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.)