Highly confined spectrally selective absorber-emitter for effective solar thermophotovoltaics

Abstract

A high efficiency in solar thermophotovoltaic (TPV) systems is achieved by recycling photons to the emitter in a cell with a high out-of-band reflectivity. Ideally, if all out-of-band photons can be recovered in a blackbody emitter, the effect would be equivalent to that when using a spectrally selective emitter. However, in practice, it is difficult to fully recover large number of out-of-band photons emitted from blackbody emitters. Meanwhile, spectrally selective emitters generally have lower thermal stability than blackbody emitters, resulting low in-band photon flux. Considering these practical constraints, our analysis demonstrates that the combination of spectrally selective absorbers and spectrally selective emitters has a higher system efficiency than the other cases using blackbody absorbers or emitters, even in systems that exhibit a high confinement performance at practical input densities of 200 W/cm2. A confined system with a high thermal radiation transport efficiency of 0.9 is constructed in which a tungsten cube coated with a hafnium oxide layer was used as a spectrally selective absorber and spectrally selective emitter. A total experimental system efficiency of 7.5% was achieved; using the model of the experimental system, we estimated a 13% system efficiency for a system containing a cell with Roob = 0.98, and a 19.5% system efficiency for a system with improved spectral selectivity of the absorber and emitter.