Review of indirect and direct solar thermal regeneration for liquid desiccant systems

The large electricity consumption due to the extensive use of vapour compression machines in the air conditioning and industrial sectors dramatically increases the emissions of greenhouse gases. Besides, the synthetic halocarbon refrigerants used in said machines are responsible for ozone depletion. These environmental concerns give an opportunity to solar thermally-driven liquid sorption systems, whose energy consumption primarily lies on the regeneration heat of liquid sorbents. This paper surveys both theoretical and experimental studies on solar thermal regeneration methods of the hygroscopic solutions used in liquid sorption systems. A brief review of some conventional and alternative liquid desiccants utilised in said systems is firstly presented. Furthermore, information about several configurations of regenerators and their performance is covered in detail, putting special emphasis on solar collector/regenerators, which use directly solar energy to reconcentrate the weak desiccant solution. A comparative assessment of relevant thermophysical properties of some hygroscopic liquids is realised in order to determine their suitability for use in sorption systems. The results show that the aqueous solutions of potassium formate and ionic liquids are very promising desiccants due to their low vapour pressures, specific heat capacities and dynamic viscosities, as well as their non-corrosiveness. A performance comparison between typical regeneration units and solar collector/regenerators is also carried out. It could be shown that solar collector/regenerators have a high potential for enhancing the water desorption rate from a diluted desiccant solution and, consequently, the capacity of the air dehumidification process within the absorber.