ASHRAE Journal - February 2013 - 73

Data Center Definitions
What Makes Data Centers Different? (October 2012)
• Data Center Occupancy: The primary occupants in a
data center are not people and, therefore, the pattern and
variation in occupancy is significantly different than most
commercial buildings… the variation in occupancy is actually
significantly greater than in a commercial building.
• Data Center Change Frequency: The frequency of
change (plug loads, etc.) for a data center is often very rapid
compared to most commercial buildings. The magnitude of
change for a data center further compounds this difference.
• Data Center Power Density: Typical data centers are
often between 50 to 200 W/ft2 (538 to 2153 W/m2) of
plug load, although some data centers are more than 1,000
W/ft2. Even modern high-tech offices with all of their electronics are only around 10 to 15 W/ft2 (108 to 161 W/m2).
center may only see high Internet traffic for eight hours of any
given day.
Weather-related events such as natural disasters seem to
have more of a roller coaster type of impact on Internet traffic.
Typically, surges occur before and after the event as people seek
information, but drop during the event, presumably caused by
interruptions to service because of significant surges in demand.
In summary, each data center has its own unique traffic profile, and each of those profiles can be subject to fluctuations of
disruptive proportions. These fluctuations often are accounted
for in the installed capacity of the server systems, specific to
each site, and are something that is constantly managed.

Imminent Arrival of the Internet of Things
The speed that hardware requirements and/or use (or even both)
can change represents a speed of change profile that is not normal
in the building industry. Making this an even bigger challenge is
that although the change can be incremental, it can also double or
more in its thermal and power load demand. These are unusual
extremes compared to a building industry requiring a somewhat
unique and more detailed approach to load profiling and forecasting. This challenge, more than likely will only get worse.
The “Internet of Servers” is transitioning into the “Internet of Things.” In the context of this article, the Internet of
Things can be thought of as virtual/digital representations of
uniquely identifiable physical objects interconnected via the
Internet, enabling activities such as sharing information and
interoperability.
Smart grid, smart city, smart buildings are all tied into the
concept of the Internet of Things, which creates some interesting potential from an overall global load profile improvement.
However, it makes the load profile for data centers even more
unpredictable. Essentially, it means many new applications will
dilute the accuracy of historical load profile data.
February 2013

Data Centers: Cooling as a Service (November 2012)
• Cloud Computing: IT operations departments are
migrating towards considering computing as a service (i.e.
cloud computing). Cloud computing provides a more fluid
IT environment that allows customers to modulate their IT
usage in a manner more resembling a utility service. Cloud
computing can be thought of as “on demand computing,”
which makes the load difficult to project. Depending on
the cloud provider, there may be no constraint on the
demand limit.

Data Center Energy Metric – PUE (January 2013)
• PUE Category 0 (PUE0): A power demand based
calculation measured during peak IT equipment use over a
12-month period (a simple snapshot).
• PUE Category 3 (PUE3): An energy consumption-based
calculation using a cumulative measurement over a 12-month
period at the point of connection of the IT equipment.

Hardware Virtualization
The Internet of Things and its limitless potential load impact
notwithstanding, Internet traffic fluctuations and the idle server
power of 50% still being the maximum server power has meant
that it was common to see peak demands in a data center as a
small percentage of the connected load such as 25% (similar to
other areas of the building industry). In other words, to make
the Internet work for the fluctuations and customer demands,
it has been built with large margins so that it works under all
operating conditions. However, this has proved an inefficient
use of energy, as well as capital.
As a result, there has been an overall trend towards virtualization of servers and network making them far more flexible for
fluctuating demands, but also reducing the stranded capacity
associated with servers with one or more applications that are
seldom used or at low demands.
Virtualization started on the mainframe computers back in
the 1960s. Essentially virtualization at the server level means
a change from physical servers to virtual servers. From the
perspective of the software applications, virtualization means

Nameplate – 920 W (1 kVA w/PF = 0.92)
ASHRAE Thermal Report – 420 to 600 W

Figure 1: Comparison of thermal report and nameplate.
ASHRAE Journal

ASHRAE Journal - February 2013

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