Thermoresponsive liquid desiccants for dehumidification cycles

We describe a liquid desiccant dehumidification cycle that regenerates via liquid–liquid phase separation instead of the typical liquid–vapor phase separation. Liquid-liquid phase separation is enabled by the use of thermoresponsive liquid desiccants that exhibit a “U-shaped” immiscibility curve in their phase diagram with water. Crossing this immiscibility curve causes a single-phase liquid to transform into a two-phase liquid mixture, which then facilitates the liquid-state removal of the absorbed humidity. In order to systematically explore this cycle's dehumidification coefficient of performance (COP_deh), we employ the Flory-Huggins theory of mixing to systematically vary the immiscibility curve. The Flory-Huggins theory uses two key parameters, the lower critical solution temperature (LCST) and the enthalpic interaction parameter (χ_H) to define the position and shape of the immiscibility curve, respectively. We show that LCST temperatures close to the cycle's heat rejection temperature improve COP_deh by reducing the necessary sensible heat during regeneration. We also show that enthalpic interaction parameters with larger negative magnitudes lead to flatter immiscibility curves that improve COP_deh. This improves COP_deh by better directing regeneration heat toward liquid–liquid demixing as opposed to increasing temperature. Our modeling indicates that COP_deh values up to ∼4 are possible using this cycle whereas traditional non-thermoresponsive liquid desiccant cycles would achieve COP_deh values of <1. This large increase in COP_deh is due to the fact that the demixing enthalpy associated with liquid–liquid phase separations is much smaller than the enthalpy of vaporization associated with liquid–vapor phase separations.