Towards a comprehensive model for characterising and assessing thermoelectric modules by impedance spectroscopy

Abstract

Thermoelectric devices have potential energy conversion applications ranging from space exploration through to mass-market products. Standardised, accurate and repeatable high-throughput measurement of their properties is a key enabling technology. Impedance spectroscopy has shown promise as a tool to parametrically characterise thermoelectric modules with one simple measurement. However, previously published models which attempt to characterise fundamental properties of a thermoelectric module have been found to rely on heavily simplified assumptions, leaving its validity in question. In this paper a new comprehensive impedance model is mathematically developed. The new model integrates all relevant transport phenomena: thermal convection, radiation, and spreading-constriction at junction interfaces. Additionally, non-adiabatic internal surface boundary conditions are introduced for the first time. These phenomena were found to significantly alter the low and high frequency response of Nyquist spectra, showing their necessity for accurate characterisation. To validate the model, impedance spectra of a commercial thermoelectric module was experimentally measured using a new and parametrically fitted. Technique precision is investigated using a Monte-Carlo residual resampling approach. A complete characterisation of all key thermoelectric properties as a function of temperature is validated with material property data provided by the module manufacturer. Additionally, by firstly characterising the module in vacuum, the ability to characterise a heat transfer coefficient for free and forced convection is demonstrated. The model developed in this study is therefore a critical enabler to potentially allow impedance spectroscopy to characterise and monitor manufacturing and operational defects in practical thermoelectric modules across multiple sectors, as well as promote new sensor technologies.

Experimental conditions required for accurate measurements of electrical resistivity, thermal conductivity, and dimensionless figure of merit (ZT) using Harman and impedance spectroscopy methods

“…6 Impedance spectroscopy has recently attracted attention as a suitable alternative method for the characterization of TE materials and devices. [8][9][10][11][12][13] This technique, in contrast to the Harman method, measures a wide range of frequencies and not only ac and dc conditions. However, it has similarities with the Harman method, in the sense that ZT can be directly obtained from the high frequency and low frequency limits of the impedance spectrum.…”

“…The impedance method also requires similar experimental conditions to determine ZT accurately, and can be affected by the Joule effect, 12 and convection and radiation losses as well. 13,14 Here, we have evaluated the effect on the accuracy of both Harman and impedance methods of several experimental conditions, which have not been clearly identified previously. This investigation has been performed employing a skutterudite sample, whose TE properties were previously determined by established commercial instruments.…”

European Radioisotope Thermoelectric Generators (RTGs) and Radioisotope Heater Units (RHUs) for Space Science and Exploration

“…A number of generations and variations of bismuth telluride based thermoelectric converters or generators (TEGs) have been produced for the RTG system (Mesalam et al 2018Williams et al 2014). Americium-based nuclear power systems have lower power densities and occupy larger volumes for equivalent power outputs than Pu-based systems and offer the option of targeting lower operational temperatures Barco et al 2019a).…”

“…An impedance spectroscopy technique that has been developed at Leicester by Mesalam et al (2018) for the rapid evaluation of modules has been reported in detail in Mesalam et al (2018. This method enables rapid diagnosis and isolation of material or manufacturing effects on module properties and performance.…”

Non-linear impedance spectroscopy for complete thermoelectric characterization: Beyond the zT estimation

“…The current excitation is given by Eq. (15) which is equivalent to the current density given in Eq. (2).…”