Pressure retarded osmosis (PRO) is a process where water flows naturally from a low salinity stream (feed water) at an ambient pressure across a semi-permeable membrane to a pressurized high salinity stream (draw solution). This process is driven by the osmotic pressure difference across the membrane. The increased volume of pressurized solution can be utilized to drive a turbine for power generation. As such, PRO process is considered as a promising approach to harvest salinity gradient energy.
In this invention, PRO composite hollow fiber membranes with an ultra-thin RO-like skin layer on the inner surface of a UF hollow fiber substrate have been successfully developed using a two-step preparation. The advantage of this approach is that it allows the use of different materials and different methods for the preparation of the skin and the hollow fiber substrate separately. Pilot-scale PRO modules (2-inch) have been fabricated and operated in a long run of 6 months for validation test.
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The global fresh water run-off is about 1.1 × 106 m3/s that contains salinity gradient power of 2.6 × l012 watts when pairing with seawater as draw solution in PRO process. In theory, up to 0.8 kW per cubic meter of fresh water is extractable in controlled mixing with seawater — equivalent to the potential energy contained in water of 280 m high hydraulic head. It has been predicted that the osmotic potentials derived from the salinity gradient represent an energy potential of 2000 TWh across the globe, which corresponds to about 1% of the world energy consumption. The PRO technology is thus a promising renewable energy harvesting technique in the intensified energy market.