Australian Potential for PRO-Assisted Desalination

Seawater desalination is an energy intensive process. In Australia, the desalination industry heavily relies on the energy from fossil fuel combustion, and as such, the industry is deemed to significantly contribute to climate change. Incorporating forms of renewable energy in seawater desalination to combat increasing CO ₂ emissions and to help plants achieve better energy efficiency is becoming a necessary condition, but more research is needed, particularly on what sources of renewable energy would best suit desalination in terms of technical and economic aspects. This study investigates a form of reducing the excessive consumption of energy in seawater desalination based on the utilization of the energy harnessed from the mixture of brine (the main seawater desalination byproduct) and seawater. When seawater is mixed with brine, a great amount of energy is released from the salinity gradient between these two solutions. The power generated from salinity gradients is referred to as 'osmotic power' and is completely renewable and greenhouse gas emission free. Pressure-Retarded Osmosis (PRO) is a technology used to generate osmotic power. In this study, we suggest the use of PRO to generate power for the desalination process. Two major Australian desalination plants are used as case studies - the Perth Seawater Desalination Plant and the Southern Seawater Desalination Plant. Calculating the energy released from the salinity gradient between brine and seawater, estimating the quantity of energy that could be potentially harnessed using the PRO technology and calculating the energy offset in the desalination process, were the main objectives of this study. The maximum energy extractable during the mixing of solutions with concentrations similar to brine and sweater in a PRO system is approximately 1.1 MW per m ³ s ^-1 of seawater (Helfer and Lemckert, 2015). In this current study, a plant efficiency of 70% was adopted (Loeb, 2001), making the extractable energy 0.77 MW per m ³ s ^-1 of seawater. For Perth Seawater Desalination Plant and the Southern Seawater Desalination Plant, which produce about 60 GL and 140 GL of brine per year, respectively, the combined amount of osmotic power that could be generated is 43 GWh. This would correspond to approximately 7% of the requirements of energy for the seawater desalination process. The preliminary results showed in this study are promising, but an investigation to estimate the costs involved in building a PRO plant is required in order to determine the financial viability of PRO-assisted desalination. In a future study, a different scheme will be investigated, using Australian desalination plants operating in “stand-by mode” as case studies. It has been suggested that, for such plants, the membrane modules of the RO process could be used under PRO, rather than RO conditions, during times when the plant is not being required for freshwater production. Under this proposed configuration, the utilization rate of the desalination structure would be significantly augmented, as the plant would be generating energy instead of producing freshwater in periods of high water availability; conversely, it would be producing freshwater, instead of generating energy, in periods of severe water scarcity. Low plant utilization rates have been a common public concern in several coastal cities in Australia and discussions towards this issue has sparked debate of how to justify costs of constructing and maintaining desalination infrastructure. Designing these plants in such a way that they could be utilised for dual purpose (ie, PRO power and RO desalination) would be one way to justify these high investments. The results of this investigation will be reported in a future publication.