ASHRAE Journal - September 2013 - 38

technology AwArd

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38

ASHRAE JouRnAl

ashrae.org

Septem ber 2013

the rest of the building. It is operated
at room temperature, which is well
within the limits of manufacturers’
recommendations but higher than
most conventional industry practice.
Hot air discharged from equipment
is controlled through containment

lighting system and plug load reductions decrease much of the internal
loads, further improving energy performance. Lastly, thermal comfort is
addressed using an integrated system
of thermal mass, radiant slabs, night
purging, and natural ventilation
that completes the whole-building
approach to energy conservation.
The mechanical configuration
of the main system, a decoupled
hydronic thermal slab with a dedicated outdoor air system (DOAS), is
based on our past successful work.
At NREL the campus central plant
supplies the hot and chilled water
to the RSF. The air system consists
of heating and cooling water coils
as well as evaporative coils and heat
recovery. The air is delivered through
a raised floor as an underfloor airdistribution system, providing ventilation air as well as humidity control.
The thermal slabs in turn manage the
building’s thermal comfort and consist of embedded cross-linked polyethylene (PEX) tubes in concrete floor
slabs with water running through the
tubes, and are installed to operate as
radiant ceiling slabs.
Approximately 42 miles (68 km)
of radiant piping runs through all
floors of the building, using water
as the principal cooling and heating
medium as opposed to forced air.
There are five thermal slab zones,
grouped in terms of perimeter
exposure and interior spaces. All
perimeter zones have switchover
capabilities that could operate either
in heating or cooling mode. Spaces
of high load requirements, such as
conference rooms, are equipped
with supplementary hydronic fancoil units.
The data center has its own dedicated conditioning system built
around free cooling, while also exporting useful heat to be reclaimed for

The lobby features a multistory wall faced with pine,
which was milled from wood killed by a beetle infestation
that has devastated forests in the Rocky Mountain region.

racks, protecting the supplied cool
air. These provisions allow a relatively
warm supply air temperature that
can be met for most of the year in
Colorado’s dry climate by either direct
outside air through economizers or
evaporative cooling.
Any air above 90°F (32°C) discharged
from the server racks is used directly
as transfer air for ventilating other
parts of the building when outside
conditions are cool. The data center air
handlers include chilled-water coils as
backup to provide cooling for a handful of hours each summer. Between
the free cooling and IT efficiency measures, the NREL data center is one of
the most efficient in the world. During
the cooler months, the data center
has been running a power utilization
effectiveness (PUE) of 1.10 to 1.15, and

ASHRAE Journal - September 2013

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