Pressure retarded osmosis (PRO) is an osmotically driven membrane process that utilizes the energy of mixing between streams of high and low chemical potential to generate electrical energy. High power density of a PRO membrane is essential to maximize process efficiency and minimize the capital and operating costs. Thus, robust PRO membranes that can support high pressure and have high water flux, low reverse salt flux, low structural parameter, and a good membrane support structure are needed. In this study, four commercial forward osmosis (FO) membranes for use in PRO were compared. The effect of operating pressures, membrane spacers (type, orientation, and arrangement), and flow velocities on process performance were investigated. A thin film composite polyamide membrane from Hydration Technology Innovations was found to be the most robust and selective membrane. Compared to other spacer configurations, the use of unique feed channel spacer orientations was found to increase PRO power density by up to 46%, yielding a power density of 22.6 W/m(2) (3 M NaCI draw solution and 4.1 MPa hydraulic pressure). However, membrane deformation was observed when operating pressures exceeded 3.5 MPa. The use of unique spacers coupled with decreased draw solution cross-flow velocities was found to increase PRO process efficiency. (C) 2015 Elsevier B.V. All rights reserved.