Thermoelectrics versus thermophotovoltaics: Two approaches to convert heat fluxes into electricity

Abstract

Direct conversion of heat fluxes into electricity is usually done by thermoelectric generators (TEGs). For hot-side temperatures above 1000 K, thermal radiation carries a high energy density, comparable with the energy density extracted from TEGs and therefore a direct conversion of thermal radiation into electricity, named thermophotovoltaics (TPV), would also be an option. This paper compares both methods with respect to efficiency and extractable power density. The physical limits are estimated under simplified but realistic boundary conditions. For TPV the radiative detailed balance limit under black body radiation, which was calculated for different hot-side temperatures from 310 K to 3000 K for an optimized bandgap of the applied material was used. But, since very narrow bandgaps leading to strong non-radiative recombination mechanisms, the bandgap was limited to Eg ≥ 0.5 eV. The effect of suppressing sub-bandgap radiation as well as an enhanced radiation density in the nearfield (near-field TPV) were also included. The TEG efficiency and power density was calculated under thermal matching conditions with a heat transfer coefficient of Kcont = 250 W m-2 K-1 and an average device ZT̄ = 1. The results are compared with experimental data for TPV and TEGs from literature. It is shown, that up to 600 K hot-side temperature TEGs are superior to TPV, due to a significant higher power density. Above 1000 K TPV profits from higher efficiency by a similar power density. But above 2000 K TPV suffers from cell heating. The range 600 K to 1000 K is currently captured by high temperature thermoelectrics, but near-field TPV (NF-TPV) has good chances to compete with TEGs in this temperature range in the future.