Electrical properties of phase change materials for electrical impedance-based sensing of the liquid fraction

Abstract

Detailed real-time monitoring of solid-liquid phase change processes can provide invaluable insights for the optimal design and operation of latent thermal energy storages (LTES) with phase change materials (PCMs). Among possible monitoring approaches for solid-liquid phase change, electrical impedance-based sensing is advantageous in terms of accuracy and ease of implementation in real-world LTES. However, the electrical behavior of most PCMs has not yet been characterized in detail, which hinders the development of electrical impedance-based sensing technologies. In this study, a wide range of organic PCMs and sodium acetate trihydrate (SAT) are characterized via electrical impedance spectroscopy between 4 Hz and 100 kHz. The electrical behavior of organic PCMs is classified as non-polar and polar. Non-polar paraffin-based PCMs behave like dielectrics with very little loss and have a relative permittivity between 3 % to 8 % higher in solid phase than in liquid phase. For the tested frequency range, polar organic PCMs in the liquid phase behave like dielectrics with a moderate conductive loss, while in the solid phase the occurrence of a relaxation process accompanied by conductive loss is observed. For polar PCMs, the high-frequency relative permittivity of the liquid phase is between 1.21 and 2.71 times larger than in the solid phase. This moderate contrast in permittivity could be used to monitor the liquid fraction via electrical impedance measurements. However, the use of large electrodes with a small space in between them may be necessary to keep the impedance inside adequate measuring ranges. On the other hand, SAT behaves as an electrolyte and the impedance is mostly resistive at high frequency. SAT has an innate contrast in ionic conductivity of almost two orders of magnitude between the liquid and solid phase, which can be exploited for phase detection. The significantly lower electrical resistivity of SAT, as compared to organic PCMs, could allow for the implementation of smaller electrodes, which would favor electrical impedance tomography approaches to determine the phase distribution with good spatial resolution. • Organic and inorganic PCMs are characterized via electrical impedance spectroscopy. • Paraffin-based non-polar organic PCMs behave like dielectrics with minor contrast in permittivity (∼0.94). • Polar organic PCMs display moderate contrast in permittivity between liquid and solid phase (1.20–2.71). • Polar organic PCMs present some conductive loss, which is more noticeable in liquid phase. • Sodium acetate trihydrate behaves like an electrolyte with a 100× contrast in conductivity.

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