Experimental Results of a Large Scale Single Tank Thermocline

Abstract

For large scale high temperature thermal energy storage, two-tank molten salt systems mark the current state-of-the-art. New fields of application are emerging, for example, industrial furnaces can shift their energy consumption or conventional power plants could reach lower part load levels. In volatile electricity markets, like in Germany, this increased flexibility enables significant cost reduction potentials. The two-tank concept comprises potential for further cost reductions. A first step is storing the hot and cold phase inside a single tank. The separation of both phases is achieved by their different density or by a floating barrier. Gaining operational experience and deriving optimized operation strategies, promise additional cost reduction potentials. A final step is the thermocline with filler concept, where a large fraction of the salt can be substituted by a low-cost solid material. The “test facility for thermal energy storage in molten salts” (TESIS:store) has been set up at DLR in Cologne. The facility operates at temperatures up to 560°C and a mass flow up to 4kg/s. The storage volume is 22m³. The plant allows the investigation of the thermocline concept with and without filler and gaining widespread operation experience. In this work first experimental results of the thermocline concept without filler are presented. As HTF, solarsalt at maximum temperature is used for several consecutive charging and discharging cycles. 150 thermocouples measure the temperature distribution along the middle axis and additionally at four planes in two radial directions each. The experiments help to understand and improve the molten salt distribution at the inlet regions as well as the development of the thermocline zone. Interest lies in achieving a stable thermocline zone where energetically adverse mixing should be minimized. The development of the thermocline zone and its movement speed through the storage volume are compared to a 1D numerical model.