Question:
How can we improve the efficiency of a chiller plant that serves an office building?
Answer:
Chillers are big consumers of energy in the commercial and industrial sectors. Energy savings from chilled-water systems can come from two sources:
- Improving the coefficient of performance (COP) of the chiller by reducing the temperature difference between condensing and evaporative temperatures, and through efficient equipment selection (if applicable).
- Efficiency improvements in the auxiliary systems. These include the cooling tower/condenser water loop and chilled water distribution networks. Recent advances in technology have impacted both chiller and chilled-water distribution network design.
Improving COP
The easiest way to improve the COP of a chiller is to buy an efficient chiller in the first place. Many owners had this opportunity when older refrigerants were phased out several years ago, but there are probably still thousands of older chillers that will need to be replaced. Chiller efficiency has increased markedly in the last twenty years—from over 1 kW/ton, down to about 0.58 kW/ton. From the Flex Your Power Chillers website: "Chillers have been significantly reengineered in recent years to have full-load efficiencies of 0.47 kilowatts (kW)/ton in the 170- to 2,300-ton range. " Also, during that time, a number of new, non-halogenated refrigerants have been introduced that meet EPA standards.
For comparisons of different chillers, the U.S. Department of Energy’s Federal Energy Management Program offers simple screening tools that estimate a product's lifetime energy cost savings at various efficiency levels. Because the calculation depends on a very rough estimate of annual cooling hours, the energy cost estimate is also very rough, although it would be reasonable for comparing the relative costs of two chillers with different efficiencies. :
Improving Auxiliary Systems
There are a number of ways to improve the COP of the chilled water system without buying a new chiller, including:
- Clean the water and air sides of chilled-water coils—dirty coils reduce heat transfer effectiveness.
- Clean chiller condenser and evaporator heat exchangers to improve heat transfer.
- Clean cooling towers and replace fill to restore cooling tower capacity.
- Increase chilled water delta-T by modifying the chilled water distribution system.
One of the most significant changes in chilled water system design over the last twenty years has been the replacement of three-way control valves (which use constant-volume chilled water pumping) with two-way control valves with variable-speed chilled water pumping. This innovation reduced an old and chronic chiller efficiency problem called "low delta-t.". Another Q&A from the Energy Solutions Database on this subject is Cooling system Control Valves. An article on the subject is
"With Low Delta T, Where Does the Excess Flow Go?" from Flow Control Industries.
Identifying Energy Saving Opportunities
The U.S. Department of Energy’s Energy Efficiency and Renewable Energy Program provides a handy tool called Chilled Water System Analysis Tool (CWSAT). The CWSAT helps you estimate the annual cost of running a chilled water plant and includes the energy used by the condenser pumps, chilled water pumps, and the cooling tower. With this information you can easily decide when it is economical to replace an older, inefficient chiller, upgrade the chilled water distribution system or condenser water pumps, replace or enlarge a cooling tower, and compare different chilled water plant designs.
Additional Resources
Articles on chiller sizing and chilled water system design in the Trane Company's engineers newsletter:
- "Series Chillers and Variable Primary Flow (VPF) Chiller Plants". Vol. 38, No. 4, 2009.
- ""free" cooling using
Water Economizers" Vol. 37, No. 3, 2008.
- "Don't Overlook Optimization Opportunities in ‘Small’ Chilled Water Systems,", Vol. 32, No. 4, 2003.
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"Variable-Primary-Flow Systems Revisited," Vol. 31, No. 4, 2002.
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"Variable-Primary-Flow Systems," Vol. 28, No. 3, 1999.
Mick Schwedler, P.E., and Beth Bakkum. "Upgrading Chilled-Water Systems". ASHRAE Journal. November 2009.
Piper, James. "HVAC Efficiency: Chillers". February 2008. Maintenance Solutions. The Facilitesnet website is worth searching further for articles on chillers and chiller efficiency.
U.S. Energy Star:
U.S. DOE. Federal Energy Management Program: