Recent developments in pressure retarded osmosis for desalination and power generation

When two streams of different salinities are mixed, Gibbs free energy of mixing is released. This energy can be harvested and further converted to electric energy through pressure retarded osmosis (PRO) process. Despite several improvements on PRO over the past decades, there still exist several technical issues pertinent to its adequate implementation that remain unresolved. These issues are mainly (i) water transport in the membrane, (ii) membrane material, (iii) fouling, (iv) process efficiency, and (v) techno-economic viability. Different process parameters such as temperature, type of draw solution, feed concentration, and membrane type directly affect the efficiency and power density of PRO. In this review, major trends of PRO (and hybrid plants) are analyzed and the suggested improvements of PRO membranes are discussed. Since potential side-benefits of PRO include electricity production and the treatment of rejected brine from desalination, the process presents a unique path to utilize all these advantages. Therefore, PRO is also coupled with other osmotic, desalination processes to maximize efficiency. The amount of useful energy from PRO would probably be enormous if this was exploited globally. There are several questions remain unanswered about the overall feasibility of PRO as a stand-alone process. This paper offers a comprehensive background and overview on the developments in PRO to enhance its power density and feasibility.