Strategies to raise the off-design efficiency of chiller machines

Most HVAC systems in large buildings are equipped with centrifugal chillers which are typically designed for maximum efficiency at 70% to 80% of their full-load. But, below about 30% full-load, their part-load efficiency starts to deteriorate rapidly (increasing kilowatts per ton). For older centrifugal chillers, this rapid drop may start at even higher part-load, rendering their operation at low part-load undesirable. In many cases chillers are over-sized, forcing operation below 50% of the full-load most of the time. Furthermore, the field performance of most chiller machines is generally not documented, and there is no tradition of recording chiller performance history. There is growing incentive-driven interest in chiller monitoring. But, field visits, discussions with HVAC engineers and opinions from manufacturers have proven that virtually no chiller systems are currently monitored for kilowatts per ton. Only recently has field data gathering been initiated, and part-load performance of chillers in the field started drawing serious attention. The problem, however, still remains poorly disclosed to the end-user. As a result, chiller machines actually operate at much lower efficiency than the design values. There are opportunities for profoundly improving their field performance. Part-load operation is particularly poor for sites with single centrifugal chillers. This research is an appraisal of strategies for improving the part-load performance of chiller systems. A review of simulation results and field data indicate that chiller machines operate at higher kilowatts per ton than allowed for by current technology. Energy consumption of various chiller types and configurations for a generic building indicate that there are possibilities for lowering chiller energy consumption, and these opportunities can be harvested with simple measures.