Modelling of osmotic energy from natural salt gradients due to pressure retarded osmosis: Effects of detrimental factors and flow schemes

In this investigation, a simplified pressure retarded osmosis (PRO) model incorporating the detrimental effects of internal polarization concentration (ICP), external polarization concentration (ECP) and reverse salt permeation (RSP) is proposed and verified using published data. The results demonstrate the accuracy of the model to address decreased water flux and power density due to the performance limiting effects. Based on the model, the discharge behaviour of a PRO process is reported with respect to different applied pressures on the draw solution and two flow schemes, co-current and counter-current flows. In the co-current flow PRO process, from the flow profiles in the draw and feed channels, it is found that the adverse effects on the process dynamics, such as water flux and power density, and required membrane area, can be regarded as a further retardation by applying 'an extra applied pressure' on the draw solution. In addition, the capacity of extractable energy of the full scale PRO discharge is significantly reduced due to the ICP, ECP and RSP effects. Furthermore, the termination conditions of the PRO discharge and its effects on the dynamics of the PRO process are also investigated in the case of the counter-current flow PRO process.