Air-Side EconomizerDescription An air-side economizer (see Figure 13 below) brings outside air 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 outside air is particularly cold, the economizer may mix it with the exhaust air so its temperature and humidity fall within the desired range for the equipment. The air-side economizer is integrated into a central air handling system with ducting for both intake and exhaust; its filters reduce the amount of particulate matter, or contaminants, that are brought into the data center. Because data centers must be cooled 24/7, 365 days per year, air-side economizers may even make sense in hot climates, where they can take advantage of cooler evening or winter air temperatures. For various regions of the United States, Figure 14 shows the number of hours per year with ideal conditions for an air-side economizer.

Figure 13: Diagram of an air-side economizer

Figure 14: Hours with ideal conditions for an air-side economizer. (Courtesy of DOE and The Green Grid)Savings and Costs Intel IT conducted a proof-of-concept test that used an air-side economizer to cool servers with 100% outside air at temperatures of up to 90F. Intel estimates that a 500kW facility will save $144,000 annually and that a 10MW facility will save $2.87 million annually. Also, the company found no significant difference between failure rates using outside air and an HVAC system.38 A San Jose, California, data center estimates it can reduce its cooling costs by 60% through air-side economization. A Sacramento, California, data center projects a 30% savings over conventional data centers.39 PG&E's experience with air-side economizer retrofits indicates that paybacks are greater than two years.40 Red Rocks Data Center in Morrison, Colorado retrofitted their facility with an air-side economizer and estimates that it will be able to utilize mountain air for 80% of the year.41 Retrofitting an existing data center may not be simple or inexpensive. Software giant Oracle did not find an economizer to be a practical retrofit.42NetApp Data Center with Air-Side Economizers Earns Energy Star Label A large part of the operational savings at NetApp's Global Dynamic Laboratory (GDL) the first data center to earn the Energy Star label is due to air-side economizers. Air-side economizers allow NetApp to make use of free cooler air from outdoors for much of its equipment cooling needs. Using outside air allows the GDL to operate without a chilled water plant for more than 75 percent of the year. 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. This and other design approachessuch as elevated supply-side air temperatures, cold-aisle containment, pressure-sensors to minimize fan use, and the elimination of most ductworkreduced building costs by over 66 percent and ongoing operating costs by roughly 60 percent.Considerations Control systems are very important to the operation of the air-side economizer and must be properly maintained. Excessive humidity control can cut into the savings achieved by the economizer. In certain geographic locations, for example, air can be very cool but very dry, and the system may spend excessive energy humidifying the air. Users will need to consider ASHRAE's recommendations, studies of their ambient climate, and their humidity preferences before considering implementation. If desired humidity ranges are too restrictive, net energy savings from an economizer can be limited. Proper management and controls are imperative to ensure that correct air volume, temperature and humidity are introduced.43 PG&E, along with Lawrence Berkley National Labs, found that data centers with economizers have higher particle concentrations than those that introduce minimal, if any, outside air. However, an improved filter design may mitigate any contamination concerns.44 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).4538 http://www.intel.com/it/pdf/reducing_data_center_cost_with_an_air_economizer.pdf39 http://www.42u.com/cooling/air-side-economizers.htm40 Conversation with Mark Bramfitt, former Principal Program Manager, High Technology Energy Efficiency Team, PG&E, July 14, 2010.41 http://www.intel.com/it/pdf/reducing_data_center_cost_with_an_air_economizer.pdf42 http://www.intel.com/it/pdf/reducing_data_center_cost_with_an_air_economizer.pdf43 http://www.42u.com/cooling/air-side-economizers.htm44 Tschudi, 2007.45 http://www.42u.com/cooling/air-side-economizers.htm The function of the economizer is, as its name implies, to "economize" or save on cooling costs. Obviously, it costs money to operate the compressor. If the compressor can be shut down and the system still provide adequate cooling, energy savings can be realized. Heat internal to the building, such as people, lights, computers, copy machines, motors, and other machines, causes the temperature inside a structure to increase. Heat soaked up by the building structure may also continue to heat the building long after the temperature outside the building has dropped. There are times when the temperature outside a building is lower than the temperature inside. Whenever the HVAC system is calling for cooling and the temperature outside is cool enough, it is economical to shut off the compressor and bring in cool outside air to satisfy the cooling needs of the building. Such is the function of an air economizer system. There is one drawback to this type of control system. Even though the thermostat acknowledges that the outside air temperature is low enough to cool the building, the outside air may be too humid to provide adequate comfort for the building occupants. The occupants will feel cool but clammy. The solution is an economizer that adds a second control, which works in harmony with the outdoor thermostat and measures the outdoor air humidity. Such a control is called an enthalpy control. The term enthalpy means total heat. The enthalpy control measures both sensible and latent heat in the air and only allows outside air to be used for cooling if the air is both cool and dry enough to satisfy the space conditions.

Shown is a rooftop economizer, the Carrier EconoMi$er+ OA hood design. (Photo courtesy of Carrier Corp.)If the indoor thermostat calls for cooling and the outside air enthalpy (total heat) is low enough, then the economizer brings in this cooler and less humid air and uses it for cooling instead of operating the compressor. Using the outside air for cooling is less expensive than operating the compressor to provide cooling. So an enthalpy control is a control which checks to see if both the temperature (sensible heat) and the humidity (latent heat) are low enough to be used for cooling. This combination provides for the greatest comfort at the least cost. Not all economizers use enthalpy controls. Some just check the outside air temperature and do not check the outside air humidity. Those controls do not provide the same levels of comfort as enthalpy controlled economizers. Economizers can save a great deal of energy. They can also waste energy if they are not operating properly or are improperly adjusted. For example, if the outside air dampers are not closing properly when the outside air temperature is high, then hot air is unnecessarily entering the building and causing the air conditioning compressor to operate longer and under higher loads, thus consuming a great deal more energy than necessary. If the dampers are open too far during the heating season, the heating system must heat the extra outside air entering the structure. Such extra heating and cooling costs can be quite high. The cost of a service call to repair such a problem is often less than the cost of one or two months of energy wasted. Many economizers are not functioning at all or are out of service because they are not well understood by some service technicians. In fact, some service technicians simply disable them. It is essential that economizers are working properly and saving energy rather than increasing costs. Since air economizers control and vary the amount of outside (fresh) air brought into a structure, they play an integral role in maintaining the quality of indoor air. A properly operating economizer can greatly improve indoor air quality (IAQ) and reduce air quality-related illnesses. Therefore, it is important for the service technician to have at least some knowledge of indoor air quality and its relationship to heating and cooling system operation. Air economizers are available for residential and commercial systems and can be retrofitted to most systems as energy conserving devices. Most packaged light commercial systems (rooftop systems) have an economizer add-on package as an option which can be installed when the system is new or added to the system later.Economizer Maintenance The following items should be checked at least annually to ensure the air economizer is operating properly: Setting and operation of the outdoor thermostat or enthalpy control; Condition of the outdoor thermostat or enthalpy control; Proper setting and operation of the economizer mixed air thermostat; Proper damper operation and lubrication; Minimum damper position adjustment; Correct operation of the system when a call for cooling comes from the thermostat; Function and condition of the economizer damper motor; and Condition of the wiring and electrical terminations. Since the enthalpy control is located in the outdoor air airstream and is a relatively sensitive control, it is not uncommon to have to replace it every few years depending upon the location of the equipment and the weather extremes in the area. The cost of a replacement control is usually recovered quickly through the energy saved. Economizer service should be a part of the scheduled maintenance performed at least on a yearly basis. Just as our automobiles need regular service, so do residential and commercial heating and cooling systems. Like automobiles, the frequency of service depends upon how it is operated, how often and long it operates, and the environment where it operates. Like automobiles, well-maintained systems operate more efficiently, last longer, and fail less often. Norm Christopherson is a former HVACR instructor, a technical writer, and a seminar presenter. He is currently seeking seminar and training opportunities. He can be contacted at nchristo@juno.com. Publication date: 05/03/2004Did you enjoy this article? Click here to subscribe to The NEWS Magazine