Pressure‐Retarded Osmosis

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.

Experimental Results from RO-PRO: A Next Generation System for Low-Energy Desalination

“…While solar and wind are among the most well-known renewable energy resources, another nearly untapped resource exists within salinity gradients. Salinity gradient power potential is significant as it could increase current renewable energy production by 20%. , One method to harvest salinity gradient power is through pressure-retarded osmosis (PRO), a process where a pressurized higher salinity (draw) solution is used to extract fresh water through a semipermeable membrane from a lower salinity solution. , This increases the available mechanical (pressure–volume) work in the draw solution, which can be used to spin hydro turbines for electrical power production or can be used directly to supplement the mechanical load required for reverse osmosis (RO) desalination systems …”

“…1,2 One method to harvest salinity gradient power is through pressure-retarded osmosis (PRO), a process where a pressurized higher salinity (draw) solution is used to extract fresh water through a semipermeable membrane from a lower salinity solution. 3,4 This increases the available mechanical (pressure− volume) work in the draw solution, which can be used to spin hydro turbines for electrical power production or can be used directly to supplement the mechanical load required for reverse osmosis (RO) desalination systems. 5 RO desalination is currently the most efficient widely adopted commercial desalination technology; however, it still requires a great deal of energy to create the high pressures necessary to overcome the osmotic pressure of saline waters.…”

River-to-sea pressure retarded osmosis: Resource utilization in a full-scale facility

“…This system, in addition to the elevated draw solution salinity would also have reduced parasitic loads compared to the river-to-sea configuration as the draw solution is already pre-treated before RO and the turbine-generator is not employed. PRO may also have application in a closed-loop configuration to convert waste heat into useful work [49]. In this configuration, elevated draw solution salinity would be beneficial to convert energy from the waste heat at higher rates, making the system more compact and more efficient.…”

The osmotic membrane bioreactor: a critical review

“…Fig.4Schematic drawings illustrating two potential integrated FO systems in which OMBR can be incorporated in an osmotic dilution configuration: (a) OMBR-RO-OMBR hybrid system for enhanced water recovery and reuse in seawater desalination, and (b) PR-OMBR-RO hybrid system for water desalination and energy recovery. Adapted from Achilli et al84 …”