Heat flow measurement as a key to standardization of thermoelectric generator module metrology: A comparison of reference and absolute techniques

Abstract

Traceable and standardized metrology for thermoelectric generator modules (TEM) is a mandatory element for an industrialization of thermoelectric applications. High measurement deviations > 20% have been observed in inter-laboratory tests on TEM properties. Such uncertainties are too high for scientific studies and seem insufficient to validate industrial benchmarks. Particularly, works on high-temperature TEM have to be supported by appropriate characterization techniques. This shall accelerate progress towards product releases for thermoelectric energy conversion.In order to overcome the deficits this article suggests a generic procedure for TEM characterization. Such guideline has not been established or purposed so far and there may be other metrological approaches to conclude on performance characteristics of TEMs. The described procedure does not constitute per se a guarantee for lower uncertainties compared to alternative approaches. The objective of this work is to describe the underlying metrological procedures in detail and to link them to analytical descriptions for the derivation of the TEM properties. The procedure and given analytic expressions for particular measurands cover all functional TEM properties needed for a meaningful benchmarking.The definition of procedures and analytical expressions for the determination of measurands enable the determination of uncertainties. This will be given in this work for the heat flow measurement, which is a mandatory part for TEM characterization and which poses certain metrological difficulties particularly at higher temperatures. Uncertainty of heat flow determination by means of a reference principle and an absolute guarded hot plate (GHP) technique will be deduced. Based on a standard reference material for the thermal conductivity full uncertainty budgets are derived for both methods between 373 K and 1023 K. Best estimates of heat flow show deviations lower than3% compared to a comparative FEM simulation. However, significant differences are found for the underlying uncertainty of heat flow. The combined uncertainty of the reference principle lies between 10% and 13%, whereas a significantly lower uncertainty between 0.2% and 0.75% is observed for the GHP-based absolute method for heat flow determination.