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AC and DC Battle For Data Center Efficiency Crown

Posted by samzenpus on 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."

8 of 168 comments (clear)

  1. Makes sense. by Anonymous Coward · · Score: 5, Interesting

    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.

    1. Re:Makes sense. by betterunixthanunix · · Score: 4, Informative

      AC is better than DC for transporting electricity because you can convert between voltages with just a transformer.

      Which was a winning argument in the 19th century, but not anymore. The use of AC entails significant power loses, especially for cables that are immersed in salt water, which is why DC is used in such situations:

      https://en.wikipedia.org/wiki/High_voltage_direct_current

      --
      Palm trees and 8
    2. Re:Makes sense. by tlhIngan · · Score: 4, Informative

      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.

      It depends.

      AC wins out because of ease of conversion, becaues the higher the voltage, the lower the current, and lower the current, the lower the IIR losses in the wire. DC didn't win because at the time, efficient (and cheap) voltage converters didn't exist. These days, a switching DC-DC supply can easily exceed 90% efficiency, and you can get solid-state converters that can handle transmission line powers easily. Hence the launching of HVDC transmission lines which don't have resonant losses and no phasing issues

      In a datacenter, you'd probably take the incoming power and turn it into an intermediate voltage like 48VDC per rack or something - something that minimizes IIR losses (you want high voltages) and DC-DC converter losses (ideally you want output voltage and no converter).

      It will have to be per-rack at the minimum purely because of the losses - if we did 12V lines and a few servers take 1200W total, we're talking 100A in current If we bump it to 48V, we're dealing with 25A (maybe 30A after inefficiencies), and IIR losses at 25A are lower than at 100A (it increases with the square of the current).

      Also, the 100A cables are big and chunky (which you need because they reduce the "R" part of IIR losses).

    3. Re:Makes sense. by effigiate · · Score: 5, Informative

      One of the challenges of HVDC, especially in the transmission/distribution world, is that normal switching happens on the line and not at the breaker. If you can switch futher down the line, you can leave all the people closer to the breaker with power. The issue is that this switching happens while current is flowing which requires that the device interrupts real current. In the AC system this is relatively easy because the arc created by opening a high voltage circuit under load goes out at every current zero. There is no current zero on DC, so you force the interrupting device to break current. An similar situation can be seen if you look at relay contacts. They may be rated at 20A @120VAC but only 0.5A at 12VDC.

  2. Yay, another volt standard... by mlts · · Score: 4, Informative

    There was an article about using 380 volts a couple weeks ago on /. in the data center.

    Having DC brings some benefits, mainly just needing to step down voltage and not have to rectify it smoothly with capacitors to even out the output current.

    However, there are some downsides:

    1: AC power supplies in devices tend to be more tolerant of power fluctuations. An all DC shop might completely be halted by a power surge/spike that wouldn't bother a data center on AC.

    2: DC sparks a lot when connecting/disconnecting. AC has plenty of zero-crossings a second (120 or so), so it won't make the fireworks show when plugging/unplugging. This makes switches rated for DC a lot more expensive than AC.

    3: There is no such thing as a NEMA 380VDC connector. So, either items would have to be wired up to a bus bar similar to how 48VDC telco stuff gets, or it will end up like 12VDC with at least 5+ connectors (direct wires, cig lighter, airplane, marine connector, male/female combined connector, motorcycle accessory connector, banana plugs.)

    4: Safety. 12 VDC shocks are annoying; a shock from 380VDC will be fatal, especially because of DC's tendency to get muscles to "lock". (This is why stun fences uses AC, while kill electric fences use DC so they can keep the target locked on the wires long enough to get the amps across the heart.)

    5: Issues with wire length. AC, it isn't hard to use a transformer to deal with voltage drop. DC, that will be a lot harder.

    All and all, 380VDC seems like a solution in search for a problem. We really don't need another standard. Heck, just pointing out 120VAC in the US means I have to doublecheck if I'm dealing with 15 amps, 20 amps, 30 amps, or 50 amps, and the locking versions of each, which means six plug types and minimum wire gauges.

  3. 6 one way, half a dozen the other by thsths · · Score: 4, Interesting

    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.

  4. Re:Actually converting DC is pretty easy these day by marcosdumay · · Score: 4, Interesting

    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.

    --
    Rethinking email
  5. Verari Systems by sdguero · · Score: 4, Interesting

    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.