For large scale thermal energy storage at temperatures above 300°C, two-tank molten salt systems mark the current state-of-the-art as they are proven technology in parabolic trough and tower solar thermal power plants. Research is focusing on the utilization of molten salts not only as storage medium but also as heat transferring fluid (HTF) in parabolic trough plants [1]. The current two-tank concept offers serveral cost reduction possibilities. Firstly, instead of storing the hot and cold phase in two separate tanks, the salt could be stored inside a single tank to avoid a large gas volume. The separation of both phases can either be achieved by a floating insulated barrier or simply by the different densities of both phases. Secondly, a high share of the total investment costs of a molten salt storage system is caused by the molten salt itself. For the two-tank system in 50 MWel power plants, this can be as high as half of the total TES costs [2]. In the thermocline with filler concept, a large fraction of the molten salt can be substituted by a cost effective solid material, offering a significant potential for further cost reductions [3]. Finally, gaining operational experience of such systems and the ability to derive optimized operation strategies, promise an additional cost reduction potential. The “test facility for thermal energy storage in molten salts” (TESIS:store) has been set up at DLR in Cologne, Germany. An outside view of the plant can be seen in Fig. 1. The facility operates at temperatures up to 560 °C and a maximum molten salt mass flow of 4 kg/s. The storage volume has a length of 5.4 m with a total tank volume of 22 m³. The plant allows the investigation of the thermocline concept with and without filler and gaining widespread operation experience. Heat tracing along the containment walls and the piping ensures adiabatic conditions.