Seawater bitterns as a source of liquid desiccant for use in solar-cooled greenhouses

The processes of desalination and sea-salt production currently produce large quantities of by-product, in the form of brine and bitterns, which is generally regarded as waste. This article explores the scope for exploiting the hygroscopic salts occurring in these by-products - such as magnesium, calcium and sodium chloride - as desiccant solutions in a greenhouse cooling system. These solutions are compared to other liquid desiccants more conventionally used in solar-driven refrigeration: namely solutions of lithium chloride, lithium bromide and zinc chloride. To establish which properties are relevant, we discuss the relation of the properties of the liquid desiccant to the attributes of the greenhouse as a whole. A property of primary importance is hygroscopicity, as quantified by the equilibrium relative humidity (ERH). Further properties reviewed include cost, availability, density, viscosity, specific heat capacity, thermal conductivity, heat of dilution, water absorption capacity, human- and ecotoxicity, and corrosivity. Calculations based on five locations (Tunis, Jiddah, Abu Dhabi, Mumbai and Bangkok) show that the liquid desiccant should have ERH ≤50% to give improved cooling compared to both direct and indirect evaporative systems. Except for sodium chloride, all six salts considered meet this requirement. Magnesium chloride is the most abundant salt in seawater bitterns and both magnesium and calcium chloride stand out as being of low toxicity. Their hygroscopic properties, though inferior to those of the lithium and zinc salts, make them suitable for cooling of greenhouses (even if not of human dwellings). We envisage an integrated desalination and agricultural system, comprising a solar desalination plant supplying freshwater (for irrigation) and bitterns (for cooling) to greenhouses, enabling efficient water use and local crop production in hot climates.