Parametric Optimization of Exergy Efficiency in Solar Thermoelectric Generators

Abstract

This paper presents the optimization of the exergy efficiency of a solar thermoelectric generator (STEG) with respect to its hot-side temperature, length-weighted current density, thermoelement area ratio, length of the thermoelectric (TE) legs, and the thermal concentration ratio (CR). Lagrange multiplier technique was employed to perform the optimization. It was found that optimal hot-side temperatures exist at which maximum exergy efficiencies of STEGs are attained. In the absence of optical concentration, but with thermal CRs as large as 600–700, these exergy efficiencies were as much as 6–8% for a hot-side temperature range of 175–277 °C. The optimal hot-side temperatures were found to be independent of both the TE device geometry and the thermal CR, but varied with the TE device’s dimensionless figure of merit, as well as the properties of the STEG’s optical component and absorber. Moreover, optimal combination of these parameters gives the configuration that optimizes the efficiency. For instance, for a Bi₂Te₃-based TE material, with ZT_m = 1, and under vacuum conditions, the optimal configuration is: a solar surface absorber area of 1300 mm²; cross-sectional area of 1 mm² and height of 1 mm for both p and n legs; and an emittance of 0.05, giving a conversion efficiency of about 7%. So with the optimized configuration, complicated optical systems (which usually include tracking mechanisms) may be avoided, and the required amounts of TE material minimized. This ultimately results in substantial reductions in material, manufacturing and system costs.