Impact of the Control Variables on the Efficiency and Operating Range of a Francis, Kaplan, and Pump-as-Turbine for Hydropower

Hydropower is a mature technology that is installed globally for energy generation and storage. In this study, the impact of the variable control of different control variables on the efficiency and operating range is analyzed for three different turbine types: a Francis pump-turbine, a horizontal Kaplan bulb turbine, and a pump-as-turbine. For each turbine, an efficiency map is generated that stores the most efficient combination of the control variables for the full operating range in terms of varying fallhead and power. To quantify the impact of each control variable, the methodology is repeated with one or more variables fixed. The results show that the variable control of an inlet valve in the system can significantly increase the operating range (respectively by 32.9%, 53.8%, and 72.3%). However, in a system with guide vanes already present, no efficiency increase is obtained. Furthermore, it is found that implementation of a variable speed drivetrain also increases the operating range, although less pronounced. In addition, the efficiency is increased, especially for subnominal operating points, with average increases of 1.3%, 3.8%, and 1.0%, respectively. Finally, it is found that neither a variable speed drivetrain nor a variable inlet valve can compensate for the fixing of the guide vanes and/or blade angle, as these retain the highest impact on the system efficiency and its high-efficiency operating range.