Progress in pressure retarded osmosis (PRO) membranes for osmotic power generation

The rapid increases in global energy consumption and greenhouse gas emissions have stimulated the exploration of renewable energy sources as alternative fuels. Osmotic pressure gradient energy released from the mixing of water streams with different salinities is an unexploited resource of renewable energy. By employing a semipermeable membrane to control the mixing process, the osmotic pressure gradient energy can be harvested in terms of electrical power via pressure retarded osmosis (PRO) without causing adverse environmental impacts. The ideal of harvesting osmotic power via PRO was proposed in the early seventies; however, the absence of effective membranes with desirable structure and performance hindered further advancement of the PRO technology. During the last few years, a significant progress in PRO technology has been achieved. Novel flat-sheet and hollow fiber polymeric membranes with desired structure, mechanical robustness and permeation characteristics have been developed for PRO applications. Membranes with a target power density of 5 W/m² to produce commercially viable PRO processes have been achieved. At this point of time, a comprehensive review is imperative in order to summarize what we have accomplished and provide insights for the development of next generation PRO membranes. After a brief introduction of the PRO process and the early PRO development using the existing RO/NF and FO membranes, this review focuses primarily on novel and the state-of-the-art PRO membranes. Furthermore, the requirements for fabricating effective PRO membranes will be discussed and future perspectives will be presented.