Data Centers And DC Power 379
mstansberry writes "In the final article in a series on the price of power in the data center, IT pros weigh the pros and cons of direct current-powered servers. A limited number of companies make servers with the power supplies removed with DC power distributed to multiple machines from a single unit. It saves power by skipping an extra conversion from alternating current (AC). Telcos have been using this method for years, but some data center pros are leery of taking on the new systems. It's not something people are familiar with and if they break down, you have to hire a specialized engineer to come fix them. But if they're saving even half of what they're reported to save on the electric bill, companies could afford to hire the engineers." We've reported on previous articles in the series.
Forgot DC and AC power (Score:3, Interesting)
Proprietary connectors (Score:2, Informative)
Re:Forgot DC and AC power (Score:2, Informative)
Re:Forgot DC and AC power (Score:2)
Re:Forgot DC and AC power (Score:5, Informative)
Yes. Mainframes, large UNIX systems, and the storage boxes that connect to them still require three-phase. (I am a storage specialist.)
SirWired
Re:Forgot DC and AC power (Score:3, Interesting)
Re:Forgot DC and AC power (Score:5, Informative)
Yes, real big iron still uses 3-phase power. I can only speak on behalf of large IBM system (zSeries, etc). These systems will accept 192VAC to 508VAC on the input, 3-phase Delta. This means no neutral required. Additionally, they will even run with one phase totally missing.
The first power conversion stage in any piece of their 'big iron' is a very large AC to DC converter, rated for a 350VDC output at over 42kW. Actually it's six 7.5kW converters paralled, and these are redundant/hot swappable. Totally modular, with no cable connections. This block is about 95% efficient.
This DC is then distributed to the rest of the system power supplies, with redundant cabling supplying all point of load converters. All point of load converters are also redundant and hot swap. These converters have a range of efficiencies, but are typically much better than industry standards. A DC/DC converter in the z9 can source 1000A alone on the CPU Vcore level (12 of these supplies are in the machine). Supplies are used for CPU nodes, I/O cages, blowers and refrigeration.
All blowers are 3-phase DC-brushless type, with the 3-phase synthesized off the 350VDC feeds. The blowers are usually 300W or larger, each.
The CPU refrigeration is also run by 3-phase compressors, this power also being synthesized off of 350VDC. This is done to allow a conventional off-the-shelf compressor to be run off any line voltage, and ride through phase losses (as this is seen by the bulk AC/DC converter instead).
The 'big iron' also supports built in UPS cabability, allowing you to connect battery packs directly to the bulk AC/DC converters. A machine will handle six 400V@2.5Ah battery packs connected to it. This feature is used to ensure a system such as a z9 has true 100% availability, and won't suffer a hard shutdown due to careless datacenter workers or electricians.
In short, the article is intend to address small white box systems that use $12 power supplies with very poor reliability and efficiencies.
And to another poster that brought up 3-phase being more efficient for power conversion...that's not really true these days, as everything requires power-factor correction. Nothing in the IT uses huge three-phase bridge rectifiers and phase-regulated primaries anymore.
See, I told you so (Score:5, Funny)
Re:See, I told you so (Score:5, Informative)
But you're sentiment is correct. Edison never really believed in AC power.
Re:See, I told you so (Score:3, Informative)
But you're sentiment is correct. Edison never really believed in AC power.
Actually he was a pretty firm supporter of AC where the electric chair was concerned.
The original example of FUD!
Re:See, I told you so (Score:3, Interesting)
But later on he screwed Tesla anyway.
Though admittedly, Nikola was not much of a businessman, which is why while he was perhaps the most brilliant scientist to ever exist on this planet, he died virtually pennyless.
Re:See, I told you so (Score:5, Informative)
Westinghouse didn't give Tesla a job, he contracted with Tesla Electric Light & Manufacturing for R&D and licensed the AC patents. Eventually Tesla released Westinghouse from paying royalties to prevent the company from going under. (The AC/DC wars nearly bankrupt both Edison and Westinghouse.)
Though admittedly, Nikola was not much of a businessman
Indeed. He was always a little too paranoid. Instead of learning how to properly use the laws and courts to protect his work, he felt that the only option was to keep his work super-secret. The sad part about this is that we still don't fully understand some of his inventions. For example, take his electric car. How did he manage to power that thing at such high velocities given the technology of the day? The answer is still a mystery even today. (And a favorite of the free energy quacks, I might add.)
which is why while he was perhaps the most brilliant scientist to ever exist on this planet, he died virtually pennyless.
At least in part, that had to do with all the equipment he was purchasing to perform his grounded power experiments. He had this idea that he could run power through the Earth itself, allowing anything that touched the surface of the Earth to tap into the grid. Such a concept would have been a boon for electric vehicles. Sadly, his theories on the subject were later proven incorrect, meaning that he wasted his money and time on a dead end.
Re:See, I told you so (Score:3, Informative)
What about houses? (Score:5, Insightful)
Jolyon
Re:What about houses? (Score:5, Interesting)
If there were several pins, many different voltages would be possible, and a device could even use more than one voltage from one plug (eg, it could draw 2V for a relay, 4V for a power indicator, and the standard AC for the actual thing it's powering.)
By not having to have transformers and big resistors inside all the household devices, there would be huge savings in power, things wouldn't get so hot, wouldn't need such big heatsinks, there would be far less electromagnetic radiation around the place (which is probably responsible for a lot of people getting sick etc), and it's safer too (devices that only need a small DC power source won't electrocute you when you drop them in some water.)
Re:What about houses? (Score:4, Informative)
Granted, you may not need to carry a lot of amps at 2V. However, no matter what voltage/current you pick, it's much easier (in terms of wiring cost) to use higher voltage for electricity distribution.
I think what the main article was discussing is changing 120 AC into 120 DC centrally, but still having the 120 DC => 2v DC conversion done right where it's needed.
Re:What about houses? (Score:3, Informative)
The section that reads "These devices, a kind of switched-mode converter [wikipedia.org], generally perform the conversion by the following steps" is misleading. While it says "AC", if you click through the link, it actually says "The inverter stage converts DC ... to AC by switching it on and off ('chopping') at a frequency of tens or hundreds of kilohertz (kHz)". In other words, it's nothing like the orignal AC (60Hz), and the kHz chopping is necessary even if you start off with AC. So it's not redu
Re:What about houses? (Score:3, Interesting)
there would be far less electromagnetic radiation around the place (which is probably responsible for a lot of people getting sick etc)
Since when was non-ionizing EM radiation dangerous? Besides, even if it was, I highly doubt that your computer PSU or little brick power supply for your cordless phone are wasting that much energy as EM. When you consider that most little brick power supplies are running most of the waste is lost to heat and not EM I highly doubt it amounts to much. Even your typical P
Re:What about houses? (Score:3, Insightful)
It isn't.
There are three parts to assessing something like this.
First, is there some physical method consistent with known science by which injury could occur? In this case, no.
Second, do controlled experiments on human or lab animals show an effect? In this case, no.
Third, do statistical studies show a correlation? In this case, not when they are done competently. The problem is that a lot of studies have been done by people who don't understan
Re:What about houses? (Score:4, Interesting)
don't be so sure... Living bodies are incredibly complex electrical devices. I think it arrogant to assume artificial electric fields cannot have an effect on their proper operation.
(emphasis added) From an interview of Robert O. Becker, M. D. [energyfields.org], who was a pioneer in the study of natural electrical currents in the human body.
Re:What about houses? (Score:3, Insightful)
Of course it would be. That is why I explained things the way I did. There is a difference between saying "It has been proven that 60 Hz fields do not affect humans" and saying "There is no evidence that 60 Hz fields affect humans."
We can speculate all day about what could conceivably affect a human body, given the limits to our understanding. But that leads to tinfoil-hat country unless we look fo
Re:What about houses? (Score:5, Informative)
That said, there's no reason why the power couldn't come to your house as AC and then be turned into DC centrally by an efficient PSU in the basement (or wherever). The only minor problem is that DC is somewhat more dangerous than AC - if you touch a live AC wire you can pull away from it more easily than if you are in a DC circuit due to the effect on nerves.
Re:What about houses? (Score:4, Informative)
AC prevents that galvanic(?) effect to occur on the house outlets.
Re:What about churches? Very small rocks? (Score:3, Interesting)
I've heard this before, but I haven't heard a terribly good explanation for why.
HVDC Pacific Intertie between Oregon and California:
http://en.wikipedia.org/wiki/Pacific_Intertie [wikipedia.org]
http://www.transmission.bpa.gov/cigresc14/Compendi um/PACIFIC.htm [bpa.gov]
I worked on a transmission sales automation project at BPA, and I seem to recall some very good explanations for why they had a 2000+ MW, 400kV transmission hop.
AC vs. DC (Score:5, Informative)
Easy.
First, DC actually is better for transmitting power over long distances. AC current tends to concentrate in the surface of the conductor, leading to higher current densities and larger ohmic losses.
So, why do we use AC almost everywhere? Transformers. It is relatively easy and efficient to use a transformer to change voltages of AC power. For large electrical lines, the voltage is cranked way up, which means the current is reduced. The less current, the smaller the losses due to resistance in the wires. So power is transmitted at high voltages, so the current and hence losses are low. Then, near the place where power is needed, transformers change the power to lower voltage, higher current. (This is because you can't have house wiring and appliances that won't arc or explode when hit with 13,800 V.)
Converting between high and low voltages with DC power is much more difficult, and requires more complex equipment. (An AC transformer is two pieces of wire wrapped around a chunk of iron.)
Re:AC vs. DC (Score:4, Informative)
A good DC-DC converter is actually a DC to AC to DC converter. It can be more efficient that a 60/50Hz AC to DC converter as a high frequency AC is used. High frequency transformers are smaller and can be more efficient the low frequency transformers. Some AC to DC converters are actually 60/50Hz AC to DC to high frequency AC to DC converters.
The question of AC vs DC power is complex. There are advantages and disadvantages to both. You can't just count the number of conversion steps, or guess the efficiency of the converters.
Re:What about houses? (Score:3, Interesting)
Yeah, Electric Chairs used AC power because it's LESS DANGEROUS, right?
Re:What about houses? (Score:2)
Re:What about houses? (Score:2)
If you think of the elecricioan falling off the ladder, let me tell you that a 100V jolt will make you jump as well.
Efficiency. DC loses more energy per foot / mile than AC.
Untrue since AC has higher peak current and there fore higher losses in ohmic conducting situations.
Conversion capability. It's easy to
AC conversion... (Score:2)
Actually the 50-60Hz of household AC isn't all that great for power conversion by today's standards. People either rectify it and throw it through a second higher frequency oscillator or use a custom active-switching DSP solution.
Re:AC conversion... (Score:3, Insightful)
What about the big power guzzlers in the house: Refrigerators and Air conditioners? Those AC motors suck power directly from 220/110 VAC, and isn't AC better for these cheap induction motors?
Re:What about houses? (Score:2)
Efficiency: Low voltages (generally what your home devices use as DC) loses more energy per foot/mile than higher voltages.
This because if your home appliance needs 50W to work, sending 110VAC to it will require small currents through the power cable (~0.5A). Then it's converted to, say, 12VDC and travels a few inches (~4A). (using ~ not to engage in pedantic discussions).
The more current you
Re:What about houses? (Score:2)
Well, I simplified it. Check out the wikipedia article [wikipedia.org] for the whole story.
Essentially, since we can use high-voltage for transmission and then convert it to low-voltage on the poles, we can take advantage of the greater efficiency achieved by using a hi
Re:What about houses? (Score:2, Interesting)
I think the best way to do this is to use a smart bus, reminiscent of USB. You plug in the device, and it indicates "I'd like 5V DC", or whatever, and the other side provides the appropriate voltage. A powerbar would have to say "I'd like 120 VAC" or "I'd like raw AC power", then be smart enough to switch that to the desired voltage for each device.
You could only have one plug on each wire from the smart hub.
Costs would be higher due to all the electronics involved, but they'd come down with mass produc
You mean, like PoE? (Score:2)
http://www.poweroverethernet.com/ [poweroverethernet.com]
Re:What about houses? (Score:2)
Also keep in mind that the switching power supplies used in computers are not usefu
The first step (Score:4, Interesting)
At this point, we could start to build it into houses and other buildings.
Voltage Regulation (Score:2)
I doubt many people would be happy with the large copper bus bars that would be needed to distribute low voltage DC at any reasonable power level.
Re:What about houses? (Score:2)
Easy: use PoE (Score:2)
PoE compliant switches are actually rather complex and do a great job at providing safe DC electricity and cutting off shorts. As an EE I read the standard expecting it to be a total bodge-job like all else ethernet related, but I was actually impressed and wouldn't hesitate to use it.
Now if we could only get all our gadgets and gizmos to burn less than 14W...
Re:What about houses? (Score:2)
how does it save a conversion (Score:5, Insightful)
The question is of the efficiency saving of doing all the converting in a big box against the efficiency loss of piping it around the data centre as dc, and wether you get a large total net saving (which I suspect that you do, since even inside the data centre, it's not going far)
Re:how does it save a conversion (Score:4, Insightful)
Power gets converted to DC anyhow to keep the UPS batteries charged. If the lights go out, the DC from the batteries is converted back to AC to go to the power supplies and back to DC inside each system.
No, it doesn't take as much power to keep the batteries charged as it would to run the center off DC, but that's not the point. Anyone with a large UPS already has a beefy AC/DC and DC/AC conversion system in place.
I would also assume one large converter / power supply would be more efficient for power and heat than hundreds (in my data center) or thousands (in a big one) of little power supplies. Any thoughts on that?
The ups AND cooling! (Score:3)
By saving 20% on the conversion you also save 20% on the cooling. But also the power can now run in a different room where the temperature condition might be less demanding, so even more cooling might be saved.
Re:how does it save a conversion (Score:2)
Think of the UPS (Score:2)
If main power fails in an AC data center, the UPS systems need to take the DC from the batteries and convert to AC, then distribute, then each machine needs to convert back to DC. That's terribly wasteful, since neither of these conversions is anywhere near perfect efficiency. In a DC data center, the UPS systems are just the batteries, so th
Re:Think of the UPS (Score:2)
Regardless of incoming power status, almost all large UPS's are always in the electricity path, so there is always AC-->DC-->AC-->DC.
Re:Think of the UPS (Score:3, Informative)
My understanding is that the UPS's will typically have a power source switch in front of them, not behind, and when the emergency generator kicks in, its power goes through the UPS just like the normal utility power.
There's a very good reason for that, too. Virtually every UPS will clean up the power feed, and backup generators are usually 'dirtier' power than mains power - the last thing you want is spikes and droops from the backup genny
Re:Think of the UPS (Score:2)
The system would then basically be:
External AC
-> Generator unit (can replace external AC with generator AC, but needs some time to start the generator)
-> AC/DC unit
-> Battery unit (can buffer short power out
Re:Think of the UPS (Score:2)
Rectifiers are cheap (Score:2)
Re:how does it save a conversion (Score:2)
Later on, the article talks about batteries, so I'm guessing that the "power distribution unit" is actually a UPS, and that's why they need the extra conversions:
The
Absolutely - there is ALWAYS AC-DC conversion (Score:3, Informative)
Where there *could* be some benefit is where you have larger, more efficient converters very near the point of use. If you figure each power supply inside each box is 50% efficient, but a single big one is 75%, then you reap a net benefit (totally rhetorical - I have no i
Here's how it saves (Score:2)
AC -> Building -> UPS -> DC converter -> Battery -> AC converter -> Server Power Supply -> Components
DC-only
AC -> High efficiency DC conversion -> Battery -> Server Components
Re:Here's how it saves (Score:2)
Re:how does it save a conversion (Score:2)
Data center UPS's do always on converstion. (Score:2)
Re:how does it save a conversion (Score:2)
There's more to it than that. The problem is that essentially all of these systems run on UPSs. A UPS takes the AC in and converts it to DC to charge a battery. It then takes the DC from the battery and converts it back to AC
Re:how does it save a conversion (Score:2)
DC power for data centers (Score:4, Insightful)
Re:DC power for data centers (Score:2)
I wish homes were wired for DC (Score:3)
Re:I wish homes were wired for DC (Score:2)
That's the wrong question. The right question is: What type of devices in house actually run on AC (or don't care) ? And the answer is: Anything that requires _lots_ of power (and/or has a simple motor in it). Hot pot, hairdryer, iron, dishwasher, washing machine, dryer, lamps, vacuum cleaner, etc.
Re:Twice the cables! (Score:3, Insightful)
It's different, however, in a data center, where you have hundreds of computers, network switches, etc., each with their own power supply, and also importantly, all in a relatively small confined space. Here, I think (once standards
They should have already hired such engineers. (Score:4, Funny)
If such a data centre is just now considering bringing such people in, then they have serious operational problems. They're not getting professionals in to do the jobs that professionals must do.
The answer is in the Racks, young Jedi (Score:3, Funny)
Wait. Did I just reinvent a 64 way Sun server? Imagine that.
Bring on DC! (Score:2)
specialized engineers? (Score:3, Insightful)
Re:specialized engineers? (Score:2)
Re:specialized engineers? (Score:2)
It should be noted that there will still be power supplies in the computers. 48 volt is the distribution system, and then your computer uses 12 volt and 5 volt for main stuff... and then the processor usually does its own thing off in lala land. It's just a lot more efficient conversion than the mess we have in most places with AC->DC->AC->DC
Re:specialized engineers? (Score:3, Interesting)
No problem at all. Turns immediately to metal vapour and will likely not even interrupt server operation.
And with todays computers that would more likely be 12V/3000A
Of course the human being standinge besides this events may suffer some serious damage....
wasted servers (Score:3, Interesting)
Why such low utilization ?
Any other industry would scrap 80% of that equipment to save costs and power.
DC in Telco (Score:5, Interesting)
One thing telco companies do well is DC power, they have alot of skill in providing multiple DC feeds from DC power systems, with battery backup and generators all in line.
I would imagine that any big server farm would benefit from this kind of setup. Especially when you have people runnnig the lines that are as good as some of the guys in the telo world, they can really make the wiring look like a art in some places.
Depending on your UPS configuration... (Score:5, Informative)
However, it does provide a few significant advantages.
Telcos use DC because it's easy to battery-back. Since all your gear is already running from the DC supply, there's no guesswork about whether your UPS will be able to handle the load. Each piece includes its own converters, so all you have to do is size the battery bank. Since most telcos aim for 8-hour runtimes on battery (long enough to discover and fix a generator problem), overkill is the order of the day.
There's also the point that you can run several small generators, instead of one large one. In an AC world, keeping multiple generators syncrhonized is nearly impossible on a small scale, so you just run one big one. If your setup grows, you rip out the old generator and replace it with a larger one. In DC, since all your generators feed the same battery bank, you can just tack on more capacity without trashing your original investment.
Using multiple generators provides cheaper redundancy too. In an AC setup if you wanted to be protected against a generator failure, you'd need two identical gensets, each large enough to run the whole load. With DC, say you had 5 generators but 4 could power the load. You still have no single point of failure, and you don't have to buy *double* the generating capacity.
Oh, and if a second generator fails, say you're down to 3, you're below the break-even point, but you're still limping along, with the operating generators assisting the batteries, extending your battery runtime long enough that you can probably fix one of the failed gensets. Oh, you found a spare generator at the rental place down the street? Switch a few rectifiers onto it and watch your charge status come back into the green. You just don't have that sort of versatility with AC.
DC is easier to noise-filter than AC. Keeping the high-frequency noise from switching converters off the AC input is something of a black art, and is hard to do effectively. You also have Power Factor (PF) issues when running large numbers of computers (or anything that uses switch-mode power supplies) from AC. Hence, your supplies have to be PF-corrected, which adds bulk and complexity, and reduces efficiency.
A DC-DC converter suffers none of those problems, going from your 48v battery bank down to the 12, 5, and 3.3 levels in your servers. It's easy to filter the switching noise because the input is DC, a big L-C filter works quite well. There's no such thing as power factor on DC, so the converters themselves are simpler and smaller, and run cooler.
One other huge benefit is that 48 volts is "low voltage" according to the NEC, so you can wire it yourself. You'll never have to let pole-climbers into your server room again.
Another advantage is that most DC-input equipment has a telco heritage, and supports dual inputs. Everything in telco has an "a-side" and a "b-side" power supply. It's only relatively recently that high-end datacomm gear has started to support multiple AC power inputs. History and experience are on your side with DC.
DC {ower Makes Sense but... (Score:2)
DC power in the data center unquestionably makes sense. Higher density, less heat and if it is on UPS, a lot off efficiency gains both at the computer part and the power distribution systems. Power distribution from the UPS is generally cheaper not having to transform from battery to A/C current. It would even save on air conditioning costs through lower heat on the computer and UPS electronics/transformers. In the long term, such a data center would be much less expensive to operate.
But the only draw
electrical isolation (Score:2, Informative)
Cons of DC power (Score:4, Informative)
1) Contacts tends to rust on the positive side.
2) Lower voltage means bigger current for the same power. This would require thicker, more expensive cables
3) DC-DC voltage conversion is, somewhat less efficient... Ok, I know switching mode power supplies are efficient, but this leads me to the last point:
4) No insulation between systems. That way, systems get more prone to ground loops...
Cons of DC power, debunked. (Score:3, Informative)
True, the effect is called "galvanic corrosion". That's why the entire telco network is negative with respect to ground. It's been that way since the days of Western Union. Already solved, sorry.
True. But low voltage (under 50vDC nominal) doesn't require licensed electricians to run it. Clearly the extra buck for thicker copper outweighs the cost of paying
Gonna need big copper cables (Score:2)
Yes, central DC is attractive from a number of viewpoints: easier redundancy, higher efficiency, integratable into UPS and perhaps most important these days -- removing headload from CPU bays. But you're gonna need big busbars, especially if your boards need 3.3V. I can't see using some sort of non-custom miniPSUs to do12VD
DC can be really annoying... (Score:5, Informative)
I've worked in DC powered labs.
There isn't really any concept of 'plug' in the DC powered world. Powering up a device usually entails reading it's current draw off the equipment, selecting the correct gauge of wire, cutting the correct length of wire, strip both ends, hook up to your DC distribution on one end and your equipment on the other, select about the right size fuse, plug it in... etc. It's a royal pain. Oh, and make sure you do it correctly, because it's not that hard to electrecute yourself...
Nearly every engineer I've ever worked with whose been exposed to DC powered labs has begged to return to the AC powered world... it's just MUCH easier to work with.
On the flip side though... telco racks rock! Nothing beats hex head rack screws... you can literally drive them in at a 45 degree angle with a power drill and it's OK. It makes going back to the world of crappy philips head data wrack screws that you occasionally have to drill out because the head has stripped very annoying.
Re:DC can be really annoying... (Score:3, Interesting)
The world could use a set of powerpole orientation and color standards. Hmm.
What everyone seems to forget (Score:3, Insightful)
Re:What everyone seems to forget (Score:3, Informative)
For Christ's sake, this guy doesn't know what he's talking about! "DC power is naturally unstable", "unclean power" WTH?
Back to the original topic, the article is, as other mentioned already, 100% pure dribble. The major advantage of AC input power is that the power conversion (AC to DC system and from there down to 5V/3.3V/VCore/DDR/IO/etc), happens close to the loads.
AC voltage is 110V or mor
Don't you still need a PSU for 48v DC? (Score:3, Interesting)
The downside with DC is that lower voltages require much thicker wires, and you're at much greater risk for fire. Circuit breakers and other things are also more complicated and expensive since DC tends to weld things together.
An EE I know just built a data center supplying 208v (2 branches of a 3 phase iirc) to all the racks. Almost all existing power supplies can take it, and it saves a bundle in wiring costs. I'm not sure about servers, but most desktop power supplies operate at a power factor of
How does it save power? (Score:3, Insightful)
Assume that an AC-to-DC conversion causes a loss of 10% (just to have a number).
If we bring in AC, convert it to DC in one location, and then distribute it as DC to all the computers, we've lost 10%.
If we bring in AC, distribute it to all the computers, and convert to DC at each computer, we lose 10%. The conversions are independent and parallel, and so the loss is not additive. (After all, if we have 10 computers, it doesn't mean we are losing 100% of the power). I can see how we might save money, as we no longer would need a complicated power supply at each computer. Also, we wouldn't have a hot power supply in each computer, and this could reduce cooling costs. But I don't see where the power savings comes from.
Saving a conversion step isn't the issue. (Score:5, Insightful)
save a conversion step. Well, maybe you do, maybe you don't, but
counting the number of AC-to-DC and DC-to-AC conversions is very
misleading.
Converting 50 or 60 Hertz to DC is much more costly and less efficient
than converting in either direction at a higher frequency. Low
frequency rectification requires large filter capacitors, complex and
expensive inrush current limiting, and active power-factor correction.
By doing that front-end work in one place only, preferably from a
3-phase source, you save power and increase reliability. You probably
still want multiple 50/60Hz to DC rectifier stages, of course, but now
they can be in parallel (for redundancy), rather than each one
downstream of the other where a failure of either one will bring down
the system.
Just because you're distributing DC to the racks, doesn't mean you
don't have to convert it again. It typically gets converted to AC and
back to DC at least once, usually twice before it reaches CPU and
memory chips. That's equally true in data centers that distribute AC
or DC. The fact is, memory and CPU devices want very low DC voltages
and very high currents. To make matters worse, not all parts of the
system want exactly the same DC voltage, you almost always have to
have multiple supply rails. You can't distribute very low voltages,
because it would require wires as thick as your arm and they'd still
be too resistive and inductive, so instead you distribute the DC at,
typically, 48 volts. The subsequent conversion to low DC voltages has
to happen via an intermediate AC, but it's a high frequency AC, so it
can be done much more efficiently using ferrite magnetic components,
active rectification, and often resonant mode filters. This high
frequency AC is confined to the internals of a power supply unit, it
never travels over wires or between boxes, thus reducing typical
high-frequency problems such as RFI.
I haven't mentioned battery-backup (i.e. UPSs). They make the system
more complex, but don't change any of the fundamental concerns. Even
on a DC distribution system, the UPS system requires it's own
additional stages of DC->AC->DC conversion, both while charging
(standby) and while discharging (during AC power failure). This is
because battery charging has to have a precisely controlled current
envelope. And batteries don't discharge at the uniform and
well-regulatted voltage that your DC distribution wants. They need
regulators, and switchmode regulators (typically DC->AC->DC) are the
most efficient choice.
Re:Saving a conversion step isn't the issue. (Score:3, Interesting)
50/60Hz originally was chosen because the low frequencies were easier to generate with the mechanical equipment available 100 years ago but also to avoid a lot of indu
This article and the raised-floor article both bad (Score:4, Insightful)
That article talked just like some "Intelligent-Design" moron. Just because HE can't figure out how to properly model raised-floor airflow, it must not be possible to do it at all. Wrong. There are any number of companies that will do this for you.
The solution to raised floor airflow is proper modeling of the equipment, vent tiles, and blowers, and relatively unobstructed floor plenum. The solution is NOT air-cooled equipment on bare floor and overhead cable runs. If cooling is still a problem, then use liquid-cooled racks and equipment. (This is where things seem to be going right now.) While overhead cable runs may work fine for some dinky test lab, "real" equipment requires power cables of a size that would quickly fill most overhead runs.
This article proposing DC power is equally stupid.
An enterprise storage box, fully configured that I looked at requires 13,800 kVA of 208V three-phase power (100A inrush current). My mind can barely fathom the completely unbendable copper "wire" that supplying that much juice at 40-ish volts would require.
Telco's switches have a far lower power density than modern servers, and the DC power was made to correct for different problems.
If this guy's ideal data center is overhead cable runs, ceiling blowers, bare floor, and DC power, I'd run away fast.
SirWired
puzzling move for a datacenter (Score:3, Funny)
Ok, I'll order more HP Xeon servers. They use more power, cost more, perform worse, and have a limited upgrade path. But Intel sends me cool swag so I use them.
Johnson, your fired! We are going Opteron, and raising our capacity 30%!
Engineers or Technicians? (Score:3, Insightful)
I guess using the term "engineer" sounds better though since it tends to scare the corporate fat-cats away from a technology because of the implied additional cost from hiring an engineer as compared to a technician.
Some clues on power distribution (Score:3, Informative)
There are several approaches to power distribution. One is "telco type" -48VDC distribution. [telephonyonline.com] This is most appropriate when the configuration doesn't change much. Wiring usually involves big cables and screw lugs. Plugs aren't standardized. More importantly, there's no set of simple rules, like the UL/NEMA/NEC standards that govern plugs, outlets, wiring, and circuit breakers, that make 120V power distribution safe without having to measure everything.
In the 120VAC world, everything has been designed so that end users don't have to worry much about overloading the wiring. If they do, a circuit breaker will trip. An ordinary power plug, a "5-15P", can handle 15A, so if you have an outlet strip, there is a breaker to protect the plug and cord from overload, should the total load on the power strip exceed 15A. A 20A power strip must have a "L5-20P" plug, the big twist-lock type. As soon as you get away from 120VAC, you lose that designed-in idiot-proofing. (Europe is still struggling in this area, with too many different connectors [pandora.be], so you don't get the same level of idiot-proofing in the 220VAC part of the world.) So once you leave 120VAC, you're going to need power engineering skills. (Clamp-around ammeters are very useful, and yes, you can get them for DC.)
There's also 400Hz AC distribution, which allows for smaller transformers and filter caps in power supplies. 400Hz rackmount servers are available. [rave.net] Aircraft, military, and some mainframe systems use 400Hz. It's not a big win in this era of switching power supplies.
There's 3-phase power distribution. Here's a 3-phase outlet strip. [servertech.com] More to the point, there's an efficiency gain in running a UPS from 3-phase power, and big UPSs are usually 3-phase, at least on the input side. Arguably, power should be 3-phase down to the point where it's rectified to DC, because 3-phase rectifiers need far less filtering, but nobody does this for small loads.
American Power Conversion [apc.com] has been pushing the idea of integrating power conversion, cable management, and cooling into standard racks. Classically, those are the big problems in big computer systems. Seymour Cray used to say that the big problems were "the thickness of the (wiring) mat" and "getting rid of the heat". By that standard, APC is now as much of a computer manufacturer as, say, Dell; neither makes motherboards or ICs, they just package gear from others. Which is a wierd thought.
All of this power is going to be converted again, at least once, and probably twice, before it hits the semiconductors. That's the job of point-of-load DC to DC converters, [ti.com] usually ICs on the board that do the final conversion. Typically, when you get to the computer, there's a conversion from the line voltage (120-240VAC, 48VDC, etc) to internal distribution voltages of 5-12VDC, then another conversion and regulation just before each device, usually downward to something like 3.3VDC. This keeps transient load changes from one device from affecting others. There may be on-chip regulation, too. The losses at those last stages of conversion are usually the biggest ones in the whole chain.
Re:Some clues on power distribution (Score:3, Interesting)
Especially if you can swing a 12-pulse rectifier - which gives much smoother DC and less harmonics on the AC side. This is becoming less of an issue with PFC SMPS's.
Typically, when you get to the computer, there's a conversion from the line voltage (120-240VAC, 48VDC, etc) to internal distribution voltages of 5-12VDC, then another convers
Datacenters and DC/AC power (Score:3, Informative)
I am a datcenter manager that has had the opportunity to not only run but also build a datacenter from pretty much scratch. In my experience I have found that both DC and AC powered equipment both have their places in the environment. Neither system is perfect so by running hybrid you can get the most flexibility.
We recently moved our datacenter form a 10K sq ft facility down to a 1700 ft facility by doing a technology refresh and changing many of our key infrastructure methods. In the new facility I currently have 315 HP blade servers plus another 10-15 traditional rack type servers running. I have the capacity to add up to another 144 blades (assuming they are 1U) before I run out of floor and HVAC capacity. The power delivery method is hybrid. I run DC for the blades which are fed by Emerson Energy's Candeo XL rectifier stacks (originally designed for telco) and AC for everything else. To eliminate a lot of the under floor clutter I use a trough system instead of conduit for the various AC circuits. HVAC is provided by 4 Liebert 22TON units which keep my room at a comfy under floor temp of 66 degrees.
Adequate airflow is critical so we spent a lot of time planning tile placement. The key for proper cooling in this scenario was a high volume of airflow pushing the cooling to about 5.5ft up from the raised floor. This way my cooling isn't being sucked up by just the bottom half of the rack. Low voltage cabling is overhead.
We chose to power the blades DC for two reasons. First was the limited space I had for installing breaker boxes on the walls. The number of AC circuits I had was limited so I pulled fat feeds directly to the Candeo systems. A full rack of HP p-class blades would require 4 x 3phase 208 circuits per rack. My initial installation of blades would have consumed 144 of my 168 circuits leaving next to nothing to power my SAN/Network/Tape Library/etc equipment. The other reason was power supply efficiency. In the conversion of power from AC to DC the efficiency of the power supply must be taken into consideration. It's not just the number of conversions you do but the loss at each. Typical power supplies in servers run about 80% efficient while my Candeo as they are setup gets about 90%. For me this ultimately meant less heat and more available cooling, therefore I could bring in more servers under the existing HVAC.
I prefer a best of class mentality. IMHO there is no best universal solution. For those of you that use traditional rack mounts servers like Dell you can purchase these units with a DC option. I am not sure if HP offers a similar option but they might.
Len
p.s. Someone also made the comment about DC not generating noise in network cabling while AC does. This is not a totally true statement. Anytime you run a current through a conductor you will generate a magnetic field. Put this in parallel to another conductor and you will further induce a mag field (this is why any power runs that have to intersect low-voltage cabling should only intersect at 90 degree angles to avoid inductance). The big difference is the way DC cabling runs. In most DC circuits the feed and return lines run together so the proximity of the out of phase magnetic fields will cancel each other out. Don't believe me? I had this problem when we intially wired these Candeo systems up. The small feeds to the racks and the big mains that connected to the common buss bar were about a foot apart. Because the fields weren't cancelling, we were getting enough noise on the lines that it looked like there was AC leaking through the circuits (6volts p-p in some cases). By simply wire tieing the lines together, the proximity cancelled the fields out and everything was peachy.
Re:correction (Score:2)
That big and efficient power supply generates a lot less heat.
Re:correction (Score:5, Interesting)
Nope, you save two conversions.
Without DC distribution, you have AC->DC->AC in the central UPS, and then AC->DC in each computer's power supply.
With DC distribution, you have AC->DC in the central UPS, and no conversion in the computers.
You get down from 3 conversions to 1.
Solar power for telcos! (Score:2)
I'm not going to suggest that a telco CO could run entirely from rooftop solar -- far from it! But they'd see a much faster return on investment, compared to residential systems, because they wouldn't have to buy inverters, charge controllers, or any of that crap. (One advantage of an undersiz
Re:What am I missing? (Score:4, Interesting)
Re:What am I missing? (Score:2)
Re:What am I missing? (Score:2)
I guess you mean the usual server setup with an uninterruptible power supply (UPS) between AC line voltage and server. With a standard computer that runs off AC line voltage, it looks like this:
-The UPS uses internal batteries => DC. To charge them, the UPS needs to conver
Re:DC power outlets (Score:3, Interesting)
Well, none. Assuming you still have a washing machine, dryer, refrigerator, (possibly) electric heating, (possibly) electric oven, hair dryer, (possibly) electric water heater, and air conditioning, which in total consumes almost all of the power that is used in your house, and all of which is more efficient in AC. DC would benefit your electronics, which make up a much smaller portion of the total electric con