Theoretically comparative study of spectrally selective solar absorbers in concentrated solar-thermoelectric...

A spectrally selective solar absorber (SSA) made from alternating tungsten and SiO₂ layers is an important component in a concentrated solar-thermoelectric generator (CSTEG). However, the highly efficient SSA consisting of a high number of tungsten and SiO₂ layers may require complicated fabrication procedures, thereby raising the overall cost of the CSTEG. In this study, we have theoretically optimized SSAs consisting of a different number of layers, and the performances of CSTEGs installed with different optimized SSAs and thermoelectric materials (Si₈₀Ge₂₀ alloys, SnSe crystals) are theoretically investigated. It is found that two SSAs, one with only one layer of SiO₂ glass (88 nm) coated on a tungsten reflective plane, and the other with three alternating layers of SiO₂(92nm)/tungsten(7nm)/SiO₂(78nm) coated on a tungsten reflective plane, have the highest average solar absorptance equal to 0.7050 and 0.9999, respectively. To reach the same steady-state temperature (T_h), the one-layer SSA requires higher optical concentrations (C) than the three-layer SSA. The CSTEG with Si₈₀Ge₂₀ alloys requires much higher optical C than the CSTEG with SnSe crystals in order to reach the same T_h because of much a higher rate of heat flux. At T_h=800deg⁡C, the CSTEG with Si₈₀Ge₂₀ alloys have the output electrical power per unit cross-sectional area of TEG about 6 times higher than that of the CSTEG with SnSe crystals. The net efficiency of CSTEG with SnSe crystals could reach 25.6% if the electrical conductivity of its p-type thermoelectric leg could be enhanced to be comparable to the electrical conductivity of its n-type thermoelectric leg.