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Data Centers And DC Power
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.
To be pedantic for a moment, Tesla quit after Edison screwed him over on a $50,000 bonus he was promised.
But you're sentiment is correct. Edison never really believed in AC power.
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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.
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...this may or may not save a step.
:)
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.
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...
"There is always an easy solution to every human problem -- neat, plausible, and wrong."
H. L. Mencken
There are definite advantages to DC power... but it can also be *hugely* annoying.
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.
Does big iron still use 3-phase power?
Yes. Mainframes, large UNIX systems, and the storage boxes that connect to them still require three-phase. (I am a storage specialist.)
SirWired
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.)
But westinghouse did believe in it .. who then gave Tesla a job.
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.
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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.