PRO Power Generation with Natural Salinity Gradients: How Much Energy Is Available? and What Power Densities Can Be Achieved?

PRO Power Generation with Natural Salinity Gradients: How Much Energy Is Available? And What Power Densities Can Be Achieved?

Ngai Yin Yip and Menachem Elimelech

Department of Chemical and Environmental Engineering

Yale University, New Haven, Connecticut 06520-8286

Phone: 203-432-8110

ngaiyin.yip@yale.edu

Abstract. The free energy of mixing dissipated when fresh river water flows into the sea can potentially be harnessed for sustainable power generation with pressure retarded osmosis (PRO). In this study, we look at two questions that are at the core of the technology's viability: the amount of energy available for extraction and the power density attainable with current membranes. In the first part, we present a reversible thermodynamic model for pressure retarded osmosis.  The model verifies that the maximum theoretical extractable work in a PRO process is identical to the Gibbs free energy of mixing.  Analysis is carried out for pressure retarded osmosis operated at a constant pressure to determine the extractable work.  Inefficiencies in the extraction of work are attributed to i) frictional losses required to drive water permeation across the membrane and ii) unutilized energy due to the termination of permeate at the flux reversal point. We show that the highest extractable work in constant-pressure PRO with a seawater draw solution and river water feed solution is 0.75 kWh/m³ while the free energy of mixing is 0.81 kWh/m³ — a thermodynamic extraction efficiency of 91.1%. The actual work extraction efficiency in a practical PRO process is less than the thermodynamic limit due to operational constraints and losses from the system components. In the second part, we investigate the phenomena that limit performance in pressure retarded osmosis membranes: external concentration polarization, internal concentration polarization, and reverse draw salt flux. Our analysis shows that power density is constrained by the trade-off between permeability and selectivity of the membrane active layer. This behavior is attributed to the opposing influence of the beneficial effect of membrane water permeability and the detrimental impact of reverse salt flux coupled with internal concentration polarization. We demonstrate that membrane performance is maximized by tailoring the water and salt permeabilities to the structural parameters. By thoughtful selection of the membrane properties, the detrimental effects that limit productivity in a pressure retarded osmosis power generation process can be methodically minimized to achieve projected power densities of ~10 W/m³. This study aims to provide insights on the viability of sustainable power generation with natural salinity gradients by pressure retarded osmosis.