Green Grid Enters AC vs. DC Power Debate

The Green Grid says international AC power standards can improve on current data center efficiency, but predicts the greatest long-term gains can be found in a new DC configuration.

In a white paper issued this week, The Green Grid has entered the debate about the merits of DC power for improving the efficiency of power distribution within data centers. The group has evaluated seven power distribution configurations - four using AC power, and three using DC power.

The conclusion? The Green Grid predicts that international AC power standards can improve on the current U.S. standard of 480V and will gain traction in the short-term, but says the greatest long-term gains can be found in a currently unused 380 volt DC configuration. The carefully crafted paper highlights the benefits and drawbacks of AC and DC power without fully endorsing either approach, which is not entirely surprising for a 102 member consortium on an issue that generates lively debate among data center managers.

The Green Grid white paper, titled "Qualitative Analysis of Power Distribution Configurations for Data Centers," begins by noting that the U.S. standard of 480V and Canadian standard of 600V will experience only incremental efficiency improvements. It then examines five alternate configurations: 277V AC, 400V AC, 48V DC, 550V DC and 380V DC.

Here's a brief summary of the evaluations of each configuration. The Green Grid emphasizes that these are qualitative analyses:

  • 480V AC (US standard): "Because it is so commonly used, it is likely to be used well into the future."
  • 600V AC (Canadian): "It is unlikely this system will ever become popular in the US."
  • 277V AC: This is described as the "US version of a 400V/230V AC configuration." The Green Grid says that 277V AC will be "less prevalent than 400VC AC systems in the near term" because it exceeds the 250V AC rating of most IT equipment, but may become more mainstream in the future.
  • 400V AC: "Can be implemented today, is compatible with a wide variety of power distribution and IT equipment, and has the potential to increase system efficiency ... it could be expected to be widely used in coming years."
  • 48V DC: The paper notes that this configuration is the standard for the telecom industry, and thus widely compatible. "However, in very large data centers with centralized distribution systems, 48V DC will have higher cable and/or bus bar costs associated with high currents and long runs."
  • 550V DC: This approach "overcomes the distribution losses associated with centralized 48V DC" but would likely not be the most efficient approach.
  • 380V DC: The Green Grid notes that there is "a great deal of research being done" on this configuration. "380V DC appears to promise the highest efficiency but will require the introduction of new products, including UPSes, and changes to IT equipment power supplies."

The Green Grid white paper won't likely to settle the AC vs. DC debate, which has raged since the War of the Currents in the late 1880s in which Thomas Edison (who favored DC) contested with AC advocates George Westinghouse and Nicolas Tesla. But it provides an informed overview of the leading options. The group noted that "efficiencies of the various configurations are within 5-10 percent of each other."