Solar Energy Introduction Solar energy refers to energy derived from the sunlight that reaches the earth. Solar energy can be converted directly or indirectly into other forms of energy, such as heat and electricity. The major drawbacks to the extensive application of solar energy are (1) the intermittent and variable manner in which it arrives at the earth's surface and (2) the large area required to collect energy at a useful rate. Solar energy is used for a number of applications including heating water for domestic use, space heating of buildings, drying agricultural products, and generating electrical energy. Electricity can be produced directly from solar energy using photovoltaic devices or indirectly from steam generators using solar thermal collectors to heat a working fluid. Photovoltaic Energy Photovoltaic energy is the conversion of sunlight into electricity through a photovoltaic (pv) cell, commonly called a solar cell. A pv cell is a solid state device (nonmechanical), usually made from silicon alloys. Sunlight is composed of photons, or particles of solar energy. These photons contain various amounts of energy corresponding to the different wavelengths of the solar spectrum of which only a small range of wavelengths is in the "color" spectrum. When photons strike a pv cell, they may be reflected, pass right through, or be absorbed. Only the absorbed photons provide energy to generate electricity. When enough sunlight (energy) is absorbed by the material (a semiconductor) electrons are dislodged from the material's atoms. Special treatment of the material surface during manufacture makes the front surface of the cell more receptive to free electrons, so the electrons naturally migrate to the surface. When the electron leaves its position it causes a hole to be formed. When many electrons, each carrying a negative charge, travel toward the front surface of the cell, the resulting imbalance of charge between the cell's front and back surfaces create a voltage potential like the negative and positive terminals of a battery. When the two surfaces are connected through an external load, electricity flows. The pv cell is the basic building block of a pv system. Individual cells can vary in size from about 1 cm (1\2 inch) to about 10 cm (4 inches) across. However, one cell only produces 1 or 2 watts, which isn't enough power for most applications. To increase power output cells are electrically connected into a packaged weather-tight module. Modules can be further connected to form an array. The term array refers to the entire generating plant, whether it is made up of one or several thousand modules. As many modules as needed can be connected to form the array size (power output) needed. The performance of a pv array is dependent upon sunlight. Climatic conditions (e.g., clouds, fog) have a significant effect on the amount of solar energy received by a pv array and, in turn its performance. Most "current technology" pv modules are about 10 percent efficient in converting sunlight to electricity with further research being conducted to raise this efficiency to 15 percent. The pv cell was discovered in 1954 by Bell Telephone researchers examining the sensitivity of a properly prepared silicon wafer to sunlight. Beginning in the late 1950s, pvs were used to power U.S. space satellites. The success of pvs in space generated commercial applications for pv technology. The simplest pv systems power many of the small calculators and wrist watches used everyday. More complicated systems provide electricity for pumping water, power communications equipment, and even providing electricity to our homes. Photovoltaic conversion is useful for several reasons. Conversion from sunlight to electricity is direct, so that bulky mechanical generator systems are unnecessary. The modular characteristic of photovoltaic energy allows arrays to be installed quickly and in any size required or allowed. Also, the environmental impact of a pv system is minimal, requiring no water for system cooling and having no generation by-products. Photovoltaic cells, like batteries, generate direct current (DC) that is generally used for small loads (electronic equipment). When DC from photovoltaic cells is used for commercial applications or sold to electric utilities using the electric grid, it must be converted to alternating current (AC) using inverters, solid state devices that convert DC power to AC. Historically, pvs have been used at remote sites to provide electricity. However, a market for distributed generation that could be provided from pvs may be developing with the unbundling of transmission and distribution costs due to electric industry deregulation. The siting of numerous small-scale generators in electric distribution feeders could improve the economics and reliability of the distribution system. Solar Thermal Heat The major applications of solar thermal energy at present are heating water for domestic use and space heating of buildings. For these purposes, the general practice is to use flat-plate solar-energy collectors with a fixed orientation (the angle formed between the earth's surface and the direction of the collector). Where space heating is the main consideration, the highest efficiency with a fixed flat-plate collector is obtained if it faces approximately south and slopes at an angle to the horizon equal to the latitude plus about 15 degrees. Solar collectors fall into two general categories: nonconcentrating and concentrating. In the nonconcentrating type, the collector area (i.e., the area that intercepts the solar radiation) is the same as the absorber area (i.e., the area absorbing the radiation). In concentrating collectors, the area intercepting the solar radiation is greater, sometimes hundreds of times greater, than the absorber area (analgous to the behavior of a magnifying glass directed at the sun). With concentrating collectors much higher temperatures can be obtained than with the nonconcentrating type. Where temperatures below about 200o F are sufficient, such as for space heating, flat-plate collectors of the nonconcentrating type are generally used. There are many flat-plate collector designs but generally all consist of (1) a flat-plate absorber, which intercepts and absorbs the solar energy, (2) a transparent cover(s) that allows solar energy to pass through but reduces heat loss from the absorber, (3) a heat-transport fluid (air or water) flowing through tubes to remove heat from the absorber, and (4) a heat insulating backing. Solar space heating systems can also be classified as passive or active. In passive heating systems the air is circulated past a solar heat surface(s) and through the building by convection (i.e., less dense warm air tends to rise while more dense cooler air moves downward) without the use of mechanical equipment. In active heating systems, fans and pumps are used to circulate the air or the heat absorbing fluid. Solar Thermal Power Plants Solar thermal power plants use the sun's rays to heat a fluid, from which heat transfer systems may be used to produce steam. The steam in turn is converted into mechanical energy in a turbine and into electricity from a conventional generator coupled to the turbine. Solar thermal power generation is essentially the same as conventional technologies except that in conventional technologies the energy source is from the stored energy in fossil fuels released by combustion. Solar Thermal technologies use concentrator systems due to the high temperatures needed for the working fluid. The three types of solar-thermal power systems in use or under development are: parabolic trough, solar dish, and solar power tower. Parabolic Trough The parabolic trough is the most advanced of the concentrator systems. This technology is used in the largest grid connected solar-thermal power plants in the world. The largest solar power complex in the U.S. is a parabolic trough, the Luz in California with a capacity of over 300 megawatts. A parabolic trough collector has a linear parabolic-shaped reflector that focuses the sun's radiation on a linear receiver located at the focus of the parabola. The collector tracks the sun along one axis from east to west during the day to ensure that the sun is continuously focused on the receiver. Because of their parabolic shape, troughs can focus the sun at 30 to 100 times its normal intensity (concentration ratio) on a receiver pipe located along the focal line of the trough achieving operating temperatures over 400o C. A collector field consists of a large field of single-axis (e.g., east to west) tracking parabolic trough collectors. The solar field is modular in nature and is composed of many parallel rows of solar collectors aligned on a north-south horizontal axis. A working (heat transfer) fluid is heated as it circulates through the receivers and returns to a series of heat exchangers at a central location where the fluid is used to generate high-pressure superheated steam. The steam is then fed to a conventional steam turbine/generator to produce electricity. After the working fluid passes through the heat exchangers, the cooled fluid is recirculated through the solar field. The plant is usually designed to operate at full rated power using solar energy alone given sufficient solar energy. However, all plants are hybrid solar/fossil plants, that have a fossil-fired capability that can be used to supplement the solar output during periods of low solar energy. The Luz plant is a natural gas hybrid. Solar Dish A solar dish/engine system utilizes concentrating solar collectors that track the sun in two axes, concentrating the energy at the focal point of the dish because it is always pointed at the sun. The solar dish's concentration ratio is much higher than the solar trough, typically over 2,000 and a working fluid temperature over 750o C. The power-generating equipment used with a solar dish can be mounted at the focal point of the dish making it well suited for remote operations, or as with the solar trough, the energy may be collected from a number of installations and converted to electricity at a central point. The engine in a solar dish/engine system converts heat to mechanical power by compressing the working fluid when it is cold, heating the compressed working fluid, and then expanding the fluid through a turbine or with a piston to produce work. The engine is coupled to an electric generator to convert the mechanical power to electric power. Solar Power Tower A solar power tower or central receiver generates electric power from sunlight by focusing concentrated solar energy on a tower-mounted heat exchanger (receiver). This system uses hundreds to thousands of flat sun-tracking mirrors called heliostats to reflect and concentrate the sun's energy onto a central receiver tower. Concentration ratios are as high as 1,500. Energy losses from thermal-energy transport are minimized as solar energy is being directly transferred by reflection from the heliostats to a single receiver rather than being moved through a transfer medium to one central location, as with parabolic troughs. Power towers must be large to be economical and is a promising technology for large-scale grid-connected power plants. Though power towers are in the early stages of development compared with parabolic trough technology a number of test facilities have been constructed around theworld. Solar One, near Barstow, California which operated from 1982 to 1988, at about 10 megawatts was the world's largest power tower plant. In Solar One, water was converted to steam in the receiver and used directly to power a steam turbine. The heliostat field consisted of approximately 1,800 heliostats. The storage system stored heat from solar-produced steam in a tank filled with rocks and sand using oil as the heat-transfer fluid. A consortium comprising the U.S. Department of Energy, and a number of electric utilities led by Southern California Edison redesigned Solar One to a more advanced molten-salt technology which started operation in 1996, Solar Two. The molten-salt storage system allows for several hours of base load power generation when the sun is not shining. The Solar One heliostat field, tower, and turbine/generator required only minimal modifications. New stretch-membrane heliostats were added to the existing heliostat field to increase the system's energy output. The plant consists of over 1,900 motorized heliostats focused on a 300-foot-high central receiver generating station rated at 10 megawatts. Additional Information The goal of the Office of Energy Efficiency and Renewable Energy (EE) of the Department of Energy is to develop cost-effective energy efficiency and renewable energy technologies that protect the environment and support the nation's economic competitiveness. EE achieves this goal through a strong and balanced program of research, development, and market deployment through private sector partnerships. For additional information see their Web Site at WWW.EREN.DOE.GOV/ SELECTED RENEWABLE ENERGY TABLES Table A. Number of Companies and Annual Shipments of Solar Thermal Collectors, 1974-1997 (Thousand Square Feet) Number of Solar Thermal Collectors Year Companies Total Shipments Import Shipments Export Shipments 1974 45 1,274 NA NA 1975 131 3,743 NA NA 1976 186 5,801 NA NA 1977 321 10,312 NA NA 1978 340 10,860 396 840 1979 349 14,251 290 855 1980(a) 233 19,398 235 1,115 1981(a) 203 20,133 196 771 1982(a) 265 18,621 418 455 1983(a) 203 16,828 511 159 1984 225 17,191 621 348 1985 NA NA NA NA 1986 98 9,360 473 224 1987 59 7,269 691 182 1988 51 8,174 814 158 1989 44 11,482 1,233 461 1990 51 11,409 1,562 245 1991 48 6,574 1,543 332 1992 45 7,086 1,650 316 1993 41 6,968 2,039 411 1994 41 7,627 1,815 405 1995 36 7,666 2,037 530 1996 28 7,616 1,930 454 1997 29 8,138 2,102 379 (a) = Includes imputation of shipment data to account for nonrespondent. NA = Not Available. The data reported for 1985 are incomplete. Note: Total shipments as reported by respondents include all domestic and export shipments and may include imported collectors that subsequently were shipped to domestic or foreign customers. Source: Energy Information Administration, Form EIA-63A, "Annual Solar Thermal Collector Manufacturers Survey." Table B. Companies Involved in Solar-Thermal Activities by Type, 1996 and 1997 Type of Activity 1996 1997 Collector or System Design 20 22 Prototype Collector Development 15 12 Prototype System Development 7 7 Wholesale Distribution 19 20 Retail Distribution 12 14 Installation 9 11 Noncollector System Component Manufacture 7 9 Source: Energy Information Administration, Form EIA-63A, "Annual Solar Thermal Collector Manufacturers Survey." Table C. Solar Thermal Collector Shipments by Type, 1987-1997 (Thousand Square Feet) Low-Temperature Medium-Temperature High-Temperature(a) Average per Average per Year Shipments Manufacturer Shipments Manufacturer Shipments 1987 3,157 263 957 19 3,155 1988 3,326 416 732 16 4,116 1989 4,283 428 1,989 55 5,209 1990 3,645 304 2,527 62 5,237 1991 5,585 349 989 24 1 1992 6,187 387 897 26 2 1993 6,025 464 931 28 12 1994 6,823 426 803 26 2 1995 6,813 487 840 32 13 1996 6,821 487 785 41 10 1997 7,524 579 606 29 7 (a) For high-temperature collectors, average annual shipments per manufacturer are not disclosed. Source: Energy Information Administration, Form EIA-63A, "Annual Solar Thermal Collector Manufacturers Survey." Table D. Distribution of Solar Thermal Collector Shipments, 1996 and 1997 Shipments (thousand square feet) Recipient 1996 1997 Wholesale Distributors 4,843 4,446 Retail Distributors 1,655 2,491 Exporters 372 417 Installers 529 585 End Users and Other(a) 217 199 Total 7,616 8,138 (a) Other includes minimal shipments not explained on Form EIA-63A. Note: Totals may not equal sum of components due to independent rounding. Source: Energy Information Administration, Form EIA-63A, "Annual Solar Thermal Collector Manufacturers Survey." Table E. Solar Thermal Collector Shipments by Type, Quantity, Value, and Average Price, 1996 and 1997 1996 1997 Quantity Value Average Price Quantity Value Average Price (thousand (thousand (dollars per (thousand (thousand (dollars per Type square feet) dollars) square foot) square feet) dollars) square foot) Low-Temperature Liquid and Air 6,821 18,227 2.67 7,524 19,584 2.60 Medium-Temperature Air 9 139 15.83 54 2484 45.75 Liquid ICS/Thermosiphon 343 7,424 21.63 33 773 23.28 Flat Plate 431 3,697 8.57 516 5,835 11.30 Evacuated Tube 1 110 75.10 2 99 49.27 Concentrator 0 -- -- 0 -- -- All Medium-Temp. 785 11,369 14.49 606 9,191 15.17 High Temperature Parabolic Dish and Trough 10 180 18.75 7 180 25.00 Total 7,616 29,776 3.91 8,138 28,970 3.56 (s) = Less that 500 square feet. ICS = Integral Collector Storage -- = Not Applicable Note: Totals may not equal sum of components due to independent rounding. Source: Energy Information Administration, Form EIA-63A, "Annual Solar Thermal Collector Manufacturers Survey." Table F. Shipments of Solar Thermal Collectors by Market and Type, 1996 and 1997 (Thousand Square Feet) Low-Temperature Med Temp Medium Temp Medium Temp Medium Temp Medium Temp High Liquid/Air Air Liquid Liquid Liquid Liquid Temperature Metallic and ICS/ Flat-Plate Evacuated Parabolic 1997 1996 Market Sector Nonmetallic Thermosiphon (Pumped) Tube Concentrator Dish/Trough Total Total Residential 6,791 53 24 491 1 0 0 7,360 6,873 Commercial 726 1 9 25 0 0 7 768 682 Industrial 7 0 0 0 0 0 0 7 54 Utility 0 0 0 0 0 0 0 1 (s) Other(a) 0 1 0 0 1 0 0 2 7 Total 7,524 54 33 516 2 0 7 8,138 7,616 End Use Pool Heating 7,517 0 7 4 0 0 0 7,528 6,787 Hot Water 0 52 26 509 1 0 7 595 765 Space Heating 7 2 0 0 0 0 0 10 57 Space Cooling 0 0 0 0 0 0 0 0 0 Combined Space & Water Heating 0 0 0 3 0 0 0 4 3 Process Heating 0 0 0 0 0 0 0 0 4 Electricity Generation 0 0 0 0 0 0 0 0 (s) Other(b) 0 0 0 0 1 0 0 2 0 Total 7,524 54 33 516 2 0 7 8,138 7,616 (a) = Other Market Sector includes shipments of solar thermal collectors that are manufactured for private contractors for research and development projects. (b) = Other End Use includes shipments of solar thermal collectors for other uses such as cooking foods, water pumping, water purification, desalinization, distilling, etc. (s) = Less that 500 square feet. ICS = Integral Collector Storage. Note: Totals may not equal sum of components due to independent rounding. Source: Energy Information Administration, Form EIA-63A, "Annual Solar Thermal Collector Manufacturers Survey." Table G. Number of Companies and Total Shipments of Photovoltaic Cells and Modules, 1985-1997 (Peak Kilowatts) Number of Photovoltaic Cell and Module Shipments Year Companies Total Imports Exports 1985(a) 15 5,769 285 1,670 1986(a) 17 6,333 678 3,109 1987(a) 17 6,850 921 3,821 1988(a) 14 9,676 1,453 5,358 1989(a) 17 12,825 826 7,363 1990(a) (b)19 (b)13,837 1,398 7,544 1991(a) 23 14,939 2,059 8,905 1992(a) 21 15,583 1,602 9,823 1993(a) 19 20,951 1,767 14,814 1994(a) 22 26,077 1,960 17,714 1995(a) 24 31,059 1,337 19,871 1996(a) 25 35,464 1,864 22,448 1997(a) 21 46,354 1,853 33,793 (a) = Does not include shipments of cells and modules for space/satellite applications (b) = Includes imputed data for one nonrespondent, which exited the industry during 1990. Note: Total shipments as reported by respondents includes all domestic and export shipments and may include imports that subsequently were shipped to domestic or foreign customers. Source: Energy Information Administration, Form EIA-63B, "Annual Photovoltaic Module/Cell Manufacturers Survey." Table H. Total Shipments of Photovoltaic Cells and Modules, 1995-1997 Shipments (peak kilowatts) Item 1995 1996 1997 Modules 19,627 24,534 33,645 Cells 11,432 10,930 12,709 Total 31,059 35,464 46,354 Source: Energy Information Administration, Form EIA-63B, "Annual Photovoltaic Module/Cell Manufacturers Survey." Table I. Distribution of Photovoltaic Cells and Modules, 1995 - 1997 Shipments (peak kilowatts) Recipient 1995 1996 1997 Wholesale Distributors 16,413 21,424 31,385 Retail Distributors 1,181 1,457 424 Exporters 321 367 4,081 Installers 4,098 4,860 1,236 End Users 458 1,048 1,522 Module Manufacturers 5,794 5,528 5,247 Other(a) 2,793 781 2,459 Total 31,059 35,464 46,354 (a) Other includes categories not identified by reporting companies. Note: Totals may not equal sum of components due to independent rounding. Source: Energy Information Administration, Form EIA-63B, "Annual Photovoltaic Module/Cell Manufacturers Survey." Table J. Shipments by Type of Photovoltaic Cell and Module, 1995-1997 Shipments (peak kilowatts) Percent of Total Shipments Type 1995 1996 1997 1995 1996 1997 Crystalline Silicon Single-Crystal 19,857 21,742 29,997 64 61 65 Cast and Ribbon 9,883 12,255 14,317 32 35 31 Subtotal 29,740 33,996 44,677 96 96 96 Thin-Film Silicon 1,266 1,445 1,886 4 4 4 Concentrator Silicon 53 23 154 * * * Other(a) 0 0 0 0 0 0 Total 31,059 35,464 46,354 100 100 100 (a) Other includes categories not identified by reporting companies. (*) Represents less than 0.5, rounded to zero. Note: Data do not include shipments of cells and modules for space/satellite applications. Totals may not equal sum of components due to independent rounding. Source: Energy Information Administration, Form EIA-63B, "Annual Photovoltaic Module/Cell Manufacturers Survey." Table K. Value and Average Price of Cells and Modules by Type, 1996 and 1997 1996 1997 Value Average Price Value Average Price (thousand (dollars per peak watt) (thousand (dollars per peak watt) Type dollars) Modules Cells dollars) Modules Cells Crystalline Silicon Single-Crystal 75,043 3.97 2.81 108,226 4.08 2.81 Cast and Ribbon 46,646 3.92 2.73 55,701 4.03 2.59 Subtotal 121,689 3.95 2.80 163,927 4.06 2.78 Thin-Film Silicon W W W W W W Concentrator Silicon W W W W W W Other(a) 0 0 0 0 0 0 Total 131,066 4.09 2.80 175,089 4.16 2.78 (a) = Other includes categories not identified by reporting companies W = Withheld to avoid disclosure of individual company data. Notes: Data does not include shipments of cells and modules for space/satellite applications. Totals may not equal sum of components due to independent rounding. Source: Energy Information Administration, Form EIA-63B, "Annual Photovoltaic Module/Cell Manufacturers Survey." Table L. Shipments of Photovoltaic Cells and Modules by Market Sector, End Use, and Type, 1996 and 1997 (Peak Kilowatts) Crystalline Thin-Film Concentrator Other 1997 1996 Type Silicon Silicon Silicon Total Total Market Sector Industrial 11,244 504 0 0 11,748 8,300 Residential 10,691 300 2 0 10,993 8,475 Commercial 7,621 340 150 0 8,111 5,176 Transportation 3,378 196 0 0 3,574 3,995 Utility 5,331 320 0 0 5,651 4,753 Government (a) 3,772 135 2 0 3,909 3,126 Other (b) 2,276 91 0 0 2,367 1,639 Total 44,314 1,886 154 0 46,354 35,464 End Use Electricity Generation Grid Interactive 7,402 871 0 0 8,273 4,844 Remote 8,433 195 2 0 8,630 10,884 Communication 7,289 94 0 0 7,383 6,041 Consumer Goods 72 275 0 0 347 1,063 Transportation 6,645 60 0 0 6,705 5,196 Water Pumping 3,748 35 0 0 3,783 3,261 Cells/Modules to OEM (c) 4,984 261 0 0 5,245 2,410 Health 1,267 36 0 0 1,303 977 Other (d) 4,473 59 152 0 4,684 789 Total 44,314 1,886 154 0 46,354 35,464 (a) = Includes Federal, State, and local governments, excluding military. (b) = Includes shipments that are manufactured for private contractors for research and development projects. (c) = OEM is original equipment manufacturers (d) = Other End Use includes shipments of photovoltaic cells and modules for uses such as cooking food, desalinization, distilling, etc. W = Data withheld to avoid disclosure. Note: Total may not equal sum of components due to independent rounding. Source: Energy Information Administration, Form EIA-63B, "Annual Photovoltaic Module/Cell Survey."