Limiting performance mechanisms in desiccant wheel dehumidification

Abstract

Desiccant cooling is an environmentally attractive alternative to conventional mechanical air-conditioning. The heart of the process is the rotary desiccant wheel which is used to dehumidify air. Recent experimental measurements of wheels with proposed alternative materials at low regeneration temperatures (<80C) have shown fewer benefits than anticipated based on the material adsorption characteristics.Here a numerical model of a desiccant wheel was used to investigate the specific influence of the desiccant equilibrium adsorption isotherm on the overall wheel performance. The heat of adsorption, moisture diffusion rate, desiccant specific heat capacity and density were varied to provide further insight into the limiting heat and mass transfer mechanisms for low temperature regeneration. In addition, an optimization analysis of the desiccant adsorption isotherm shape was performed for a range of process conditions.The results show that the extent of dehumidification is limited primarily by a combination of thermal affects caused by both the exothermic adsorption process and the carryover of heat from the regeneration stream. Braunuer Type 1 isotherms increase supply air dehumidification over a linear shape, though this is mostly due to the air inlet conditions which are more typically in the lower relative humidity range. The tendency toward Type 1 behaviour is greater when the heat of adsorption is a stronger function of the adsorbed moisture content. At moderate to high face velocities desiccant layer moisture diffusion kinetics also become important. Critically, the absolute moisture capacity has very limited influence on the performance. These findings have important implications for the design of desiccant wheels.