No, PUE is not dead. It’s alive and well, and the fact that an ASHRAE committee backed away from using a version of PUE in the new data center efficiency standard that’s currently in the works hasn’t changed that.
The Green Grid Association, the data center industry group that championed the most widely used data center energy efficiency metric, has found itself once again in the position of having to defend the metric’s viability after the ASHRAE committee struck PUE from an earlier draft of the standard.
Roger Tipley, Green Grid president, said an important distinction has to be taken into account. The type of PUE ASHRAE initially proposed, Design PUE, is not the PUE Green Grid has been championing. “This Design PUE concept is not a Green Grid thing,” he said.
Green Grid’s PUE is for measuring infrastructure efficiency of operational data centers over periods of time. Design PUE is for evaluating efficiency of the design of a new data center or an expansion.
New Data Center Standard
ASHRAE Standard 90.4 is being developed specifically for data centers and telecommunications buildings. The standard that’s in place today, 90.1, covers buildings of almost every type – the only exception is low-rise residential buildings – and 90.4 is being developed in recognition that data centers and telco buildings have certain design elements that are unique and require special treatment.
ASHRAE’s efficiency standards are important because local building officials use them extensively in inspections and permitting, and non-compliance on a building owner’s part can be costly.
During the course of a standard’s development, the responsible ASHRAE committee puts out multiple drafts and collects comments from industry experts. Every draft is made public and open for comment for a limited period of time, and the draft that follows takes the feedback that has come in into consideration.
The latest draft of ASHRAE Standard 90.4 was released for comment on April 29, and the comment period will close on May 29. To comment or learn more, visit www.ashrae.org/publicreviews.
Green Grid Not Opposed to Design PUE
While Green Grid had little to do with Design PUE, the organization is not opposed to it, Tipley said. “It makes certain sense for the design community to have some [energy efficiency] targets to go for.”
The 90.4 committee struck Design PUE from the initial draft after some prominent data center industry voices spoke out against its inclusion in the standard. The argument against it was that it would put colocation providers at a disadvantage.
PUE compares the amount of power used by IT equipment to the total amount of power the data center consumes. PUE gets lower (which means better) as the portion of total power that goes to IT gets higher. More often than not, colo providers launch new data centers at very low utilization rates.
They have to keep an inventory of available capacity to serve new or expanding customers, which means they theoretically cannot get close to ideal PUE just because of the nature of their business.
Different Metrics, Similar Goals
The committee has replaced Design PUE with more granular metrics that in some ways resemble PUE but focus separately on electrical infrastructure efficiency (Electrical Loss Component, or ELC) and on mechanical infrastructure efficiency (Mechanical Load Component, or MLC). They have also proposed a third metric that combines the two.
The committee’s new approach to measuring efficiency is somewhat similar to Green Grid’s Data Center Maturity Model, which also takes into consideration contributions of individual infrastructure components to the facility’s overall PUE, Tipley pointed out.
In fact, Green Grid is planning to evaluate ELC and MLC for inclusion in the second version of the maturity model, which is targeted for release in 2017, he said.
There is value to such levels of granularity, and at the end of the day, ASHRAE’s metrics have the same goal as Green Grid’s: higher data center efficiency. “The end result is they’re trying to get to a low PUE,” Tipley said.
The comment period for the latest draft of ASHRAE Standard 90.4, Energy Standard for Data Centers, ends on May 29. To review the draft and to comment, visit www.ashrae.org/publicreviews.
