Energy conversion efficiency of a welded Cu/Bi–Te/Cu composite under periodically alternating temperature gradients

Abstract

It was reported recently by us that the energy conversion efficiency η ATG of thermoelectric (TE) generators working under the alternating temperature gradient (ATG) produced by switching at an optimum period of τ = 120 or 240 s is several times higher than η STG of them operating under the steady temperature gradient (STG), where the generators are sandwiched between two Peltier modules. However, the reason for η ATG > η STG has not yet been explained explicitly. In order to clarify the relation between the optimum τ and η ATG or η ATG/ η STG in the wide range of τ, long p- and n-type Cu/Bi–Te/Cu composites with t Bi–Te = 4.0 mm and t Cu = 5.0 mm were fabricated so that they have an optimum τ longer than the previous periods. The maxima η ATG of the p- and n-type composites were 0.073% at τ = 960 s and 0.057% at τ = 480 s, respectively, and their η ATG/ η STG were approximately 1.4. Owing to this experimental result, η ATG/ η STG is related closely to the thermal diffusion time τ D of a composite or a generator, so that η ATG and η ATG/ η STG were found to be in inverse proportion to τ D. The reason for this low η ATG results from the high external resistance and the long τ caused by a thick t Bi–Te, in addition to too fat Bi–Te sample giving the high thermal conductance. The maximum η ATG was also found to be expressed successfully by a theoretical expression.