Right Definition of Thermoelectric Figure of Merit toward the Thermoelectric Efficiency Prediction for Segmented Devices

The dimensionless figure of merit zT has been considered as a good metric for thermoelectric efficiency. But, due to the strong temperature dependence of material properties, the relation between the material properties and device performances is not clear. Here, we generalize the definition of device figure of merit Z_genT. Since the thermoelectric leg acts as a electric and a thermal circuit, the Z_gen should be written as Z_gen = (V/ΔT)²/RK, where V, R, and K are generated voltage, electric resistance, and the thermal conductance. Thus V, R, and 1/K should be computed as integrals of material properties (-α.gradT, ρ, κ) on position x, not on temperature T. Using the temperature approximation of zero-electric-current condition, the Z_gen is simply approximated as Z_gen⁽⁰⁾=<α>²/<ρκ>, where <> is a temperature average. Also we reveal that the Z_gen⁽⁰⁾ is a good rank-preserving-parameter for efficiency prediction even for the segmented devices.