Assessing high-temperature photovoltaic performance for solar hybrid power plants

Hybrid solar photovoltaic/thermal power systems offer the possibility of dispatchable, low-cost, efficient and reliable solar electricity production. A key design strategy capable of fully exploiting the heat generation stemming from both solar cell thermalization and sub-bandgap photons involves an integrated photovoltaic/thermal absorber operated under concentrated sunlight, at temperatures conducive to efficient turbine operation, to wit, hundreds of degrees C. A pivotal aim is attaining the highest efficiency possible while ensuring a substantial fraction of the total power derives from the turbines, with gas-fired backup heating and/or thermal storage mitigating the ephemeral character of solar availability. However, the performance of solar cells at unprecedented elevated temperatures remains an open question. Key issues include (a) whether the efficiency loss stemming from high-temperature solar cell operation can be maintained acceptably small, as well as how optical concentration affects it, and (b) whether the solar thermal contribution can constitute a significant fraction of total electricity production. Here, we try to establish upper bounds on photovoltaic and system performance, covering a broad range of cell temperature and concentration levels, for single- and multi-junction cells operating at the radiative limit. We demonstrate that (1) the use of highly concentrated sunlight markedly diminishes photovoltaic - as well as thermal - efficiency losses at high temperature, and (2) the extent to which high operating temperature affects cell efficiency strongly depends on cell architecture. The implications for future generations of high-temperature/high-concentration solar cells are also addressed.