Assessing variable speed pump efficiency in water distribution systems

Energy savings and greenhouse gas emission reductions are increasingly becoming important design targets in many industrial systems where fossil fuel based electrical energy is heavily utilised. In water distribution systems (WDSs) a significant portion of operational cost is related to pumping. Recent studies have considered variable speed pumps (VSPs) which aim to vary the operating point of the pump to match demand to pumping rate. Depending on the system characteristics, this approach can lead to considerable savings in operational costs. In particular, cost reductions can take advantage of the demand variability and can decrease energy consumption significantly. One of the issues in using variable speed pumping systems, however, is the total efficiency of the electric motor/pump arrangement under a given operating condition. This paper aims to provide a comprehensive discussion about the components of WDS that incorporate variable speed pumps (including electric motors, variable frequency drives and the pumps themselves) to provide an insight of ways of increasing the system efficiency and hence to reduce energy consumption. In addition, specific attention is given to selection of motor types, sizing, duty cycle of pump (ratio of on-time and time period), losses due to installation and motor faults. All these factors affect the efficiency of motor drive/pump system.