Energy conversion efficiency of a thermoelectric generator under the periodically alternating temperature gradients

Abstract

The thermo-emf ΔV and thermoelectric current ΔI generated by imposing the temperature gradient (TG) alternating at a period of T on a thermoelectric (TE) generator were measured as a function of t, where t is the lapsed time and T was varied from 8s to ∞. The alternating TG was produced by switching a voltage of 2.6V across two Peltier modules (20×20×4.0mm3) connected in series. Two different types of TE generators were employed as a generator and sandwiched between Peltier modules. The dimensions of the generators (1) and (2) were 15×15×4.0 and 20×20×3.8mm3, respectively. The input power Winput fed to two Peltier modules increases abruptly with an increase of 1∕T, but the effective temperature difference ΔTeff produced between two Peltier modules has a local maximum at 1∕T=1∕240s−1 for the generator (1) and at 1∕120s−1 for the generator (2), whose maximum values are 2.59 and 3.65 times as large as those obtained at 1∕T=0s−1. The local maxima of ΔVeff, ΔIeff, and ΔWeff, generated by a generator, appeared at the period T giving the local maximum ΔTeff. The maximum energy conversion efficiency ΔW∕Winput obtained at an optimum period is 0.37% for the generator (1) and 0.27% for the generator (2), which are 3.54 and 5.66 times as high as those obtained at 1∕T=0s−1, respectively. The application of the periodically alternating TG to a TE generator was thus found to be very effective to enhance the energy conversion efficiency of a generator. However, the present low conversion efficiency is attributed not only to both the small ΔTeff (<22K) and low ZT of generators (<0.76 at 298K) and two Peltier modules (=0.75 at 298K) but also to much heat transfer to or from the surroundings other than at the hot and cold sides of a TE generator or two Peltier modules.