Planning for seawater desalination in the context of the Western Cape water supply system

[摘要] ENGLISH ABSTRACT: South Africa has historically been reliant on inexpensive surface and groundwater resources; however, as pressure on these resources continues to grow, seawater desalination has begun to emerge as a potential future supply source. One of the towns earmarked as a candidate for large-scale seawater desalination is Cape Town. In order to maximise the benefits and minimise the costs of such a scheme, the desalination plant needs to be considered as an integral part of the current system. Integrated planning has been lacking at the existing seawater desalination plants in South Africa, most of which were constructed as emergency schemes and are financially cumbersome for the municipalities to operate and manage.Recent research related to inter-basin water transfer schemes has shown that a comprehensive approach is required in assessing water supply from a new scheme in which the system as a whole is modelled stochastically and the estimated water transfer extracted. This comprehensive approach was the foundation of the modelling undertaken in this research. Existing models of the Western Cape system were adapted to include a seawater desalination plant, and short-term and long-term analyses were completed for a variety of possible desalination plant operating scenarios and capacities. The increase in system yield and the annual supply from the desalination plant were determined. First-order capital and operating costs were estimated, and these costs were combined with the annual supply values to calculate and compare unit reference values.The maximum increase in yield was found to occur when the seawater desalination plant is used as a base supply, operational all the time. There was little benefit, in terms of system yield, in using the desalination plant as an emergency supply source only. Unit reference values for the desalination plant decrease as the percentage supply from the desalination plant increases, meaning that the lowest possible cost per cubic metre of water supplied is when the desalination plant is used as a base supply. It was also apparent that the unit reference values decrease with an increase in desalination plant capacity, suggesting that, from an economic perspective, the optimal solution would be to have one large desalination plant operational immediately. The lower the reservoir trigger level at which the desalination plant becomes active, the larger the stochastic variation in the supply from the desalination plant and hence the larger the variation in the costs. Hence, using stochastic modelling to calculate unit reference values is particularly important for integrating a desalination plant into an existing conventional supply system when used as a peak or emergency supply source.

[发布机构] Stellenbosch University