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Using Airside Economizers to Chill Data Center Cooling Bills

One of the lowest cost options for significantly cutting the cooling bill for your data center is an airside economizer. Depending on the climate, the steady, 24-hour cooling load of a typical data center is well suited to take advantage of seasonal and nighttime temperature variations to provide a source of nearly zero-cost cooling. In many cases, economization can reduce annual cooling energy consumption by more than 60 percent.

There are additional benefits beyond annual energy cost savings, such as lower run hours on cooling equipment and improved system redundancy. Savings from economization have been realized for decades in office spaces. With proper design attention to a few key issues, economization offers even larger savings benefits to data centers.

Principles of Airside Economizers

An economizer can cut data center cooling costs by more than 60 percent using standard, commonly available low-cost equipment.

A standard data center cooling system can remove heat from the data center it serves only by running compressor(s), which is a major electrical cost. With an economizer, when the outside air is cooler than the return air, the hot return air is exhausted and replaced with cooler, filtered outside air - essentially "opening the windows" for free cooling.

Economization must be engineered into the air handling system and controls.

In dry climates, controls should include redundant outdoor air humidity sensors to stop economization when the absolute humidity (or dewpoint) is too low. This will prevent unnecessary large, and expensive, humidification loads on very dry cold days. There also are ways to harvest savings during dry, cold periods, with some increasing total annual cooling energy savings by 70 percent to 90 percent.

Using a partition-defined hot aisle / cold aisle configuration significantly increases economizer savings, and in some cases creates a de facto heat exhaust that saves energy even when outside air temperatures are above 80 F.

Over-specifying the space temperature and humidity tolerance severely limits economization savings and should be avoided. Actual manufacturer requirements should be followed, especially in regard to humidity where the 40 percent to 55 percent operating region suggested by ASHRAE is often found to be conservative.

In small data centers located in mixed-use buildings, some energy savings may be realized by maximizing the use of a house, office or support area system that is equipped with an economizer.


The savings from economization depend on the local climate. A good way to evaluate the local climate is to plot the outside air conditions for each hour of a typical year, as was done in Figures 1, 2, and 3 for Chicago, Dallas, and San Jose, respectively.
The chart is divided into four regions:

Region A: Outside air is cool and humid enough for economization to 55 F supply air.

Region B+A: Outside air is cool and humid enough to be used for economization in a typical hot aisle/cold aisle system with 65 F supply air.

Region A+B+C: Outside air is cool enough to provide partial cooling in a partition-defined hot aisle/cold aisle configuration with 90 F return air temperature, essentially economizing up to 90 F outside air temperature.

Region D: Hours excluded due to low humidity.

Regions A, B, and C correspond to the hours that can yield energy savings, depending on economizer and data center configuration. Estimated energy use was calculated for a typical data center in each of the three climate zones graphed using a typical year of hourly weather data. The table below summarizes the results.
The last two columns show that the greatest savings come from economizing up to 90 F outside air temperature, which can be achieved with a hard-partitioned hot aisle / cold aisle configuration that creates a heat exhaust from the racks, collecting waste heat at the source and directly exhausting the heated air at temperatures of 90 F or higher. In such configurations, the economizer also provides an additional measure of redundancy to the mechanical cooling equipment, since it is able to carry the full data center load in an emergency situation whenever outside air temperature is below the maximum allowable equipment operating temperature, which is often over
85 F.

A higher supply air temperature also greatly increases energy savings, as seen in the difference in savings between economizing to 55 F versus economizing to just 10 degrees higher at 65 F. It is assumed that a higher supply air temperature is achieved by a hot aisle/cold aisle arrangement that increases the return air temperature and therefore allows a correspondingly higher supply air temperature without increasing total airflow and fan power. If the supply air temperature is increased without a corresponding return air temperature increase, the additional fan power penalty must be calculated to determine the final energy savings. A hot aisle / cold aisle configuration always increases capacity, but the airflow control must be done properly to fully capture energy savings potential.

The area labeled Region D is assumed to be too dry for economization, resulting in 20 percent RH at 70 F, and is not included in the above savings. Economization is simply locked out in this region. If an adiabatic humidification system is implemented using waste heat in the exhaust stream, or a free cooling system utilizing a cooling tower (essentially indirect evaporative cooling) is used, then this region can also provide economization cooling. Any reduction in the allowable minimum space humidity can also move this line downward, allowing for a significant increase in economizer savings.

Design Approach

An outdoor economizing system is best implemented starting at the schematic design stage, where any required architectural accommodations can be made at little or no additional cost. The key objective is for all data center air handlers to have access to 100 percent outside air as well as return air. While this is typically easiest with a central air handling system, several Computer Room Air Conditioner (CRAC) manufacturers now offer economization packages.

Data centers in very temperate climates with no space humidity control concerns may be able to use standard economizer controls, which do not consider humidity and operate based only on the drybulb temperature.

However, it is far more common that an economizer lock out on low OSA humidity as needed. The minimum humidity setpoint in a data center is typically the most critical control parameter influencing the savings from economization yet it is often set by arbitrary rules of thumb. This approach leads to unnecessarily tight, and expensive to maintain, ranges. ASHRAE Technical Committee 9.9 recommends a range of 40 percent to 55 percent. ANSI/ESD-S20.20-1999 [PDF] recommended a range of 30 percent to 70 percent for electrostatic discharge (ESD) control, focusing primarily on grounding techniques (ANSI/ESD-S20.20-2007 dropped the specific humidity recommendation).

In practice, many large data center facilities have a minimum humidity setpoint of 30 percent RH with no ill effects observed. The actual requirements of the installed computer equipment should be evaluated when setting the data center humidity control band, and a minimum humidity higher than the equipment's minimum requirement should be explicitly justified. Any legacy humidity criteria should be re-evaluated in light of modern tape drive use and even, in some extreme cases of design inertia, the elimination of punch card handling equipment.

A recent study [PDF] by Pacific Gas and Electric Co. and Lawrence Berkeley National Laboratory looked at the impact of particle levels in data centers with airside economization. They found that particle levels in a typical data center, with ASHRAE 40 percent filters, did increase from economization use but that the annual average remained within ASHRAE standards. Increasing the filtration to ASHRAE 85 percent or better resulted in particle counts that were almost as low as data centers with no economization. While specific contamination concerns such as salt or corrosives entrainment may require special filtration, standard filtration treatment can usually meet or exceed particulate control requirements.

A Small Data Center Case Study

A 1,400 square foot data center located in a large office building converted to a system with full economization capability when its dedicated 25-ton air cooled chiller required replacement. Due to load growth and reliability problems, portable air conditioners were also being used to maintain control in the data center space. The existing system used chilled water fed CRAC units located in the data center. During normal operating hours, a large house chilled water plant system served the space, while the dedicated chiller was used for off hour (night and weekend) cooling and backup.

An air-cooled package unit was chosen to replace both the air-cooled chiller and the associated chilled-water CRAC units. The package unit, shown in Figure 4, included full airside economization capability.
Package unit with airside economizer

With this unit, exhaust air is ejected from one end of the air handler, while outside air is drawn in from the opposite end of the unit through louvers in the screenwall. Figure 5 shows the final system layout.
Data center converted to use an airside economizer

This system had multiple benefits for the data center: it replaced the failing chiller, allowed for airside economization, and eliminated all chilled water piping from within the data center envelope. The main house air handler system, based on a large water cooled chiller plant that would be grossly oversized if used for just the data center space alone, is used during the day since the chilled water plant is more efficient than the air-cooled DX package unit system. The house system can also be started and used at anytime to provide emergency backup, a rare occasion where efficiency is of little concern. At night, the DX package system carries the load, although in practice it operates on weekends since nighttimes are typically cool enough to allow for full economization cooling.

This system also freed up floor space in the data center by removing the CRAC units. The removal of the CRAC units effectively enlarged the space available for computing equipment, reducing the need for costly future expansions of the space. The data center can still be served from the central plant, but now through cooled air from the house air handler rather than chilled water from the house loop. The data center uses a hot aisle/cold aisle configuration to increase the return air temperature, which both extends economizer operation into correspondingly higher outdoor air temperatures and increases the amount of heat that is exhausted during economizer operation.

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