Data Center Power

December 5, 2011

Back in the early days of electric power distribution, there was an ongoing conflict, sometimes called the “War of the Currents“, between proponents of direct current [DC] for power distribution. led by Thomas Edison, and advocates of alternating current [AC], led by George Westinghouse and Nikola Tesla.   The controversy was eventually resolved in favor of using AC for power distribution, mainly because AC voltage can be easily stepped up or down using transformers.  Since power transmission is more efficient at high voltages, this allowed transport of electricity over long distances.   AC power works fine for incandescent lighting and for the AC induction motor, patented by Tesla in 1888.  Here in the US, the ordinary domestic power supply has been standardized on three-phase AC at 120 volts, 50-60 Hz, for a long time.

Recently, though, there has been a resurgence of interest in local DC distribution networks, specifically for data centers.  The reasoning behind this interest is straightforward enough: most electronic devices work on DC.  Traditionally, data centers have taken in AC power, converted it to DC to charge the batteries in a UPS [uninterruptible power supply], then converted the DC from the UPS back to AC for distribution within the data center, finally converting it back to DC in the equipment power supplies.   Since none of these conversions can be done with perfect efficiency, some energy is lost, mainly in the from of heat — which of course requires more energy to remove via the air conditioning.

The “Wired Enterprise” blog at Wired has a recent post describing some of the recent experiments on the relative efficiency of AC vs. DC power.   The results show that a data center using DC power distribution would achieve a meaningful increase in efficiency.

The results of six different tests show that the DC power system exhibited an energy savings ranging from 14.9 percent (when all servers and storage arrays were run at full power) to 15.8 percent (when the servers were turned on, but idle). On average, DC improved efficiency about 15.3 percent over AC systems.

A DC distribution system would also, potentially, be an easier fit with alternative energy sources, like photovoltaic {solar} cells, which produce DC.

Some new data centers being built by firms like Facebook and Google are using some DC distribution elements.  For existing data centers, the near-term savings may not justify the cost of retrofitting DC technology, especially when other opportunities for savings exist,  Still, doing this kind of analysis is worthwhile.  Those existing centers will have to be replaced or rebuilt someday.


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