Compatibility Testing on a New Ceramic Filler Material for Packed Bed Thermal Energy Storage

Thermal storages can contribute to decarbonization in storing energy of fluctuating renewables and waste heat of industrial processes by decoupling supply and demand. A great opportunity to decrease the cost of high temperature storage is given by single tank thermocline storage systems, where large fractions of cost-intensive high-temperature fluids are replaced by a low-cost filler material. Besides low cost, a suitable filler material must meet various other criteria such as suitable thermal conductivity, high heat capacity, high volumetric density of the packed bed of fillers, sufficient mechanical stability and compatibility with high temperature fluids in the temperature range of the application. For one promising type of ceramic filler, the compatibility with a solar salt mixture of 60 wt% sodium nitrate (NaNO3) and 40 wt% potassium nitrate (KNO3) was investigated. The filler is composed of additives, such as metallic slag and various other low-cost recycled materials, and a phosphatic binder resistant to the high temperatures. For testing, fillers were immersed in the corresponding fluids in a crucible under air atmosphere. The samples were thermally cycled in an oven up to the maximum foreseen temperature (250-550°C for the salt). Overall, the ceramics tested show good compatibility with solar salt and have the potential to significantly reduce the cost of the storage systems.