Boiling sodium and melting salt: Evaluation and optimisation of a novel solar–thermal system concept

The phase-change phenomenon facilitates efficient heat transfer and the storage of a large amount of heat close to an upper-bound temperature limit. In this paper, a novel solar–thermal system configuration is evaluated and optimised, where liquid sodium is boiled in a receiver and energy is stored through the melting and freezing of NaCl salt. Polar and surround field configurations of a 1 MWe sodium-boiler plant were investigated where co-optimisation of solar multiple, field parameters, receiver geometry and storage parameters was performed to obtain lowest possible levelised cost of electricity (LCOE); this compared to a 100 MWe two-tank molten salt plant optimised via the same method. The LCOE values obtained for the 1 MWe sodium-boiler plant was 128.3 USD/MWh for a polar field configuration and 144.9 USD/MWh for a surround field configuration, compared to 107.1 USD/MWh for the 100 MWe two-tank molten salt system. Whilst promising for smaller-scale systems, the analysis identifies limitations and challenges associated with high operating temperatures and low receiver flux limits, which opens the possibilities for the investigation of lower-temperature phase change materials (PCMs) and alternative PCM geometries.