Viability of pressure-retarded osmosis for harvesting energy from salinity gradients

Pressure-retarded osmosis (PRO) has been widely investigated as a source of clean energy. Several configurations and hybrid processes have been suggested and deeply studied, namely, standalone open-loop PRO (OLPRO), standalone closed-loop PRO (CLPRO), and PRO coupled to seawater reverse osmosis (SWRO-PRO). Until now, no study has provided a definite decision on the viability of this process. In this study, the feasibility of PRO as a renewable energy source was studied. First, SWRO-PRO was energetically and economically investigated by developing the overall specific energy consumption and the levelized cost of electricity (LCOE). Energy and economic analyses revealed that SWRO-PRO is unviable, even with ideal membrane performance and an optimized process. OLPRO and CLPRO were also economically evaluated. The analysis showed that OLPRO operating with hypersaline sources is economically viable only with very low-concentration feed solution. CLPRO displayed better economic and energy performance than OLPRO, but its economic viability is still strongly dependent on the energy consumption of the draw solute regeneration system. The lower bounds of power density (PD) that should be produced to ensure economic competitiveness with wind and solar energy were identified with good accuracy. Finally, practical recommendations were suggested toward a viable PRO-based power plant.