Assessment of thermal energy storage options in a sodium-based CSP plant

Sodium has proven to be an efficient heat transport fluid in a central receiver system in the IEA-SSPS facility in Almeria in the 1980s with a 5 MWhth direct two tank storage system. In recent years the interest in liquid metals, particularly sodium, as heat transfer fluids for concentrating solar power has reawakened. However, an assessment of thermal energy storage options in a liquid metal-based concentrating solar power system has not been performed yet. In this paper sensible, latent and thermochemical systems, described in the literature and potentially suitable for a solar power plant using sodium, are investigated. As sensible systems, direct sodium two tank and one tank thermocline storage systems with filler are considered, as well as indirect molten salt systems. Latent systems include configurations with finned tubes, packed beds of phase change material capsules and active screw type systems. In addition, metal hydride dehydrogenation, ammonia dissociation and hydroxide dehydration are considered as thermochemical storage systems. The presented storage systems are discussed and compared on the basis of the following criteria: Storage medium cost, storage density, cycling behaviour, maturity level and suitability for sodium as heat transfer fluid. As a result, the direct sodium thermocline one tank storage with filler material represents a promising direct storage option. It could be further enhanced by a cascaded arrangement of phase change capsules. Moreover, the thermochemical storage systems based on ammonia dissociation or hydroxide dehydration are identified as best indirect storage options.