Experimental and numerical analysis of a PCM-supported humidification-dehumidification solar desalination system

This work discusses the results of experimental and theoretical studies on a humidification-dehumidification (HDH) water desalination system equipped with phase change material (PCM) as a packing media in all its main components. The PCM-supported HDH system consists of an evaporator and condenser comprised by two cylindrical direct-contact packed beds filled with spherical PCM packing elements. Moreover, a solar collector and an external PCM thermal storage are used to drive the HDH plant. The external PCM thermal storage is used to guarantee continuous operation of the plant day and night round the clock under the transient behaviour of solar irradiation. The objective of using PCM elements in the evaporator and condenser was for heat storage as a back up during cloudy hours or for part-time night operation. During analysis of steady state conditions, it was discovered that the PCM packing media seem to enhance their thermal performance through locally establishing multiple-effects of heating/humidification (MEHH) and cooling/dehumidification (MECD) while air passing through the successive packing layers in the evaporator and condenser respectively. The multiple-effect phenomena are attributed solely to existence of conductive packing media, which act as heat and mass exchangers. Thus, the focus of the study lies on the thermal conductivity rather than the thermal capacity or solid-liquid phase change processes of the packing in the two columns. The major aim of this study is to examine systematically the effect of MEHH and MECD at enhanced thermal conductivity of the filling material and to determine the technical and economic feasibility of applying these concepts in HDH desalination plants under steady state operation conditions. Transient simulation models for the components in the HDH system have been established and validated against experimental measurements. Using both experiment and simulation, a detailed heat and mass transfer analysis for the performance of the evaporator and condenser over a wide range of operation conditions under steady state has been performed using different types of packing materials. Furthermore, a yearly parametric analysis for the whole HDH plant has been performed under real weather conditions for two locations in Egypt. The overall performance analysis focuses on the optimum operation conditions of the HDH system, with optimum conductive filling material, with and without external PCM thermal buffer.