Theoretical limits of thermophotovoltaic solar energy conversion

Theoretical efficiencies are derived in a detailed balance calculation for thermophotovoltaic solar energy conversion, where solar radiation is absorbed by an intermediate absorber, which emits radiation inside an evacuated housing towards a solar cell. For ideal components with no optical losses and only radiative recombination in the solar cell, maximal efficiencies are found of 85% for full concentration of the incident sunlight on a black absorber, and of 54% for no concentration and a selective absorber absorbing only for (h) over bar omega > 0.92 eV. This is considerably larger than the efficiency for directly illuminated solar cells with also only radiative recombination, the Shockley-Queisser limit, which is 41% for full concentration and 30% for no concentration. In order to approach efficiency limits for real TPV systems, several non-idealities have been introduced: (a) realistic assumptions about the geometry of the intermediate absorber, (b) optical losses of 5% for photons with energy below the band gap of the solar cell and (c) non-radiative recombination in the solar cell of the same amount as radiative recombination. This reduces the efficiency for non-concentrated sunlight to only 32.8%, but for very high concentrations of 10 000 and above suitable absorber geometries still seem to allow efficiencies close to 60%.