Renewable and Sustainable Energy Reviews xxx (xxxx) xxx Please cite this article as: Muhammad Tawalbeh, Renewable and Sustainable Energy Reviews, https://doi.org/10.1016/j.rser.2020.110492 1364-0321/© 2020 Published by Elsevier Ltd. Recent developments in pressure retarded osmosis for desalination and power generation Muhammad Tawalbeh a , Amani Al-Othman b , Noun Abdelwahab b , Abdul Hai Alami a, c , Abdul Ghani Olabi a, d, * a Sustainable and Renewable Energy Engineering Department, University of Sharjah, P.O. Box 27272, Sharjah, United Arab Emirates b Department of Chemical Engineering, American University of Sharjah, P.O. Box 26666, Sharjah, United Arab Emirates c Centre for Advanced Materials Research, University of Sharjah, P.O. Box 27272, Sharjah, United Arab Emirates d Mechanical Engineering and Design, Aston University, School of Engineering and Applied Science, Aston Triangle, Birmingham, B4 7ET, United Kingdom A R T I C L E INFO Keywords: Pressure retarded osmosis Osmotic power Salinity gradient Desalination Power generation ABSTRACT 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 effciency, and (v) techno-economic viability. Different process parameters such as temperature, type of draw solution, feed concentration, and membrane type directly affect the effciency 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-benefts 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 effciency. 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. 1. Introduction Fossil fuels impose several adverse impacts on the environment through releasing different pollutants such as the oxides of sulfur and nitrogen, volatile organic chemicals and greenhouse gases [1,2]. Therefore, new sustainable and clean energy sources are currently explored. Researchers are investigating the feasibility of generating energy from wind, sun, water, biomass and internal heat of earth to minimize the usage of the conventional fuels [3,4]. Recently, electricity is produced from the osmotic pressure using an emerging technology for power generation called pressure retarded osmosis (PRO). The mixing of two aqueous solutions with different salinities releases the free energy of mixing, which can be transferred to hydraulic pressure using pressure retarded osmosis (PRO). After that, the electric power can be obtained via hydro-turbines [5,6]. In general, PRO can be operated using the discharge of any desalination technology such as the reverse osmosis [7]. Therefore, it can potentially decrease the ecological effect of the concentrated brine of a desalination technology [8]. The way that PRO operates is when the water moves to the side of high salt concentration to increase the volumetric fow rate of the pressurized higher salinity water, therefore increasing the workable energy of the draw water [9,10]. The concept of generating energy from the osmotic pressure was frst described in 1954 [11]. Twenty years after, the PRO process was frst implemented experimentally to evaluate the viability of generating en- ergy from salty water at Israel’s Ben-Gurion University of the Negev [9]. More than thirty years separate the frst conducted experiment and the frst installed power plant of PRO [9]. These thirty years witnessed numerous developments attempting to promote the PRO process from the bench scale to the pilot scale [12]. PRO however, faced several challenges in the market of power generation due to the high cost of membranes and the low power output. The cost of membranes and their * Corresponding author. Sustainable and Renewable Energy Engineering Department, University of Sharjah, P.O. Box 27272, Sharjah, United Arab Emirates. E-mail address: aolabi@sharjah.ac.ae (A.G. Olabi). Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: http://www.elsevier.com/locate/rser https://doi.org/10.1016/j.rser.2020.110492 Received 20 October 2019; Received in revised form 6 September 2020; Accepted 19 October 2020