An air-side economizer (see Figure 1 below) brings cool air from outdoors into a building and distributes it to the servers. Instead of being re-circulated and cooled, the exhaust air from the servers is simply directed outside. If the air outside is particularly cold, the economizer may mix warm exhaust air with incoming air so its temperature and humidity fall within safe operating ranges for the data center equipment. An air-side economizer is integrated into a central air handling system with ducting for both intake and exhaust; its filters reduce the amount of airborne particulate matter, or contaminants, that are brought into the data center.1 Both air-side and water-side economizers save enormous amounts of energy because they cool the data center without operating an air conditioner or mechanical chiller. That’s why data center operators may refer to the use of economizers as “free cooling.”
Not Just for Cold Climates
Because data centers must be cooled 24/7, 365 days per year, air-side economizers may make sense in hot climates, where they can take advantage of cooler evening and winter air temperatures. Even in northern Florida, an air-side economizer can be utilized for about half the year. In some cooler climates, such as San Francisco, air-side economization is possible for just about every day of the year! For various locations in the United States, Figure 2 shows the expected number of hours per year with ideal weather conditions for air-side economizer operation -- when the dry bulb temperature is less than 81 degrees Fahrenheit and the dew point, a measure of humidity, is less than 59 degrees Fahrenheit.
For Technology Leaders, Air-side Economizers are Yesterday’s News
Some of the most prominent technology companies in the world have been using air-side economizers for years. In 2008, Google began operating a data center in Belgium that used only outside air for cooling.2 Microsoft opened a 300,000 square foot facility in Dublin in 2009 that uses only air-side economizers for cooling -- and allows for server inlet temperatures as high as 95 degrees.3 In 2010, Yahoo opened its “chicken coop” data center in upstate NY. It relies entirely on outdoor air for cooling and consists of long, narrow building, like a chicken coop – a shape that makes it easier to circulate air and take advantage of prevailing winds.4
High Density? Not a Problem
Historically, a data center manager would simply add floor space to accommodate more racks and servers in order to meet new demand for computing services. This approach eventually got very expensive, so data center managers responded by increasing the amount of computing equipment contained in a given rack. This reduces the physical footprint but increases the amount of exhaust heat in a server rack. Fifteen years ago, the power draw per rack averaged 2-4 kW in data centers.9 With high density computing, the power draw today can be as high as 60 kW per rack.
Data center operators have successfully deployed air-side economizers in high density settings. Intel used free cooling at a data center with 240 servers and 960 cores per rack, running up to 43 kW of power per rack. Overall, the data center’s IT load averaged 1100 watts per square foot. With server inlet temperatures set at 95 degrees Fahrenheit, free cooling was possible for all but 39 hours in an entire year! With air-side economizers, the Data Center’s Power Usage Effectiveness (PUE) was a mere 1.07.5
Side Benefit: Risk Reduction
Air-side economizers, like water side economizers, offer cooling redundancy because they can provide some cooling in the event that mechanical cooling systems go offline. This can reduce the risk of data center down time.
Savings and Costs
A number of case studies document air-side economizer savings.
- In 2013, the U.S. Headquarters of Marvell Semiconductor6 in Santa Clara, California added an air-side economizer to their 5,000 square foot data center with 720 kW of IT load. Chilled water is supplied by three air-coiled chillers of 340, 340, and 310 tons. As shown in Table 1, the air-side economizer paid for itself in 2 years without the utility incentive and in 1.5 years with the utility incentive.
Table 1: Air-side Economizer Data Center Retrofit Costs and Savings
Design and Construction of Economizers Cost ($)
City of Santa Clara Municipal Electric Utility Rebate ($)
Ave. Monthly Energy Use 12 Months Prior to Economizer Install (kWh)
Ave. Monthly Energy Use 12 Months After Economizer Install (kWh)
Monthly Energy Use Reduction (kWh)
Monthly Savings ($)
Annual Savings ($)
Payback without Incentive (years)
Payback with Incentive (Years)
- A 1,400 square foot data center in a large office building added air-side economizers when it replaced its air cooled chiller. The facility subsequently realized total energy savings of 45%.7
- NetApp's Global Dynamic Laboratory (GDL) – the first data center to earn the Energy Star label – uses air-side economizers to operate without a chilled water plant for more than 75 percent of the year (i.e., “full free cooling”).8 In addition, the company is able to use outside air for partial free cooling more than 98 percent of the time. Since the “return air” (air exhausted from server racks) is 90 degrees, any outdoor temperature below 90 degrees allows for at least partial free cooling. As a result, the data center's water plant operates nearly 90 percent less frequently than that of a typical data center. (The diagrams in Figure 3 show air flow and temperature tolerances for free cooling, partial free cooling, and “pay” cooling modes). Air-side economizers and other design approaches (e.g., elevated supply-side air temperatures, cold-aisle containment, pressure-sensors to minimize fan use, and the elimination of most ductwork) reduced building costs by over 66 percent and ongoing operating costs by roughly 60 percent.
Tips and Considerations
- Integrated air-side economizers provide additional savings compared to non-integrated packages. Integrated air-side economizers take in fresh air when the temperature outside falls below the temperature of return air (air exhausted from server racks) rather than waiting until the outside air temperature falls to the supply temperature (server intake) set-point. The total cooling load required of the compressors is reduced by cooling outside air rather than the warmer return air.10
- Retrofitting an existing data center is not always simple or inexpensive. Software giant Oracle did not find an economizer retrofit to be an economical as there was no convenient way to bring the large amounts of outdoor air needed on raised data floor with down flow design. There were also concerns about humidity control problems and other contaminants11
- Humidity control can cut into the savings achieved by an air-side economizer. In certain geographic locations, for example, air can be very cool but also very dry; consequently it may be necessary to expend a lot of energy humidifying the air. Data center operators will need to weigh ASHRAE's recommendations for data center humidity, ambient climate data, and their IT equipment’s humidity tolerances prior to implementation. Proper management and controls are imperative to ensure that correct air volume, temperature and humidity are introduced.12 If target humidity ranges are too restrictive, net energy savings from an economizer can be disappointing.
- Some studies have argued that humidity controls perhaps are unnecessary. However, others point to increased equipment failure rates at high humidity. For example:
- In August 2008, Intel conducted a 10-month study13 to assess the effectiveness of using only outside air to cool a data center. The temperature range was 64°F to 92°F. Humidity varied from 4% to over 90% and changed rapidly at times. No increase in server failure was observed
- A 2016 study concluded that data center managers should be aware of humidity levels in their data centers when operating air-side economizers. Researchers collected data from nine Microsoft data centers worldwide that host over 1 million disk drives. Six were located in areas characterized as hot and humid, while three were located in cool and dry conditions. The dry data centers had the lowest failure rates regardless of cooling technology used. Data centers in hot, humid areas with the highest internal relative humidity had failure rates 107-260% higher than the lowest failure rate (observed at a chiller-cooled data center in a cool and dry area.) Internal relative humidity seemed to have the biggest impact on disk lifetime, with average temperature exerting a lower (but still significant) impact, the researchers note.14
- Utility PG&E, along with Lawrence Berkley National Labs, found that data centers with air-side economizers had higher particle concentrations than those that introduce minimal, if any, outside air. However, an improved filter design may mitigate any contamination concerns.15 Using ASHRAE Class II, 85% filters, the authors of the report estimated that the particle counts in facilities with economizers would rival the small amount found in data centers without economizers. (In their study, data center CRACs and air handlers were equipped with ASHRAE Class I, 40% filters).
1 PG&E. Data Center Best Practices Guide. Available online at: http://www.pge.com/includes/docs/pdfs/mybusiness/energysavingsrebates/incentivesbyindustry/DataCenters_BestPractices.pdf (PDF, 2.3 MB)
2 Miller, Rich. Google’s Chiller-less Data Center. Data Center Knowledge. July, 2009. Available online at: http://www.datacenterknowledge.com/archives/2009/07/15/googles-chiller-less-data-center/
3 Miller, Rich. Microsoft’s Chiller-less Data Center. Data Center Knowledge. September, 2009. Available online at: http://www.datacenterknowledge.com/archives/2009/09/24/microsofts-chiller-less-data-center
4 Yahoo Unveils Chicken Coop Data Center. Environmental Leader. September, 2010. Available online at: https://www.environmentalleader.com/2010/09/20/yahoo-unveils-chicken-coop-data-center/
5 Musilli, John. Vaccaro, Paul. Intel IT Redefines the High Density Data Center: 1,100 Watts/Sq. Ft. Presented at SVLG Data Center Efficiency Summit. November 5, 2014
6 Alipour, Mehrdad. Economizer for Data Center. ASHRAE Journal. October 2013.
7 PG&E. Data Center Best Practices Guide. Available online at: http://www.pge.com/includes/docs/pdfs/mybusiness/energysavingsrebates/incentivesbyindustry/DataCenters_BestPractices.pdf (PDF, 2.3 MB)
8 Skiff, Mark. NetApp Data Center Design. Presented at the SVLG Data Center Efficiency Summit. November, 2014. Available online at: http://svlg.org/wp-content/uploads/2014/11/NetApp-Data-Center-Design-SVL...http://svlg.org/wp-content/uploads/2014/11/NetApp-Data-Center-Design-SVLG.pdf (PDF, 3.9 MB)
9 What Does High Density Mean Today? by Jeff Clark, November 8, 2011, The Data Center Journal. http://www.datacenterjournal.com/what-does-high-density-mean-today/
10 Data Center Best Practices Guide, PG&E. Available online at: http://www.pge.com/includes/docs/pdfs/mybusiness/energysavingsrebates/incentivesbyindustry/DataCenters_BestPractices.pdf
11 EPA Conference on Enterprise Servers and EPA Conference on Enterprise Servers and Data Centers: Opportunities for Energy Data Centers: Opportunities for Energy Savings, January 31 to February 1, 2006. Available online at: https://www.energystar.gov/sites/default/files/asset/document/MKhattar_Case_Study.pdf (PDF, 2.3 MB)
12 Utilize “Free Air” to significantly reduce your operation costs by implementing Air-Side Economizers by Data Center Solutions. http://www.42u.com/cooling/air-side-economizers.htm
13 Intel: Servers Do Fine with Outside Air, by Rich Miller, Data Center Knowledge. September 18, 2008. http://www.datacenterknowledge.com/archives/2008/09/18/intel-servers-do-fine-with-outside-air
14 Beware of Humidity in Free-Cooled Data Centers, Buildings, April 2016. Available online at: http://www.buildings.com/article-details/articleid/20218/title/bewae-humidity-in-free-cooled-data-centers.aspx
15 Data Center Economizer Contamination and Humidity Study (PDF, 3.4 MB) by A. Shehabi, W. Tschudi, and A. Gadgil, Pacific Gas and Electric/Lawrence Berkeley National Laboratory, March 2007.