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Power Hardware

Are Data Centers Finally Ready For DC Power? 462

Posted by samzenpus
from the edison-wins-again dept.
1sockchuck writes "It's been five years since a landmark study outlined the potential benefits of DC power distribution in data centers. But adoption of DC in data centers remains limited, even as the industry aggressively pursues a wide array of other energy savings strategies. Advocates of DC distribution are hoping a new study will jump start the conversation about DC distribution, which can save energy by eliminating several wasteful AC-to-DC conversions within a data center. Meanwhile, an industry association for DC power adoption, the EMerge Alliance, has formed a new technical standards committee for data centers, and is advancing a 380-volt DC power standard. Will DC distribution ever gain momentum in data centers?"
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Are Data Centers Finally Ready For DC Power?

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  • by rolfwind (528248) on Wednesday November 30, 2011 @05:51PM (#38219476)

    Not really. AC was the answer to how to transport electricity long distances.

    Currently, it is still converted to DC in a huge amount of devices, in the computer at the PSU. Few devices use AC iirc, something like a fan/ceiling fan probably has an AC motor because a DC motor would slice your finger off if you decided to play with the blades. So, the question then just remains how to optimize the point of conversion. It's rather like the electric car-fossil-fuel-electric-plant/gasoline car debate: have a bunch of small inefficient combustion engines or a large efficient one but deal with transport losses (along with a bunch of other issues).

    In this case, just where along the line do you convert the AC to DC. Since DC can't really travel far at all without significant losses, I guess that would be at the rack level?

  • by bigtrike (904535) on Wednesday November 30, 2011 @05:53PM (#38219500)

    Lower voltages require larger conductors to carry the same current. Copper isn't that cheap.

  • by Shatrat (855151) on Wednesday November 30, 2011 @05:54PM (#38219510)
    I work with DC power in Telecom and it has 3 huge advantages I can think of off the top of my head:

    1) You centralize your rectification. Instead of having hundreds of power supplies running at 80% efficiency, you can have a large rectifier system running at up to 96%.
    2) Lead Acid batteries are hugely more reliable and less expensive than equivalent UPS systems, and provide more holdover time. They're still expensive and finicky, but many times less so than a UPS.
    3) Any old technician with a brain in their head can run DC power feeds to equipment relatively safely due to the low voltages involved. AC power work of any kind should have a qualified electrician involved.

  • by Wonko the Sane (25252) * on Wednesday November 30, 2011 @05:57PM (#38219578) Journal

    AC was the answer to how to transport electricity long distances

    AC was used because using transformers to convert between voltage levels was more efficient than motor-generators and solid state electronics hadn't been invented yet. All other things being equal, DC is always more efficient than AC for long distance transmission.

  • Re:why 380v? (Score:4, Informative)

    by Urban Garlic (447282) on Wednesday November 30, 2011 @06:01PM (#38219652)

    Basic Ohm's law -- the resistive loss through a DC wire is the voltage drop across the wire, times the current through the wire. But the voltage drop across the wire is proportional to the current, it's just I*R, so the total power dissipated in the wire itself (i.e. not transferred to the load) is I*I*R. So, you want the current going to the load to be as small as possible. But, of course, the load still needs to get all the power it needs, so the operating voltage (which is distinct from the through-the-wire voltage *drop*, of course) needs to be higher if the current is lower.

    So, high operating voltages reduce distribution losses.

    The same analysis works for AC too, and is the reason that trans-continental transmission wires have such crazy-high voltages. AC has additional losses due to radiation and induction, of course.

  • Re:Why 380v? (Score:5, Informative)

    by RichMan (8097) on Wednesday November 30, 2011 @06:02PM (#38219664)

    440 * sin(120) = 381.05 ....

    3 phase has 2 ways of looking at the voltages, Y or delta.
    The 3 phase delta is 440v when you measure between any pair of the 3 wires. The center point is ground. You don't see that in delta, but you do when measuring it in Y form. The same signals that are 440v when measured as a pair are 3 x 380v when looked at in the Y configuration.

    So 3 phase 440v gives you 3x 380v to ground.

    As to the 12v/5v/1.5v/ whatever you are going to have to do DC to DC all over the place. Better to have as high a voltage as possible for less current and less losses.

  • by Anonymous Coward on Wednesday November 30, 2011 @06:03PM (#38219682)

    The positive ground in telco systems is not bizarre at all: one end of the twisted pair is grounded, and, being at zero volts does not suffer galvanic corrosion. The other end is at -48V and benefits from cathodic protection: it's the anode that generally gets corroded in a galvanic cell.

  • by Elder Entropist (788485) on Wednesday November 30, 2011 @06:04PM (#38219718)
    Very high voltage was the answer to how to transport electricity long distances. AC was the answer to how to convert that high voltage to safer/useful low voltages cheaply. Very high voltage DC can lose less power over distance than AC. On smaller/cheaper wires too due to the AC skin effect.
  • by Mr Z (6791) on Wednesday November 30, 2011 @06:05PM (#38219738) Homepage Journal

    The problem with DC back in Edison's day was that you couldn't easily step it up or down. DC doesn't have higher losses than AC at the same voltage. In fact, DC radiates less energy away than AC does, and is therefore more efficient [wikimedia.org].

    Ohmic losses all come down to I^2 * R. R is the resistance of the cable, and I is current. To deliver a given amount of power, you have to have a certain V*I. To reduce Ohmic losses, then, you have to reduce the amount of current, which means going up in voltage.

    Incidentally, that's also what's driving automobile manufacturers toward 48v instead of 12v [automotive-eetimes.com], since it would cut the current from the battery by a factor of 4, thereby reducing the amount of loss in the wiring by a factor of 16. That means you can use smaller wires to deliver the same amount of power, safely.

  • by Mr Z (6791) on Wednesday November 30, 2011 @06:10PM (#38219812) Homepage Journal

    The current carrying capacity of the wires would need to be about 30 times larger, though, to deliver the same amount of power. That's pretty huge. To go to 12v everywhere, you'd need huge current-carrying wires everywhere (think "as big as your car battery cables or bigger"). To carry 1kW through a 380V line, you only need to handle 2.6A. To carry 1kW through a 12v line, you need to handle 83A. And that's just one beefy server.

    Now think of your house wiring. Outside of your major appliances, where do you see runs higher than 15A or maybe 30A? There's a reason high voltage is good.

  • by nzac (1822298) on Wednesday November 30, 2011 @06:13PM (#38219862)

    No the skin effect is for (High frequency) AC.

    For DC impedance is determined by the material and the cross section area.

    It does make the cables easier to bend though.

  • by vlm (69642) on Wednesday November 30, 2011 @06:19PM (#38219984)

    The original low speed USB electrical spec was pretty much classical 5 volt NRZI TTL. So insisting on 12 volt supply would mean every USB device would require a 5 volt regulator inside it to talk to the data lines, and the data lines would need protection circuitry on both momma boards and all USB devices because TTL traditionally gets really pissed off when an input voltage rises about its power voltage in case of a short. CMOS gets pissed off too at over voltage. It would just be a bad scene.

    Something like RS-485 but really faster would have been "better", but ...

  • by Wonko the Sane (25252) * on Wednesday November 30, 2011 @06:29PM (#38220128) Journal

    Yes, the current will be higher at lower voltage. This does NOT correlate to needing thicker wires, as the wire has to withstand not current but power which is the result of multiplying voltage with current.

    You've managed to be right while also being wrong at the same time.

    You could use voltage*current to calculate the thermal losses in a conductor but what you've done incorrectly is assume that "voltage" in this equation is the voltage between the conductor and ground.

    The correct way to calculate losses in a conductor is current * end-to-end voltage difference

    The end-to-end voltage difference is directly proportional to the current so the most efficient way to calculate the losses is current squared times resistance.

    Since the surface area of a wire is proportional to the square of the wire diameter and the conductivity required is proportional to the square of the current carried it ends up that wire diameter is directly proportional to the current.

  • by Relayman (1068986) on Wednesday November 30, 2011 @06:45PM (#38220342)
    At 60 Hz, the skin effect is virtually nonexistent.
  • by Obfuscant (592200) on Wednesday November 30, 2011 @06:57PM (#38220506)

    What bothers me is all the new LED bulbs that have transformers in them (guessing, because they get hot! ... feels like wasted energy)

    High power LEDs get hot because you are running good amounts of current through them, not because there is a transformer. Transformers are pretty much useless with the DC current that runs LEDs.

    I'd think it would be more efficient to run DC to lighting and certain outlets like those where small devices would sit ...

    The problem comes in deciding what voltage to use. 12V means you need rather hefty wires to get the required current for some devices. A 6W LED needs half an amp at 12V. If you use a voltage that makes the current resonable, then you need to convert that voltage to what your device needs, every place you have a device.

    Sending 380V means you can use the same or smaller wires than you'd use for 120V systems, but you'll be busy converting that 380V DC to 12V DC or 5V DC or 1.2V DC -- and while DC-DC conversion has gotten a lot better, it is still more complicated than a simple transformer.

  • Re:Power monitoring (Score:5, Informative)

    by blair1q (305137) on Wednesday November 30, 2011 @08:08PM (#38221046) Journal

    Hall effect. [yeint.fi]

    In the presence of a static magnetic field (as around a conductor carrying a constant current), electrons in the clamp circuit, which also carries a DC current, will be pushed to one side of the clamp conductor, inducing a voltage relative to the other side. Measure the voltage and you know the current in the wire it's clamped around.

  • by WaffleMonster (969671) on Wednesday November 30, 2011 @08:22PM (#38221128)

    I told you bitches I would prevail one day!

    There seems to be a popular/fundemental misunderstanding of the tesla/edison debate.

    DC is MORE effecient on the wire than AC given the same voltage, amperage and wire gauge.

    The reason for this is in AC systems eddy currents induced by changing electric fields at 50/60hz cause electrons to migrate away from the core effectivly reducing wire size.

    Why AC has been the choice for so long is an engineering problem.

    Building rectifiers to convert AC to DC from huge AC generators which produce virtually all of our electricity with the kinds of voltages needed to carry massive quantities of volumes of energy is difficult, unreliable and ineffecient..even today.

    Back then it was practically impossible. The choice between Tesla and Edison really boiled down to high vs low voltage. Low voltage transmission required impossible quantities of copper or decentralized generation.

    Tesla wanted larger more centralized generation which given what we use for fuel these days is an exceedingly smart move.

  • by ktbaia (2457938) on Wednesday November 30, 2011 @09:07PM (#38221532)
    I beg to differ. The Electronic Power Steering in my 1995 NSX decidedly does NOT feel like crap. When EPS systems were first put on production cars (the NSX was the first), it was on sports cars, where good steering feel is very important. See this link for information on how a decent EPS system should work. Just because Toyota can't figure out how to do a decent EPS doesn't mean that it hasn't been done. http://www.nsxprime.com/FAQ/Technical/eps.htm [nsxprime.com]
  • by DragonHawk (21256) on Wednesday November 30, 2011 @10:44PM (#38222414) Homepage Journal

    Only very stupid engineers design power connectors that can fit both ways.

    The DC power supply connections in telco equipment is generally screw terminals and spade connectors.

  • by LinuxIsGarbage (1658307) on Wednesday November 30, 2011 @11:12PM (#38222598)

    Computer fan motors are brushless DC. But really they are permanent magnet AC motors with a simple VFD (variable frequency drive) in it. I suspect RC planes are the same.

    In the industrial world VFDs are very popular. On anything from a 2HP conveyor to a 1000HP+ piece of equipment. They rectify three phase input to DC, then convert it back to AC at the desired speed. Some are setup so you can have one central rectifier, and multiple inverter sections for your different loads.

  • by bkcallahan (2515468) on Thursday December 01, 2011 @01:28AM (#38223406)
    A/C skin effect applies *based on frequency*; I assure you skin effect at 50/60Hz, even at high voltages, is negligible -- Doesn't matter if it's 420kV or 4.2V. A circuit at 29.350 MHz at low voltages has to worry a LOT more about it than a 50/60Hz line voltage. (And it really starts kicking off at the start of the microwave range, 300MHz, and is exceptionally important by the end of the microwave range 3GHz.) The reason A/C is used is based off of Ohm's law and is based on current and resistance; Jack up the voltage 1,000x and you can reduce the current (and therefore, heat loss) 1,000x.

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