Practical Approach to Water System Optimal Operation

Abstract

Optimal pump scheduling is a major consideration when dealing with minimizing operational costs of a water distribution system. Pump operation must balance between three factors. Water balance constraints, including consumer demand and water tank volumes. Hydraulic constraints determining water pump operating point. Electrical tariff rate effecting energy cost.Optimization models may assume linear or discrete pump operation, depending on type and accuracy of the model in use. Linear operation assumes the pump may operate during part of the time step while discrete operation requires the pump to be either on or off during the entire time step. Linear optimization models commonly have short solution times, but cannot contain non-linear constraints such as hydraulic headloss. By such, linear model results may be difficult to implement in a real water system as the hydraulic behavior of the system may render the optimal solution impractical. Likewise, if the pump operation partially uses the time step the pump may be forced to come in and out of duty often causing mechanical ware and tare.Discrete operation provides smooth pump operation and may contain non-linear hydraulic constraint to calculate a more realistic working point for the pump. Discrete models have long solution times due the vast amount of pump operating combinations, which must be explored. Heuristic techniques may be used to shorten solution times but these do not assure global minimization of the solution.The goal of the research is to create a minimum cost optimal operation water distribution system model that utilizes the short solution time of a linear model but also includes non-linear hydraulic constraints effecting pump energy consumption and discrete pump operation. The motivation is to use the model for real-time pump scheduling and for water system design.