Consulting-Specifying Engineer - March 2009 - (Page 12) M/E/P Roundtable Data center discussion PARTICIPANTS Our roundtable discusses energy efficiency, technological advancements, and best practices in data centers. BY MICHAEL IVANOVICH, Editor-in-Chief, and PATRICK LYNCH, Associate Editor CSE: What problems arise due to a lack of a standard for data center energy efficiency or Power Utilization Equivalence (PUE)? Keith Lane: There have been many unsubstantiated claims of very low PUE levels from data center operators and salespeople. PUE is a good matrix to identify energy efficiency, but there must be a standard to make sure that you are comparing the same thing. Also, the PUE levels are going to be dependent on the climate. There has been some discussion in the industry about providing a coefficient based on climate and location to provide more meaning to PUE. In some way it would be a method of comparing efficiency of data centers around the world. Very low PUE numbers are suspect. By the time you take the UPS losses, transformer losses, I square R losses of the cables, and a few lights, you will be over 1.1 PUE before any mechanical or plumbing systems are brought into the equations. Additionally, engineers and owners should ask whether PUE claims are based on theoretical evaluation, actual best case scenario, average, or peak information. Steve Berry: I agree. The main problem is an inability to make like-for-like comparisons. Other issues include setting performance aspirations too high or low, and concentrating effort on “playing the PUE game” rather than pursuing good, efficient design. Peter Babigian: With a new spotlight on carbon footprint, the Green Grid makes a very important first step toward a calculation intended to help data center managers understand the level of efficiency at which facilities are operating. There are several areas in which the methods for computing this efficiency must be further refined. At the simple level, PUE is computed by dividing total facility power by IT equipment power. The areas in which further refinements to PUE are required and underway include the following: • Geographic location: Better PUE numbers are more easily achieved in temperate areas than in areas with an extreme hot or cold climate. • Distinctions between facility and IT equipment: From internal server fans to cooling elements (such as pumps, refrigeration, blowers, and heat exchangers within the IT equipment itself), it is not yet clear how we can distinguish between IT equipment and cooling equipment. In addition, the categorization of certain components, such as facility equipment versus IT equipment, skews the PUE calculation. • Dynamic nature: Simply dividing the total power by the IT power would produce a result, but there is no formal definition for measurement frequency or for averaging requirements. If measurements are taken on a day of extreme hot weather and low IT equipment usage, the results would be far different than if the measurements were taken on a day of temperate weather and high equipment use. In reality, PUE is not constant. Rather, it is always changing and computations should be adjusted to reflect this. • Compute cycles: Present calculations do not account for the number of compute cycles or the actual work produced by energy that is expended. Data centers are encouraged to compare PUE numbers and discuss the measures they have taken to increase the operating efficiency of the facility. In addition to the variables above, the PUE calculation can be skewed by, for example, running a data center with higher server inlet and outlet temperatures or Peter Babigian, LEEP AP, RCDD Principal WB Engineers Jersey City, N.J. Steve Berry Associate Director Arup Building Engineering Group 3 London Keith Lane, PE, RCDD, LEED AP Partner/Principal Lane Coburn & Assocs., Seattle Michael Ryan, PE Senior Staff Engineer Global Lab & Data Center Design Services Org. Sun Microsystems Santa Clara, Calif. 12 Consulting-Specifying Engineer • MARCH 2009
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