A novel solar-powered thermal desalination unit coupled with a reverse osmosis plant to increase overall water recovery

Abstract

The global water crisis could be alleviated by converting saline water to potable water through desalination technologies. Reverse osmosis is the dominant technology in the market. However, its brine disposal is challenging. This paper proposes and investigates a novel hybridization between a solar-driven reverse osmosis plant and a solar-driven thermal desalination unit to enhance the water recovery ratio (freshwater produced/feed saline water). The thermal desalination unit combines an adsorption cycle, ejectors, and a humidification-dehumidification cycle. The brine from the reverse osmosis unit is used as a feed for the thermal desalination unit to produce higher amounts of freshwater. The extra-high saline water discharged from the proposed system can be used for salt extraction in a solar pond as a byproduct. Theoretical and economic models are developed to evaluate the performance and cost of the proposed system. It is found that the recovery ratio of standalone reverse osmosis and the proposed hybrid desalination plant is 40.8% and 70.5%, respectively, having a feed salinity and brine salinity limit of 32,500 ppm and 110,000 ppm. The recovery ratio of the hybrid system could reach 84%, with brine disposal of 220 ppm (which is valid for salt crystallization processes). The reduction in freshwater cost by using the hybrid plant is 23% to 27% compared to the standalone reverse osmosis plant for 40,000 to 50,000 ppm feed salinity and 10 m3 plant capacity. The suggested system can produce freshwater of 83 and 47 m3 per ton of silica gel per day at the gained output ratio of 2.71 and 2.6 in June and December, respectively.