The major concern for data centers is to ensure the continuity of energy supply and improve the energy efficiency. The tier systems with different typologies contribute to improve the reliability and availability. The comparison of AC and DC energy distribution in the data center gives some guidance on the electrical configuration design.
A. Tier Classifications
The Uptime Institute defines four tier system typologies for
describing the availability of system as shown in the Figure 1. It can be seen that the difference between Tier I and Tier II
is the number of generator and UPS. In Tier II, additional
generators and UPS provide backup for the most critical
components. The significant difference between Tier II and Tier
III is the number of delivery path. In Tier III, the alternative
power from a second utility provides the parallel power support
for the critical IT load, in case of power failure of the primary
path. However, there is no requirement to install UPS in the
passive path. Therefore, Tier III system is vulnerable to utility
conditions. Tier IV provides a complete redundant system by
adding two active power delivery paths. It can enable dual
systems to run actively in parallel. In both power paths, it
contains N+1 UPS and generator sets. The comparison of
performance in different Tier systems is shown in Table II. It
shows that higher level of Tier system has greater system
availability
Figure 1. Typologies of different Tier systems.
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Table 1. Comparison of Tier systems
B. AC and DC Energy Distribution in Data Centers
Typically the discussion of AC versus DC in the data center
starts with efficiency. Since a data center draws a significant
amount of power, a relatively small increase in efficiency can
lead to a reduction in operating costs. The advantages of DC
data center over AC are energy efficiency, reliability, smaller
carbon footprint, lower installation and maintenance costs,
scale ability, easier integration of renewable energy, utility rebates
and credits, and safety. Total energy savings can reach upward
of 30% for both mechanical and electrical power savings.
Because of this efficiency, DC systems can use various utility
rebates and credits available for corporations. There are fewer
power components in a DC system, making it more reliable than
an AC system. With fewer power conversions, there is also less
heat to affect the electronic equipment. It is because every
conversion wastes energy and produces heat. And while any
single conversion may be in the mid 90 percent range, they don’t
add together, they multiply. When energy to remove the
heat is considered, it is only about 50% of the energy that
actually gets used by the processors in the servers.
