Water-Side Economizers

Utilize "Free Cooling" to reduce carbon emissions while lowering operational cost with Water-Side Economizers

Limiting chiller operation-a practice dubbed "free cooling"-is intriguing, for "reducing the number of hours of chiller operation has a larger impact on lowering energy use in a facility than by selecting a more energy-efficient chiller" (Sorell, 2007). Users with an existing chilled water infrastructure can accomplish "free-cooling" via a supplemental heat exchanger called a Water-Side Economizer.

What is a Water Side Economizer?

Using this economizer, there are no noticeable changes on the data center floor. The same collection of air handlers, raised floors, and fans move air as they normally would. The change occurs behind the scenes in the production of the chilled air and the removal of waste heat.

A water-side economizer eliminates the need for cooling via compressors. When environmental conditions are optimal, the warm return water from the data center is routed to the economizer. There, condenser water accepts this heat and ultimately rejects it to the atmosphere via a dry cooler or evaporative tower. Returned to its desired temperature, the chilled water supply then returns to the data center air handlers.

Water-side economizer operation depends on ambient conditions. The outside air must sufficiently cool the condenser water to allow for proper heat exchange between the two loops.

Water-Side Economizer: Configuration

During the EPA's 2006 Conference on Enterprise Servers and Data Centers, Oracle addressed an economizer retrofit, where space was a particular concern. The economizer is installed between the chiller and the data center. The additional parts- new "pipes, valves, and controls,"- require physical space which existing facilities may not have (Oracle, 2006). In addition, the controls- switching between economizer and mechanical cooling, were not well defined. Any irregularities could comprise cooling on the data center floor.

Their experience stresses the importance of design. A data center at the design stage can allocate the space and engineer the controls in advance. This initial investment positions a data center to take advantage of a favorable climate for years to come. Wells Fargo, for instance, built an economizer into its new Minneapolis facility at a cost of ~$1 million and saved $150,000 during its first year of operation. The bank envisions saving up to $450,000 annually on its energy costs, taking advantage of the cold seasonal temperatures in Minnesota (Mitchell, 2007).

These advantages are not exclusive to northern geographies. Despite a milder, more humid climate, UPS achieves some measure of free-cooling from November through April. A robust building management system eliminates the complexity of switching between chiller and economizer operation. This facility is also an example of a successful retrofit. UPS built the data center in 1995 and brought the economizer online in 2000 (Stansberry, 2008).

Water Side Economizer: Environmental Considerations

Below you will find ASHRAE's recommended envelope for temperature and humidity on the data center floor:

The objective is to use the economizer and comply with ASHRAE's envelope. To demonstrate an ROI, users must approximate the economizer's availability, a calculation that involves climate, chilled water supply temperature, and the ultimate method of heat rejection.

Dry coolers depend on dry bulb temperatures, making lower fluid temperatures harder to achieve. Evaporative coolers use wet bulb temperatures and can attain greater availability as "cooling towers evaporate water to lower temperatures (Equinix, 2008). With each method, users must consider an approach temperature. With a 60 degree chilled water supply and a seven degree approach temperature, the economizer can kick in when ambient conditions are 53F. The type of technology (dry coolers vs. evaporative coolers) will impact the approach temperature.  With this data, users can approximate their operating hours.

Maximizing Economizer Efficiency

Water-side economizers allow users to flick the switch on the compressors for a finite period of time. To demonstrate a quick ROI, however, they must take additional steps to maximize the technology.

Containment Strategy
Data centers usually supply very cold water to air handlers on the data center floor. To consistently produce 42 degree water, for example, the economizer would require ambient conditions in the 30s. For most data centers, that's "possible only during relatively brief periods of extremely cold weather, making it hard to justify the expense of investing in an economizer" (Intel, 2007). The cold water creates cold air, which counteracts recurrent mixing of the supply and return airstreams.

If data centers use containment (either cold aisle or hot aisle), they can potentially raise their chilled water set point. With intermixing eliminated, perhaps users can now achieve their desired server inlet temperature with a 55 degree or warmer chilled water supply. Thus, containment can afford the economizer additional operating hours.

Elevated Chilled Water Temperatures
The chilled water discussion isn't as simple as raising the supply temperature. A cooling coil within any data center air handler is designed with a certain refrigerant supply temperature. Raising this temperature may be impractical if it affects the overall capacity of that air handler and the room cooling design. In addition, the chilled water plant may serve areas other than the data center; those areas may be adversely impacted with an increased chilled water temperature.

Yet, there are certain close-coupled products that address high densities with elevated water temperatures. A higher water temperature means more hours off the chiller and more hours on the economizer. A psychrometric chart of San Jose, California, for instance, shows 5180 hours per year the wet-bulb temperature falls below 55 degrees F. This temperature can result in 62 degrees F supply chilled water and 70 degrees F inlet air to the servers, well within ASHRAE's recommended envelope.

Data center provider Equinix, during a 2008 presentation, uses Baltimore as another example. A 43 degree F water setting can be achieved approximately 1,900 hours of the year. If the chilled water temperature can increase to 50 degrees, the operating window improves to 2,900 hours per year. This availability would only increase if used with a close-coupled design and an even higher inlet temperature.

Understanding the Controls
Like its counterpart (air-side economizers), this technology depends on design and controls to work optimally. Water side economizers add another level to the cooling scheme, "components that must be operated and maintained properly-if not savings will not be realized" (Equinix, 2008). For this reason, the interactions between the economizer and mechanical cooling must be seamless and well-defined to ensure on the data center floor it's business-as-usual.

Conclusion

Facilities with water-side economizers have seen considerable returns. The aforementioned Equinix study envisions a 5 megawatt data center in the Northeastern US will save $225,000 a year through the technology. Their final slide also provides an important talking point: "The computer room is a critical environment-- (we) can't allow energy efficiency goals to compromise uptime or the room environmental operating parameters."

This statement highlights concerns about the marriage of efficiency and availability. Yet the answer is always in the engineering. A user who spends a little more on initial design is poised to save even more over the lifetime of his data center. As they are accustomed, data center professionals must look beyond the short term-"the perception that designing for efficiency equates to a larger up-front investment"-and realize the big efficiency and cost picture: "the lower the PUE, the shorter the ROI on your investment" (Sun Microsystems, 2008). Circumventing the chiller, the water side economizer is an asset in designing to a low PUE, achieving availability/reliability goals, and conserving considerable energy in this process.

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