Pressure retarded osmosis (PRO) is a promising technology for extracting renewable salinity-gradient energy. However, its performance can be significantly influenced by membrane deformation at the high applied pressures when feed spacers are used in the operation. This study systematically investigated the effect of feed spacer geometry on membrane deformation and the resulted PRO performance. It was observed that feed spacers with larger openings induced more severe membrane deformation at higher applied pressures. A theoretical mechanical model further reveals that the extent of membrane deformation is proportional to the applied pressure and the square of the opening size of the feed spacer and inversely proportional to the membrane mechanical strength. Moreover, a modified RO method was developed to measure the separation properties (i.e., the water permeability (A value), solute permeability (B value) and solute/water selectivity (B/A value)) of the deformed membrane. The measured A, B and B/A values increased with the extent of the membrane deformation. It was further found that the severe membrane deformation not only drastically reduced the PRO water fluxes but also substantially increased the hydraulic pressure loss in the feed flow channel. For feed spacers with large openings, the reduced PRO water fluxes together with the increased pumping need for feed flow severely decreased the net power output. In contrast, the use of feed spacers with small openings resulted in significant improvement of PRO performance. In the current study, the use of commercial RO permeate carrier as the PRO feed spacer (opening ratio similar to 0.35) lead to a significantly enhanced power density of 5.8 W/m(2) at the optimal applied pressure of 17.2 bar due to the minimized membrane deformation. The current study can provide important implications for PRO studies. (c) 2013 Elsevier B.V. All rights reserved.