Energy efficiency optimizing speed control method for reservoir pumping applications

Variable speed operation is often the most energy-efficient flow control method for pumping systems, as the pump performance can be adjusted to meet the process demand instead of adjusting additional hydraulic losses. Often, the process demand is a direct indicator of the rotational speed to be used. However, there are several pumping applications where the applied rotational speeds can be selected more freely. A reservoir pumping application without strict time limits is the primary example of a system having degrees of freedom for the selection of the rotational speed to be applied. In this application, the rotational speed selection is also affected by the time-varying process characteristics, which can be determined by separate measurements or preferably by a frequency converter without a need for additional instrumentation. This paper proposes a frequency converter-based method for finding the most energy-efficient rotational speeds for reservoir pumping applications, where the process static head varies during the pumping task. The proposed method relies on frequency converter-based identification of the process characteristics, which is introduced in detail. The energy efficiency of the resulting rotational speed profiles is studied both by simulations and laboratory measurements. The applicability of the studied speed control method in a common waste water pumping application is also discussed. As reservoir pumping systems are an elementary part of municipal waste water treatment, these systems are globally used and thereby have a notable energy conservation potential.