When Google disclosed in October that its data centers had an average Power Usage Effectiveness (PUE) of 1.21, some people in the data center industry were impressed and others were skeptical. On Tuesday the company said its data center energy efficiency got even better, improving to 1.16 percent in the fourth quarter and 1.19 percent for the trailing 12-month period.
Google said the data reflects all Google-designed data centers with an IT load of at least 5 megawatts that have been operating for six months or more. Google Senior Vice President of Operations Urs Holzle discussed the company's PUE improvement on the Google blog, noting the impact of free cooling, which incorporates fresh air into data center cooling.
"Efficiency is affected by seasonal weather patterns - cooler weather is better than hot weather, and several of our facilities benefited from that in Q4," Holzle writes. "Also, we continually review our efficiency metrics so that we notice, for example, that one of our data centers is not performing consistently with others of similar size and locale. So we'll take a closer look at optimizing that facility. Are we using fans to cool spaces that don't need to be cooled? Is the thermostat at the right set-point? Can we reduce the time the chillers need to run while keeping the machines operational? So we apply lessons we've learned from better-performing data centers to other facilities, and several such improvements took place in Q4."
While all Google data centers showed the benefits of seasonal cooling, the two newest facilities improved dramatically to a PUE of 1.12, the lowest reading yet. A third Google facility had a PUE of 1.13 for the fourth quarter, giving the company three data centeers with a PUE of 1.13 or lower.
The two data centers with the best PUE, identified as data centers E and F, "moved through commissioning and into full operation" in the fourth quarter. That shift was a factor in their improvement, Google said.
"During commissioning, at times all chillers and air movers were run simultaneously in order to test proper operation of the facility," the company said. "Concurrently, IT equipment load was small because the facility was in the early stages of deployment. These factors caused the higher Q3'08 average PUE values as well as significant day-to-day variations. Q4 '08 saw these commissioning activities taper off, and the favorable seasonal conditions led to lower, stable PUE values."
The PUE metric (PDF) was developed by the Green Grid and compares a facility’s total power usage to the amount of power used by the IT equipment, revealing how much is lost in distribution and conversion. An average PUE of 2.0 indicates that the IT equipment uses about 50 percent of the power to the building (see more detailed explanations here and here).
Discussions of PUE have become contentious at times, with some industry observers openly challenging Google’s data and its decision to include only company-designed facilities. Google has published documentation of its process in compiling its PUE on the Data Center Efficiency section of its web site.
Holzle said Google will continue to share data and best practices, and will disclose more details on our sustainability efforts at the CeBIT conference in Germany in March.