PAT as a strategy for optimal WDS management: leakage reduction and energy production

In water networks characterized by a significant variation in ground elevations the necessity of pumping water in some areas is complicated by a conflicting requirement to reduce excess pressures in other areas. This and the increasing cost of electricity has led to the use of Pumps-operating-As-Turbines (PATs) devices that both require energy to pump water or can reduce pressure (and leakage) whilst harvesting energy. This paper presents a methodology for selecting locations and operation of PATs in a water distribution system. The optimisation is driven by the minimization of leakage (via minimization of surplus pressure at network nodes), the minimization of operational pumping costs and the maximization of the income generated through energy recovery. Decision variables are locations and types of PATs to be installed and the related pump schedules and initial tank levels. The optimization method is based on a highly-parallelized Evolutionary Algorithm, the multi-objective Omni-Optimizer algorithm, employing a pressure-driven solver to evaluate hydraulic constraints. The water demands at network nodes are considered as uncertain variables modelled by using a probabilistic approach in order to take into account unknown future demands. The approach is demonstrated on two case studies. The results obtained highlight that the economic benefits of installing PATs for energy recovery in conjunction with a combined pump-scheduling and pressure management regime is especially related to the input network characteristics.