ASHRAE Journal - February 2023 - 48

FEATURE
remain under the purview of Standard 90.1, but data
centers are covered by Standard 90.4. In short, it is the
ITE power load and density that serve as the demarcation
between the applicability of these two independent,
but interrelated standards.
What made it necessary to develop a separate energy
efficiency standard for data centers when all other
energy aspects of commercial buildings are well covered
by Standard 90.1? There are four major reasons:
1. Standard 90.1 is primarily a prescriptive standard
and mandates economizers as a major part of its mechanical
system energy saving strategy. Nothing is wrong
with economizers. Economizers are not only allowed,
but are also encouraged in the 90.4 standard. However,
if used in the same manner as in conventional office
buildings, economizers can have serious reliability
consequences for mission critical data centers. Further,
the data center industry has developed methods of
cooling efficiently that are not practical for commercial
buildings. This enables data centers to be constructed in
existing buildings where the addition of an economizer
under Standard 90.1 requirements would be impractical
or cost-prohibitive, precluding an important business
need. Standard 90.4 is first and foremost a performance
standard, enabling any available solution or solutions
to be used by the design engineer that meets the prescribed
efficiency requirements.
2. Modern data centers consume enormous quantities
of energy over relatively small footprints compared
with typical office buildings, and they do it continuously
(24/7). Where 2.5 W/ft2 (27 W/m2) is reasonable for a
typical office building during occupied hours, 250 W/ft2
(2700 W/m2) for data center server rooms is not unusual.
Conventional cooling solutions are simply not applicable
and, if used, will almost certainly fail to meet the efficiency
requirements of Standard 90.4 as well as likely
undercooling the critical computing hardware.
3. Data center ITE loads can be cooled with 80°F (27°C)
inlet air, exhaust 110°F (43.3°C) or higher air, and are
100% sensible (no latent cooling required). The environment
can also vary from a low of 8% relative humidity
(RH) to as high as 70% RH depending on facility
grounding and gaseous contamination conditions.
These design parameters are radically different than
typical " comfort cooling. " The combination of heat
levels, temperature differentials, humidity and yearround
operation does not even enable viable simula48
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F E B R U A RY 2023
tions using standard energy modeling tools. This drives
different engineering challenges and provides interesting
opportunities for the development of cooling
solutions with or without conventional economizers.
And because of the high, and continually increasing,
heat loads, the data center industry is using more and
more liquid cooling approaches of various types. The
wide range of requirements and available solutions
make it critical that Standard 90.4 be a performancebased
standard.
4. Last, but possibly most important over the long
haul, is that data center designs must quickly evolve
as the ITE technology changes. This inevitably creates
facility and infrastructure challenges to meet new and
more demanding requirements. Future changes can
be anticipated to at least some extent-and planned for
in greenfield projects. But existing facilities must also
undergo constant ITE " refreshes " to support computing
demands that weren't even in existence when the facility
was originally designed. A good historical example was
the introduction of " blade servers " in 2001, in which
boxes of very expensive hardware waited unused for
months because they couldn't be either powered or
cooled in existing facilities. Newer cooling solutions are
the use of indirect adiabatic and other compressor-less
cooling solutions, and the aforementioned trend toward
deploying direct liquid-cooled ITE in lieu of traditional
air-cooled servers.
Saving energy by improving efficiency is always a good
thing, but there is also an associated cost savings. Those
not familiar with the computing industry might not
appreciate the financial gains possible with these investments.
Smaller " enterprise " data centers, owned and
operated by individual businesses, can easily run from
1 MW to 10 MW capacities. At the industry's hyperscale
end, facilities are often designed to grow modularly,
with demand in as much as 50 MW increments.
These " hyperscale " campuses are being built around
the world and can reach over 1 gigawatt capacities. In
other words, this relatively small aspect of real estate
is consuming a disproportionate amount of energy,
so even very small improvements in efficiency have
the same disproportional benefits in both total energy
conserved and reduced operating costs. A 1% efficiency
increase in a 10 MW data center, for example, could
mean 240 kWh per day savings or more than $7 million
annually at only $.08 per kWh. For a hyperscaler,
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ASHRAE Journal - February 2023

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