On the Significance of the Thermoelectric Figure of Merit Z

When the electrical conductivity, sigma, thermal conductivity, lambda, and thermopower, S, of a material are all assumed to be constant over the temperature range of interest, then the well-known thermoelectric (TE) figure of merit, Z = sigma S (2)/lambda, arises as part of the derivation of conversion efficiency in a TE generator. However, there are an infinite number of parameter sets (sigma, lambda, S) that yield any given Z. So, are they truly equivalent? This paper reviews the historical basis for Z as a metric for TE quality and discusses results of simulations on three systems having different parameter sets but the same Z. The three systems exhibit different power generation capabilities, illustrating that Z is not sufficient to specify the likely performance of a TE material in a system. Instead, a systems analysis is required that incorporates, at a minimum, source and sink temperatures and thermal resistances.