Modelling mass transport in hollow fibre membranes used for pressure retarded osmosis

Abstract The principle of PRO is to utilise parts of the free energy of mixing when mixing freshwater with seawater for production of electrical energy. The mixing process takes place over a semi permeable membrane that will retain salts, and ideally allow only transport of water. The transport of water will occur due to differences in chemical potential across the membrane. The PRO process will be operated with elevated pressure on the seawater side of the membrane, and the net increase in volume due to osmotic transport of water across the membrane, can be utilised to run a turbine and hence produce energy. At present the biggest challenge in PRO is to develop membranes with sufficient specific power. This challenge involves optimisation of the basic membrane characteristics , i.e., high water permeability ( A ), low salt permeability ( B ) and low structure parameter ( S ). In this paper a transport model for water and salt in hollow fibres in PRO has been developed. Further, a structure parameter equivalent to the structure parameter for flat sheet membranes has been defined. The impact of the membrane parameters ( A, B and S ) on PRO performance in a hollow fibre element has been demonstrated by applying iso-watt diagrams. In general, for a given structure parameter S , increasing the A value and decreasing the B value will increase the specific power. It was also found that if the salt permeability exceeds a certain level, a further increase in the water permeability will not result in a corresponding increase in the specific power. Similarly, if the water permeability is lower than a certain level the effect of increased salt permeability will be low. Further, the PRO performance of different hollow fibre membranes has been estimated using the iso-watt diagrams. Based upon the reported membrane characteristics the PRO performance for the selected membranes ranges from less than 1 W/m 2 to above 4.5 W/m 2 . The membranes having a specific power around 4.5 W/m 2 are promising candidates for future PRO plants, given that these membranes can tolerate the typical operation pressure in a PRO plant of 12–14 bar.