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 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 carry out a thermodynamic analysis of constant pressure PRO. We show that the maximum theoretical extractable work in PRO is identical to the Gibbs free energy of mixing, and work extraction inefficiencies in constant pressure PRO are attributed to entropic losses and unutilized energy. The extractable work in constant-pressure PRO with river water-seawater can potentially reach 0.75 kWh/m3 while the free energy of mixing is 0.81 kWh/m3 � a thermodynamic extraction efficiency of 91%. The technology can potentially provide electricity for half a billion people by utilizing just a tenth of the 37,000 km3 annual global river discharge, even after taking into consideration further limitations due to operational constraints and losses from system components. In the second part of the study, we demonstrate the fabrication of thin-film composite membranes capable of achieving high power density performance in PRO energy production with natural salinity gradients. Our analysis of the performance limiting phenomena shows that power density is constrained by the trade-off between permeability and selectivity of the membrane active layer. By thoughtful selection of the fabrication conditions, the beneficial effect of membrane water permeability is balanced with the deleterious impact of reverse salt flux coupled with internal concentration polarization to methodically minimize the detrimental effects that limit productivity.  The fabricated membranes demonstrated the potential to achieve high power densities of ~10 W/m2, twice the benchmark of 5 W/m2 necessary for the process to be cost-effective.

Harnessing osmotic energy for sustainable power generation with high performance thin-film composite pressure retarded osmosis membranes

246th National Meeting (2013)

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