Modelling, optimisation and control of osmotic energy extraction from natural salinity gradients using pressure retarded osmosis
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This thesis investigates the osmotic power generation from natural salinity gradients using pressure retarded osmosis (PRO), focusing on modelling, optimisation and control of the process. In this study, first, due to the lack of the model to represent the realistic scale PRO osmotic power plant, the mathematical model of a scale-up PRO osmotic power plant is developed based on the validated non-linear transport equations of water and solute flux across the membrane, and the flows in the membrane channels. The developed PRO model considers the detrimental effects in a scale-up process, namely internal concentration polarisation in the support layer, external concentration polarisation near the active layer in the draw solution channel, and the reverse solute permeation across the membrane. Then, on the basis of the developed model, the overall performance of the scale-up PRO due to the detrimental effects of the mass transfer is addressed. In a scale-up PRO process, the performance of the scale-up PRO process is significantly dependent on the dimensionless flow rate. Furthermore, with the increase of the specific membrane scale, the accumulated solute leakage becomes important. The preferred membrane to achieve the optimal performance moves to the low permeability in order to reduce the detrimental effect of the reverse solute permeation. And counter-current flow scheme results in more evenly distributed water permeation across the membrane in a scale-up PRO process, compared to the co-current flow scheme. The counter-current flow scheme is capable to increase the process performance with a higher permeable and less selectable membrane compared to the co-current flow scheme. Moreover, different configurations and flow schemes of PRO are analysed and optimised in order to maximise the osmotic energy generation from the natural salinity gradients. Configurations includes single-stage PRO system, two-stage PRO system, hybrid reverse osmosis (RO) and PRO system, and hybrid solar photovoltaic and osmotic PRO powered RO desalination system in this work. The case study of the proposed PVROPRO plant developed based on the hourly solar data of Perth Australia in a year indicates that the highest weekly production rate is found to be almost 20 times the rate in PVRO in the same week. Annual production is increased more than nine times compared to the stand-alone PVRO plant. However, detrimental effects also potentially cause the weekly permeation production rate reduction in the range of 16-20% and the overall annual reduction is 18.07% in the case study of Perth. Moreover, in order to deal with the possible fluctuations of the 4 operating condition, maximum power point tracking (MPPT) control of a PRO osmotic power plant is studied. Two algorithms, perturb & observe and incremental mass-resistance method, are proposed and investigated. Both the algorithms are demonstrated to be capable of tracking the maximum power point. In order to improve the performance of the MPPT, furthermore, an optimum model-based controller (OMC) and the strategy to coordinate MPPT and OMC are developed and investigated by simulation. The results demonstrate the capability of OMC to deal with the rapid variations of the salinities.
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