Abstract
Thermoelectric devices are widely used as solid-state refrigerators and have potential energy generation applications. Their characterization is key to develop more efficient devices and monitor their performance. Electrical impedance spectroscopy has been proved to be a useful method for the characterization of thermoelectric modules. However, deviations from current impedance models still exist in experimental results, especially in the high frequency part of the impedance spectrum, which limits its use. Here, we present a new comprehensive impedance model (equivalent circuit) which covers all the key phenomena that affects the module performance, and it is able to explain the observed deviations. The new equivalent circuit includes, as new additions, the thermal influence of the metallic strips (electrodes), combined with the thermal contact resistance between the metallic strips and the outer ceramic layer. Moreover, a new more accurate spreading-constriction impedance element, which considers the variation of the heat flow in the radial direction at the outer ceramic surfaces, is also developed. The comprehensive equivalent circuit was used to perform fittings to impedance spectroscopy measurements of modules fabricated by different manufacturers. From the fittings, it was possible to identify, among other key properties, the internal thermal contact resistances, whose direct determination is very challenging. Thermal contact resistivities at the metallic strips/thermoelectric elements interface in the range 2.20 × 10-6-1.26 × 10-5 m2KW−1 were found. An excellent thermal contact was identified at the metallic strips/ceramic layers. This opens up the possibility of using impedance spectroscopy as a powerful tool to evaluate, monitor, and identify issues in thermoelectric devices.
Highlights
Thermoelectric (TE) devices can directly convert heat into electricity and vice versa
A new more accurate spreading-constriction impedance element, which considers the variation of the heat flow in the radial direction at the outer ceramic surfaces, is developed
A more comprehensive equivalent circuit included the radiation effect [18] in conjunction with the spreading-constriction and the convection phe nomena [13]. It has been included recently the effect of the metallic strips, assuming they behave like a capacitor, and the presence of a thermal contact resistance between the thermoele ments and the electrodes [19]. Apart from these models under sus pended conditions, we have recently reported an electrical impedance equivalent circuit of a module attached to heat sinks at both sides, which takes into account the effect of a thermal contact resistance between the outer ceramic surfaces and the heat sinks [20]
Summary
Thermoelectric (TE) devices can directly convert heat into electricity and vice versa. It has been included recently the effect of the metallic strips (electrodes), assuming they behave like a capacitor, and the presence of a thermal contact resistance between the thermoele ments and the electrodes [19] Apart from these models under sus pended conditions, we have recently reported an electrical impedance equivalent circuit of a module attached to heat sinks at both sides, which takes into account the effect of a thermal contact resistance between the outer ceramic surfaces and the heat sinks [20]. This new equivalent circuit, which includes convection and radiation effects previously developed, and the inductive phenomena existing at the highest frequencies [21], allows the analysis of the different characteristic features experimentally observed, which was not possible before with the previous models. The fact that impedance spectroscopy can identify and quantify all these phenomena, in addition to its capability to accurately measure the module zT from a single and simple measurement, opens up the possibility of using this method as a standard quality control tool, able to identify and monitor in detail issues in TE modules in energy applications
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