Humidifying solar collector for improving the performance of direct solar desalination systems: A theoretical approach

In this paper, a new type of solar collector named humidifying solar collector, that is, a solar collector with air humidification function, is proposed. The particularity of this system compared to previous systems in the literature is that the quantity of liquid water present in the collector at each instant is equal to the quantity that will evaporate within the following unit time, no longer greater. This minimizes the quantity of liquid water present in the collector at each instant, and consequently allows to reach desired evaporation temperatures in shorter times and even under low solar irradiances, and minimizes the thermal resistance between evaporation surface and absorber caused by water depth. Then, a theoretical and comparative study by simulation using Engineering Equation Solver software between the performance of the humidifying solar collector-based solar still (proposed system) and that of the solar air heater-based humidification dehumidification solar desalination system (conventional system) is conducted. The performance parameters assessed are energy and exergy efficiencies, dry air mass flow rate required and the maximum water mass flow rate that can evaporate in that air. The results reveal that, in general case, the proposed system is fundamentally more efficient than the conventional system. For instance, for the sizes and heat transfer parameters chosen in this study, the performance of the proposed system is 1.3–32.2 times higher than that of the conventional system; its freshwater productivity under incident solar irradiance of 900 W/m2 can reach 2.923 kg/h, and can be further improved by optimizing the design of the humidifying solar collector.