Water-Side Economizer


  • For data centers with water- or air-cooled chilled water plants, a water-side economizer uses the evaporative cooling capacity of a cooling tower to produce chilled water and can be used instead of the chiller during the winter months. (A chiller is a machine that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool air or equipment as required.)
  • Water-side economizers offer cooling redundancy because they can provide chilled water in the event that a chiller goes offline. This can reduce the risk of data center down time.
  • Water-side economizers are best suited in climates where the wet bulb temperature is lower than 55°F for 3,000 hours or more. This describes the majority of the United States barring areas in the extreme Southwest and portions of the Southeast. (See Figure 15 below.)
    Figure 15: Hours of ideal water-side economizer conditions in the United States. (Courtesy of DOE and The Green Grid)

    Figure 15: Hours of ideal water-side economizer conditions in the United States. (Courtesy of DOE and The Green Grid)

  • Unlike air-side economizers, water-side economizers can be economically retrofitted.

Savings and Costs

  • During water-side economizer operation, costs of a chilled water plant are reduced by up to 70%.46
  • Water-side economizer retrofits in California have shown one- to four-year paybacks.47
  • Water side economizer retrofits can sometimes be costly.


  • Water-side economizers can be integrated with the chiller or non-integrated. Integrated water-side economizers are the better option because they can pre-cool water before it reaches the chiller. Non-integrated water-side economizers run in place of the chiller when conditions allow.
  • A variable speed drive should be used on the fan motor in the free cooling tower.
  • Like air-side economizers, this technology depends on design and controls to work optimally.
  • Data centers that use containment, along with a cold aisle or hot aisle configuration, usually have much higher server inlet temperatures. These higher temperatures can be produced using a higher chilled water set point. With intermixing eliminated, data center operators may be able to achieve their desired server inlet temperature with a 55°F or warmer chilled water supply. With these higher temperatures, water-side economizers can be used more often.
  • Physical space is a major concern. Oracle attempted a retrofit and concluded that the additional parts–new "pipes, valves, and controls"–required physical space that existing facilities simply did not have.48
46 High Performance Data Centers, January 2006, PG&E, p. 38.
47 Data Center Energy Practitioner HVAC Specialist Training Day 2.
48 https://www.energystar.gov/sites/default/files/asset/document/MKhattar_Case_Study.pdf (PDF, 123.27 KB)