were recorded for concentrator cells. Single-crystal and amorphous PV module efficiencies of 12 and 5% were achieved by the early ’90s. For an average module efficiency of 10% and an insolation of 1 kW/m2 on a clear afternoon, 10 m2 of collector area is required for each kW of output. The output of a PV system is dc and inversion is required for supplying ac loads or for utility-interactive operation. While the required fuel input to a conventional power plant depends on its output, the input to a PV system is determined by external factors such as cloud cover, time of day, season of the year, geographic location, orientation, and geometry of the collector. Therefore, PV systems are operated, as far as possible, at or near their maximum outputs. Also, PV plants have inertialess generation and are subject to rapid changes in their outputs due to moving clouds. The current-voltage (IV) characteristic of an illuminated solar cell is shown in Figure 60.1. It is given as where Io and Is are the dark and source currents, respectively, k is the Boltzmann constant (1.38 × 10–23 J/K), T is the temperature in K, and e is the electronic charge. Under ideal conditions (identical cells), for a PV module with a series-parallel arrangement of cells, the IV characteristic will be similar, except that the current scale should be multiplied by the number of parallel branches and the voltage scale by the number of cells in series in the module. The source current varies linearly with insolation. The dark current increases as the cell FIGURE 60.1 Typical current-voltage characteristic of an illuminated solar cell. III eV kT s o = − − exp 1
operating temperature increases. Also, the larger the energy gap of the material, the smaller the dark current The ratio of source current to dark current should be made as large as possible for improved operation Single-crystal silicon is still the dominant technology for fabricating PV devices. Polycrystalline, semicrys- talline, and amorphous silicon technologies are developing rapidly to challenge this Highly innovative tech- nologies such as spheral cells are being introduced to reduce costs. Concentrator systems typically employ ilium arsenide or multiple junction cells. Many other materials and thin-film technologies are under inves- tigation as potential candidates PV applications range from milliwatts(consumer electronics) to megawatts(central station plants). They are suitable for portable, remote, stand-alone, and utility-interactive applications. PV systems should be con- sidered as energy sources and their design should maximize the conversion of insolation into useable electrical form. Power requirements of practical loads are met using an energy storage and reconversion system or utility interconnection. Concentrating systems have been designed and operated to provide both electrical and low- grade thermal outputs with combined peak utilization efficiencies approaching 60% The vigorous growth of PV technology is manifested by a doubling of world PV module shipments in six years-from 42 MW in 1989 to 84 MW in 1995. Tens of thousands of small(<1 kW)systems are in operation around the world. Thousands of kilowatt-size systems(1 to 10s of kw) also have been installed and are in operation. Many intermediate-scale systems(10 to 100s of kw)and large-scale systems(1 MW or larger)are being installed by utility-and government-sponsored programs as proof-of-concept experiments and to glean valuable operational data. By 1988, nearly 11 Mw of Pv was interconnected to the utility system in the United States alone. Most were the 1-to 5-kW range. The two major exceptions are the 1-MW Hesperia-Lugo project installed in 1982 and the 6.5-MW Carrisa Plains project installed in 1984, both in California. In Germany, a 340-kW system began operation in 1988 as part of a large program. Switzerland had a plan to install 1 MW of PV in 333 roof-mounted units of 3 kW each. By 1990, the installed capacity of Pv in Italy exceeded 3 MW. Many nations have recognized the vast potential of Pv and have established their own PV programs within the past decade. A view of the 500 kW flat-plate grid-connected PV system installed and operated by the city of Austin electric utility depart ment in Austin, Texas is shown in Figure 60.2. From a capital cost of $7000/kW in 1988 with an associated levelized energy cost of 32</kWh, even with a business-as-usual scenario, a twofold reduction to $3500/kw by 2000 and an additional 3-to-1 reduction to $1175/kW by 2030 are being projected. The corresponding energy costs are 15 and 5</kWh, respectively. These FIGURE 60.2 A view of the city of Austin PV-300 flat-plate grid-connected photovoltaic system. Courtesy of the city of Austin electric utility department) e 2000 by CRC Press LLC
© 2000 by CRC Press LLC operating temperature increases. Also, the larger the energy gap of the material, the smaller the dark current. The ratio of source current to dark current should be made as large as possible for improved operation. Single-crystal silicon is still the dominant technology for fabricating PV devices. Polycrystalline, semicrystalline, and amorphous silicon technologies are developing rapidly to challenge this. Highly innovative technologies such as spheral cells are being introduced to reduce costs. Concentrator systems typically employ gallium arsenide or multiple junction cells. Many other materials and thin-film technologies are under investigation as potential candidates. PV applications range from milliwatts (consumer electronics) to megawatts (central station plants). They are suitable for portable, remote, stand-alone, and utility-interactive applications. PV systems should be considered as energy sources and their design should maximize the conversion of insolation into useable electrical form. Power requirements of practical loads are met using an energy storage and reconversion system or utility interconnection. Concentrating systems have been designed and operated to provide both electrical and lowgrade thermal outputs with combined peak utilization efficiencies approaching 60%. The vigorous growth of PV technology is manifested by a doubling of world PV module shipments in six years — from 42 MW in 1989 to 84 MW in 1995. Tens of thousands of small (<1 kW) systems are in operation around the world. Thousands of kilowatt-size systems (1 to 10s of kW) also have been installed and are in operation. Many intermediate-scale systems (10 to 100s of kW) and large-scale systems (1 MW or larger) are being installed by utility- and government-sponsored programs as proof-of-concept experiments and to glean valuable operational data. By 1988, nearly 11 MW of PV was interconnected to the utility system in the United States alone. Most were in the 1- to 5-kW range. The two major exceptions are the 1-MW Hesperia-Lugo project installed in 1982 and the 6.5-MW Carrisa Plains project installed in 1984, both in California. In Germany, a 340-kW system began operation in 1988 as part of a large program. Switzerland had a plan to install 1 MW of PV in 333 roof-mounted units of 3 kW each. By 1990, the installed capacity of PV in Italy exceeded 3 MW. Many nations have recognized the vast potential of PV and have established their own PV programs within the past decade. A view of the 300 kW flat-plate grid-connected PV system installed and operated by the city of Austin electric utility department in Austin, Texas is shown in Figure 60.2. From a capital cost of $7000/kW in 1988 with an associated levelized energy cost of 32¢/kWh, even with a business-as-usual scenario, a twofold reduction to $3500/kW by 2000 and an additional 3-to-1 reduction to $1175/kW by 2030 are being projected. The corresponding energy costs are 15 and 5¢/kWh, respectively. These FIGURE 60.2 A view of the city of Austin PV-300 flat-plate grid-connected photovoltaic system. (Courtesy of the city of Austin electric utility department.)
WIND TURBINE pcoe∈EE"FNFR 05 HIGH SPEED TWO OR O I TIP SPEED RAT|0入 IGURE 60.3 Typical aeroturbine characteristics estimates put the cost of energy from PV in par with the cost of energy from conventional plants in the early part of the twenty-first century. Wind-Electric Conversion wind energy is intermittent, highly variable, and site-specific, exists in three dimensions, and is the least dependent upon latitude among all renewable resources. The power density (in W/unit area) in moving air rind)is a cubic function of wind speed and therefore even small increases in average wind speeds can lead to significant increases in the capturable energy. Wind sites are typically classified as good, excellent, or outstanding, with associated mean wind speeds of 13, 16, and 19 mph, respectively. Aeroturbine employ lift and/or drag forces to convert wind energy to rotary mechanical energy, which is then converted to electrical energy by coupling a suitable generator. The power coefficient C, of an aeroturbine is the fraction of the incident power converted to mechanical shaft power, and it is a function of the tip speed to-wind speed ratio n as shown in Figure 60. 3. For a given propeller configuration, at any given wind speed, there is an optimum tip speed that maximizes Cp. Several types of aeroturbine are available. They can have horizontal or vertical axes, number of blades ranging from one to several, mounted upwind or downwind, and fixed-or variable-pitch blades with full blade control or tip control. Vertical-axis(Darrieus)turbines are not self-starting and require a starting mechanism Today, horizontal-axis turbines with two or more blades are the most prevalent, and considerable work is underway to develop advanced ns of these The electrical output Pe of a wind-electric conversion system(WECS) is given as Pe=n n where n and nm are the efficiencies of the electrical generator and mechanical interface, respectively, A is the swept area, K is a constant, and v is the wind speed incident on the aeroturbine. There are two basic options for wind-electric conversion With varying wind speeds, the aeroturbine can be operated at a constant speed by blade-pitch control, and a conventional synchronous machine is then employed to generate constant-frequency ac. More commonly, an induction generator is used with or without an adjust ble var supply. In this case, the aeroturbine will operate at a nearly constant speed. Alternatively, the aeroturbine rotational speed can be allowed to vary with wind to maintain a constant and optimum tip speed ratio, and then a combination of special energy converters and power electronics is employed to obtain utility-grade ac e 2000 by CRC Press LLC
© 2000 by CRC Press LLC estimates put the cost of energy from PV in par with the cost of energy from conventional plants in the early part of the twenty-first century. Wind-Electric Conversion Wind energy is intermittent, highly variable, and site-specific, exists in three dimensions, and is the least dependent upon latitude among all renewable resources. The power density (in W/unit area) in moving air (wind) is a cubic function of wind speed and therefore even small increases in average wind speeds can lead to significant increases in the capturable energy. Wind sites are typically classified as good, excellent, or outstanding, with associated mean wind speeds of 13, 16, and 19 mph, respectively. Aeroturbines employ lift and/or drag forces to convert wind energy to rotary mechanical energy, which is then converted to electrical energy by coupling a suitable generator. The power coefficient Cp of an aeroturbine is the fraction of the incident power converted to mechanical shaft power, and it is a function of the tip speedto-wind speed ratio λ as shown in Figure 60.3. For a given propeller configuration, at any given wind speed, there is an optimum tip speed that maximizes Cp. Several types of aeroturbines are available. They can have horizontal or vertical axes, number of blades ranging from one to several, mounted upwind or downwind, and fixed- or variable-pitch blades with full blade control or tip control. Vertical-axis (Darrieus) turbines are not self-starting and require a starting mechanism. Today, horizontal-axis turbines with two or more blades are the most prevalent, and considerable work is underway to develop advanced versions of these. The electrical output Pe of a wind-electric conversion system (WECS) is given as Pe = ηgηm ACp Kv 3 where ηg and ηm are the efficiencies of the electrical generator and mechanical interface, respectively, A is the swept area, K is a constant, and v is the wind speed incident on the aeroturbine. There are two basic options for wind-electric conversion. With varying wind speeds, the aeroturbine can be operated at a constant speed by blade-pitch control, and a conventional synchronous machine is then employed to generate constant-frequency ac. More commonly, an induction generator is used with or without an adjustable var supply. In this case, the aeroturbine will operate at a nearly constant speed. Alternatively, the aeroturbine rotational speed can be allowed to vary with wind to maintain a constant and optimum tip speed ratio, and then a combination of special energy converters and power electronics is employed to obtain utility-grade ac. FIGURE 60.3 Typical aeroturbine characteristics
The variable-speed option allows optimum efficiency operation of the turbine over a wide range of wind speeds resulting in increased outputs with lower structural loads and stresses. All future utility-grade advanced turbines are expected to operate in the variable-speed mode and use power electronics to convert the variable-frequency output to constant frequency with minimal harmonic distortion. s. Large-scale harnessing of wind energy will require hundreds or even thousands of WECS arranged in a wind m with spacings of about 2 to 3 diameters crosswind and about 10 diameters apart downwind. The power output of an individual WECS will fluctuate over a wide range, and its statistics strongly depend on the wind atistics. When many WECS are used in a wind farm, some smoothing of the total power output will result, depending on the statistical independence of the outputs of individual wECS. This is desirable, especially with high(>20%) penetration of WECS in the generation mix. while the output of wECS is not dispatchable, with large wind farms the possibility of assigning some capacity credit to the overall output significantly improves. Although wind-electric conversion has overall minimum environmental impacts, the large rotating structures involved do generate some noise and introduce visual aesthetics problems. By locating wind energy systems sufficiently far from centers of population, these effects can be minimized. The envisaged potential for bird kills turned out to be not a serious problem. wind energy systems occupy only a very small fraction of the land. However, the area surrounding them can be used only for activities such as farming and livestock grazing Thus, there is some negative impact on land use Today, the cost of energy delivered by wind plants rivals those obtained from some nonrenewable sources. By 1990, wind became the most utilized and competitive option among all the solar energy technologies for the bulk power market at a cost of generation of about 8c/kWh(or roughly 7+10%). Permanent magnet generators provide another alternative, especially if the output is to be rectified and stored for later use in the case of very small units. Geothermal Geothermal plants exploit the heat stored in the form of hot water and steam in the earths crust at depths of 2000 to 8000 ft. By nature, these resources are extremely site-specific and slowly run down(depletable)over a period of years. For electric power generation, the resource should be at least around 250C. Depending on the temperature and makeup, dry steam, flash steam, or binary technology can be employed. Of these, dry natural steam is the best since it eliminates the need for a boiler e 2000 by CRC Press LLC
© 2000 by CRC Press LLC The variable-speed option allows optimum efficiency operation of the turbine over a wide range of wind speeds, resulting in increased outputs with lower structural loads and stresses. All future utility-grade advanced turbines are expected to operate in the variable-speed mode and use power electronics to convert the variable-frequency output to constant frequency with minimal harmonic distortion. Large-scale harnessing of wind energy will require hundreds or even thousands of WECS arranged in a wind farm with spacings of about 2 to 3 diameters crosswind and about 10 diameters apart downwind. The power output of an individual WECS will fluctuate over a wide range, and its statistics strongly depend on the wind statistics. When many WECS are used in a wind farm, some smoothing of the total power output will result, depending on the statistical independence of the outputs of individual WECS. This is desirable, especially with high (>20%) penetration of WECS in the generation mix. While the output of WECS is not dispatchable, with large wind farms the possibility of assigning some capacity credit to the overall output significantly improves. Although wind-electric conversion has overall minimum environmental impacts, the large rotating structures involved do generate some noise and introduce visual aesthetics problems. By locating wind energy systems sufficiently far from centers of population, these effects can be minimized. The envisaged potential for bird kills turned out to be not a serious problem. Wind energy systems occupy only a very small fraction of the land. However, the area surrounding them can be used only for activities such as farming and livestock grazing. Thus, there is some negative impact on land use. Today, the cost of energy delivered by wind plants rivals those obtained from some nonrenewable sources. By 1990, wind became the most utilized and competitive option among all the solar energy technologies for the bulk power market at a cost of generation of about 8¢/kWh (or roughly 7¢/kWh in 1987 dollars). Ongoing research and development work in new design tools, advanced airfoils, site tailoring, operating strategies, array spacing, and improved reliability and manufacturability is expected to bring the cost of energy further down by a factor of 2 to 3. At around 1600 MW, nearly 90% of all the WECS installed in the world are in California. They are expected to generate nearly 3 billion kWh of electricity per year to the state’s utilities to which they are interconnected. Although their lack of control and the intermittent nature of wind-derived energy are not embraced enthusiastically by electric utilities, this gap is expected to be bridged very soon with appropriate computer controls and operating strategies. Wind energy is already an economical option for remote areas endowed with good wind regimes. The modularity of WECS, coupled with the associated environmental benefits, potential for providing jobs, and economic viability point to a major role for wind energy in the generation mix of the world in the decades to come. Hydro Hydropower is a mature but neglected and one of the most promising renewable energy technologies. In the context of DG, small (less than 15 MW), mini (less than 1 MW), and micro (less than 100 kW) hydroelectric plants are of interest. The source of hydropower is the hydrologic cycle driven by the energy from the sun. Most of the sites for DG hydro are either low-head (2 to 20 m) or medium-head (20 to 150 m). The global hydroelectric potential is vast. One estimate puts it at 31 GW for Indonesia alone! The installed capacity of small hydro in the People’s Republic of China was exceeding 7 GW by 1980. Both impulse and reaction turbines have been employed for small-scale hydro for DG. Several standardized units are available in the market. Most of the units are operated at constant speed with governor control and are coupled to synchronous machines to generate utility-grade ac. If the water source is highly variable, it may be necessary to employ variable-speed operation. If the speed variations are not large, induction generators can be used. Special variable-speed constant-frequency (VSCF) generation schemes may be needed if the range of speed variations is large (> ±10%). Permanent magnet generators provide another alternative, especially if the output is to be rectified and stored for later use in the case of very small units. Geothermal Geothermal plants exploit the heat stored in the form of hot water and steam in the earth’s crust at depths of 2000 to 8000 ft. By nature, these resources are extremely site-specific and slowly run down (depletable) over a period of years. For electric power generation, the resource should be at least around 250°C. Depending on the temperature and makeup, dry steam, flash steam, or binary technology can be employed. Of these, dry natural steam is the best since it eliminates the need for a boiler
The three basic components of a geothermal plant are (1)a production well to bring the resource to the surface, (2)a turbine generator system for energy conversion, and(3)an injection well to recycle the spent geothermal fluids back into the reservoir Worldwide deployment of geothermal plants reached 5000 Mw by 1987 in 17 countries. Nearly one-half of this was in the United States. The Geysers plant north of San Francisco is the largest in the world with an installed capacity of 516 MW. In some developing countries, the Philippines for example, geothermal plants supply nearly 20% of their electrical needs The origin of tidal energy is the upward-acting gravitational force of the moon, which results in a cyclic variation in the potential energy of water at a point on the earths surface. These variations are amplified by topographical features such as the shape and size of estuaries. The ratio between maximum spring tide and minimum at neap can be as much as 3 to 1. In estuaries, the tidal range can be as large as 10 to 15 m. Power can be generated from a tidal estuary in two basic ways. a single basin can be used with a barrage at a strategic point along the estuary. By installing turbines at this point, electricity can be generated both when the tide is ebbing or flooding. In the two-basin scheme, generation can be time-shifted to coincide with hours of peak demand by using the basins alternately As can be expected, tidal energy conversion is very site-specific. The largest tidal power plant is the single basin scheme at La Rance in Brittany, France. It is rated at 240 MW and employs 24 vane-type horizontal turbines and alternator motors, each rated at 10 MVA. The plant has been in operation since 1966 with good technical and economic results. It has generated, on the average, around 500 GWh of net energy per year. The Severn estuary in the southwest of England and the Bay of Fundy in the border between the United States and Canada with the highest known tidal range of 17 m have been extensively studied for tidal power generation. There are several other possible sites around the world, but the massive capital costs required have delayed their exploitation Fuel cells A fuel cell is a simple static device that converts the chemical energy in a fuel directly, isothermally, and continuously into electrical energy Fuel and oxidant(typically oxygen in air)are fed to the device in which an electrochemical reaction takes place that oxidizes the fuel, reduces the oxidant, and releases energy. The ener released is in both electrical and thermal forms. The electrical part provides the required output. Since a fuel cell completely bypasses the thermal-to-mechanical conversion involved in a conventional power plant and ince its operation is isothermal, fuel cells are not Carnot-limited. Efficiencies in the range of 43 to 55% are forecasted for modular dispersed generators featuring fuel cells The low(500 C, respectively. Technical feasibility of the central receiver system was demonstrated in the early 80s by the 10-MWe Solar One system in Barstow, California. Over a six-year period, this system delivered 37 GWh of net energy to the e 2000 by CRC Press LLC
© 2000 by CRC Press LLC The three basic components of a geothermal plant are (1) a production well to bring the resource to the surface, (2) a turbine generator system for energy conversion, and (3) an injection well to recycle the spent geothermal fluids back into the reservoir. Worldwide deployment of geothermal plants reached 5000 MW by 1987 in 17 countries. Nearly one-half of this was in the United States. The Geysers plant north of San Francisco is the largest in the world with an installed capacity of 516 MW. In some developing countries, the Philippines for example, geothermal plants supply nearly 20% of their electrical needs. Tidal Energy The origin of tidal energy is the upward-acting gravitational force of the moon, which results in a cyclic variation in the potential energy of water at a point on the earth’s surface. These variations are amplified by topographical features such as the shape and size of estuaries. The ratio between maximum spring tide and minimum at neap can be as much as 3 to 1. In estuaries, the tidal range can be as large as 10 to 15 m. Power can be generated from a tidal estuary in two basic ways. A single basin can be used with a barrage at a strategic point along the estuary. By installing turbines at this point, electricity can be generated both when the tide is ebbing or flooding. In the two-basin scheme, generation can be time-shifted to coincide with hours of peak demand by using the basins alternately. As can be expected, tidal energy conversion is very site-specific. The largest tidal power plant is the singlebasin scheme at La Rance in Brittany, France. It is rated at 240 MW and employs 24 vane-type horizontal turbines and alternator motors, each rated at 10 MVA. The plant has been in operation since 1966 with good technical and economic results. It has generated, on the average, around 500 GWh of net energy per year. The Severn estuary in the southwest of England and the Bay of Fundy in the border between the United States and Canada with the highest known tidal range of 17 m have been extensively studied for tidal power generation. There are several other possible sites around the world, but the massive capital costs required have delayed their exploitation. Fuel Cells A fuel cell is a simple static device that converts the chemical energy in a fuel directly, isothermally, and continuously into electrical energy. Fuel and oxidant (typically oxygen in air) are fed to the device in which an electrochemical reaction takes place that oxidizes the fuel, reduces the oxidant, and releases energy. The energy released is in both electrical and thermal forms. The electrical part provides the required output. Since a fuel cell completely bypasses the thermal-to-mechanical conversion involved in a conventional power plant and since its operation is isothermal, fuel cells are not Carnot-limited. Efficiencies in the range of 43 to 55% are forecasted for modular dispersed generators featuring fuel cells. The low (500°C, respectively. Technical feasibility of the central receiver system was demonstrated in the early ’80s by the 10-MWe Solar One system in Barstow, California. Over a six-year period, this system delivered 37 GWh of net energy to the
Southern California Edison's grid with an overall system efficiency in the range of 7 to 8%. With improvements in heliostat and receiver technologies, annual system efficiencies of 14 to 15% and generation cost of 8 to 12</kWh have been projected. Parabolic-dish electric-transport technology for DG was under active development at the Jet Propulsion Laboratory (PL)in Pasadena, California, in the late 70s and early80s Prototype modules with Stirling engi reached a record 29% overall efficiency of conversion from insolation to electrical output. Earlier parabolic dish designs collected and transported thermal energy to a central location for conversion to electricity. Advanced designs such as the one developed at JPL employed engine driven generators at the focal points of the dishes, and energy was collected and transported in electrical form. By far the largest installed capacity (nearly 400 MW)of solar-thermal-electric DG employs parabolic-trough collectors and oil to transport the thermal energy to a central location for conversion to electricity via a steam Rankine cycle With the addition of a natural gas burner for hybrid operation, this technology, developed by LUZ under the code name SEGS(solar electric generating system), accounts for more than 90% of the world solar electric capacity, all located in Daggett, Kramer Junction, and Harper Lake in California. Generation costs of around 8 to 9/kWh have been realized with SEGS. This technology uses natural gas to compensate for the mporal variations of insolation and firms up the power delivered by the system. This compensation may me during 7 to 11 P.M. in summer and during 8 A.M. to 5 P. M. in winter. SEGS will require about 5 acres/Mw or can deliver 130 MW/mi2 of land area Biomass energy Biological sources provide a wide array of materials that have been and continue to be used as energy sources her d, wood wastes, and residue from wood processing industries, sewage or municipal solid waste, cultivate herbaceous and other energy crops, waste from food processing industries, and animal wastes are lumped together by the term biomass. The most compelling argument for the use of biomass technologies is the inherent recycling of the carbon by photosynthesis. In addition to the obvious method of burning biomass, conversion to liquid and gaseous fuels is possible, thus expanding the application possibi In the context of electric power generation, the role of biomass is expected to be for repowering old units and for use in small (20 to 50 MW)new plants. Several new high-efficiency conversion technologies are either Iready available or under development for the utilization of biomass. The technologies and their overall conversion efficiencies are listed below FBC (fluidized-bed combustor), 36-38% EPS(energy performance system) combustor, 34-36% BIG/STIG(biomass-integrated gasifier/steam-injected gas turbine),38-47% Acid or enzymatic hydrolysis, gasification, and aqueous pyrolysis are some of the other technology options available for biomass utilization Anaerobic digestion of animal wastes is being used extensively in developing countries to produce biogas, hich is utilized directly as a fuel in burners and for lighting. An 80-20 mixture of biogas and diesel has been used effectively in biogas engines to generate electricity in small quantities Biomass-fueled power plants are best suited in small(<100 MW)sizes for DG to serve base load and intermediate loads in the eastern United States and in many other parts of the world. This contribution is clean renewable, and reduces CO, emissions. Since biomass fuels are sulphur-free, these plants can be used to offset CO2 and SO2 emissions from new fossil power plants. Ash from biomass plants can be recycled and used as rtilizer. a carefully planned and well-managed SRwC (short-rotation woody crop) plantation program with yields in the range of 6 to 12 dry tons/acre/year can be effectively used to mitigate greenhouse gases and ntribute thousands of Mw of DG to the U.S. grid by the turn of the century Thermoelectri Thermal energy can be directly converted to electrical energy by using the thermoelectric effects in materials. Semiconductors offer the best option as thermocouples since thermojunctions can be constructed using a p-type e 2000 by CRC Press LLC
© 2000 by CRC Press LLC Southern California Edison’s grid with an overall system efficiency in the range of 7 to 8%. With improvements in heliostat and receiver technologies, annual system efficiencies of 14 to 15% and generation cost of 8 to 12¢/kWh have been projected. Parabolic-dish electric-transport technology for DG was under active development at the Jet Propulsion Laboratory (JPL) in Pasadena, California, in the late ’70s and early ’80s. Prototype modules with Stirling engines reached a record 29% overall efficiency of conversion from insolation to electrical output. Earlier parabolicdish designs collected and transported thermal energy to a central location for conversion to electricity. Advanced designs such as the one developed at JPL employed engine driven generators at the focal points of the dishes, and energy was collected and transported in electrical form. By far the largest installed capacity (nearly 400 MW) of solar-thermal-electric DG employs parabolic-trough collectors and oil to transport the thermal energy to a central location for conversion to electricity via a steamRankine cycle. With the addition of a natural gas burner for hybrid operation, this technology, developed by LUZ under the code name SEGS (solar electric generating system), accounts for more than 90% of the world’s solar electric capacity, all located in Daggett, Kramer Junction, and Harper Lake in California. Generation costs of around 8 to 9¢/kWh have been realized with SEGS. This technology uses natural gas to compensate for the temporal variations of insolation and firms up the power delivered by the system. This compensation may come during 7 to 11 P.M. in summer and during 8 A.M. to 5 P.M. in winter. SEGS will require about 5 acres/MW or can deliver 130 MW/mi2 of land area. Biomass Energy Biological sources provide a wide array of materials that have been and continue to be used as energy sources. Wood, wood wastes, and residue from wood processing industries, sewage or municipal solid waste, cultivated herbaceous and other energy crops, waste from food processing industries, and animal wastes are lumped together by the term biomass. The most compelling argument for the use of biomass technologies is the inherent recycling of the carbon by photosynthesis. In addition to the obvious method of burning biomass, conversion to liquid and gaseous fuels is possible, thus expanding the application possibilities. In the context of electric power generation, the role of biomass is expected to be for repowering old units and for use in small (20 to 50 MW) new plants. Several new high-efficiency conversion technologies are either already available or under development for the utilization of biomass. The technologies and their overall conversion efficiencies are listed below. • FBC (fluidized-bed combustor), 36–38% • EPS (energy performance system) combustor, 34–36% • BIG/STIG (biomass-integrated gasifier/steam-injected gas turbine), 38–47% Acid or enzymatic hydrolysis, gasification, and aqueous pyrolysis are some of the other technology options available for biomass utilization. Anaerobic digestion of animal wastes is being used extensively in developing countries to produce biogas, which is utilized directly as a fuel in burners and for lighting. An 80–20 mixture of biogas and diesel has been used effectively in biogas engines to generate electricity in small quantities. Biomass-fueled power plants are best suited in small (<100 MW) sizes for DG to serve base load and intermediate loads in the eastern United States and in many other parts of the world. This contribution is clean, renewable, and reduces CO2 emissions. Since biomass fuels are sulphur-free, these plants can be used to offset CO2 and SO2 emissions from new fossil power plants. Ash from biomass plants can be recycled and used as fertilizer. A carefully planned and well-managed SRWC (short-rotation woody crop) plantation program with yields in the range of 6 to 12 dry tons/acre/year can be effectively used to mitigate greenhouse gases and contribute thousands of MW of DG to the U.S. grid by the turn of the century. Thermoelectrics Thermal energy can be directly converted to electrical energy by using the thermoelectric effects in materials. Semiconductors offer the best option as thermocouples since thermojunctions can be constructed using a p-type
and an n-type material to cumulate the effects around a thermoelectric circuit. Moreover, by using solid solutions of tellurides and selenides doped to result in a low density of charge carriers, relatively moderate thermal conductivities and reasonably good electrical conductivities can be achieved. In a thermoelectric generator, the Seebeck voltage generated under a temperature difference drives a dc current through the load circuit. Even though there is no mechanical conversion, the process is still Carnot mited since it operates over a temperature difference. In practice, several couples are assembled in a series- parallel configuration to provide dc output power at the required voltage Typical thermoelectric generators employ radioisotope or nuclear reactor or hydrocarbon burner as the heat source. They are custom-made for space missions as exemplified by the SNAP (systems for nuclear auxiliary power)series and the rtG (radioisotope thermoelectric generator)used by the apollo astronauts. Maximum formance over a large temperature range is achieved by cascading stages. Each stage consists of thermocouples electrically in series and thermally in parallel. The stages themselves are thermally in series and electrically in parallel. Tellurides and selenides are used for power generation up to 600C Silicon germanium alloys turn out better performance above this up to 1000.C. with the materials available at present, conversion efficiencies in the 5 to 10% range can be expected. Whenever small amounts of silent reliable power is needed for long periods of time, thermoelectrics offer a viable option Space, underwater, biomedical, and remote terrestrial power such as cathodic protection of pipelines fall into this category. Thermionics Direct conversion of thermal energy into electrical energy can be achieved by employing the Edison effect-the release of electrons from a hot body, also known as thermionic emission. The thermal input imparts sufficient energy(2 work function) to a few electrons in the emitter(cathode), which helps them escape. If these electrons are collected using a collector(anode) and a closed path through a load is established for them to complete the circuit back to the cathode, then electrical output is obtained. Thermionic converters are heat engines with electrons as the working fluid and, as such, are subject to Carnot limitations Converters filled with ionizable gases such as cesium vapor in the interelectrode space yield higher power densities due to space charge neutralization. Barrier index is a parameter that signifies the closeness to ideal performance with no space charge effects. As this index is reduced, more applications become feasible A typical example of developments in thermionics is the TFe (thermionic fuel element)that integrates the converter and nuclear fuel for space nuclear power in the kw to Mw level for very long(7 to 10 years)duration missions. Another niche is the thermionic cogeneration burner module, a high-temperature burner equipped with thermionic converters. Electrical outputs of 50 kW/MW of thermal output have been achieved. High(600 to 650C) heat rejection temperatures of thermionic converters are ideally suited for producing flue gas in the 500 to 550C range for industrial processes. A long-range goal is to use thermionic converters as toppers for conventional power plants. Such concepts are not economical at present. Integrated System Concepts DG technologies offer many possibilities for integrated operation Integrated systems may be stand-alor energy storage and reconversion or include grid connection. Also, both renewable and conventional an be integrated to achieve the required operational characteristics. Integrated renewable energy systems that harness several manifestations of solar energy to supply a variety of energy and other needs have many advantages and applications worldwide. The complementary nature of some of the resources(insolation and wind, for example)over the annual cycle can be exploited by iRES to decrease the amount of energy storage necessary and lower the overall cost of energy. System Impacts sponse of distribution systems to high penetrations of dg is not yet fully understood. Also, the nature of the response will depend on the Dg technology involved. However, there are some general areas of potential impacts common to most of the technologies: (i) voltage flicker, imbalance, regulation, etc. (ii)power quality; e 2000 by CRC Press LLC
© 2000 by CRC Press LLC and an n-type material to cumulate the effects around a thermoelectric circuit. Moreover, by using solid solutions of tellurides and selenides doped to result in a low density of charge carriers, relatively moderate thermal conductivities and reasonably good electrical conductivities can be achieved. In a thermoelectric generator, the Seebeck voltage generated under a temperature difference drives a dc current through the load circuit. Even though there is no mechanical conversion, the process is still Carnotlimited since it operates over a temperature difference. In practice, several couples are assembled in a seriesparallel configuration to provide dc output power at the required voltage. Typical thermoelectric generators employ radioisotope or nuclear reactor or hydrocarbon burner as the heat source. They are custom-made for space missions as exemplified by the SNAP (systems for nuclear auxiliary power) series and the RTG (radioisotope thermoelectric generator) used by the Apollo astronauts. Maximum performance over a large temperature range is achieved by cascading stages. Each stage consists of thermocouples electrically in series and thermally in parallel. The stages themselves are thermally in series and electrically in parallel. Tellurides and selenides are used for power generation up to 600°C. Silicon germanium alloys turn out better performance above this up to 1000°C. With the materials available at present, conversion efficiencies in the 5 to 10% range can be expected. Whenever small amounts of silent reliable power is needed for long periods of time, thermoelectrics offer a viable option. Space, underwater, biomedical, and remote terrestrial power such as cathodic protection of pipelines fall into this category. Thermionics Direct conversion of thermal energy into electrical energy can be achieved by employing the Edison effect—the release of electrons from a hot body, also known as thermionic emission. The thermal input imparts sufficient energy (≥ work function) to a few electrons in the emitter (cathode), which helps them escape. If these electrons are collected using a collector (anode) and a closed path through a load is established for them to complete the circuit back to the cathode, then electrical output is obtained. Thermionic converters are heat engines with electrons as the working fluid and, as such, are subject to Carnot limitations. Converters filled with ionizable gases such as cesium vapor in the interelectrode space yield higher power densities due to space charge neutralization. Barrier index is a parameter that signifies the closeness to ideal performance with no space charge effects. As this index is reduced, more applications become feasible. A typical example of developments in thermionics is the TFE (thermionic fuel element) that integrates the converter and nuclear fuel for space nuclear power in the kW to MW level for very long (7 to 10 years) duration missions. Another niche is the thermionic cogeneration burner module, a high-temperature burner equipped with thermionic converters. Electrical outputs of 50 kW/MW of thermal output have been achieved. High (600 to 650°C) heat rejection temperatures of thermionic converters are ideally suited for producing flue gas in the 500 to 550°C range for industrial processes. A long-range goal is to use thermionic converters as toppers for conventional power plants. Such concepts are not economical at present. Integrated System Concepts DG technologies offer many possibilities for integrated operation. Integrated systems may be stand-alone with energy storage and reconversion or include grid connection. Also, both renewable and conventional systems can be integrated to achieve the required operational characteristics. Integrated renewable energy systems (IRES) that harness several manifestations of solar energy to supply a variety of energy and other needs have many advantages and applications worldwide. The complementary nature of some of the resources (insolation and wind, for example) over the annual cycle can be exploited by IRES to decrease the amount of energy storage necessary and lower the overall cost of energy. System Impacts Response of distribution systems to high penetrations of DG is not yet fully understood. Also, the nature of the response will depend on the DG technology involved. However, there are some general areas of potential impacts common to most of the technologies: (i) voltage flicker, imbalance, regulation, etc.; (ii) power quality;
