Quantitative comparison of heat flow, guarded-heater and AC Harman methods for thermoelectric module efficiency

TL;DR

This study benchmarks three thermoelectric efficiency measurement methods, revealing their relative accuracy, limitations, and the importance of accounting for heat losses to standardize module-level thermoelectric testing.

Contribution

It provides a quantitative comparison of heat flow, guarded heater, and AC Harman methods, highlighting the limitations of the AC Harman technique and proposing correction strategies.

Findings

Heat flow and guarded heater methods agree within uncertainty for temperature differences up to 70 K.

AC Harman method underestimates efficiency by about 30%.

Boundary effects and radiative heat dissipation cause the underestimation in the Harman method.

Abstract

The evaluation of thermoelectric conversion efficiency remains challenging owing to the lack of internationally standardized measurement protocols. Commonly used techniques -- including the heat flow, guarded heater, and AC Harman methods -- differ fundamentally in their operating principles and sensitivity to heat losses. In this study, we benchmark three module-level efficiency measurement techniques -- the heat-flow, guarded heater, and AC Harman methods -- using commercial Bi$_2$Te$_3$-based modules with different substrates materials. The conversion efficiencies obtained using the heat flow and guarded heater methods showed good agreement within experimental uncertainty for temperature differences up to 70 K. In contrast, the AC Harman method underestimated the conversion efficiency by approximately 30 %. Through systematic measurements on modules with different substrates and detailed finite element simulations, this underestimation was attributed to boundary-condition effects and radiative heat dissipation, which significantly reduce the effective temperature difference developed across the module in the Harman configuration. These results highlight the limitations of the AC Harman method for quantitative conversion-efficiency evaluation under non-ideal thermal environments and emphasize the necessity of accounting for radiative and substrate-related heat losses. Nevertheless, with appropriate modeling and correction, strategies, the AC Harman method remains a viable tool for rapid performance screening. Our results provide a quantitative benchmark of major measurement techniques and contribute to clarify best practices for module-level thermoelectric metrology and guide method selection, fully supporting future efforts toward methodological standardization.