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Cloud Power Hardware

AC and DC Battle For Data Center Efficiency Crown 168

Posted by samzenpus
from the who-made-who dept.
jfruh writes "AC beat DC in the War of the Currents that raged in the late 19th century, which means that most modern data centers today run on AC power. But as cloud computing demands and rising energy prices force providers to squeeze every ounce of efficiency out of their data centers, DC is getting another look."
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AC and DC Battle For Data Center Efficiency Crown

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  • Makes sense. (Score:5, Interesting)

    by Anonymous Coward on Wednesday March 14, 2012 @03:30PM (#39357313)

    AC is better than DC for transporting electricity because you can convert between voltages with just a transformer. But in a data centre, when all the equipment will be powered by the same voltage, it makes sense to use one good efficient power supply for multiple computers, so that all the components don't have to be duplicated for each computer.

  • Re:Makes sense. (Score:4, Interesting)

    by Imagix (695350) on Wednesday March 14, 2012 @03:40PM (#39357429)
    Because you've immediately forgotten the concept of redundant power supplies? In a rack of 48 1U computers, that could be 96 AC-DC converters. Or replace those 96 with 2 (or 3, or 4, depending on risk tolerance) big, high-efficiency AC-DC converters. Better efficiency, easier to cool.
  • by thsths (31372) on Wednesday March 14, 2012 @03:46PM (#39357535)

    AC, DC, it does not make a difference any more. Yes, you have to rectify AC before it powers a computer, but the rectification costs less than 1% of the energy. Power factor compensation can be more costly, but it could be avoided by going to a 3 phase rectifier. There are also serious distribution advantages in 3 phase electricity, but it is not used because of the extra complexity, despite being cheap.

    DC distribution is expensive, and 1% gain is just not enough to pay for it. Once we have intelligent grids, the situation may be different, but for now there is just no business case.

  • by ZorinLynx (31751) on Wednesday March 14, 2012 @03:47PM (#39357549) Homepage

    Standard -48VDC current distribution requires four times the current as 208V AC distribution for the same amount of power. Have you seen DC cabling at data centers that use it? If we're going to start using DC in data centers we need to come up with a higher voltage standard, otherwise we're going to spend all the savings on more copper (which is expensive!) to carry those extra amps.

  • You are getting that wrong. DC can be transmitted farther than AC. DC has only resistive losses, while AC also has capacitive and inductive ones.

    I'd sumarize it as the following:

    DC is slighlty (just slightly) better for transmitting;
    AC was easier to convert from one tension to the other (currently, we have the oposite situation);
    AC is better to use on motors (it was much better, now it is just slightly better);
    AC is easier to generate (it was much better, now it is just slightly better - except on photovoltaics);
    AC is easier on the connectors (hight current DC connectors are a hell to maintain)

    It is easy to see why AC won. I bet AC would win again just because of the connectors and generators, after all, converting it to DC is relatively cheap. The only problem is the low frequencies we currently use, it would be better to increase them a lot now that we have better materials.

  • Re:Makes sense. (Score:4, Interesting)

    by Skapare (16644) on Wednesday March 14, 2012 @04:33PM (#39357995) Homepage

    And a big disaster waiting to happen with such large DC currents available on all the busses going all over the room. FYI, telco 48VDC systems addressed the dangers with resistive busses. But that was a huge efficiency loss. They didn't care so much about efficiency back then as all they wanted was a reliable battery backed up system. Making DC efficient is also making DC unsafe, at data center scale. AC is safer on that scale. Then do the conversion to DC at no larger than one rack, and put ride-through (2 minute) backup batteries in each rack (just need to be long enough for slow start generators or maybe a little longer for diversity loading systems so you don't slam the generators with load). I'd have a separate AC distribution system for the generator power and have each (two input) power converter switch over at randomized times over a 2 minute interval.

  • Verari Systems (Score:5, Interesting)

    by sdguero (1112795) on Wednesday March 14, 2012 @04:49PM (#39358165)
    I worked there for 7 years. I'm not going to get into specifics but I will say:

    Verari tried to take advantage of the efficiency gains in DC with exotic power supplies etc... And that company went the way of the dodo bird after trying to force 800V, 48V, and 12V DC power distribution systems in customer data centers. The fact is, everything already out there (switches, routers, servers, etc) uses AC-DC power supplies in each unit and it works in 99% of power outlets with pretty good uptime. The added complexity of running DC infrastructure isn't worth the efficiency gains (which on paper sound like a lot but theory rarely translates to reality the way we think it will), and when one DC rectifier burns up and takes down a hundred servers (vs 1 server with an AC-DC supply), customers aren't happy. Between the uptime issues and employee safety concerns (high amperage DC power is more dangerous than AC for a variety of reasons) it's also a liability nightmare

    Again, I don't feel like getting into specifics but modern datacenters != underground telco installations and DC power distribution has a LOT of challenges that are often overlooked when marketing types start squawking about efficiency gains.
  • by Anonymous Coward on Wednesday March 14, 2012 @10:18PM (#39360615)

    Actually, 240V, 1kHz would be a lot more efficient, and make power supplies cheaper and more robust to boot.

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