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Content Type:Q&A

Revised November 2013

Question:

How can we improve the efficiency of a chiller plant that serves an office building?

Answer:

Before making any changes to the chiller system, it is important to discuss your ideas and options with the chiller manufacturer and a design professional. The chiller system is a complicated system and many site specific variables will be part of any decisions made.

Measuring Chiller Efficiency

In the past, the way chiller efficiency was compared among types and manufacturers was by looking at the power needed per ton of cooling, or kW/ton, at full load. The more efficient the chiller is, the lower the kW per ton will be. However, chillers have evolved and now have a new unit of measure for comparative analysis, the Integrated Part Load Value. The IPLV provides a way to also consider performance at part load. This is important because chillers rarely operate at full load, usually less than 5% of the hours per year. The primary feature in chillers today that allows for part load operation is Variable Frequency Drive technology.

Variable Frequency Drives

Equipment that can operate at part load, instead of a bypass system, is more efficient. A chiller compressor with variable frequency drives (VFD), or staging, allows the equipment to operate only as much as needed to match the load. VFDs can also be added for the pumps that serve the evaporators and condenser, and for the cooling tower blowers. It will be important to discuss these options with a design professional because this measure will lead to:

  • Elimination of most of the 3-way valves
  • Control systems that can provide proper set-points
  • Pressure independent flow control valves

Improving COP (Coefficient of Performance)

A chiller system is comprised of the chiller, the heat rejection system, and the pumping systems. In an office building, these systems need to meet a variable load, operating at close to 100% capacity in the summer, and down to less than 10% in the winter. There are a number of ways to optimize the chiller plant, including installation of high IPLV equipment, cooling water with an oversized tower, modularizing for load matched design, chiller type selection based on load profile, and/or installing variable flow.

Water Cooling: Water cooled chillers that use a cooling tower are much more efficient than air cooled chillers. In fact energy codes now limit the size of air cooled chillers allowed at a site. Water cooled chillers offer the opportunity to bypass the chiller on cooler days when evaporative cooling from the cooling tower can meet the cooling load in the building. Also, over-sizing the cooling tower will not only extend the hours that evaporative cooling can occur, but will also provide a lower condenser water temperature when used with the chiller, allowing the chiller to operate more efficiently. The additional cost for over-sizing the cooling tower is usually recovered in less than 5 years due to energy savings. Dry climates with low wet bulb temperatures will see an even shorter time before payback. All water systems need to have water treatment for good transfer of heat.

Proper water treatment: Proper water treatment will prevent scale, corrosion and biological matter from negatively impacting the water's capacity to transfer heat.

Set-point adjustments: Adjusting the difference in supply and return water temperatures will allow the systems to operate more efficiently. If the temperature difference in the water can be increased from the typical 12 to 15 degrees, to 20 degrees F, pumping energy can be reduced. Also, reducing the supply chilled water temperature a few degrees will allow the pumps to deliver the same cooling capacity at over 30% less flow. Operating the pump at lower flow is like increasing the size of the piping. There is lower resistance to flow, requiring less power to deliver the water to the zones.

Maintenance: A good maintenance team will enable peak performance from equipment. Maintenance personnel will have operating logs. Operating logs highlight anomalies such as refrigerant leaks, refrigerant quality, energy use trends, sensor calibration, pressure and temperature changes, etc. A 1 degree error in water temperature can affect the system efficiency proportionally. Therefore, it is important to use quality sensors and meters.

Information Resources

Mick Schwedler, P.E., and Beth Bakkum. "Upgrading Chilled-Water Systems " . ASHRAE Journal. November 2009.

Piper, James. "HVAC Efficiency: Chillers". Maintenance Solutions. February 2008. The Facilitesnet website is worth searching further for articles on chillers and chiller efficiency.

U.S. DOE. Federal Energy Management Program:

Topic: Building/Space Type--Office
Topic: Heating/Cooling/Ventilation--Chillers
Topic: Heating/Cooling/Ventilation--Cooling Towers
Sector: Commercial
Sector: Industrial
Content Type: Q&A
Keywords: Centrifugal chillers
ID:  4368