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

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