Experimental Innovation and Performance Assessment of a Solar HDD System

The solar humidification dehumidification desalination (HDD) systems described in the literature have four main components, namely, the air heater, the water heater, the humidifier, and the dehumidifier. The heating of air and water are carried out in thermal solar collectors. These components physically occupy a considerable amount of space and the performances of the HDD systems still possess room for improvement. In the present work a novel humidification mechanism is developed, in which the processes of water and air heating and humidifying are contained in the same unit, giving opportunity to designing smaller sized HDD systems. The unit is essentially a solar collector filled with water. Air is driven in the form of bubbles while some water vapor mixes with it. By the time bubbles reach the outlet of the unit at the top, it is possible to have almost 100% relative humidity. The solar unit was designed and constructed using the experience gained from a preliminary experiment conducted in the laboratory. In this experiment air was charged through a sparger that was immersed in water and located at the bottom of the humidifier. The experiment was performed under different configurations. Water temperature, air mass flow rate, water level, and the pitch distance between the holes in the spargers are the variables under which the humidification unit was tested. Higher water temperature, higher air mass flow rate, and higher water level resulted in the highest performance when the pitch distance was 10 mm. In an attempt to increase the performance of the unit, intermediate bubble regeneration stages were added. As a result, significant increase in the humidification effectiveness of the unit was observed. The number of bubble regeneration stages for obtaining the maximum humidification effectiveness at any temperature and air flow rate was found to be eight. The effectiveness of the humidification process at a water level of 40cm with 8 bubble-regeneration stages was 100%. Experiments of the solar unit were conducted under the weather conditions of North Cyprus. Outlet air approaches saturation and its temperature is found to reach the hot water temperature in the collector (thus increasing the vapor carrying capacity). Moreover, the effectiveness of the humidification process is found to be maximized. The system is capable of producing 15kg/m2 of fresh water per day. The normalized production of the system is compared with different pilot and commercial scale solar systems. It is found that the present design has a superior performance. Keywords: Solar Desalination, HDD, Bubble regeneration, Heat and Mass Transfer, GOR