Experimental characterization and simulation of a hybrid sensible-latent heat storage

Versatile and economically competitive thermal energy storages are necessary to fulfill the widely differing requirements for storages applied in renewable energy systems, process heat, district heating, power generation and domestic heating. We present the concept of a hybrid sensible-latent heat storage based on an adapted commercial shell-and-tube heat exchanger. The phase change material (PCM) is encapsulated within the tubes and thermal oil serves as sensible heat storage as well as the heat transfer medium. We designed and built a prototype using high density polyethylene (HDPE) as PCM and characterized the storage on a dedicated test rig at AIT. Energy capacities and power profiles are presented for different mass flows and (dis)charging temperatures. Two physical models were developed and implemented using the Modelica language. Dymola was used to simulate the behavior of the prototype storage. Very good agreement was achieved between simulation and experiment. Using the models, we studied the heat transfer within the storage in detail, which enabled us to present how to adapt the storage geometry and PCM properties to cover a broad range of applications. We discuss storage costs and calculate material costs per stored kilowatt-hour for different PCM-thermal oil volume ratios as a function of the tube outer diameter. Finally, we highlight the main advantages and design freedoms of our concept and describe concrete application scenarios in district heating and process heat.