used to generate electricity. These systems are expected to have conversion efficiencies similar to those of solar tower concentrators, although dish concentrator systems have not yet been successfully commercialized.
Dish concentrators with a combined capacity of more than 1,700 MW are being developed; over 35 percent of them are in the Asia Pacific region.42 These systems require that an efficient and inexpensive generator be developed on a much smaller scale than that needed for tower systems. These systems lack the centralized storage capability of the power tower approach, and thus alternate storage methods must be employed to store power.
Some research is being conducted on the possibility of integrating CPV and CSP systems. In a photovoltaic system, the energy that is not converted into electrical power is dissipated as heat, which has the potential to be recycled, especially in a concentrated photovoltaic system. This waste heat can either be recycled to generate more electricity or can serve a variety of other uses, such as the heating of water.43 The primary difficulty is that most PV chips become less efficient as they increase in temperature. Wide-bandgap materials will work better for solar cells at higher temperatures, but there will still be a loss of efficiency.44 Researchers at Stanford University are conducting research on a “photovoltaic-like” device that becomes more efficient at higher temperatures, which would be ideal for integration into a hybrid system.45 The Stanford approach, also discussed in a paper by Andrews et al.,46 has calculated efficiencies for an idealized device, which can exceed the theoretical limits of single-junction photovoltaic cells. The device is based on thermionic emission of photoexcited electrons from a semiconductor cathode at high temperature. Temperature-dependent photoemission-yield measurements from gallium nitride (GaN) show strong evidence for photon-enhanced thermionic emission. As mentioned by CleanEnergyAuthority,47 the proposed solar converter
42 Frost and Sullivan Research Service. 2011. Global Solar Power Market.
44 Landis, G.A., D. Merritt, R.P. Raffaelle, and D. Scheiman. 2005. High-Temperature Solar Cell Development. Washington, D.C.: National Aeronautics and Space Administration, pp. 241-247. Available at http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050206368_2005207966.pdf. Accessed July 24, 2012.
45 Schwede, J.W., I. Bargatin, D.C. Riley, B.E. Hardin, S.J. Rosenthal, Y. Sun, F. Schmitt, P. Pianetta, R.T. Howe, Z.-X. Shen, and N.A. Melosh. 2010. Photon enhanced thermionic emission for solar concentrator systems. Nature Materials 9:762-767.
46 Andrews, J.C., F. Meirer, Y. Liu, Z. Mester, and P. Pianetta. 2011. Transmission x-ray microscopy for full-field nano imaging of biomaterials. Microscopy Research and Technique 74(7):671-681.