Microwave Power-to-Heat for Solar Salt: Multiphysics Analysis and Design Constraints

Thermal energy storage using suitable materials is a strategic solution for integrating renewable energy and decarbonising industrial processes. Current Power-to-Heat systems using solar salt rely on electric heaters; however, the low thermal conductivity of molten solar salt promotes localised hot spots, leading to material degradation and reduced performance. Microwave heating is a promising alternative due to its volumetric heating capability and compatibility with renewable electricity. Nevertheless, dielectric characterisation shows that molten solar salt behaves as a highly conductive ionic medium with significant dielectric losses, limiting microwave penetration and resulting in predominantly surface-localised heating. To investigate this limitation, two cavity configurations were analysed using multiphysics simulations and parametric design studies: a single-mode elliptical cavity operating at 915 MHz with an iris, and a quasi-cylindrical multimode cavity operating at 2.45 GHz for scalable applications. The coupled electromagnetic, fluid-flow, and thermal behaviour was evaluated through the resulting field distributions and heating patterns. Complementary experiments assessed microwave-transparent container materials and determined the emissivity of molten solar salt from thermographic measurements, highlighting key engineering considerations for integrating microwave heating into next-generation Power-to-Heat technologies. The results demonstrate that microwave heating of highly conductive molten solar salt is fundamentally constrained by the limited electromagnetic penetration depth, defining practical design limits for its integration into next-generation Power-to-Heat systems.