Description:

This measure applies to centrifugal chillers, which is a common type of cooling plant for large office buildings. At low load, the efficiency of these chillers is less than the full load efficiency. Also, these chillers have limited turndown capability. Part-load efficiency and turndown can be improved by converting the compressors from constant speed to variable speed through the addition of a VFD.

For cooling plants with multiple chillers, one chiller can be retrofit with a VFD and used as the lead, since it will operate most efficiently at part load. The other chillers in the plant can come online when the load increases beyond the load of the one chiller. Then the VFD-equipped chiller modulates to maintain the setpoint while the other chillers operate fully loaded.

Special Considerations

• Not all centrifugal chiller compressors can be retrofitted with a VFD, due to refrigerant performance and torsional resonance. The chiller manufacturer should be consulted before adding VFDs to the compressors.
• The VFD retrofit should be designed for the particular chiller model, and installed by a qualified person. To maximize energy efficiency, the existing chiller staging strategy should be modified to allow part load operation of the VFD-equipped chiller and full load operation of the constant speed chillers.
• The energy savings related to this measure depends on the building cooling load profile and the number and size of chillers in the plant. The more variable the cooling load profile and the fewer number of chillers, the greater the energy savings potential. It may be more efficient to install a small chiller to handle low loads than to install a VFD on one of the existing compressors.

Estimates for this measure come from the Department of Energy's Advanced Energy Retrofit Guide.