High performance thin-film composite pressure retarded osmosis (PRO) hollow fiber membranes for renewable osmotic energy generation

Worldwide energy crisis has become an inevitable problem due to the explosive increase in energy demand and the shrinking reserves of traditional fossil fuels. The global environmental concerns are calling for the production of energy in environmentally friendly ways. By utilizing the renewable osmotic energy discharged when two solutions of different salinities were mixed, pressure retard osmosis (PRO) has attracted rapid attention ever since Statkraft built the first prototype osmotic power plant in Norway in 2009. The estimated global osmotic energy that can be generated from the mixing of ocean and river waters is around 1750-2000 TWh/year. More energy could be expected when high salinity retentates from desalination plants are purposely mixed with recycled water. However, the current PRO technology is constrained by the lack of high performance semipermeable membranes. Flat-sheet membrane has been observed to be easily deformed in high pressure PRO processes due to membrane-spacer interactions that may lead to severe salt leakage, structure parameter enhancement and hydraulic pressure loss in the feed flow channel. Moreover, the shadow effects from the spacer in the feed channel further reduces the overall water flux and power density. In contrast, the self-supported hollow fiber membranes are of great interest due to the spacer-free module fabrication. Not only could this minimize the membrane deformation owing to membrane-spacer interactions, but also eliminate the extra energy loss in the feed flow channel of flat-sheet modules. The purpose of this presentation is to illustrate the science and engineering of fabricating thin film composite PRO hollow fiber membranes with desirable robust strength and power density for osmotic power generation. The newly developed TFC PRO hollow fiber membranes exhibit a power density as high as 16.5 W/m² and a very low specific reverse salt flux (J_s/J_w) of 0.015 mol/l at a hydraulic pressure of 15 bar using synthetic seawater brine (1.0 M NaCl) as the draw solution and deionized water as the feed. In addition, systematically investigations in the effects of membrane stretch resistance and acceptable ductility, membrane structures and morphology, and the kinetics of phase inversion during spinning on the PRO performance have been presented. The obtained conclusions will provide useful perspectives on the design criteria of a desirable PRO hollow fiber membrane by revealing the influence of the support layer properties.