Dielectric characterisation of solar salt for volumetric heating applications in Power-to-Heat-to-Power systems

Carnot batteries, or Power-to-Heat-to-Power systems, rely on solar salt as a thermal energy storage medium and require efficient and controllable heating technologies. However, conventional resistive heating is constrained by the low thermal conductivity of solar salt, leading to temperature gradients, local overheating, and material degradation, which motivates the exploration of alternative volumetric heating approaches. In this context, this study evaluates the feasibility of microwave-based volumetric heating of solar salt by analysing its dielectric behaviour across both solid and molten states. Dielectric properties were measured using the cavity perturbation method at 912 MHz and 2.45 GHz with different sample volumes and electromagnetic field configurations. Under these conditions, the sharp increase in electrical conductivity in the molten state results in high effective dielectric losses that violate the small-perturbation assumption underlying this technique. Consequently, the microwave measurements were complemented by four-electrode electrochemical impedance spectroscopy from 100 Hz to 1 MHz up to 550°C to confirm the dominance of ionic transport mechanisms. ... mehrThe results show activation energies of 0.810 eV in the solid state and 0.148 eV in the liquid state, while extrapolated conductivities of approximately 160–170 S m$^{−1}$ correspond to microwave penetration depths of about 1.3 mm at 912 MHz and 0.8 mm at 2.45 GHz, providing an application-relevant measure of the interaction between molten solar salt and electromagnetic fields. These findings indicate that accurate dielectric characterisation of molten solar salt at microwave frequencies requires measurement systems specifically adapted to highly conductive liquids and suggest that effective microwave heating strategies may rely on solar salt-compatible ceramic materials combined with appropriately tailored electromagnetic field distributions.