REI's retrofit led to a 93 percent reduction in the energy needed to cool the data centre. The company also makes better use of the energy it does consume to run its machines. Power usage effectiveness (PUE) is a metric organisations use to gauge data centre energy efficiency, with the goal of moving the dial as close to 1 as possible. Myers said REI's data centre had a PUE rating of 2.4 prior to the project. It now aims for a PUE value of well below 1.4, he adds.
REI's energy savings stems from a number of sources — among them an evaporative cooling tower installed on the roof of the building housing the data center. Before the retrofit, the data centre relied on mechanical cooling, specifically indoor, closed-loop dry coolers, noted Michael Stachowiak, senior energy engineer at CLEAResult Consulting, a company that provides energy efficiency programs and services and worked with REI on its energy-saving data center initiative.
The evaporative cooling unit, however, is open to the outdoor air, so provides the efficiency of "free cooling." Here's how it works:
- A pump moves water to the top of the cooling tower.
- The water passes through a filter and the water, as it's exposed to the air, is cooled through evaporation.
- A small amount of water evaporates during this process; the rest of the water runs down the tower and into a coil inside the building.
- Air is pushed through the coil and cools the data center.
- The water is then pumped back to the tower.
The cooling tower, Myers says, takes advantage of the Seattle region's temperate climate. Temperatures are relatively mild year round, but it's the typically low humidity that makes evaporative cooling a good fit. (Water-side free cooling approaches such as evaporative cooling are most effective in low-humidity areas. Less water evaporates in high humidity conditions.)
Airborne contaminants are one downside of evaporative cooling systems exposed to the elements. Stachowiak says the REI cooling tower's filter works around this issue, screening out cottonwood seeds, bird feathers and other matter. The filter is self-cleaning and flushes itself once a day.
Myers says evaporative cooling handles 99 percent of the data center's cooling needs, with mechanical cooling automatically taking over for the evaporative system during particularly hot and humid conditions. "We need an extra kick in the summer for a few hours," he says.
Chilled Water Offers Opportunity for 'Free Cooling'
DuPont Fabros Technology, a Washington, D.C.-based real estate investment trust that develops and operates wholesale data centers, is incorporating mechanical and evaporative cooling technologies in a new data center under construction in Northern Virginia.
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