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 [tau] 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 tBi-Te = 4.0 mm and tCu = 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 tBi-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.