Theoretically comparative study of spectrally selective solar absorbers in concentrated solar-thermoelectric generators working at high temperature

Abstract

A spectrally selective solar absorber (SSA) made from alternating tungsten and ${{\rm{SiO}}_2}$ 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 ${{\rm{SiO}}_2}$ 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 (${{\rm{Si}}_{80}}{{\rm{Ge}}_{20}}$ alloys, SnSe crystals) are theoretically investigated. It is found that two SSAs, one with only one layer of ${{\rm{SiO}}_2}$ glass (88 nm) coated on a tungsten reflective plane, and the other with three alternating layers of ${{\rm{SiO}}_{2}}({{92}}\;{\rm{nm}}){\rm{/tungsten}}\;({{7}}\;{\rm{nm}})/{{\rm{SiO}}_{2}}({{78}}\;{\rm{nm}})$ 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 (${C_{\rm{opt}}}$) than the three-layer SSA. The CSTEG with ${{\rm{Si}}_{80}}{{\rm{Ge}}_{20}}$ alloys requires much higher optical ${C_{\rm{opt}}}$ than the CSTEG with SnSe crystals in order to reach the same ${T_h}$ because of much a higher rate of heat flux. At ${T_h} = {{800}}\;\deg {\rm{C}}$, the CSTEG with ${{\rm{Si}}_{80}}{{\rm{Ge}}_{20}}$ 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.