Energy consumption of HVAC variable-speed pumping systems

Variable speed drives are widely used in heating ventilation and air-condition (HVAC) pumping systems in order to save energy under part load conditions. However, the efficiency of variable speed pumping systems has not been well investigated. The effects of system configuration, control method, and load profile on the energy consumption has not been well examined either. Three stages of research were carried out to explore these issues.; First, the efficiencies of the motor and drive at different speeds, torques and drive switching frequencies were measured. The test results show that the motor and drive efficiencies vary with motor torque and speed. The combined motor-drive efficiency is poor at low motor speeds and torques. The drive switching frequency has a significant impact on both motor and drive efficiencies. The drive efficiency drops significantly with increasing switching frequency.; Second, a real variable speed pumping system was built with similarity to an actual HVAC pumping system. The overall pumping system efficiency (wire to water efficiency) was measured as well as the efficiencies of each component over a wide range of operating conditions. The test results show that at a fixed system flow rate, the overall system efficiency decreases with increasing pump speed. At a fix pump speed, the overall system efficiency decreases dramatically when the flow rate is reduced to below 50% of the maximum flow rate under that pump speed.; Third, energy consumption simulations of the model pump system were performed. Different load profiles, system configurations and control parameter settings were applied to the model to evaluate their impacts on pump energy consumptions. The simulation results show that the load distribution affects the dynamic system curve and pump energy consumption dramatically. A smaller pressure offset reduces the pump energy consumption. A significantly oversized pump causes the pump to work less efficiently. Based on the simulation results, some control concepts were also proposed to improve the energy efficiency of variable speed pumping systems. Parallel pump operation can increase the system efficiency at part load conditions, especially when the system load is below 50% of the full load. Variable pressure offset control and direct digital control can control the pump output to match the system load more closely than conventional differential pressure control resulting in greater energy savings.