Smart Approaches to Free-Cooling in Data Centers

Ron Spangler is Senior Product Manager for the Liebert Precision Cooling business of Emerson Network Power. Ron has more than 24 years experience in application engineering and product marketing of data center cooling products. He is a registered professional engineer in the State of Ohio and a Certified Energy Manager.

RonSpangler

If your data center is located where weather conditions are favorable for using an economizer system, doing so can be an extremely effective strategy for reducing energy consumption. However, when selecting an economizer system, care should be taken to weigh the benefits of different types of economizers against associated costs and risks.

Economizer systems use - when cold enough - outside air to help meet cooling requirements and provide “free-cooling” cycles for computer rooms and data centers. Using outside air when conditions are favorable reduces or eliminates compressor operation in Computer Room Air Conditioning units, or reduces chiller operation on chilled water systems. This enables economizer systems to lower precision cooling system energy usage from 30 to 50 percent, depending on the average temperature and humidity conditions of the installation site. Two types of economizers are commonly available for data centers: fluid and air economizers.

Fluid Economizers: A fluid-side economizer system (often called water-side) works in conjunction with a heat rejection loop consisting of an evaporative cooling tower or drycooler to satisfy cooling requirements.

Air Economizers: An air-side economizer system serves as a control mechanism to regulate the use of outside air for cooling in a room or building. It utilizes a system of sensors, ducts and dampers to allow entry of the appropriate volume of outside air to satisfy cooling demands.

Economizers for Data Centers
When making the choice to improve data center energy efficiency with an economizer system, consider the impact to sensitive electronic equipment as well as the effective hours of operation, based on the weather profile of the specific geography.

Maintaining consistent, acceptable temperature levels in the data center can be achieved using both air and fluid economizers. ASHRAE generally recommends a temperature range of 68 to 77 degrees F for class 1 and 2 data centers. However, it has provided guidelines for expanding the upper limit to 80.6 degrees F to increase the number of hours economizers can be used without compromising availability. However, increasing the inlet temperatures to the server can offset economizer system efficiency gains, because sever fan power increases as temperatures rise.

Ignoring the impact of humidity can result in serious short- and long-term problems, including damage to equipment and to the facility’s infrastructure. In most cases, the optimal relative humidity range for a data center environment is 45 to 50 percent. ASHRAE recommends a data center dew point between 41.9 and 59 degrees F, and a maximum relative humidity of 60 percent.

Introducing outside air via an air-side economizer system in the winter months is fine from a temperature standpoint, but unless the air is further treated, it can lower RH to unacceptable levels, causing electrostatic discharge that interferes with normal equipment operation. A humidifier can compensate for this, but its operation offsets some of the energy savings provided by the economizer.

Air economizers reduce mechanical cooling, however increased energy use due to filtration and exhaust of outside air will offset some of the mechanical systems savings.

Fluid-Side Economizers
In contrast, fluid-side economizer systems use the cold outside air to cool the water/glycol loop, which in turn provides fluid cold enough for the cooling coils in the air conditioning system. This keeps the outside air out of the space and eliminates the need to condition that air.

On chilled water systems two types of fluid economizers are commonly employed: 1) an air-cooled chiller with a drycooler providing cold water, when available; and, 2) a water-cooled chiller with a cooling tower providing cold water during cold temperatures. How these solutions operate determines their effectiveness in achieving the goal of free-cooling.

Assuming a chilled water temperature leaving from the chiller of 50 degrees F, full-economizer cooling can be achieved when the outdoor temperature is less than 35 degrees F. In this case, the drycooler fans provide energy to make cold fluid, removing the load from the chiller.

An added benefit of an economizer operating as part of a closed loop is that water is not consumed during heat rejection, as is the case on open cooling towers.

The second fluid-economizer solution is a water-cooled chiller that uses a cooling tower to reject the heat picked up by Computer Room Air Handling units, in addition to chiller compressors. It is customary for a cooling tower to produce water at a temperature of 85 degrees F during summertime operation. However, in the wintertime the cooling tower can provide cold water, as low as 45F, to offset chiller operation. Partial free-cooling can also be obtained during mild temperatures to reduce chiller operation. By increasing the leaving chilled water temperature and associated water temperature rise results in more economizer hours and potentially less pump energy.

Conclusion
Economizers are attracting attention for their ability to reduce energy usage. However, when selecting an economizer system, care should be taken to weigh the benefits of different types of economizers against their associated costs and risks, so that availability is not compromised to achieve energy efficiency.

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