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

The thermo-emf Delta V and thermoelectric current Delta I 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 8 s to infinity. The alternating TG was produced by switching a voltage of 2.6 V across two Peltier modules (20 x 20 x 4.0 mm(3)) 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 x 15 x 4.0 and 20 x 20 x 3.8 mm(3), respectively. The input power W-input fed to two Peltier modules increases abruptly with an increase of 1/T, but the effective temperature difference Delta T-eff produced between two Peltier modules has a local maximum at 1/T = 1/240 s(-1) for the generator (1) and at 1/120 s(-1) for the generator (2), whose maximum values are 2.59 and 3.65 times as large as those obtained at 1/T = 0 s(-1). The local maxima of Delta V-eff, Delta I-eff, and Delta W-eff, generated by a generator, appeared at the period T giving the local maximum Delta T-eff. The maximum energy conversion efficiency Delta W/W-input 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 = 0 s(-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 Delta T-eff (< 22 K ) and low ZT of generators (< 0.76 at 298 K ) and two Peltier modules (= 0.75 at 298 K) 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. (c) 2007 American Institute of Physics.