Design of a Helical Coil Dehumidifier for a Novel Gravity-driven Solar Distillation Unit

Abstract

Solar distillation is a widely researched option for producing potable water in water-starved regions of the globe. Despite its technical simplicity and easiness-to-use, its efficiency has not sufficiently improved to promote its widespread use. The reasons for relative inefficiency of solar distillation are poor energy harvest at solar collectors, poor utilization of the harvested energy for evaporation of the water and suboptimum condensation of the humidified air in the condenser. A novel concept of solar still attempts to optimize the humidification-dehumidification cycle of a solar distillation unit to address these issues related to solar distillation units. The system works in a closed air open water humidification-dehumidification cycle. The energy source is solar irradiation, which heats up an air-water mixture. The driving force in the system is a small suction pressure induced in the system due to an aerator attached to the water inlet. A helical coil dehumidifier achieves efficient dehumidification in the system. The current paper describes experiments conducted to improve the efficiency of the dehumidifier attached to the novel solar distillation unit by optimizing the design parameters. The dehumidifier has the shape of a helical coil heat exchanger, which is cooled by the relatively cold water from a reservoir, which conversely, is heated by the heat of condensation. Even though there are sufficient studies on the design parameters of general helical coil condensers in literature, few studies are done to study their use in a humidification-dehumidification cycle involving non-condensable air. Further, a detailed parameter study of the helical coil dehumidifier is not yet done. The current paper discusses the various design parameters of a helical coil dehumidifier working in conjunction with a solar distillation unit as described above. Different design parameters of a helical coil dehumidifier discussed in this paper are pitch and diameter of the helical dehumidifier. Design insights for helical coil dehumidifier are thus obtained. The study reveals that the overall efficiency of a solar humidification-dehumidification cycle of a solar distillation unit could be considerably improved by optimizing the design of the helical coil dehumidifier used therein.