Optimizing Hydropower Reservoirs with Limited Flexibility on the Columbia River

Chelan County Public Utility District in central Washington owns and operates the Rocky Reach and Rock Island hydropower dams on the Columbia River. The dams have a combined generating capacity of 1924 MW but very limited reservoir storage. This presents significant challenges when managing the cascading reservoirs given variable inflows, a combination of licensing and environmental obligations, and the requirement to meet highly variable and uncertain power demand, shaped in part by a high level of variable wind generation in the region. Without careful management, reservoirs can quickly empty, forcing cuts in generating capacity, or they can quickly fill, leading to excessive spill, which not only represents lost generating potential but also produces high levels of total dissolved gas (TDG) resulting in high fish mortality. We will describe the RiverWare Optimization model Chelan uses to guide operations. The optimization meets formal operational requirements while positioning the reservoirs with flexibility to respond to forecast deviations. The model combines RiverWare’s Rulebased Simulation and Goal Programming Optimization solutions over three forecast horizons at a five-minute timestep. After meeting formal requirements, the priority-based solution uses remaining degrees of freedom for desirable operations, such as preventing spill, keeping reservoirs relatively balanced and preventing large changes in flows. Simulation of TDG formation and transport allows the model to alert the system of anticipated TDG problems. The model runs automatically every five minutes with results feeding into Chelan’s real-time control system.