oa Dual Stage Pressure Retarded Osmosis: Potential and Applicability

A dual stage PRO process has been proposed for power generation from a salinity gradient across a semi-permeable membrane. Both closed-loop and open-loop dual stage PRO system were evaluated using 2 M NaCl and Dead Sea as draw solutions, whereas the feed solution was either fresh water or seawater. The impact of feed salinity gradient resource and feed pressure on the net power generation and water flux were evaluated. The results showed that power density in stage one reached a maximum amount at, but the maximum net power generation occurred at. This result was mainly attributed to the variation of net driving pressure in stage one and two of the PRO process. The dual stage PRO process was found to perform better at high osmotic pressure gradient across the PRO membrane, for example when Dead Sea brine or highly concentrated NaCl was the draw solution. Total power generation in the dual stage PRO process was up to 40% higher than that in the conventional PRO process. This outcome was achieved through harvesting the rest of the energy remaining in the diluted draw solution. Therefore, a dual stage PRO process has the potential of maximizing power generation from a salinity gradient resource by 20%. DSPRO can be combined with desalination plant using seawater brine as the draw solution either in closed-loop or open-loop. This hybridization has multiple applications such as reducing the impact of discharging concentrated brine to sea, energy storage, and increase the recovery rate of the desalination. Power generation by DSPRO will reduce the energy consumption by the desalination processes. Waste heat from power plants can be used for the regeneration of the draw solution in the closed-loop DSPRO. Process modelling has been performed and shown promising results for DSPRO application for power generation. The impact of module configuration, area and length, with relation to draw solution concentration have shown to have significant impact on osmotically driven processes and should be counted for.