Airside Economizers: Free Cooling and Data Centers

By Marcus Hassen  
OTHER PARTS OF THIS ARTICLEPt. 1: Designing a More Energy Efficient Data CenterPt. 2: This PagePt. 3: The Role of a UPS in Efficient Data Centers

The concept of an airside economizer, where the air handler serving a space is used to draw outside air, is commonly being applied in today's commercial office buildings. Though geography and site conditions can present hurdles, this concept can be used in data centers to provide a low-cost alternative for cooling the data center both at night and during winter days. Assuming standard operating conditions, using an airside economizer can lower cooling costs by more than 50 percent.

During economization mode, the return air is exhausted. At outside air temperatures below 55 F (depending upon the computer room load) the compressors are not required to run, which conserves energy. In turn, cooler outside air is used to cool the space, hence the term "free cooling." The steady round-the-clock load profile of a data center generally makes it a good candidate to capitalize on the night and seasonal temperature variations available in most climates. (See "Free Cooling Opportunities Exist in Many Locations" below.)

Re-evaluating HVAC system operating fundamentals can significantly improve performance. The early design phase is where chiller system configurations, pumping system topologies, chilled water loop supply temperatures and selection of energy efficient equipment can best be coordinated for efficiency.

Modern chillers can be selected for as low as 0.4 kW/ton electrical usage, a significant improvement over older chillers that perform at 0.6 KW/ton or higher. Equipping the chiller with a VFD and condenser water reset is usually the most efficient option, as the VFD will adjust the chiller performance as the compressor load and condenser water supply temperature varies. Newer, highly efficient cooling towers with drift eliminators can be selected to reduce water usage by as much as 40 percent to 50 percent.

Because of the emphasis on reliability and planning for staged growth, chiller systems for data centers have historically been oversized, resulting in less efficient operation until full design loads are realized. Chillers are designed to operate at a specific difference between supply and return temperature or "delta-T." A 48 F supply and 58 F return water temperature therefore would be denoted as a 10 F delta-T. An efficient and properly designed system will allow the chillers to operate closer to the design delta-T over the entire range of load conditions. There are two design strategies that can help. They deal with pumping system configuration and reducing system bypasses.

Variable, primary-only pumping eliminates unnecessary chilled water bypass and allows the chiller to operate nearer to its optimized delta-T both during full- and part-load conditions. A traditional chiller design, in which a constant flow through the chiller is maintained, results in the delta-T staying proportional to the load. In a variable primary-only flow pumping system, the flow tends to stay proportional to the load while the delta-T stays closer to the optimized condition for the selected chiller. Also, a variable primary system requires fewer chilled water pumps (lower first cost), has fewer single points of failure (more reliable) and simplifies the chiller plant installation and controls.

Free Cooling Opportunities Exist In Many Locations

Available Hours of
Full Economizer
Available Hours of
Partial Economizer
No Economizer
San Francisco, Calif. 8,563 197
New York, N.Y. 6,634 500 1,626
Dallas, Texas 4,470 500 3,790
London 8,120 300 340

Potential availability for airside economizer, using a target supply air temperature of 68 degrees F at 50 degree dew point as the setpoint.
Source: Syska Hennessy Group

CFD Modeling: A Crucial Step

Computational fluid dynamic (CFD) modeling has become an integral design tool for applying best practices and arriving at energy efficient computer room cooling solutions. Using numerical methods and algorithms to analyze and solve fluid flow issues, CFD modeling is a well-suited design engine for optimizing air distribution within the data center.

CFD modeling is particularly useful in evaluating multiple HVAC design approaches, as the designer can render computer room temperatures, pressures and air flow profiles in three dimensions and fully analyze potential systems performance. CFD models can accurately predict what would be expected to occur under specific operating conditions, with adjustments interactively applied allowing for "what if" scenarios to be posed and evaluated. Ultimately, CFD modeling speeds up the design process and allows the design approach to be validated before construction begins. — Marcus Hassen

Continue Reading:

Designing a More Energy Efficient Data Center

Airside Economizers: Free Cooling and Data Centers

The Role of a UPS in Efficient Data Centers

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  posted on 11/4/2009   Article Use Policy

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