Investigation of water evaporation for closed sorption storage systems

Thermochemical storage systems, like closed adsorption systems, are promising and proven technologies that enable to cover the heat demands over the whole year with solar thermal energy. However, further improvements on the material and the components are necessary to increase the performance and lower the system cost. One of the important components is the evaporator, which is needed during discharging the storage. For this purpose, measurements on a falling film evaporator with water at 20 and 23 mbar have been conducted on a copper tube bundle (3x8 tubes). The measurements show that the heat transfer coefficient for evaporation strongly depends on the irrigation density and the overheating temperature. An increase of the temperature difference with more than 2 K leads not to further improvements. In addition best results are achieved with an irrigation Reynolds number ReΓ ˙ of 40. Overall the measured correlations are in good agreement with other measurements.