1. Title, Journal and Authors

Title: Thermodynamic limits of extractable energy by pressure retarded osmosis

Journal: Energy & Environmental Science

Authors: Shinhong Lin, Anthony P. Straub, and Menachem Elimelech*

* Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286

 

2. Summary

The concept of harnessing the energy released when two solutions of different salinities are mixed has been proposed as a promising source of clean energy.
Pressure retarded osmosis (PRO) has been identified as one of the furthest developed methods to extract this energy of mixing and many studies have looked at the performance of small-scale membrane coupons in PRO. However, these coupon-sized experiments cannot give insight into the amount of energy extractable in the full-scale PRO process since, in practice, it is operated with membrane modules at a constant applied hydraulic pressure. The authors theoretically evaluate the PRO process to understand flow and concentration behavior in constant-pressure modules. Using this analysis, the authors establish the thermodynamic limit of extractable energy at the module scale. For river water mixing with the sea, one of the most widely considered source water pairings, the authors identify the maximum extractable energy in the process to be 0.192 kW h per cubic meter of mixed total solution. Even though this is only 25% less than the Gibbs free energy of mixing, the authors discuss additional efficiency losses and energy costs that may substantially reduce the net specific energy available from the river water and seawater. The authors highlight other source water pairings for the PRO process which, in some cases, can circumvent the need for pretreatment and still offer relatively high efficiencies in a constant-pressure module.
 

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