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

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 (Th), the one-layer SSA requires higher optical concentrations (Copt) than the three-layer SSA. The CSTEG with Si80Ge20 alloys requires much higher optical Copt than the CSTEG with SnSe crystals in order to reach the same Th because of much a higher rate of heat flux. At Th=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.

Analytical solutions of electromagnetic waves in focusing and magnifying cylindrical hyperlenses: Green's function approach.

The analytical solutions of the electromagnetic waves in the inhomogeneous cylindrical hyperlens (CH) comprising concentric cylindrical layers (CCLs) with multiple point sources located either outside the structure in the focusing process or inside the core in the magnifying process are obtained by means of Green's function analysis. The solutions are consistent with FDTD simulation in both processes. The sub-wavelength focal spot λ/16.26 from two point sources with wavelength 465 nm is demonstrated in the CH made by alternating silver and silica CCLs. Our solutions are expected to be the efficient tools for designing the sub-wavelength focusing and imaging cylindrical hyperlens.