A new scientific publication led by researchers at CIEMAT – Plataforma Solar de Almería, within the framework of the GREENOLIVE project, has been released in the journal Energy Conversion and Management. The paper, titled “Dielectric characterisation of solar salt for volumetric heating applications in Power‑to‑Heat‑to‑Power systems”, presents the first study to systematically investigate the behaviour of solar salt under microwave radiation, paving the way for more efficient and controllable charging of thermal energy storage systems.
Solar salt, a mixture of sodium nitrate and potassium nitrate (60% NaNO₃, 40% KNO₃), is the most widely used storage medium in concentrated solar power plants, accounting for approximately 99.8% of installed thermal energy storage (TES) capacity worldwide. It is typically heated by electric resistance heaters. However, the low thermal conductivity of the salt leads to temperature gradients, local overheating and material degradation – problems that become more severe as the system scales up.
The study, carried out by CIEMAT in collaboration with the Karlsruhe Institute of Technology (Germany), Universitat Politècnica de València, University of Nottingham (UK), and CNRS – Université d’Orléans (France), explores microwave‑based heating as an alternative. Microwaves deposit energy directly inside the material, potentially offering more uniform heating and faster response times compared to conventional surface‑based methods.
The researchers measured the dielectric properties of solar salt at two microwave frequencies (912 MHz and 2.45 GHz) using the cavity perturbation method, with varying sample volumes and electromagnetic field configurations. In the molten state, however, the sharp rise in electrical conductivity leads to high dielectric losses, partially violating the small-perturbation assumption and introducing uncertainty in the results.
To overcome this, microwave measurements were complemented by four-electrode electrochemical impedance spectroscopy (100 Hz – 1 MHz, up to 550°C), confirming a transition from capacitive behaviour in the solid state to ionic conduction in the liquid phase.
The activation energy for ionic transport decreases markedly upon melting, from 0.810 eV to 0.148 eV. In the molten state, conductivities of ~160–170 Sm⁻¹ correspond to shallow microwave penetration depths (~1.3 mm at 912 MHz and ~0.8 mm at 2.45 GHz), indicating predominantly surface heating. These results highlight the need for measurement systems specifically adapted to highly conductive liquids for accurate dielectric characterisation.
These findings provide a quantitative basis for the design of microwave‑based heating systems for Carnot batteries (Power‑to‑Heat‑to‑Power systems). While the short penetration depths indicate that effective heating will rely on tailored electromagnetic field distributions and salt‑compatible ceramic materials, the work opens a pathway towards volumetric heating strategies that could mitigate the limitations of conventional resistive heating.
The publication contributes to GREENOLIVE’s broader goal of demonstrating scalable, cost‑effective solutions for industrial decarbonisation in the Mediterranean region.
Read the full article: https://doi.org/10.1016/j.enconman.2026.121205
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