Performance of molten sodium vs. molten salts in a packed bed thermal energy storage

Concentrating solar power plants are currently working with Solar Salt and conventional Rankine steam power cycles with upper temperatures of 565 °C. To achieve higher efficiencies, advanced power cycles are currently investigated (500-700 °C). As heat transfer fluids, both molten sodium and three types of molten salt are considered in this study. For power tower plants, the heat transfer fluid is typically also the storage medium. This is the case for state-of-the-art commercial plants using molten salt, and past and present pilot plants using sodium. However, this work shows for both cases that a packed bed arrangement, where the heat transfer fluid is replaced by a filler material, may be a technically feasible and economically viable alternative. Furthermore, for sodium there are additional safety concerns related to having a large sodium inventory, which the packed bed arrangement can help alleviate. In this study, a 40 MWhth storage system with quartzite as filler material is numerically investigated with a one-dimensional model. The results are evaluated in terms of discharge efficiency, pumping power, storage cost and thermocline degradation during standby to assess the potential of this storage solution for future scientific investigations. The packed bed system with sodium shows slightly higher discharge efficiencies (96.8%) than with molten salt (95.2-95.7%) and also lower required pumping power. However, the thermocline region expands faster during standby due to the high thermal conductivity of sodium. The influence of porosity, tank diameter-to-height ratio and filler particle diameter is analysed in a parametric study. Highest discharge efficiencies are achieved for both sodium and molten salts with small tank diameter-to-height ratios and small filler particles. For sodium, low porosities are preferable, while for molten salts, high porosities lead to better discharge efficiencies.