cooling
revisit the data centre cooling
process. Twenty years ago that was
what the cooling infrastructure did: it
cooled the IT space. In general DX
CRAC units were used, continuously
running its compressors to push cold
air of about 12°C into the raised
floor, ensuring the temperature in the
room at no point exceeded 22°C.
Whenever a chilled water system was
used, the chillers would provide 6°C
chilled water all year round.
The cooling process
Today, the term ‘cooling’ does not
give the process the credit it deserves.
We are actually talking about heat
removal. One could argue that the only
cooling process required in the data
centre occurs within the IT equipment
itself, where the hot processing
cores are kept at their correct
operating temperatures by heat
sinks supported by fans. Normally,
by using air, the excess heat is then
rejected into the data centre white
space to be removed. Within this
realm a system needs to be in place
that removes this heat and provides
the space in front of the servers with
air of the correct conditions.
Until around 2005 the
requirements for data centre
temperatures changed focus from the
room temperature to the server inlet
temperature. This opened up great
opportunities for smart designers and
cooling vendors to think up new and
clever ways to remove the IT heat
from the white space. Suddenly, there
was not only the choice between
DX or CHW. We had kyoto cooling,
(other) indirect fresh air cooling, direct
fresh air cooling, storage of cold in
underground aquifers. And to the
surprise of many, chilled water systems
could be designed to great efficiencies
too, now that an air temperature set
point of 25°C was sufficient.
Today, many heat removal
systems and technologies are
available that can all achieve partial
PUE values of 1.1-1.2, at least
in most of the temperate world. A
nominal data centre design should
see the energy consumption of the
‘cooling system’ equal to that of the
electrical system.
But the call for social responsibility
actions does not end here, and data
centre stakeholders and the industry
are constantly working on producing
even better designs and facilities.
Recently, data centre waste heat
reuse has become a hot topic and the
author has personally been involved in
several initiatives to this end.
Is there a need
to focus on PUE?
Interestingly, the current focus on
PUE is not good news for heat reuse.
Harvesting the energy inevitably costs
additional energy, which increases
the PUE which is not necessarily
bad news. Nearly every bit of
electrical energy consumed by the IT
equipment is transferred into thermal
energy. When this thermal energy is
utilised in a useful way, the ‘cooling’
system is finally serving society in
a direct way. Who then really cares
about the PUE when you get to
spend your kWh twice?
What are the opportunities
for data centre heat reuse?
So why is it not mainstream yet?
Typical issues encountered are
concerned with the impact on
the infrastructure (availability and
reliability), the heat collection
effectiveness and the value of the
heat (the business case). To start
with the latter, although sustainability
ambitions play an important role, the
venture still needs a solid business
case. Compared to your current
state-of-the-art data centre, you need
additional investments, for instance
for heat exchangers, water buffers,
storage systems, etc. There must
be a return on these investments.
There are several ways that heat can
be reused. In a paper from 2012,
Zimmermann et al investigates the
economic value of recovered heat.
Next to the obvious use for space
heating, there are also other possible
The cooling
infrastructure is
a vital element
in any data
centre, providing
essential support
to the business
applications
running in them.
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