Performance analysis of functioned brackish water for lighting-induced evaporation type solar desalination system

Fresh water demands are increasing day by day because of industrialization, motorization and increased life standards of mankind. Fresh water reserves available naturally are not capable of meeting the fresh water demands because of their less availability. Desalination is not only necessary to overcome the issue of fresh water shortage in the future, but also important for the oil-gas-coal industry which generates substantial amount of waste water during the production of oil, natural gas and coal. Compared to conventional water treatment technology, solar brackish water desalination has advantages of cleanness and sustainability. However, high cost and small scale have become the biggest obstacles for solar brackish water desalination technology, which is caused by the high-cost solar collector, the structural mismatch of the optimum working ranges between the solar collector system and the brackish water desalination system, and the large heat transfer resistance between them. To solve this problem, this paper has proposed a strong light-concentrating direct evaporation type solar brackish water desalination system. It utilizes the concentrating solar energy which is directly shined into the functioned brackish water to produce steam for repeated usage. It should be noted that solar collector is introduced to collect much more solar energy and produce steam efficiently to improve the performance of solar desalination units. The light-induced evaporation type solar desalination units show small heat transfer resistance, heat capacity small and small cavity evaporation, which can largely reduce the cost and be beneficial for the economic performance of the solar desalination system. Generally, the functioned brackish water is in the boiling state during the system working. The transmissivity of the functioned brackish water in the boiling state was measured in optical darkroom. At the same time, thermal energy utilization efficiency of functioned brackish water was tested in dish type concentrating system under actual weather condition. The objective of the work was to determine the influence of abundance and size of particles on the optical and thermal performance of the system. The variations of transmissivity for brackish water through different optical paths were recorded under the same conditions. The dish type concentrating solar desalination device was then constructed according to the optical analysis results. The characteristics of temperature and performance ratio (PR) of the water were obtained through outdoor experiment. A dimensionless parameter of "efficiency rate" was used in this paper to evaluate the extinction performance of the functioned brackish water (30-60℃). The results indicated that functioned water could be obtained through adding black particles in brackish water. In addition, the transmissivity of the functioned brackish water decreased with the increasing of abundance, and with the decreasing of particle size. With the smallest particle size of 0.72 mm used in the experiment, the transparency rate of the brackish water reached the lowest value, and the transmissivities through 2 different optical paths decreased by 75.83% and 77.08%, respectively. When the particle abundance increased from 6.25 to 50.00 mg/L, the increase rate of thermal energy utilization efficiency of the functioned brackish water could reach 41.3%. Finally, it was found that the trend of thermal energy utilization efficiency was consistent with the transmissivity.