Energy and thermodynamic analysis of power generation using a natural salinity gradient based pressure retarded osmosis process

• Power density and energy generation by PRO processes are mathematically interpreted.
• Discharge behaviour of PRO processes based on natural salinity is obtained and analysed.
• Membrane consumption of a full scale PRO discharge is studied.
• Configurations of two-stage PRO process are proposed and discussed.
• Energy generation is compared for different configurations.

This study presents a thermodynamic and energy analysis of the discharge behaviour of a single-stage pressure retarded osmosis (PRO) process which is then expanded into a proposed two-stage process to enhance total energy extraction in a practical application. A thermodynamic model describing the operational conditions for the optimal power density and the extraction of energy from a single-stage PRO process is introduced. The discharge behaviour of the power generated from the process is analysed and the profiles of water flux, power density, and extracted energy are obtained. The membrane consumption is also studied with respect to different hydraulic pressures on the draw solution, and the flows of both the draw and feed solutions. The inherent inconsistencies in the operational conditions with regard to achieving maximal power density and available energy is discussed and interpreted based on the discharge behaviour. A two-stage PRO process with two alternative feed arrangements (continuous feed and divided feed) is then proposed and its operations are simulated and analysed. The results indicate favourable energetic performance of the two-stage versus the one-stage PRO process in terms of the reduced frictional loss and unused energy involved in the process.