the overall efficiency of the system,
perhaps associated with losses in
ducts and with the efficiency of the
air-pumping system. It is worth noting
that we would expect this headline en-ergy-consumption figure to be greater
than but of comparable magnitude to
the energy required to supply and mix
air directly from the outside in the case
that direct air-exchange ventilation is
possible without a heat exchanger.
Given the inefficiencies in practical
implementation of this approach, we
may expect the energy cost of direct
heat-exchange cooling to be in excess
of 10%–20% of the cooling load; this
can be compared with the energy loss
in a cooling device—for example, a heat
pump, which might operate with a COP
factor of 1: 4 or 1: 5, representing about
20%–25% of the cooling load (Figure 5).
This system of direct mechanical air
recirculation, with heat rejection to the
exterior through a heat-pump or chiller
system, will need to be used when the
exterior is warmer than the interior
(that is, > 25°C, 77°F), so that direct
exchange is no longer viable. In run-
ning a refrigeration system, there is a
balance between the amount to which
the air temperature is reduced and the
flow rate of the air, so as to achieve the
same overall cooling flux. The optimal
balance between the reduction in tem-
perature and the flow rate depends on
the energy required by the fans, which
figure 3. Example data on the number of hours the temperature exceeds different values
per year in Berkeley, California.
hours Per year Greater than
568
1200
1005
1000
800
780
600
400
200
402
277
198
125
84
0
70 72 74 76
49 27 10 2 0
90 92 94
temperature (f)
78 80 82 84 86 88
figure 4. illustration of the fan power required for a given flow rate of air through
a building. this is relevant for the power consumption in a direct heat-exchange system,
where the power to drive air through the heat exchanger is the main load on the system.
— 8000 — 2000 — 500 — 100
1000
100
Power consumption (k W)
10
1
0, 1
0,01
0,001
100
increases as the flow rate increases,
and the efficiency of the cooling heat
pump, which typically decreases as the
temperature difference across the heat
pump increases. By optimizing the couple system, the most energy-efficient
mode of operation of the mechanical
cooling may be found.
These simple observations, based
on the external climatic conditions
and the internal heat loads, identify
the main challenge in data centers to
be the circulation of large volumes
of air (or cooling fluid) to remove the
heat; this uses a considerable amount
of energy, and in hot conditions, the
need to chill the recirculated air in addition to the pumping work increases
this energy load considerably.
The typical savings that may be
achieved by adopting the above ideas,
relative to use of a refrigeration cycle
throughout the year, are possibly very
substantial; using direct heat exchange
when external conditions are colder
could represent a large percentage of
savings in the energy for cooling. The
ideas presented here may be able to
reduce that energy consumption substantially, but this depends on the ambient temperature; some schemes that
adopt this general approach have been
installed, including those by Intel and
the KyotoCooling system, but there
seems to be tremendous potential for
much more widespread adoption of
the design principle.
This analysis, however, does point
to the need to locate data centers in
cooler climatic zones, where the exterior temperature may allow for direct
heat exchange for a greater part of the
year. Indeed, as the number of hours
for which cooling is required increases,
the energy consumed in the data center
increases. If the number of hours increases by a fraction x of the total year,
then the energy consumption may increase by an amount on the order of
0.2x of the total energy consumed in
powering the IT equipment, assuming
the use of direct cooling accounts for
about one-quarter of the energy consumption. This points to the benefits
of locating data centers in more northerly or colder climatic zones.
1000 10000
Air volume (m3/h)
100000
interior Design Considerations
for Air Cooling
Let’s now turn to the design of the in-
