Research On The Low Temperature Multi-effect Distillation Desalination Driven By Solar

Global water crisis is becoming more and more serious due to the uneven distribution of water, unreasonable development and utilization of water resources and serious water pollution. China is facing both the problems of serious shortage of water resources and their uneven distribution between the North and the South. Water scarcity in northern China is due to the lack of water resources while water scarcity in southern China is because of the water pollution. In order to alleviate the water crisis, it is necessary to realize comprehensive control and rational use of the water resources, to promote the water-saving technology and to develop stable freshwater resources. Seawater desalination is an effective measure to ease the water crisis. Now, because of energy shortage and serious environmental pollution from using fossil fuels, new energy and waste heat utilization has become the main direction of development of desalination technology. Using solar power, a clean energy, as thermal source of low-temperature multi-effect desalination technology is an important way to mitigate the freshwater crisis.In this paper, based on a set of low-temperature four-effect distillation solar desalination plant, a detailed study on the basic theory of desalination, experimental methods and performance optimization was carried out. During the study, mathematical models of the solar collector system, the distillations, the flash tanks, the preheater and the condenser were established. Besides, some equations about the equipment were also obtained, including the energy conservation equation, the mass conservation equation, the material conservation equation, the heat transfer equation and the formula of water production.After the theoretical calculations and experimental studies, the optimum operating parameters for the running of the device was obtained. When the collector tank temperature is below60℃, the desalination system should be suspended. The upper limit water temperature of the automatic control system is set at85℃. The vacuum pump connected to the end effect should maintain the minimum operating pressure of the system in the range of8KPa to12KPa. When the solar collector system is completed, water temperature will changes with time and sunshine situation, thus the hot water flow control is used to ensure the balance of pressure and temperature of each effect under a long period. Take the Pearl River Estuary in southern Guangzhou as an example, the hot water influent flow rate should be set at1400kg/h-1600kg/h when sunshine is adequate in June, July and August; the hot water influent flow rate needs to be properly reduced to1200kg/h~1400kg/h in May, September and October; In April and November, the hot water influent flow rate is about1000kg/h～1200kg/h. When the hot water influent flow rate is lower than1400kg/h, the seawater flow rate of first effect was set at200kg/h～260kg/h; the first-efficient seawater flow was set at240kg/h～300kg/h when the hot water influent flow rate reaches1500kg/h. The flow rate of the condensation water excreted from the system was set at1700kg/h～1800kg/h.Using the operating parameters measured above, the daily water production reached763.6kg under a typical clear day in April with the temperature of23～29℃and the total radiation exposure of25.849MJ/m2. Further, in the typical sunny days in July and August with the total radiation exposure reaching40MJ/m2and the inlet hot water temperature remaining between75～90℃for a long period, the hourly water production was able to reach200kg/h.Finally, though economic calculation, some effective methods for reducing the cost of water production (COP) were obtained, namely, increasing the heat collection performance of the solar collector system to improve the annual production of fresh water; reinforcing the operation and maintenance work to lengthen the service life of the equipment; improving the design of the desalination distillation system to reduce costs as well as to enhance desalination capacity; adjusting the operating parameters reasonably to achieve the maximum water production.