Experimentation of a Membrane-based Concentration Gradient Energy Storage of Liquid Desiccant Solutions Driven by Solar Energy

Solar energy storage is an indispensable and sustainable utilization mode of renewable energy; environment friendly, large-capacity, low heat loss, and long-term storage are critical to improving the integration of solar energy supply. Traditional thermal energy storage mode cannot achieve long-term storage due to the heat loss even under the excellent thermal insulation measures. In this work, a solar-powered membrane-based concentration gradient energy storage of liquid desiccant solutions is presented. In the membrane distillation process driven by solar energy under the right solar radiation conditions, the liquid desiccant solution is concentrated gradually and long-term stored as the concentration gradient energy. To this end, the measured temperature of solar hot water is in the range of 40°C to 90°C from May to September, 2018, in Xi’an, China. And then, the LiBr solution (50 wt%), the LiCl solution (35 wt%), and the CaCl₂ solution (40 wt%) were membrane-based concentrated in the temperature range of 42°C to 63°C, separately. The results showed that the water vapor pressure difference decides the water vapor transferred across the membrane pores from the liquid desiccant side to the air side. The energy storage density of liquid desiccant solutions increases along with the increases in temperature and the membrane area. Consequently, when the LiBr, LiCl, and CaCl₂ solutions are concentrated from 50% to 55%, from 35% to 40%, and from 40% to 45%, separately, the concentration energy storage density is 245 kJ/kg, 350 kJ/kg, and 306 kJ/kg, which is equivalent to or even higher than ice storage capacity. Due to the two independent closed cycle of the liquid desiccant solution and air, the liquid desiccant solution’s concentration gradient energy storage can be long-term stored environment-friendly without any insulation measures.