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 (COPdeh), 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 (chi(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 COPdeh 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 COPdeh. This improves COPdeh by better directing regeneration heat toward liquid-liquid demixing as opposed to increasing temperature. Our modeling indicates that COPdeh values up to similar to 4 are possible using this cycle whereas traditional non-thermoresponsive liquid desiccant cycles would achieve COPdeh values of <1. This large increase in COPdeh 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.