Effectively using heat to thermally enhance pressure retarded osmosis

Abstract

Thermal energy from solar radiation and industrial waste heat is abundant and can be used to improve the performance of membrane processes for water treatment and power generation. In this study, we investigate the concept of using heat to thermally enhance pressure retarded osmosis for improved energy recovery from salt gradients. A laboratory apparatus is used to demonstrate performance at temperatures between 20 and 50 °C, and improvements in water flux and power density of up to 110 % are observed. Temperature of the feed solution is found to be more important than draw temperature. A finite element model is experimentally validated and used to identify what transport dynamics are responsible for the priority to heat feed. It is found that a hot feed solution produces a warmer membrane (thereby encouraging higher permeability and mitigated polarization), and this principally because the feed boundary offers little thermal resistance as it is typically well-mixed during pressure retarded osmosis. Finally, a new metric of effectiveness is introduced to evaluate power increases relative to the change in temperature. It is found that although operation at higher temperatures may yield higher power, operation at low temperatures of ~30 °C may represent a more effective use of thermal energy.