Multiple staging of pressure retarded osmosis: Impact on the energy generation

As pressure retarded osmosis (PRO) is getting more attention for sustainable energy generation, further investigation to enhance its efficiency is required. Applying a multiple-stage PRO scenario is one suggestion to boost the generated energy in the PRO process. This research evaluates the benefit of multiple stages of pressure retarded osmosis on specific energy generation. Single, dual, ternary, and quaternary pressure retarded osmosis stages were investigated for power generation using four membrane modules in the PRO process. Eight designs were proposed to cover the arrangement of the membrane modules in a single, dual, ternary and quaternary PRO process and estimate the net-specific energy generation of pressure retarded osmosis. For each design, the variation in the total energy input and the total energy output of single and multistage pressure retarded osmosis processes were calculated. The water flux and the power density enhanced with the number of stages for the investigated multistage PRO scenarios. The maximum water flux and power density resulted in four PRO stages of values equal to 6 L/m².h and 15 W/m², respectively. For the Dead Sea-reverse osmosis brine salinity gradient, a 33.96 % rise in the net specific energy generation is observed in the quaternary-stages pressure retarded osmosis scenario. The Dead Sea and wastewater salinity gradients produced the highest net specific energy generation of 0.82 kW.h/m³ over the other investigated types of salinity gradients.