Pressure‐Retarded Osmosis

Abstract

Abstract There is a natural tendency for salt solutions to be diluted by freshwater. This natural force appears as osmotic pressure. One process of capturing the energy released from the mixing of freshwater with saltwater is pressure‐retarded osmosis (PRO). In PRO, water from a low salinity solution permeates through a membrane into a pressurized, high salinity solution; effectively transforming water chemical potential into hydraulic head (i.e., potential energy). The combination of increased interest in renewable and sustainable sources of power production and the recent progress in membrane science has led to a spike in PRO interest in the past decade. This interest culminated in the first experimental “river‐to‐sea” PRO installation, which opened in Norway in 2009. Additional applications of PRO, including reverse osmosis (RO)–PRO systems and osmotic heat engines have since gained attention. Although river‐to‐sea PRO has the potential to be a reliable source of base‐load power, the higher salinity gradient available for power production in RO–PRO systems is likely to make them a more promising component of an alternative energy portfolio. While RO–PRO systems are in the early stages of development, the possibility of PRO to enable significant energy reductions in RO energy costs are driving interest in these systems. For all applications, development of optimized and inexpensive PRO membranes is necessary. This article will provide the theoretical foundation of the PRO process as well as the state of the art of the membranes used in this process. It will also outline the development of the river‐to‐sea PRO, RO–PRO, and osmotic heat engine systems and discuss their capabilities, limitations, and future as alternative energy sources.