Experimental investigations on the performance of an air heated humidification-dehumidification desalination system

摘要

Experiments on a single- and two-stage air-heated humidification-dehumidification desalination system (HDD) driven by solar energy are conducted. The system is built on the seashore of Dhahran, Eastern Province of the Kingdom of Saudi Arabia. In this harsh climate, natural water sources are absent. Currently, Saudi Arabia uses desalination to augment its water supply. It is ranked the first worldwide in water desalination. However, the current large-scale desalination plants are fossil-fuel-driven and consume large amount of energy. Since there is abundance of solar energy in the region, attempts are made to utilize the solar energy to produce fresh water on a small scale for remote areas. The HDD systems have received considerable attention as an effective way to produce fresh water in remote areas where receiving water through pipelines is a challenge. The system used in this study is a solar air heated, closed-water closed-air cycle that can be adjusted to operate in a single-stage or two-stage mode. In this system, evacuated tube solar heaters are used to heat the air. The heated air is humidified by passing it over a spray of seawater in a countercurrent direction inside a humidifier. Additional heating and humidification processes follow in case of operating the two-stage system mode. The humidified air is then dehumidified inside a water-cooled dehumidifier (condenser). Air leaving the dehumidifier is circulated again to the solar heaters. Due to the relatively elevated temperature of incoming water to the dehumidifier, the water cycle is divided into two. In the first cycle, water leaving the cooling water tank flows to the condenser and then returns back to a tank. In the other one, water leaves a small tank and is then distributed to the humidifiers where it is sprayed. The rejected brine returns to the tank. The reason for using a small tank is to keep the water in the cycle warm due to its interaction with hot air in the humidifiers. A make-up tank is connected to this tank through a float valve to compensate for the evaporated water. Measuring sensors are connected to various locations within the system to measure dry- and wet-bulb temperatures, flow rates, and solar radiation flux. Data acquisition system is used to record the reading every 10min. Results show the effect of the main controlling parameters on the system performance in terms of produced distillate.