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While each study created scenarios that
allowed for direct comparison of different
configurations, many aspects of the power
distribution system are complex and vary
by application, making general compari-
sons difficult. Aspects which were not
considered in either study include the
degree of desired redundancy, the impact
of harmonics and load variations, and the
added load on cooling created by inefficient
systems. In cases where these factors are
present, the gaps between DC and AC
would likely be wider, as the following
analysis reveals.
Redundancy
The degree of redundancy in the
power distribution system can
impact both availability and
efficiency. In the case of AC power
distribution, these two factors may
be mutually exclusive—greater
redundancy increases availability
but lowers overall efficiency.
The highest availability is achieved
through 2N redundancy in which
each UPS module is designed to run
at no more than 50 percent load.
However, since the data center is
rarely operating at peak capacities,
the redundant systems are almost
always operating at less than 50
percent capacity. With derating,
maximum capacity may be closer to
40 percent, and, in normal operation,
each unit may operate at 20 percent
load or less. Most equipment
efficiency curves drop dramatically
below 30 percent load, resulting in
actual operating efficiencies signifi-
cantly less than ideal conditions.
In the case of DC power, redundancy
can be seamlessly integrated into the
DC UPS. In addition, Emerson Network
When selecting an optimal power architec-
ture for a particular facility, the number
of factors data center designers and
managers have to consider is substantial.
Understanding the impact of each factor
is essential to making an informed choice
for your data center.
Efficiency
End-to-end power system efficiency
can be difficult and time consuming to
accurately calculate due to the complexity
of variables involved, but it is still useful to
consider and approximate efficiency based
on specific site conditions and priorities.
In light of recent improvements in the
efficiency of both AC and DC architectures,
two vendor-neutral studies have been
published by The Green Grid and Intel
to provide an objective, yet simplified,
comparison of the efficiency of various
power system configurations.
The results from The Green Grid’s study,
“Quantitative Efficiency Analysis of Power
Distribution Configurations for Data
Centers”2, for the power architectures
discussed here, evaluated at 50 percent
operating load, are summarized below:
480V AC to 208V AC (legacy):
63 percent efficient [Figure 1]
480V AC to 208V AC (current):
88 percent efficient [Figure 1]
480V AC to 277V AC:
89 percent efficient [Figure 3]
480V AC to 48V DC:
90 percent efficient [Figures 4 & 5]
Intel conducted a similar analysis,
“Evaluation of 400V DC Distribution in
Telco and Data Centers to Improve Energy
Efficiency”
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and found that the highest
overall system efficiencies are achieved
with a 400V DC system; however, this tech-
nology is not yet commercially available.
Overall, this study concluded that DC
power (both 400V and 48V) was consis-
tently more efficient than AC at all voltages.
Factors Affecting Power Distribution Selection
Power offers an efficiency-optimizing
feature that maintains near-peak effi-
ciencies at system loads as low as 5%,
allowing the system to achieve both
high availability and efficiency in real
world conditions.
Harmonics
Harmonics are a distortion of the
normal AC waveform generally trans-
mitted by non-linear loads. In the
data center, the switch-mode power
supplies used in AC servers represent
a non-linear load that can create
harmonics. Harmonic currents accu-
mulate in the neutral wire, causing
distribution losses and increased heat
generation. If the cumulative level of
harmonics—known as total harmonic
distortion—becomes too high, damage
to sensitive electronics and reduced
efficiency can result and may require
equipment derating to overcome.
These losses are difficult to predict in
complex AC distribution systems, but
could be quite significant. Harmonics
are not present in DC systems because
there are no waveforms to contend with.
Data Center Load Variations
Many data centers are operating at
significantly less than 100 percent of
design load much of the time, with
real-time loads changing frequently.
This variation further complicates
efficiency modeling. As servers are
turned on/off and utilized at different
rates, the loading on each phase of
a 3-phase power system changes,
making alignment between the three
phases difficult. Unbalanced loads
lower total system efficiency and
produce additional heat.
Intel concluded that DC power (both 400V and 48V) was
consistently more efficient than AC at all voltages.