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)eta(ATG) of thermoelectric (TE) generators working under the alternating temperature gradient (ATG) produced by switching at an optimum period of tau = 120 or 240 s is several times higher than eta(STG) of them operating under the steady temperature gradient (STG), where the generators are sandwiched between two Peltier modules. However, the reason for eta(ATG) > eta(STG) has not yet been explained explicitly. In order to clarify the relation between the Optimum T and eta(ATG) or eta(ATG)/eta(STG) in the wide range of tau, 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 tau longer than the previous periods. The maxima eta(ATG) of the p- and n-type composites were 0.073% at tau = 960 s and 0.057% at tau-480 s, respectively, and their eta(ATG)/eta(STG) were approximately 1.4. Owing to this experimental result, eta(ATG)/eta(STG) is related closely to the thermal diffusion time tau(D) of a composite or a generator, so that eta(ATG) and eta(ATG)/eta(STG) were found to be in inverse proportion to tau(D). The reason for this low eta(ATG) results from the high external resistance and the long tau caused by a thick t(Bi-Te), in addition to too fat Bi-Te sample giving the high thermal conductance. The maximum eta(ATG) was also found to be expressed successfully by a theoretical expression. (C) 2008 Elsevier Ltd. All rights reserved.