Towards a Comprehensive Model for Characterising and Assessing Thermoelectric Modules by Impedance Spectroscopy

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

Highly‑efficient sustainable ionic thermoelectric materials using lignin‑derived hydrogels

The efficient and economical conversion of low-grade waste heat into electricity has promising potential to combat the greenhouse effect and expedite the shift towards sustainable development. This study presents an innovative and appealing approach through the utilization of lignin, an abundant waste product derived from the paper and pulp industry, to develop hydrogels as compelling and sustainable materials for application in ionic thermoelectricity. Various compositions were evaluated to examine the impacts of varying lignin concentrations, types of electrolytes, concentrations of crosslinkers, and electrolyte concentrations on the ionic thermoelectric performance of the hydrogels. The optimized lignin-derived hydrogel, infiltrated with a 6 M KOH electrolyte, demonstrates high ionic conductivity (226.5 mS/cm) and a superior Seebeck coefficient of 13 mV/K. This results in a remarkable power factor (3831 µW/m·K2 ) that leads to an impressive Figure of merit (ZTi ) (3.75), surpassing most of the existing state-of-the-art materials and making it the most efficient sustainable ionic thermoelectric material reported until now. These findings underscore the exceptional performance of lignin-based hydrogels in the realm of low-grade waste energy harvesting applications. The present study contributes to address the challenges posed by waste heat through effectively harnessing low-grade waste heat through the utilization of sustainable lignin-based hydrogels while reducing the reliance on fossil fuels and minimizing greenhouse gas emissions.</p