Solar energy storage by a two grade phase change material

A two grade phase change material (PCM) based on CaCl_2-water system has been investigated in a PCM--water heat exchanger. The crystallization of different hydrates of CaCl_2 extended the temperature range of heat storage and the storage stability was achieved by the application of wood chips as thickening agent. A short section of the heat exchanger tube (a few cm) can be characterized by a maximum curve of heat transfer coefficient in function of cooling time. Longer sections (a few meter) showed nearly constant heat transfer coefficient in the range of 115 +/- 25 W/m2K. The heat storage system is to be applied to store solar energy and the stored heat is used to preheat the water input of domestic hot water supply system