Powering servers requires copious energy; is AC or DC best?

In February 2008, the Green Grid released a study evaluating the efficiency and viability of using direct current (DC) power in the data center. Beaverton-based Green Grid launched in February 2007 and brings together manufacturers with a stake in the data center to find industry-wide solutions to data center energy consumption. As part of its foray into the AC vs. DC question, the consortium conducted a peer review of a 2007 study conducted by the government-funded Lawrence Berkeley Lab which promoted DC over alternating current (AC) power.

In the course of its research, Green Grid reignited a debate that had cooled in recent months by raising concerns over the study’s conclusions, noting that researchers may have compared old AC equipment to new DC equipment. While DC proponents complained about the involvement of Green Grid board member John Tuccillo, whose company American Power sells AC equipment, the results actually shed light on opportunities for gains in both AC and DC power, with the greatest long-term gains from DC.

"Essentially The Green Grid is in the process of analyzing the benefits associated with both AC and DC," explains Green Grid spokesman Forrest Carman. "TGG does not advocate the use of one over the other, since each offers benefits in certain use cases. Both solutions will need standards building work to enable them to be brought to market in volume," he concludes.

A whitepaper on the Green Grid Web site tones down the Lawrence Berkeley Lab study’s conclusions, citing fewer efficiency gains and noting an increase in the amount of copper wiring needed to power the data center. The paper predicts an improvement in international AC power standards, which could improve on the current U.S. 48-volt standard and gain traction in the short- term, but says the greatest long-term gains could come from an existing 380-volt DC configuration.

“The bottom line is the equipment in data centers are DC already, so using a DC system cuts down on power conversions,” says Jack McCarthy, principal at Integrated Design Group.