Utilization of renewable energy sources, as an approach to reduce Greenhouse Gas (GHG) Emissions, has been globally popular in the last few decades. Among renewable energy sources, pressure retarded Osmosis (PRO) has been scrutinized by scientists since the mid-70's. However, even today, the existing PRO systems can only marginally meet the generally approved criterion of 5 W/m(2) power density, a threshold for an economically feasible river-sea PRO system. As an approach to increase the performance of PRO systems, multi-staging of PRO modules is investigated in this paper. For this purpose, a number of models for scaled up dual stage PRO power plants with different configurations and target functions are presented and compared to a single stage system with the same membrane area. These models consider the pressure and flow drop as well as the salinity change along the membrane. The results indicate that overall performance of the system could be improved by up to 8% with a dual stage PRO in the case of specific energy. Finally, a thermodynamic analysis addresses the sources for irreversible losses, and the contribution from each source.