Solar Photovoltaic and Solar Thermal Electric Technologies 

Solar power is one of the fastest-growing sources of renewable energy worldwide. Many nations, concerned about the environmental impacts of electricity generation from fossil fuels or from large-scale hydroelectric plants, have been turning to solar power as an environmentally benign alternative. The solar energy that reaches the earth can be harnessed to generate electric power, and the potential for large-scale applications of solar power has improved markedly in recent years. Two solar power technologies—solar photovoltaic and solar thermal—are widely employed today, and their use is likely to increase in the future. 

Solar photovoltaic technologies convert sunlight directly into electricity by using photons from the sun’s light to excite electrons into higher states of energy. The resultant voltage differential across cells allows for a flow of electric current. Because individual solar cells are very small and produce a few watts of power at most, they are connected together in solar panels that can be arranged in arrays to increase electricity output. The arrangement of arrays is one major advantage of photovoltaic technologies, because they can be made in virtually any size to fit a specific application. 

One popular application of solar photovoltaics is in solar panel installations on residential roofs, which can be scaled to accommodate house size and electricity needs. Although the technology now is used most often in small residential applications, it can be scaled up to create larger power plants, such as the 14-megawatt Nellis solar plant in Nevada with some 70,000 panels and the 11-megawatt solar plant in Serpa, Portugal, with 52,000 panels. 

At present, the cost of electricity produced from solar photovolatics generally is too high to compete with wholesale electricity. In sunny locations, however, the cost can be as low as 23 cents per kilowatthour,^a which may be competitive with the delivered price of electricity to retail customers in areas where electricity prices are high, as they are in California, Southern Spain, and Italy.^b On the basis of installed cost per megawatt, solar photovoltaic installations are relatively costly, because the panel components are expensive and the conversion of solar energy to electricity in the cells still is inefficient. From conversion efficiencies of 5 to 6 percent for the first solar cells built in the 1950s, there has been an improvement to efficiencies of 12 to 18 percent for modern commercial wafer-silicon cells.^c 

Figure data

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Efficiency gains, coupled with other technological advances, have reduced the cost of solar photovoltaic capacity from approximately $300 per watt in 1956^d to less than $5 per watt in 2009.^e EIA’s Annual Energy Outlook 2009 projects that, by 2030, overnight capacity costs for new generating plants using solar photovoltaics will be 37 percent lower than the 2009 costs. In addition, the efficiency of solar photovoltaic applications is expected to improve as the technology continues to be developed. As a result, U.S. solar photovoltaic generating capacity is projected to increase from 30 megawatts in 2006 to 381 megawatts in 2030 (see figures on right). 

Although prices for electricity from photovoltaics may not become widely competitive with wholesale prices for electricity from conventional generating technologies within the next 25 years, they may be competitive with high retail electricity prices in sunny regions. Already, photovoltaic technology is gaining market share in countries where declining prices and government-backed financial incentives have led to increased usage. In Germany, for example, a feed-in tariff of 27 cents per kilowatthour^f has produced an explosion in the use of solar photovoltaics, and in Japan the government has set a target for 30 percent of all households to have solar panels installed by 2030. 

Solar thermal technologies produce electricity by concentrating the sun’s heat to boil a liquid and using the steam to rotate a generator turbine, in much the same way that electricity is produced from steam plants powered by coal or natural gas. There are two main types of solar thermal power plants: towers and parabolic troughs. A solar power tower consists of a large array of sun-tracking mirrors, which are used to reflect the sun’s rays onto a central tower. When the rays hit the tower’s receiving panel, their heat is transferred to a fluid medium that is boiled to produce steam. Solar power towers have been demonstrated successfully,  but they still are in the early stages of technology development. The world’s largest solar power tower, located in Spain, is the 15-megawatt Solar Tres Power Tower. 

The most commonly used solar thermal technology is the parabolic trough, in which a parabolic reflector focuses the sun’s rays on a heat pipe that runs the length of the trough and transports heated fluid to a central power station. Most parabolic trough installations consist of a field of reflectors concentrated on a central location, where the working fluid is heated to produce steam. The world’s largest parabolic trough installation is the Kramer Junction Solar Electric Generating System in California, which consists of five 30-megawatt parabolic trough arrays. Total U.S. installed solar thermal capacity, currently 400 megawatts, is projected to increase to 859 megawatts in 2030. 

Solar thermal power plants are designed to be large-scale grid-connected plants, but at present they generally cannot be used as baseload generators, because they do not produce heat at night or during the day when clouds block the sun. Some advances have been  made in storing solar energy by using it to heat liquid sodium, which can be used later to boil water and produce the steam needed to power a generator turbine. The process is time-limited, however, and can extend a plant’s operations by only a few hours at best. In some cases, storage times of 4 to 16 hours have been achieved, sufficient to allow electricity from solar thermal generators to be sold when it is more valuable, during the peak demand hours of 7-9 am and 5-7 pm. 

Solar technologies have benefited from much research and development over the past two decades, bringing down the delivered price of solar electricity. Today, electricity from residential photovoltaics is marketed to compete with high-priced retail electricity. In the future, it is possible that utility-scale photovoltaic plants will compete with wholesale electricity generation, provided that further technological advances are achieved. Solar thermal power plants are intended to compete with wholesale generation, especially from peaking plants, and they may become more competitive over time if heat storage technologies improve, costs decrease, and/or policies to mitigate carbon dioxide emissions are adopted.

^aU.S. Department of Energy, Solar Energy Technologies Program Multi Year Program Plan 2008-2012 (Washington, DC, April 18, 2008), p. 18, web site www1.eere.energy.gov/solar/pdfs/solar_program_mypp_2008-2012.pdf. In comparison, wholesale electricity prices averaged 5.42 cents per kilowatthour in California in 2009. 

^bBecause retail electricity prices in the United States include generation, transmission, and distribution costs, prices paid by consumers are higher than average retail prices per kilowatthour. 

^cU.S. Department of Energy, Solar Energy Technologies Program Multi Year Program Plan 2008-2012, p. 119. 

^dSouthface Energy Institute, “History of Solar,” web site www.southface.org/solar/solar-roadmap/solar_how-to/history-of-solar. htm. 

^eSolarbuzz LLC, “Solar Module Retail Price Environment,” web site www.solarbuzz.com/Moduleprices.htm. 

^fUnder a feed-in tariff structure, regional or national electric utilities are obligated to purchase renewable electricity at a higher rate than retail, in order to allow renewable energy sources to overcome price disadvantages.