Developing Highly-Permeable Membranes for Air Dehumidification

Air conditioning in buildings consumes a large amount of electricity globally, which presents significant application opportunities for advanced water-selective membranes. The energy saving values are worth tens of billions $/year. Energy efficiency of the conventional vapor compression cooling is low in warm and humid climates due to water condensation. Greater than 50% energy efficiency may be achieved by removing moisture through a membrane without changing air temperature and pressure. The membrane dehumidifier also enables evaporative cooling with even higher efficiency and sustainable water. In addition, a membrane can be used to build a mass and heat transfer device for enthalpy recovery of ventilation, i.e, recovery of both latent and sensible energies.

The critical requirements for these applications are a low-cost membrane with high water permeance in addition to adequate H₂O/air selectivity. In this presentation, air dehumidification performances of different membranes will be compared that include polymeric membranes, ionic liquids and other immobilized solutions, graphene oxide, and A-type zeolite membranes. The water permeance and H₂O/air selectivity measured under the same conditions differ dramatically among those membranes. More importantly, impacts of separation conditions (temperature, feed air humidity) on separation performances are highly dependent on specific membrane material, which can be explained by different separation mechanisms. Overall, the NaA-type zeolite membrane prepared in this work exhibits the highest water permeance (~1×10^-5 mol m^-2 Pa^-1s^-1) among the membranes evaluated here and among the literature reports as well. The zeolite membrane also shows very desirable performance attributes for air dehumidification applications, such as increase of permeance and H₂O/air selectivity with air dehumidify and stable performances over a range of temperature.