Thermal Energy Storage in Molten Salts: Overview of Novel Concepts and the DLR Test Facility (TESIS)

Kurzfassung

At present, two-tank molten salt storage systems are the established commercially available concept for solar thermal power plants. Due to their very low vapour pressure and comparatively high thermal stability, molten salts are preferred as the heat transfer fluid and storage medium. Therefore, the development of alternative, more cost-effective concepts is an important step in making thermal energy storage more competitive for industrial processes and solar thermal applications. The paper gives an overview of various storage concepts such as thermocline, floating barrier or embedded heat exchanger that have been developed in recent years. In this context, a description of functionality, a summary of the technical specification and the state of development of each concept is given. The latter includes the next steps and open questions, respectively. Additionally, further research topics, such as the stability of material selections or the functionality of necessary components are presented. In order to answer many of the open questions, a new molten salt test facility called “Test facility for thermal energy storage in molten salts (TESIS)” is under construction at DLR. This facility has two separate sub-systems, which can be operated independently from each other. One system is designed to investigate new single tank storage concepts focusing on the thermocline-filler concept. The other system is a flexible test section for molten salt process technology and component tests, such as valves, solar receiver tubes and instrumentation. Both sub-systems can operate at temperatures up to 560 °C, using nitrate salt as the storage and heat transfer media. Experimental results on the compatibility of filler materials and molten salt will be presented. In addition, the current construction and development status of the facility will be reported. Finally, an overview of current research topics and an outlook related to the test facility is presented.