AC and DC Battle For Data Center Efficiency Crown

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."

Comments

Makes sense.

[Anonymous Coward]

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.

[Microlith]

As opposed to the transformer coming into your building? How about the UPS and HVAC units supporting your server room?

Obviously, you'll have redundant DC power supplies, just like you do now. Except instead of having two AC->DC power supplies per PC, you'll route two room-level DC power supplies to each machine in the room. Lots of little, less efficient, lower quality power supplies replaced by a pair of high quality, high efficiency supplies.

Re:Makes sense.

[Imagix]

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.

Re:Makes sense.

[betterunixthanunix]

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

Re:Makes sense.

[tlhIngan]

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).

Re:Makes sense.

[effigiate]

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.

Re:Makes sense.

[Skapare]

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.

Re:Makes sense.

[nschubach]

I'm more concerned that I convert AC to DC to charge a battery, then convert it back to AC to power a power supply in my machine that outputs DC voltage. (Or, taking the DC battery output and inverting it to AC to run a computer.) Why can't I just run my PC off a battery that's kept charged by a DC current from a single power supply? I mean, I don't need the efficiency of AC for long distance transfer (we're talking maybe 3 feet) so why convert it back to AC?

Re:Makes sense.

[Nethead]

The last two data centers (Clearwire) I built out were DC. The only AC in the cage was for a video monitor and for the tech's wifi router. Very standard stuff, the telcos have always done it that way. Any bit of Cisco/Juniper/whatever kits can be ordered with DC power supplies. I see DC plants as more the standard now. And yes, the are still built using waxed string.

Even Power over Ethernet has it roots in telco -48VDC power. All the WAPs and fiber converters at a Lowe's are powered by a Valare DC power supply ( http://www.power-solutions.com/dc-power-systems/eltek-valere.php ).

One nice thing about DC plants are the power cables are cut to length so you don't have all that extra line cord to bundle and hide.

Yay, another volt standard...

[mlts]

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.

6 one way, half a dozen the other

[thsths]

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.

You're gonna spend your savings on copper

[ZorinLynx]

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.

Re:Yes, but

[KBehemoth]

The appropriate demo of the dangers of AC data center power will be to show an elephant losing his entire database due to a power failure. Ominous voiceover: "Unlike an elephant... AC-driven data centers always forget!"

Edison invented FUD

[Thud457]

Thank god, I'm so tired of stray dogs wandering into my datamacenter and getting electromacuted by deadly AC currents!
Peta's really on our asses about that.

Re:Edison invented FUD

[Mashiki]

Well if you live in Toronto, there's a very good chance that simply walking down the street you could get electrocuted by well anything. I'm not actually kidding, they had a serious problem with live plates and poles all over the city for the last couple of years.

Slashad

[Shadowhawk]

Articles main source says modern AC and theoretical DC are about the same. By the way, he has a product to sell...

Re:Actually converting DC is pretty easy these day

[marcosdumay]

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.

Verari Systems

[sdguero]

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.

Did anyone RTFA?

[goombah99]

Uh the article the post links to supports AC more than DC in case no one noticed. The article is about DC being hyped beyond the facts and that AC is claimed to be just as good. Sort of reverses the whole discussion here making it AD, alternating discussion. Edison gets the carbonite filament..

DC advocates trying to get on the same page

[1sockchuck]

The effort to gain acceptance for DC distribution in data centers is being helped by a series of investments by ABB, and the growth of the EMerge Alliance, which is trying to unify DC proponents around a 380V standard. The challenge for DC is that customers don't ask for it, meaning multi-tenant facilities aren't likely to offer it. Also, Schneider says it is "not aware of any data centers moving off of their established, traditional power distribution to DC." In fact, NTT has at least five DC data centers in Japan, and ABB is backing a DC distribution project at a Swiss hosting company. In the US, there are numerous sites testing DC power, which is widely used in telecom infrastructure.