Experimental Investigation of Water Evaporation for Closed Adsorption Storage Systems
Abstract
Closed adsorption for thermal energy storage is a promising technology, which was recently proven on real scale. Nevertheless further improvements of its material and components are necessary to enhance the system performance and to make it economically viable. One of the issues is the evaporation of water at low pressure during discharging (“adsorption”) of the storage. The current lack of accurate experimental data and correlations for the heat transfer coefficient makes the design of the evaporator difficult. Therefore the goal of the work is to gather deeper knowledge in order to design the evaporator more properly and thus improve the sorption storage efficiency. In this work a test rig was designed and constructed for the experimental investigations of water evaporation at sub-atmospheric pressure. For the commissioning, first experiments with falling film evaporation were conducted on a copper tube bundle (3 x 8 tubes) to determine the relation of the outer heat transfer coefficient to different parameters. In all experiments the observed flow patterns between the tubes consist of columns. Varying the saturation pressure from 20 to 23 mbar or the range of the ̇ 29 to 109 shows no effects. Apart from that, the measured results were compared to values from two correlations. It is concluded that the Nu numbers from the measurements were in good agreement with those from the Lorenz and Yung model.
Keywords
Closed adsorption thermal storage, Falling film evaporation, Water