A sodium boiler and phase-change energy storage system

A novel concentrating solar thermal power system is described, in which a tubular sodium boiler receiver is coupled to a latent heat salt storage system using NaCl. The isothermal liquid-gas phase change of sodium is matched to the isothermal solid-liquid phase change of NaCl, at an appropriate temperature (around 800°C) for a range of industrial process applications, as well as power generation using the Stirling engine. A storage configuration based on direct contact between the sodium and NaCl in shallow trays within a tank is proposed. In the first part of this paper, a detailed dynamic model of the storage system, coupled to simplified models of the receiver and power cycle, was developed to simulate performance over a year. For a case study based on the PS10 heliostat field, a 5.9 MWe system with annual capacity factor of 59.9% was determined to have a levelised cost of energy of 0.23 USD/kWh, using cost assumptions largely based on the 2017 System Advisor Model. Importantly, storage costs, including salt containment in the tank and trays, were a small fraction (8.7%) of total plant capital costs, an indication of the potential feasibility of this form of storage. A key technical challenge for a sodium boiler operating on sun is expected to be boiling stability. Evaporation of the microlayer, the thin layer of fluid formed below a growing bubble, is primarily responsible for bubble growth. The second part of this paper describes initial work to develop fundamental understanding of the causes of boiling instability. Results of modelling indicate the strong dependence of the microlayer heat transfer on the thermophysical properties of the liquid, which may significantly affect bubble characteristics and influence stability.