Wiedemann-Franz Law and Thermoelectric Inequalities: Effective ZT and Single-leg Efficiency Overestimation

TL;DR

This paper derives a thermoelectric inequality linking material and module ZT, revealing that single-leg efficiencies are significantly overestimated due to circuit losses, emphasizing the importance of module design and material compatibility.

Contribution

It introduces a thermoelectric inequality connecting material ZT and effective ZT, clarifies efficiency overestimation in single-leg modules, and highlights the importance of structural optimization and material compatibility.

Findings

Effective ZT in P-N modules lies between individual ZTs.

Single-leg module effective ZT is less than one-third of the material ZT.

System efficiency remains below half of the ideal material efficiency.

Abstract

We derive a thermoelectric inequality in thermoelectric conversion between the material figure of merit (ZT) and the module effective ZT using the Constant Seebeck-coefficient Approximation combining with the Wiedemann-Franz law. In a P-N leg-pair module, the effective ZT lies between the individual ZT values of the P- and N-legs. In a single-leg module, however, the effective ZT is less than approximately one-third of the leg's ZT. This reduction results from the need for an external wire to complete the circuit, introducing additional thermal and electrical losses. Multi-dimensional numerical analysis shows that, although structural optimization can mitigate these losses, the system efficiency remains limited to below half of the ideal single-leg material efficiency. Our findings explain the single-leg efficiency overestimation and highlight the importance of optimizing the P-N leg-pair module structure. They also underscore the need for thermoelectric leg-compatibility, particularly with respect to Seebeck coefficients.