Modeling and performance analysis of a fully solar-powered stand-alone sweeping gas membrane distillation desalination system for island and coastal households

Solar-powered membrane distillation is a promising desalination technology. This study newly proposed a solar-powered stand-alone sweeping gas membrane distillation desalination system to provide flexible fresh water for remote island households without reliable infrastructure, such as water and power supplies. Solar energy is the only source of energy to drive the system. A solar thermal collector is installed to provide thermal energy and a solar photovoltaic array is installed to supply direct current power respectively. To evaluate the system performance, a system-scale mathematical model was established and validated by test results under real meteorological conditions. The performance of the major components of the system, including the thermal efficiency of the solar thermal collector, the electrical efficiency of the photovoltaic array, the humidification efficiency of the hollow-fiber-membrane module and the dehumidification efficiency of the condenser, was analyzed. The effects of structural parameters, such as solar collector area, solar photovoltaic area, and desalination operating parameters, on system freshwater yield and Gained Output Ratio were investigated. In addition, the system's daily freshwater production in each month was predicted under local weather conditions of Hong Kong. In summary, for the proposed system, the average daily freshwater production is between 9.98 kg/d and 23.26 kg/d (9.98 kg/d in January and 23.26 kg/d in July), which is enough to meet the daily drinking water demand of a typical family of four (two adults and two children). The ratio of solution flow rate to air flow rate should be selected in the range of 4.0–6.0 to obtain the optimal freshwater yield. The average thermal efficiency of the solar thermal collector is 50%, which is about three times as much as the power conversion efficiency of the solar photovoltaic panels (about 15%). Thus, the solar collector area shows a more significant impact on the system performance than the photovoltaic area. The final water production cost of the system is about 18.34 $/m³. The system proposed in this paper as a small-scale fully solar-powered desalination system is attractive to provide a flexible and reliable fresh water supply for island and coastal households.