Development, validation and application of an energy model for energy efficient operation of parallel pump systems

Abstract

Parallel pumps are commonly applied in chilled water and hot water systems to handle broad water flow variation. The application of variable frequency drives leaves the sequencing control of parallel pumps a tricky problem, especially if the pumps have various sizes or/and experience various speeds. The objective of this paper is to develop and validate an energy model to optimize the operation of parallel pump systems. First a comprehensive energy model is developed based on the affinity and Kirchhoff Laws. The pump installed head is applied to eliminate the impact of the pump fittings and the drive efficiency is considered to obtain the true electrical power. Then, the energy model is constructed and validated for a parallel pump system, which has three 284 L/s pumps with one as a backup pump and each powered by a 150 kW motor. Finally, the validated model is applied to identify the energy efficient operation. The results reveal that one-pump operation outperforms two-pump operation at lower flow range with the switchover flowrate decreasing from 300 L/s to 240 L/s as the controlled pump installed head varies from a constant setpoint to a reset variable setpoint. Moreover, faulty various speed settings result in the electrical power waste of 20 kW for the lower-speed pump.