Low-temperature heat utilization with vapor pressure-driven osmosis: Impact of membrane properties on mass and heat transfer

Abstract

The emerging vapor pressure-driven osmosis (VPDO) membrane technology enables direct conversion of abundant low-temperature ( < 100 degrees C) heat resources to useful work. In this study, a theoretical model is established to understand mass and heat transfer of VPDO, and two hydrophobic nanoporous membranes, polypropylene (PP) and polytetrafluoroethylene (PTFE), of different chemistry and structural properties were evaluated. Although the PP membrane has a less effective transport pathway, the considerably larger pore size yields a much higher Knudsen diffusivity that results in consistently higher vapor fluxes across different temperature-pressure conditions. This finding provides strong evidence that mass transfer in VPDO is dominated by Knudsen diffusion. Additionally, we find that operation at higher pressurizations caused vapor flux decline that is attributed to the membrane morphological deformation. However, the PP membrane is less sensitive to the effects of compaction, underlining the importance of membrane mechanical robustness for VPDO. Lastly, the study shows that evaporative heat transfer is significantly greater than conducive losses and the PP membrane, with higher water fluxes, has better evaporation thermal efficiencies. This study provides fundamental understanding on the impacts of membrane properties on mass and heat transfer in VPDO, and highlights the centrality of vapor permeability and mechanical robustness in developing high-performance membranes.