The thermo-emf Delta Vand current Delta I generated by imposing the alternating temperature gradients (ATG) at a period of Tand the steady temperature gradient (STG) on a thermoelectric (TE) composite were measured as a function of t, where t is the lapsed time and T was varied from 60 to or infinity s. The STG and ATG were produced by imposing steadily and alternatively a source voltage Vin the range from 1.0 to 4.0 Von two Peltier modules sandwiching a composite. Delta T, Delta V Delta l and V-p oscillate at a period Tand their waveforms vary significantly with a change of T, where Delta V and Vp are the voltage drops in a load resistance R-L and in resistance R-p of two modules. The resultant Seebeck coefficient |alpha| = |Delta| I Delta T/Delta Tof a composite under the STG was found to be expressed as |alpha| = |alpha(0|) (1-R-comp/R-T), where R-T is the total resistance of a circuit for measuring the output signals and R,,,,,p is the resistance of a composite. The effective generating power Delta(Weff) has a local maximum at T = 960 s for the p-type composite and at T = 480 s for the n-type one. The maximum energy conversion efficiency eta of the p- and n-type composites under the ATG produced by imposing a voltage of 4.0 V at an optimum period were 0.22 and 0.23% at Delta T-eff, = 50 K, respectively, which are 42 and 43% higher than those at Delta T= 42 K under the STG. These maximum eta for a TE composite sandwiched between two Peltier modules, were found to be expressed theoretically in terms of Rp, RT, RL, alpha(p) and alpha, where up and alpha are the resultant Seebeck coefficients of Peltier modules and a TE composite. (c) 2007 Elsevier Ltd. All rights reserved.