“Watts per Square Meter”: the Wrong Way to Specify Density

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The traditional method for specifying power density using a single figure such as watts per square meter (or foot) is an unfortunate practice that often leads to confusion as well as a waste of energy and money.  The following bullets can summarize the problems…

• What is included in the area calculation, or how it relates to the number of IT racks or devices is not defined.
• What’s included in the power calculation is not defined.
• There’s no information about the variation in power amongst the IT racks; is it a peak number? An average over area? An average over time? Or something else???
• It’s not at all clear how this number is used for data centers that have changing growth plans or are built out over time in a modular fashion.

When important performance characteristics are left undefined in this way, considerable confusion during the specification, design, and commissioning processes will likely result – and the Facility Ops team won’t have a clear understanding of the data center’s capabilities once operational.  And beyond just confusion and the mistakes that might potentially result, an inaccurate or poor specification can lead to waste and inefficiencies.  If you specify too HIGH of a density (compared to the eventual reality in the data center), your first costs and operating expenses are needlessly increased as you will end up having more infrastructure capacity and fixed losses to pay for than needed.  And if you specify too LOW of a density, then performance becomes unpredictable with various overloads and overheating problems occurring.  So it is important that power density be specified properly using an approach that defines all the necessary parameters and which accounts for the reality of dynamic power variations – that occurs in both time and place.

White paper 155, “Calculating Space and Power Density Requirements for Data Centers” written by Schneider Electric Chief Innovation Officer, Neil Rasmussen, describes a better approach that addresses the four bullets above.  The method he developed has four main features;

• The unit of physical space in the density specification is the IT cabinet, NOT floor area.  Floor area is determined during the design as an output of the process using per cabinet power and other factors.
• The specification is hierarchical and modular, so that different rooms and zones can have different density requirements.
• The specification comprehends that IT cabinets within data centers have different power requirements, and that these requirements may not be well-defined in advance.
• The specification comprehends that IT equipment cabinets may have power require-ments that vary with time.

What really makes this white paper a practical one, is that it contains an embedded spreadsheet that makes it easy for you to use this new specification for your next data center project.  We encourage you all to check it out and would welcome any feedback.  Thanks!