A major factor that can influence power usage effectiveness (PUE) in a data center is the type of cooling system deployed and whether or not it includes free cooling.

Free cooling is the ability to leverage existing local ambient conditions to reduce energy usage. There are several latitudes and elevations around the world where data center managers can reduce the use of CRAC units and associated cooling costs by taking advantage of outside environmental conditions. According to reports by HIS, the market for free cooling technologies will grow faster than any other cooling methods over the next four years.

Air-Side vs. Water-Side

There are two forms of free cooling—air-side cooling and water-side cooling.

Air-side economizers bring cool outside air into the data center to cool servers below the specification outlined in ASHRAE’s Thermal Guidelines for Data Processing Environments, 3rd Edition. To make use of 100% air-side economization, high airflow rates are required in a data center. Also, proper filtration and some level of humidification/dehumidification are typically needed depending on humidity levels. This can be a key consideration since dehumidification requires mechanical refrigeration—exactly what free cooling is trying to avoid in the first place.

The TIA-942-A Data Center Standard for Classes A1 or A2 in ANSI/TIA-569-C recommends a temperature range of 18 to 27 °C (64 to 80°F) with a maximum dew point of 15 °C (50°F). The allowable temperature range per ASHRAE 2011 Class A2 specifications is 10 to 35?C (50 to 95°F) with a maximum dew point of 21 °C (70°F). This allows an extended number of hours that can be used for free cooling.

Water-side economizers segregate the outside air from the inside air and provide cooling through a heat exchanger, without bringing the outside air into the facility. Water-side economizers are typically not as efficient as air-side economizers, but still provide very good efficiency numbers. In some areas, sources of cold water from rivers, lakes or oceans can be brought into a data center rather than using traditional closed-loop water chiller systems.

What Are the Benefits?

The use of air-side or water-side economizers offers huge advantages. When the outside air is cold enough, a significant amount of energy can be saved through either a liquid-air heat exchanger or by just bringing outside air in. Even if free cooling options are not feasible all year round, the use of these technologies for just a few months a year can take the load off of installed CRAC units, significantly extending their life and lowering energy and maintenance costs.

An example of the improvement in PUE through the use of free cooling is shown in Figure 6 of The Green Grid white paper #49-PUE, A Comprehensive Examination of the Metric PUE. This Figure shows an improvement in PUE of approximately 0.4 when the outdoor temperature is less than 15 °C (50°F).

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Where Can You Get It?

There are many regions throughout the world where the outdoor temperature remains cool and the humidity is low enough for several months of the year to take advantage of free cooling for the data center. The Green Grid white paper #46, Updated Air-Side Free Cooling Maps: The Impact of ASHRAE 2011 Allowable Ranges, estimates the number of hours that can be used for free cooling in different regions of North America, Europe and Japan.

Areas north of the 36° N latitude (around North Carolina) have more than 200 days or about 5,000 hours of air-side free cooling, while the majority of Canada offers more than 330 days or about 8,000 hours. In fact, free cooling is available all year round in more than 75% of North America and in 97% of Europe. This makes some regions more appealing than others to big data providers. Climatic conditions in countries like Iceland and Sweden have allowed typical data center cooling systems to be eliminated all together, and a large HP data center in England uses free cooling all but 20 hours a year.

It’s important to remember that free cooling is not completely free as its name implies. Pumps, fans, filters, dehumidifiers and other air and water-handlers may be required for free cooling—all of which requires maintenance. But with cooling accounting for about 70% of a data center’s non-IT energy consumption, the efficiency that can be realized with free cooling makes this technology well worth considering.