Robust Operation of a System of Reservoir and Desalination Plant using a Multi-Objective Optimization Framework

In many cities, the water supply system is under stress due to increased competition for reliable fresh water supplies from population growth and climate uncertainties resulting in water insecurity. One method to augment fresh water supplies is seawater desalination, which converts seawater to fresh water for industrial and domestic potable and non-potable uses. We propose to address this issue of water supply scarcity and uncertainty in coastal metropolitan cities by developing a robust operating policy for the joint operation of a desalination plant with a freshwater reservoir system using a multi-objective optimization framework. Due to the unlimited availability of seawater, desalination has a strong potential as a reliable source of water in coastal cities around the world. However, being an energy intensive and expensive process, its application is limited. Reservoir water, while cheaper due to its relatively small cost of transportation to the cities, is often limited and variable in its availability. We observe that combining the operation of a desalination plant with a water supply reservoir leads to more cost efficient and reliable water production than if both were to be operated separately. We model a joint reservoir-desalination system as a multi-objective optimization problem with risk, resilience, and vulnerability as the objective functions, and cost as a constraint. In our simulations, rule curves determine the release from the reservoir as a function of existing storage level, and the remaining demand that is unmet by the release from the reservoir determines the amount of water produced from desalination. The overall cost of the system is the sum of the cost of transporting reservoir water and the cost of energy of desalinating seawater. We employ a genetic algorithm to find the optimal values of the thresholds of the reservoir rule curves and the maximum operating capacity of the desalination plant. We will discuss the tradeoffs between water supply risk, resilience, and vulnerability with and without seawater desalination to demonstrate the potential of the latter as a means of managing uncertainty. We will also discuss the additional cost necessary to improve robustness and implications for large metropolitan cities.