Ambient Air Cooling

June 11, 2012 3 Comments »
Ambient Air Cooling

It has been observed that many data center construction projects omit ambient air cooling from their efficiency strategy. Data centers collectively, however, consume a significant percentage of all energy produced in the U.S. Data center economics are changing. Research suggests that ambient air cooling is a compelling data center cooling strategy because it reduces electricity usage significantly, is a major cost savings, is proven safe and is environmentally responsible.

Ambient air cooling reduces electricity usage significantly because air conditioning units can be turned off for periods of time and because lower static pressure air delivery methods can be used; ejecting hot air before it reaches the air supply lowers the amount of cooling energy needed even when air conditioning units are required.

One common misconception with ambient air cooling is that this approach precludes the use of normal chiller units. In fact ambient air cooling, a common economizer method, can be configured to automatically step in when outside air conditions meet computer room operating parameters such as during off-summer months and nighttime. Economizing with outside air can even be used at the same time as a normal chiller. Air side economizers are used to completely replace or more commonly augment normal chillers. “While temperate environments realize the quickest ROIs, nearly all geographies in the U.S. can attain some level of free cooling through air side economizers” (42U, 2012). Every hour that normal chillers are turned off, energy savings accumulate.

Electricity is required for fans to move air through a data center. “As with any fluid dynamic, the more obstructions and constraints on the air flow the higher the static air pressure and force required to overcome this friction” (Tooley, 2010). One of the objectives in using ambient air is to reduce electricity consumption. This objective can be optimized through a well-thought-out design. Today’s high-density data centers are known to require raised floors many feet tall in order to minimize air flow restrictions. Ventilated floor tiles are notorious for restricting air flow and providing the wrong amount of air to a given computer location. Raised floor tiles for the purpose of distributing air are an unneeded expense, and they create inherent cooling problems. Worst of all, air distribution through raised floor tiles creates static air pressure. “Air flowing through perforated floor tiles must be restricted to a much greater degree than the plenum to cool equipment uniformly” (VanGilder & Schmidt, 2005). The more static air pressure, the more fan energy will be required to move the same amount of air. “Data centers can be more efficient if designed on a slab floor now that many of the benefits of a raised floor are gone” (Sty, 2012). Lower energy use, a key objective of ambient air use, can be achieved by avoiding air distribution through high-static-pressure means like a raised floor and other static-pressure-generating duct work.

A cooling strategy used in many homes today is a whole house exhaust fan. A key reason for its use is that it requires less energy to exchange the hottest inside air with cooler outside air than it does to just cool the inside hot air. The hot air created in data centers greatly exceeds that found in homes. The cost savings advantage of exhausting hot air and replacing it with cooler ambient air is likewise greater in a data center.

The size of this opportunity is more significant than many IT leaders may realize. “Another impact of higher energy densities is that server hardware is no longer the primary cost component of a data center…The purchase price of a new (1U) server has been exceeded by the capital cost of power and cooling infrastructure to support that server and will soon be exceeded by the lifetime energy costs alone for that server. This represents a significant shift in data center economics that threatens to overwhelm the advances in chip efficiency that have driven the growth of digital information during the past 30 years” (U.S. Environmental Protection Agency ENERGY STAR Program, 2007). Electricity costs have now exceeded the cost of data center equipment. IT managers put significant emphasis on price negotiations with server hardware manufacturers. This economic shift suggests that a greater amount of attention should be put on managing data center power consumption than the cost of equipment acquisition.

In 2007 the EPA produced for Congress a report that found “by 2011 DC power consumption was estimated to double from the 2007 1.5% level. Under current efficiency trends, national energy consumption by servers and data centers could nearly double again in another five years (i.e., by 2011) to more than 100 billion kWh” (U.S. Environmental Protection Agency Energy Star Program, 2007). Data center power consumption may have reached three percent of all energy generated in 2011 in the U.S. The magnitude of the data center macro opportunity cannot be ignored. At 10 cents per kWh, 100 billion kWh would equate to 10 billion dollars in data center energy consumption in 2011. Data center power efficiency is a responsibility IT managers cannot ignore.

The efficient use of power consumption can be improved just as dramatically as its growth. NREL is the only national laboratory dedicated to the advancement of research, development, commercialization and deployment of renewable energy and energy efficiency technologies. Its legacy data center had an estimated power usage effectiveness (PUE) of over 3.0. Its new facility is designed to operate at 1.08. “The commercially available technologies employed at NREL reduced energy consumption by an average of 270%” (Sty, 2012). It is important to note that ambient air cooling was the most significant efficiency-gaining technology used by NREL. The case study at NREL shows the viability and scale of efficiency improvement that can be achieved in a data center through the use of ambient air cooling.

The size of the data center efficiency problem and opportunity with ambient air cooling is great. In fact, traditionally IT leaders have spent meaningful amounts of time negotiating lower IT equipment costs, but data center efficiency is now an even greater cost reduction opportunity. Industry leaders advocate IT leaders extend their computer management practices into the data center facilities. Forrester sites this as the driver for the growing penetration of data center infrastructure management (DCIM) products (Forrester Research 2012). Realization of the cost savings opportunity within the data center can represent the greatest cost savings opportunity for IT infrastructure and operations leaders today.

Ambient air cooling is safe because modern mechanical designs do not let water through building openings, use filters to clean outside air before entering the computer room, can be used with outdoor sensors to put mechanical systems into recirculation mode in the event of dust storms similar poor conditions, and can be conditioned using humidification.

Openings in buildings to accommodate fiber, fluid cooling pipes, fresh air, electricity and other infrastructure services is commonplace. Expanding the use of fresh air with the intent of reducing energy consumption for data centers is a safe and obvious evolution for the industry. The U.S. Department of Energy recommends using economizers including outside air cooling. “HVAC system efficiency can be improved by adding equipment that can convert delivered gas or electric power efficiently or by using economizers, which allow the automatic use of outside air or allow users to regulate space conditions” (US Department of Energy, 2010). Building openings to allow for fresh air is a common practice and even recommended when done properly.

Using filters to clean outside air before entering building occupied spaces is common practice. The air conditioning industry has established specific standards to determine when and how much air cleaning is needed. “If the outdoor air contaminant levels exceed the values given in 6.1.1 (Table 1), the air should be treated to control the offending contaminants. Air-cleaning systems suitable for the particle size encountered should be used. For removal of gases and vapors, appropriate air-cleaning systems should be used. Where the best available, demonstrated, and proven technology does not allow for the removal of contaminants, the amount of outdoor air may be reduced during periods of high contaminant levels, such as those generated by rush-hour traffic. The need to control offending contaminants may depend on local regulations that require specific control measures” (ANSI/ASHRAE Standard 62-2001, 2001). This common practice can also be understood better by speaking with data center managers who have already implemented ambient air cooling. Facebook, NetApp, NREL, Oracle, The University of Utah Hospital and Yahoo, to name a few, are all examples of ambient air data centers (Mee Industries, 2011; Miller 2010, Sty, 2012). Familiarization with the common practice of treating outside air can help IT managers more widely recognize this technology as data center safe.

The latest ASHRAE air standards for data centers allow greater temperature and humidity variation. Equipment manufacturers develop their equipment to meet or exceed these data center standards. “Part of the rationale in choosing the new low and high temperature limits was based on the generally accepted practice for the telecommunication industry’s central office, based on NEBS GR-63-CORE, which uses the same dry bulb temperature limits as specified here. Most IT manufacturers start to increase air moving device speed around 25°C (77°F) to improve the cooling of the components and thereby offset the increased ambient air temperature. The concern that increasing the IT inlet air temperatures might have a significant effect on reliability is not well founded” (American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2008). This wider band of operating conditions qualifies more hours a year for ambient air cooling.  As noted, IT equipment manufacturers have developed their equipment to meet or exceed these data center ASHRAE air standards. And humidification levels can be easily raised when needed using readily available humidification systems like those used at Facebook (Mee Industries, 2011). Particulate sensors can and should be used to determine if the outside air contains an excessive amount of unwanted matter that could result in shorter filter change cycles.

Ambient air cooling is environmentally responsible because less electricity used equates to lower carbon dioxide emissions, it can equate to less water consumed and it can result in smaller air conditioner designs.

The amount of carbon dioxide emitted in electricity generation can be calculated locally and nationally. The most accurate calculation method is performed on the basis of the area where the energy is consumed. But national averages can also be used to approximate the environmental impact of energy reductions. “Most users of the Equivalencies Calculator who seek equivalencies for electricity-related emissions want to know equivalencies for emissions reductions from energy efficiency or renewable energy programs. These programs are not generally assumed to affect baseload emissions (the emissions from power plants that run all the time), but rather non-baseload generation (power plants that are brought online as necessary to meet demand) Emission Factor 6.8956 x 10-4 metric tons CO2 / kWh” (US Environmental Protection Agency, 2012). Using the EPA’s calculation, it can be determined that U.S. data centers were responsible for putting 689,560,000 metric tons of carbon dioxide into the atmosphere in 2011. Data center managers must have a response to the environmental impact and reputational risk of carbon dioxide emissions.

Fresh water is consumed by power companies in the generation of electricity. Fresh water is often depleted again by consumers to cool their data center through the use of onsite cooling towers. Fresh water is a limited resource. In many parts of the world, there is not enough fresh water to meet current demands; this is especially true in the western United States. “Because the growth of fresh water supplies is limited, growth in electricity demand can be met only by developing technologies that reduce the volume of fresh water required per kilowatt-hour of power generated” (Wolfe, 2008). Less electricity consumed equates to less fresh water consumed. Also, if a data center uses evaporative cooling, fresh water consumption can be dramatically reduced by using ambient air cooling alternatives.

When ambient air cooling is used, chillers might only be needed as trim cooling. Trim cooling can be accomplished with smaller units and less energy. This would be true in a scenario where the outside temperature is 78°F and the desired computer equipment inlet temperature is 75°F. In this scenario the ambient air only needs to be cooled by three degrees, which requires a lot less energy than would be needed to cool the 100-degree computer equipment exhaust air. At a minimum, trim cooling of ambient air may consume a lot less energy than cooling equipment exhaust air using a cooling plant only. Implementing a trim cooling strategy could even result in requiring smaller cooling mechanical equipment.

Research suggests that ambient air cooling is a compelling data center cooling strategy because it reduces electricity usage significantly, is proven safe and is environmentally responsible. Industry standards bodies, governing U.S. agencies and industry analyst firms recommend the use of ambient air cooling for data centers. Case studies have been published where the overwhelming benefits of ambient air cooling have been proven. The objectives of ambient air cooling are maximized when combined with a low-static-pressure strategy. Leaders in the data center space are implementing ambient air cooling strategies now; every data center in the US should evaluate the use of ambient air cooling as part of their cooling strategy. Ambient air cooling has become a data center strategy imperative. As data centers grow in size, electricity cost increase, fresh water supplies shrink and CO2 emissions continue to damage the environment, this solution will only become more necessary.

About the Author

Brent Elieson is Director of Infrastructure Operations at the University of Utah.

Photo courtesy of Tom Raftery


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