Recent headlines reporting sky-high natural gas prices and the possibility of $3/gallon for gasoline in the U.S. in the not-so-distant future have led many of us to add "energy prices" to our list of "things to watch." What might not be obvious is that the rising cost of energy could result in a new and, potentially, large market for compound semiconductors: solar cells for terrestrial applications. Compound Semiconductor magazine has already reported the success of GaInP/GaAs/Ge solar cells for satellites [1]. These solar cells have a higher efficiency than any other commercially available solar cell, and if they now enter the terrestrial solar cell market, the potential sales growth is even greater. Energy Generation Using Photovoltaics For decades, scientists have been projecting that solar cells (photovoltaic devices) will help to meet our world s electricity needs, but the cost of their electricity still exceeds that of conventional electricity. Nevertheless, the photovoltaic industry has been growing at a healthy rate. U.S. production was 77 MW in 1999, up 50% from 1998, but still too small to help reduce fossil fuel usage: The U.S. power industry is adding about 25,000 MW of new electricity generating capacity this year. One approach to reducing cost and rapidly increasing solar-electricity generation is to use cheap lenses or mirrors to focus light onto small solar cells. With this approach, expensive cells are cost effective if they have very high efficiency. In volume production, systems using GaAs-based solar cells in reflective dishes are estimated to be able to generate electricity at a cost of 7 to 15 cents/kWhr for a plant located in Albuquerque [2]. This is only a little greater than today s low electricity prices (average of 49 cents/kWhr in the U.S.), and these may increase; during recent shortages the selling price of electricity has spiked to more than $1/kWhr in California. Solar cells provide the highest output when the sun is shining most brightly, often close to the times of peak demand. Because it burns cleaner and generates less greenhouse gas than coal, natural gas is being used in almost all of the new electricity generation facilities. Increased demand for natural gas has quintupled its price in the past year. The natural gas industry projects that billions of dollars of investment will be required to meet the increasing demand. Although increased supply will limit price increases in the short term, higher production of natural gas implies faster depletion of one of the world s most important natural resources. Increasingly, people are demanding conversion to "green" energy. In Arizona, legislation requires that 1% of Arizona s electricity (about 75 MW) be generated by renewable sources by the year 2005. Arizona Public Service is investigating concentrator systems as one of the options for meeting this requirement. While concentrator systems have been unable to enter a photovoltaic market dominated by sales of solar cells in small products like calculators and roadside, flashing lights, concentrators can better compete when the photovoltaic market is for utility-scale power generation. Amonix, Inc. (Torrance, California) has provided more than 200 kW of concentrator systems for Arizona Public Service installations (see ), and are continuing to ship 10 kW/week, with production booked for the next year. This represents the first time that a concentrator company has started a continuous production line. Solar Systems (Hawthorn, Australia) is also starting a production line installing 52 of their 24 kW dishes (see ) in Australia. Other companies, including Entech (Keller, Texas) and SunPower (Sunnyvale, California), are also marketing concentrator products. Opportunities for GaAs-Based Concentrator Systems The growing market for the silicon-based concentrator products raises the question of when compound semiconductor solar cells might also be used in similar systems. Spectrolab recently reported a terrestrial concentrator cell efficiency of 32% for their GaInP/GaAs/Ge cell [3]. The company estimates cell costs [4] that are consistent with the projected electricity cost of 715 cents/kWhr [2]. JX Crystals and Tecstar are developing a stacked GaInP/GaAs over GaSb concentrator cell [5]. New multijunction concepts may eventually increase the concentrator cell efficiency to over 40%, increasing the advantage of compound semiconductor cells over silicon cells, which are currently 26% and may never pass 30% in efficiency. Silicon concentrator cells are likely to be cheaper than III-V cells in terms of $/cm2. However, if the GaAs-based cells are used at higher concentration, a smaller area is needed, and so they might be cheaper when priced by $/watt. This trade off is complex because, for example, increasing the concentration ratio may increase the cost of the optics and support structure. Factors that might cause one to switch from Si to III-V cells are decreasing costs of III-V cells, improvement of III-V cell efficiencies, or if higher concentration systems become practical. The biggest advantage of the III-V cells is their high efficiency, which reduces the system cost/watt. While compound semiconductor space solar cells need only minor modifications to make them suitable for concentrator systems, the development of a high-performance, reliable concentrator system will take both time and a large investment. Concentrating Technologies (Huntsville, AL) has been addressing this issue by developing a 2 kW dish concentrator system for Arizona Public Service using dense arrays from Spectrolab (GaInP/GaAs/Gesee ) and Amonix (silicon). The National Renewable Energy Laboratory (NREL) is also funding Concentrating Technologies to fabricate and install a 2 kW prototype dish system in Golden for off-grid applications. In addition, NREL is funding Amonix and Spectrolab to make improvements in their close-packed array designs and United Innovations for development of a unique cavity-based concentrating PV concept. Solar Systems Australia has completed the development of its production Dish PV concentrator system. This includes optical matching of the dish and PV receiver as well as a fully modular close-packed receiver capable of delivering 24 kW at 500 suns. Control and data acquisition are integrated so that the dish delivers optimum light flux to the PV receiver and produces maximum output at all times. Another indication of growing industry maturity is the recent development of an IEEE qualification test standard by the concentrator industry and utility representatives. The combination of rising demand for energy, increasing concern for the environment, and the likelihood that fossil fuels are not inexhaustible, implies that energy prices will eventually rise. Further development and economies of scale will reduce the price of concentrators. The convergence of these two trends implies that an opportunity for renewables, and, specifically, compound semiconductor concentrator cells, is something to be watching for. Acknowledgment We thank D. Friedman and numerous others at NREL and at the companies mentioned above for their help with this article. References [1] Compound Semiconductor 1998, 4(8), p. 32 [2] Prog. in Photovoltaics: Res. Appl. 2000, 8, p. 93 [3] Compound Semiconductor 1999, 5(9), p. 40 [4] Proc. IEEE Photovoltaics Specialists Conf. (PVSC), 2000 [5] Compound Semiconductor 2000, 6(7), p. 25