Comparison of energy efficiencies of a small centrifugal pump at constant and variable speed operations

Abstract

The objective of this study was to compare the energy efficiencies of flow rate valves used in different lines and variable speed drive (VSD) in a small centrifugal pump irrigation system. The tests were done by using an outlet valve, inlet valve, by-pass valve, and VSD. The study included four replications of constant speed and variable speed experiments, and three replications of constant pressure experiments. In each test, power consumption, inlet pressure, and outlet pressure were measured at different flow rates. During the constant speed tests at about the operating point, by-pass valve saved energy up to 66% and 5% compared to the outlet valve and inlet valve, respectively. Reducing the flow rate by 20% resulted in 7% less energy consumption with the use of both the by-pass valve and the inlet valve, and 19% more energy consumption with the outlet valve. The use of VSD showed profound advantage over the valves used in constant speed tests, with 41%, 44%, and 80% less energy demand compared to the by-pass, inlet, and outlet valve, respectively. Also, VSD and by-pass valves were compared in constant pressure operations. VSD offered 2 to 37% less energy consumption at pressures from 4.0 bar to 2.5 bar. The savings were less at high flow rates and quickly increased as the flow rate need decreased. The low system efficiency found in constant speed tests suggested that the pump was not appropriate for the hydraulic system used in low pressure applications. According to constant pressure tests, the system efficiency for VSD (26-29.1%) was greater than that of the by-pass valve (21.3-25.5%). In conclusion, the VSD was the most energy efficient method and suggested significant energy savings in small powered pump systems.