Efficiency

To comment on the efficiency of a specific distribution architecture, the power distribution and its losses need to be considered. The following example illustrates the approach.

In a typical data center implementing AC distribution, as shown in Figure 2, the incoming electrical power is distributed as shown in Figure 5 and leaves the system primarily as heat.

Figure 5 Power distribution within a typical data center implementing AC power distribution [15]

The power consumed by the UPS is primarily to compensate for the losses in the two conversion stages, first from AC to DC and then from DC back to AC. These losses are assumed to be distributed equally between the two UPS power electronic converters, resulting in approx. 9% per converter.

A typical server card (dual processor) consumes approx. 450 W of power, distributed amongst its peripherals as shown in Figure 6. It can be seen that approx. 160 W (35%) are losses in the power conversion process, according to [6].

Figure 6 Power distribution for a typical server card (dual processor) consuming approx. 450 W.

From the above loss analysis it can be seen that a DC distribution architecture already saves 131 W (29% of the server card losses) by removing the AC-DC conversion on the power card itself. This amounts to an 8% saving in the overall data center losses. A further 9% reduction in overall data center losses is achieved by removing one of the two UPS converters. A DC distribution architecture therefore saves 17% of the overall losses of a data center.

In the literature, various institutions predict similar savings in overall data center losses to those indicated in the above example. Researchers at the Lawrence Berkeley National Laboratory in the USA have experimented with a DC distribution architecture, as shown in Figure 3, with a DC bus at 380 V. They concluded that the elimination of inefficiencies caused by having to convert between DC and AC reduced the overall data center power loss by 10 to 15% [4]. Calculations by Engelen et al. [9] show that a reduction of approx. 20% in electricity charges could be expected when an Internet data center with an AC distribution bus at 100 to 200 V migrates to a DC distribution system. The saving is due to the same reason as above. An American company ‘Industrial Light & Magic’ reports savings from 10 to 20% since their migration from AC distribution, with an AC bus at 480 V, to DC distribution for their server cards, with a DC bus at 48 V [16]. Yet again due to the same elimination of inefficiencies as above.

A central AC-DC converter feeding the DC bus in a DC distribution architecture is more efficient than several smaller AC-DC converters in each server rack [16]. The amount of ‘electronic overhead’ (losses due to control and auxiliaries within the converter) is much less in such a larger converter. A single point conversion however reduces the reliability of such a system, necessitating a redundant implementation, as discussed earlier.

Another advantage of DC distribution is the lack of reactive power in the system. Reactive power results in increased losses in AC systems due to larger current magnitude for an equal amount of transferred power. Non-linear loads, such as AC-DC converters (without power factor correction (PFC)), require reactive power in an AC system. In view of the large number of converters in data centers, much is to be gained by migrating to a DC system.

A key parameter in the efficiency in a data center is the light load efficiency, as this is usually far worse than the efficiency at the rated load. This parameter is however independent of the choice of distribution architecture and therefore not addressed in detail in this paper.

A further advantage of DC distribution is that it facilitates the implementation of sustainable energy sources as well as energy storage. This is discussed in the next section.