Assessment of different PCM storage configurations in a 50 MWel CSP plant with screw heat exchangers in a combined sensible and latent storage - simulation results

Effective and economic storage solutions have an important role to enable Solar Thermal Electricity plants (STE) to generate dispatchable power. A thermal storage can increase the value of STE by providing firm electricity in times of peak load or ancillary services. Latent storages are suited optimally for direct steam generating STE systems: due to the isothermal behavior of the phase change materials (PCM) during melting /crystallization, exergy losses during storage charging/discharging are minimized. Up to now latent heat storages are, due to technical and economic issues, not used commercially in STE systems. The PCM have a very low heat conductivity - therefore a sophisticated heat exchanger design is needed to build such storages with sufficient charging/discharging power. An innovative storage concept, uses a screw heat exchanger to transport the PCM from a cold to a hot tank or vice versa during phase change. This concept separates heat exchange and storage tanks. Existing concepts do not have this ability. In annual yield calculations the use of this heat exchanger as part of a thermal storage in a 50 MWel direct steam generating solar thermal power plant has been assessed. After setting up of a cost structure, the respective levelised cost of electricity (LCOE) have been used as comparison criterion for two storage systems with different designs. It can be shown, that a storage system with three storage tanks yields lower LCOE than one with two storage tanks. There is further potential for cost reductions, such as the optimization of the heat exchanger area and the temperature difference between the heat transfer fluid and the PCM.