Multistage pressure-retarded osmosis configurations: A unifying framework and thermodynamic analysis

Pressure-retarded osmosis has enjoyed increasing research interest over the last decade. Recent studies focusing on single-stage PRO designs have raised doubts regarding the long-term economic viability of the technology. While most of the analyses are based on single-stage operation, comprehensive analysis of multistage PRO which shows promise for better energetic performance is absent. Previous studies on multistage PRO differ in their design philosophies and performance metrics, leading to an incomplete assessment regarding the potential benefits of multistaging. In this paper, we develop a unifying framework to classify several existing multistage configurations. In addition, we analyze the multistage PRO system from a thermodynamic perspective. Among the two major multistage design strategies, namely interstage pressure control and independent feed inputs to each stage, we found the latter to be more effective towards increasing net power density. In comparison to a single-stage device, a 10-stage system achieves around 9% higher net power density while using the same membrane area.