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Chill Out With Energy Savings

Key Points
  • Chillers use a substantial amount of energy, particularly during the summer peak demand period.
  • A chiller efficiency strategy should take into account the dynamics of the entire cooling system.
  • Variable fan speed and waterside economizers are commonly used energy-efficiency retrofits.
Source: www.doe.gov
Chiller

Chillers are widely used for cooling large commercial and industrial facilities. While chillers are a mature and generally efficient technology, they still use a substantial amount of energy. If your energy budget is feeling the heat, chill out with these energy-saving strategies.

View the system as a whole

A chiller upgrade should encompass the dynamics of the entire system, not just individual components. For example, decreasing the condensing water temperature is one way to increase chiller efficiency. However, this requires additional cooling tower operation, which may increase total operating costs. In humid climates, increasing the chilled water temperature can save energy; but the coil size must be adjusted to maintain the system's ability to remove humidity. 

Upgrade and retrofit strategies

A number of chiller system design changes and operational strategies will help to improve overall system efficiency and reduce energy costs. Two of the most effective include varying fan speeds and installing waterside economizers.

Fan speed. In cooling towers, the use of two-speed fan motors, in combination with fan cycling, provides improved control and efficiency over fan cycling alone. Variable speed drives (VSDs) provide the most efficient method of control. Fan speed control is most effective in facilities with small cooling loads, and in moderate or dry climates. 

Economizers. Chiller bypass systems can be retrofitted into central plants, enabling waterside economizers to cool spaces with chillers offline. In these systems, the cooling tower provides chilled water directly with filtering, or indirectly with a heat exchanger. These systems are best suited to applications with significant chilled water requirements, and in climates where the outdoor temperature is below 55°F for at least 3,000 hours per year.

Additional chiller efficiency ideas include the following:

  • Reduce the temperature set point of condenser water (cooling tower) to improve efficiency at partial load. Check with the manufacturer to make sure the chiller will operate properly at a lower temperature.
  • Replace oversized water impellers, pumps and motors with smaller, energy-efficient units. Trim the pump impeller rather than using a balancing valve to reduce flow in pump speed applications that are constant; pump power can be reduced significantly.
  • Install VSDs on water pump motors.
  • Convert single-loop chilled water and the water flow configurations of the condenser to primary/secondary loop configurations. Replace three-way valves with two-way valves on cooling coils and implement variable-flow control on the chilled water loop.
  • Chilled-water storage tanks (typically >500,000 gallons) that are cooled during off-peak hours are economically attractive in larger buildings. Check with your utility provider for rebate programs on thermal storage systems.
  • Increase wall insulation. One option for existing buildings is to add an exterior insulation and finish system (EIFS). Use systems that include a drainage layer to accommodate for small leaks that may occur over time; avoid barrier-type systems. In some cases, the savings achieved by downsizing the chiller will pay for these types of energy improvements.

Chillers that are more than 10 years old are good candidates for replacement because refrigerant replacement issues may need to be addressed. Moreover, newer models are significantly more efficient. Currently, the most efficient chillers operate at efficiencies of 0.50 kW/ton, a savings of 0.15 to 0.30 kW/ton over most existing equipment. Be careful to size new chillers correctly for your application. Oversized chillers can waste energy and reduce efficiency gains.

 

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