Membrane-based humidity pump: performance and limitations

Moisture is usually transported from a fluid of high humidity to a fluid of relatively low humidity through a membrane. Membrane-based humidity pump works in another way: moisture is transported from a dryer air stream to a humid air stream under the driving force of thermal gradients in membrane. With humidity pump, a continuous air dehumidification can be realized by the utilization of low-grade waste heat. In this article, through finite difference simulations of the coupled heat and mass transfer in the unit, the performance and limitations of a membrane-based humidity pump are analyzed. The effects of various thermodynamic and dynamic parameters of membrane on the performance of humidity pump are discussed. It is found that even though the operational parameters can be optimized to enhance the thermo-osmosis rate of water vapor, the key role is played by a factor named humidity pump. Furthermore, when the humidity pump factor is bigger than 0.02 mK−1/7.5 kJ1/3, the permeability is bigger than 1.0×10−5 kg m−2 s−1, and the humidity pump can be used in commercial applications.