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University Switches To DC Workstations
An anonymous reader writes "Researchers at the University of Bath, UK are undertaking an in-depth study of energy consumption within the new network, with the aim of demonstrating that running a large network of devices on DC rather than AC is both more secure and more energy efficient. AC electric power from the grid is converted to DC and runs 50 specially adapted computers in the University Library. Students using the system have noticed that the new computers are more compact and much quieter than the previous systems. The immediate advantages of the new system are not only for the user but for the energy bill payer and the environment."
They switched to laptops?
"I like to lick butts!" by MobileTatsu-NJG (#32700246) (Score:5, Informative)
There is no evidence or reason for DC to be more "secure". If some lame argument about it being harder to bring your own power source / utilise their outlets when you have a custom system is put forward, then, well... no.
I can understand the efficiency argument to a certain extent, although if a workstation needs enough power that a fanless AC PSU is unsuitable then the more efficient AC PSUs will be enjoying enough load to reach over 80% efficiency. Are the centralised rectifiers + wires + in-computer DC-to-DC converters as efficient?
How is DC more secure? The only reason I can think of is because of the increased difficulty connecting a stolen computer to your home power supply.
There is a small deterrance factor, but I think that by the time most theifs would have figured this out, the computer would already be stolen.
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The only thing inside a computer that actually runs on AC is the computer's powersupply. The powersupply regulates this to DC voltages! The powersupply is also quite bulky and noisy compared to the other components.
"Initial tests show that the system in Bath emits approximately half as much energy as heat than the previous AC powered system while running much faster."
Yes, I'm sure it'll generate less heat when most of that heat comes from converting AC to DC, but why the hell would it run faster when everything else in the computer is still the same?
There's something to be said for DC distribution within data center racks, but building a plug-in DC infrastructure seems like a PR stunt. They need a whole rack of power conversion gear to serve 50 desktop computers.
Google at one point proposed that rackmount computers should be built to run on 12VDC only, so you could have a single 12VDC supply in the rack and get rid of the individual power supplies for the server. Whatever happened to that?
Much industrial automation gear and military equipment runs off 24VDC. That's low enough that you don't have a shock hazard, but high enough that the wire sizes are reasonable.
AC is the tool for killing Elephants though. linky
Selective quotes from TFA:
Researchers at the University are undertaking an in-depth study of energy consumption within the new network, with the aim of demonstrating that running a large network of devices on DC rather than AC is both more secure and more energy efficient.
The new DC network also offers greater security. DC power supply units have a simpler design, with fewer parts that could fail and need replacing. The system at the University also charges a number of batteries when usage levels are low to allow the system to run independently from the grid for up to eight hours should a cut in power be experienced.
The above two paragraphs are the only I could find in TFA that mention security. I gotta ask -- can anyone speculate how centralizing the PSU would lead to a more secure system? Is it possible that there is a regional definition of "secure" to mean "very reliabile" or "very available." As in, we have "secured" a constant municipal water supply?
Westinghouse and Edison?
The major drawback to DC power is in the wiring. Direct current requires larger gauge wiring than AC power, which increases material costs considerably. In general, DC power is economical only if the wiring between the computers and the DC source is less than 35 feet in length. More than that, AC power becomes more economical.
I guess Con Edison should have waited just a few more years. Apparently 125 was not quite enough.
oh, duh! disregard.
In a single room or even perhaps a floor of an office building I guess I could see DC distribution. It would tend to reduce the power supply losses. Laptops are already doing total DC-DC conversion for the different voltages they need and there probably isn't much of a reason you couldn't run 12 volts to each computer and have it convert it over to 5 and 3.3. I would think your benefits would be significantly less if you were running 100 volts DC and requiring it all to be downconverted as DC-DC conversion isn't anywhere near as efficient as AC-DC conversion.
The big question is when do the wiring costs exceed the power supply losses. If nothing else you need to have a completely different parallel distribution network run with very, very incompatible outlets. You would NOT want to plug just anything into a DC supply, even if it was 12V. DC at 100V would destroy a lot of things and the load something like a shredder or even a desk lamp would be damaging.
Another factor I would think is redundancy. Today if you blow a power supply (one of the most common computer failures) you lose one computer. If you blow the power supply for the office floor you might lose 100 or 200 computers.
We've got USB, what I want is UDC - some kind of in-the-wall intelligent DC outlet standard so that we can get rid of all those wall-warts and reap some of the advantages this article mentions. Of course, the one-tonne AC converter is a bit of a problem...
If you mean "much cooler", you already said that. If you mean "much faster", you should probably sign up for that physics (or electronics) course.
So you don't mean "security", you mean "reliability".
I wonder how much carbon was released refining all the lead in all those batteries...
No folly is more costly than the folly of intolerant idealism. - Winston Churchill
They just move the AC/DC conversion somewhere else. All of the heat will go along with it.
:(){
He tried to tell you AC would fail. Take that, Tesla.
Warning: this article may contain humor, sarcasm, parody, and perhaps even irony. Read at your own risk.
It's not clear to me how this is any better than specing normal machines with a decent power supply. Anything rated at "Gold 80 Plus" will convert to DC with 80-90% efficiency depending on the power draw.
Is this just a matter of replacing old inefficient noisy machines with newer efficient (and thus quieter) ones?
If you're looking at the McCumber cube, then yes, availability is one of the three aspects we're trying to protect in security (along with confidentiality and integrity).
Most "security" obsessed people these days come from the "keep the bad people out" mentality, even if it's at the expense of making it so obnoxious for the authorized users to actually be able to do their job, but a complete model of security is that people who are supposed to be able to use the system are able to use it when they want.
(but I wouldn't have said 'secure' ... I'd have said 'reliable', as 'secure' has connotations of restrictiveness and secracy)
Build it, and they will come^Hplain.
Ok, so people have 'noticed' that these NEW computers are faster, quieter, smaller and just magically better than the old system.
This is obviously not very scientific, I'd venture to say if they replaced the entire old system with a new AC system, it would still be faster, quieter, smaller, etc...
Yes, DC may be great, but these observations can be said about any new vs old setup.
Do not meddle in the affairs of sysadmins, for they are subtle, and quick to anger.
My server at home is begging for a DC conversion, for example, as are my switches and other gear.
The idea is great, but like so many things, we are entrenched in our AC power systems. So until we come up with a common implementation of the "AC to DC" power supply for everything and everything comes with an option to plug in "DC" I will have to wait.
There are DC power systems for servers and such today, but such things are pretty much special order and expensive. I hope that it catches on at a level which enables me to run my stuff on DC sooner rather than later.
TFA: Initial tests show that the system in Bath emits approximately half as much energy as heat
Switched mode power supplies are on average about 94-95% efficient, so why half as much energy as heat? It should have been a mere 10% reduction.
Simple answer: without a fan, the heat is trapped in the case. Not much of an improvement.
-- Insert witty one-liner here. --
So they went AC-DC to try and get back in (the) black?
My concern is that the initial conversion would cost a touch too much, and it ain't no fun waiting for the energy savings to cover the investment - the down payment blues.
Still, in my experience the power supply is often the point of failure that finally kills the whole computer, so goodbye & good riddance to bad luck.
It has taken a while for the economical advantage of this to trickle into user space. Electronic devices are almost all DC in nature, and the efficiency improvements here are not actually the computers, they are the lack of individual power supplies. Other poster have made comments about laptops, but normal laptops are actually no more efficient than desktops. They use less power, but that power brick is not any more efficient than a good desktop power supply.
What they get to do here is run one big, presumably very efficient power supply, and run it outside of populated space, moving the noise and heat generation to where it can be more efficiently controlled.
Of course telco types will say "Umm, yeah?" because a lot of telco heavy iron has been DC forever, for the same reasons TFA is bragging up this system.
This doesn't apply very well to consumerland, because houses don't lend themselves well to special DC wiring that doesn't easily move when you rearrange the room.
And before people start asking "Why don't the power companies just use DC?" Electricity transmission over distance is much more efficient as a high voltage/low current AC than DC current, especially since you can't use transformers on DC.
But I can very easily imagine datacenters utilizing a rack sized high efficiency DC power supply to run row(s) of server racks. This would tie in very nicely to Googles battery-in-every-server method.
"Proximity to wonder has blunted our perception and appreciation of it" --Tim Hartnell in 'Exploring ARTIFICIAL INTELLI
Its the voltage that matters when transmitting over long distances, not the type of current. In fact DC is slightly more efficient for a given voltage though you'd have to ask a physicist why.
DC datacenters have been around longer than AC. All major telco's globally use DC distribution for their networking and communications equipment. They always have, and likely always will. The newer datacenter companies are just having to learn for themselves why it makes sense.
So, they run faster and produce much less heat, which means they're not running the same CPUs, etc. as before. Therefore, it's an apples-to-oranges comparison.
I have no doubt that a single large DC power supply can power multiple systems with better efficiency, especially if the AC/DC power supply is located in a separate room or outdoors where it can be cooled without contributing heat to the room containing the computers. Of course, that's not necessarily desirable in cold environments, but that's a separate issue.
AC is used for power distribution because it's far more efficient for distributing power over a distance. Even a few mi/km, DC loses are significantly worse than high voltage AC. AC is far easier and more efficient to transform to/from high voltages, making high-voltage AC far more efficient than high voltage DC for distribution. DC isn't practical until you get to the final few hundred feet.
make imaginary.friends COUNT=100 VISIBLE=false
Has electricity changed so much since then?
It's funny that you'd bring that up. Yes, actually Physicists and noted researchers are observing subtle but significant changes in the way that electricity flows through conductive materials. Electrons used to be so-called "longer" a few decades ago (end to end, mind you), so you'd actually get a fewer number of them traveling through the wire to you per second.
Now that the magnetic poles of the earth are shifting, and we're (slowly) approaching the geomagnetic "pole-arity" reversal (a little physics humor there...), electrons flowing through wires are shortening and approaching the shape of a sphere rather than an elongated sausage. You'll see this happen more strongly during the night, when your part of the Earth is facing away from the Sun and isn't experiencing as much solar radiation and bombardment of charged sub-atomic particles.
So yes, it is a good time for the University and other large groups to reconsider the way that they power their computers. Note that they'll need to install an inverter once the poles actually do reverse, however usually the reversal takes over a thousand years. That being said, taking precautions isn't a bad idea, as Wikipedia does point out:
...geologist Scott Bogue of Occidental College and Jonathan Glen of the US Geological Survey, sampling lava flows in Battle Mountain, Nevada, found evidence for a reversal that took only four years.
I'd be happy to consult with any University or Company that is considering a move to DC power or other "green power" initiatives.
For more information about the latest in advanced power tech and the influence the shifting poles has on inter- and intra-electron measure, please see the wikipedia page on such topics here.
coding is life
...running a large network of devices on DC rather than AC is both more secure and more energy efficient. AC electric power from the grid is converted to DC and runs 50 specially adapted computers in the University Library.
Computers already run on DC -- the only question is where the conversion takes place. The downside to having a single converter (rectifier) is that you have a single point of failure, but obviously you can place it away from the actual computers to reduce noise and such.
As for efficiency, it all depends how efficient the individual power supplies are/were for each PC. There's nothing inherently more efficient about converting once for all PCs vice converting once for each PC. In fact, your losses might even be higher depending on your distribution network. Additionally, there's the not-insignificant monetary and environmental cost of installing DC-DC power supplies for each PC, upgrading the transmission lines, etc. Depending on how long the network is used and the actual efficiency gains, if any, those investments may or may not pay off over time.
https://www.eff.org/https-everywhere
Too bad they didn't think to bury him in a stator.
No folly is more costly than the folly of intolerant idealism. - Winston Churchill
Update your sig. It's outdated by like 3 months.
We used to have a Bill of Rights. Now, with the rights gone, all we have left is the bill.
Just a couple of days ago I was talking with a friend about how my company already does this with some of its servers.
I know that we use AC for transmission because it loses less power over long distances than DC, but is there any reason why we don't have DC converters installed in the electrical panels of homes other than the fact that many appliances currently require AC? More and more appliances seem to be using DC lately, requiring wall warts. If we could convince more manufacturers to produce DC appliances, it would assist people transition their homes to DC power. This would be great because it is safer than AC and it would be more efficient to convert it in one place rather than through the use of the many wall warts and computer power supplies we currently have.
(just sayin)
I believe the telecom industry has known this for years. Since telecom equipment in COs can be run on both DC based and AC based power, consumption levels can be monitored between the AC and DC based offices. AC/DC - Rock on.
Not exactly - it's because of the requirement that equipment operate during power failures. Hence the large banks of lead-acid batteries in older switching centers.
Physicists and noted researchers are observing subtle but significant changes in the way that electricity flows through conductive materials. Electrons used to be so-called "longer" a few decades ago (end to end, mind you), so you'd actually get a fewer number of them traveling through the wire to you per second.
Ah! but DO the electrons flow through the wire or just shuffle back and forth as the holes flow through the wire? I've seen these very same people get into fist fights over that question too.
"A person is smart. People are dumb, panicky dangerous animals and you know it." - K
Electricity hasn't but transformers have. It used to be very cheap to do voltage conversions with AC and very expensive (and lossy) with DC. That's a lot less true now.
I am TheRaven on Soylent News
They needed to do a study to figure this one out?
I'm glad that they shared their experience with the rest of us, to save us from duplication of effort.
W..w..W - Willy Waterloo washes Warren Wiggins who is washing Waldo Woo.
Though personally, I think the article is all wet.
You do know that DC is used a lot in intercontinental and long distance high voltage power transmissions, right?
For more information: HVDC
Manuals are your last resort only
We all have houses full of DC powered devices but no DC power. How many wasted AC/DC converters is that per year? Computer power supplies, etc. wasting 30-50%+ of the input AC due to inefficiencies and poor sizing for the task. Office buildings full of DC powered computers and gear and yet we still just keep on going with pure AC power.
http://teasphere.wordpress.com - A little spot of tea
Yes. DC/DC converters have gotten a lot more efficient. When Edison and Tesla were arguing, a transformer was the only way of doing voltage conversion.
I've abandoned my search for truth; now I'm just looking for some useful delusions.
On my Atom based fileserver, with a variable speed power supply fan the only noise I could hear was the tiny fan that cools the chipset (the CPU is fanless but the chipset has its own dedicated fan). It got so annoying that I unplugged it and set up a quiet 80mm fan in the case to blow on the chipset heatsink.
There are plenty of silent PC options that don't require running special wiring just for DC power. In fact, some thin clients will power themselves over PoE (which I guess is technically a form of DC power). There's nothing magic about DC power that will make a PC silent.
Unless they are running all of the voltages required by a modern computer (12V, 5V, 3.3V, etc), then there are DC-DC converters in each computer doing the work that the power supply used to do, so it's unclear how this will dramatically reduce power usage over having an efficient fanless AC power supply.
Comm industry has been running off of -48V since, like, forever.
I want to retain the ability to arc-weld with my computer if need be!
All of my devices use DC already. They always have. AC electric power from the grid is converted to DC right after it comes out of the wall. That's how we do it in the US anyways, maybe it's different in the UK.
~Warning!~ The above is encrypted using rot676!
If you are not browsing flash websites good old thin clients do the exact same thing. the Old HP thin clients we have here that are 3 years old only draw 50 watts each WITH the 17" monitors on them.
Do not look at laser with remaining good eye.
> Researchers .. are undertaking an in-depth study of energy consumption .. with the aim of demonstrating that running a large network of devices on DC rather than AC is .. more energy efficient ..
Well, it would be as , instead of doing the conversion individually in each switch-mode power supply the conversion is done in one single unit. Of course you then need to factor in the losses in transferring DC over distances, something high-tension distribution AC was designed to overcome.
A very large cable operator has been making progress converting all its systems to DC. Right now its kinda ugly, with AC main power, AC generators, AC UPSes, lots of AC computers and network gear. Then there is the DC plant with the DC battery stack, and more and more DC computers and network gear.
DC in the data center makes sense, as if your AC, every system needs a power supply. Every power supply is inefficient. The generate heat, and use up more power then the computer needs.
AC is much easier to transport. DC resistance in the AC world is impedance. As impedance is complex, If you choose the correct frequency and voltage, you can move power very very far distances with extremely little loss. You cant do that with DC.
Also, when houses were getting power, all they had was lights, and motors, all of which run fine on AC.
When electronics started to come out, a house only had a handful, TV, Radio, that was it.
Now, I would say that a significant part of consumption is DC at a home, but there is no standard in the house for a parallel DC infrastructure. Would you do -48V like the phone companies? 12V? How would the in wall wiring work?
Three phase AC would get all the benefits of a DC system with the added bonus that it is probably available (at least it is in our library).
You could supply low voltage DC but you would go broke buying the wire to meet the additional ampacity requirement. I had a 12v DC computer and had to run 6 ga. wire from the backup battery to avoid an unacceptable voltage drop. So, you will distribute the DC at a higher voltage. That means you still have to down-convert the voltage. The power supply is still pretty complex.
Three phase power gives you all the advantages claimed for DC but costs less to install. The costs for modifying the computer would be about the same.
Yes. Of course they needed to do a study. There are all sorts of factors affecting energy efficiency. Do you want a university basing its decisions on hypotheticals?
Your post is overflowing with puns.
DC is actually more dangerous than AC. If you accidentally touch a high voltage AC source it will tend to throw you off. If you accidentally touch a high voltage DC source it can lock your muscles so you can't escape. In which case you'd better hope there's not much power available.
Quidnam Latine loqui modo coepi?
Well, this is interesting. On the workstations level it might be true that they are "quieter" but somewhere the AC is being converted to DC. Seriously, that would require a huge power supply of some kind and there will always be some inefficiency in that conversion process. So really, I don't buy that it is more efficient as a whole load on the power system. Seriously, unless they have a power supply with rectifiers that are VERY efficient (over 90% or more) then I don't see how this is an advantage. I have heard people ask me why we aren't using DC for everything. And they are people that don't know anything about electricity. Such as the fact that AC was chosen because it can be transmitted across great distances at high voltage levels with far lower losses than DC. More secure? What moron thought this up? It is a electric current. One is not more "secure" than the other. :)
Here we have an extra conversion loss but somehow it's better.
What?
The systems are on a UPS. UPSes use batteries. Batteries are DC. So, if you want to use a UPS, you have AC coming in that has to be converted to DC (to charge the batteries) and the AC power goes through the UPS to the computer, which converts it to DC. When the power goes out, the DC is taken from the battery, converted to AC, sent to the computer, then the power supply converts it to DC to run the computer.
With a DC computer, you need one conversion, AC to DC. The same DC can be used to charge the batteries and run the computer, and when the power goes out the battery can run the computer directly with no additional conversion.
So if anything the exact opposite of your assertion is true. There are either the same number of conversions (done through a central converter so you can afford a really efficient one) if you're on street power, or there are two fewer conversions and almost no loss when you're running on battery.
As for a Prius, the efficiency in those is in the fact that you can have a much smaller engine (since the electric motor and the gasoline engine can "team up" when you need a lot of acceleration) and the gasoline motor doesn't need to run when the car is moving slowly or sitting still (the battery can drive the car most of the time, and regenerative braking can reclaim a good percentage of momentum and turn it back into battery charge, so the gas motor only comes on when the battery gets low or when more acceleration than the battery can provide is needed). It's more complicated, certainly, but the whole point is to use what power the gasoline engine is generating more efficiently by storing what's not immediately used and using it later.
"This post contains words, known to the State of California to cause thought. Wash brain thoroughly after reading."
Do not use any computer, AC or DC in the bath.
DC vs. AC is a vast over simplification. At one point they talk about tying in solar cells and wind turbines that output DC. But what voltage? AC handles voltage conversion easily and efficiently with transformers. DC usually loses 10-20% going from one voltage to another (or one polarity to another), and in the middle it is converted to an AC signal then back. So while it makes a good sound bite, such stunts are a mere distraction, and reek of ignorance on the universities part.
As someone that's been an electronics hobbies for several decades (I've built radios, guitar amplifiers, clocks, you name it) I have been electrocuted numerous times. As such I can attest to the fact that getting hit with your standard outlets 120 volt AC is not that big of a deal. It hurts... somewhat like someone grabbing you in a not-very-nice way with a pair of pliers. But getting hit with DC... that's an emergency room vist level of pain. When you get hit with AC, you yell "OUCH!" and yank your hand away. When you get hit with DC, you can't yell because your mouth is clenched shut and when you yank your hand away the circuit board comes with it. Thank God for buddies near the off switch.
And before any of you get all preachy about safety, when you've been doing it for 25 years, shit happens. It's amazing how much pain a cat jumping off a shelf into your lap for petting and just the wrong time can cause.
I suspect they used PoE to provide 12.9 W to computers designed (or modded) for PoE power. SkinnyBytes has prebuilt Atom based laptops and desktops, but the library might have custom built ARM based systems.
They mention a one ton AC converter (and they had to remove walls to get it into place!), so it doesn't sound like they are using traditional PoE.
The reason for the assertion of "additional security" is not given in the article. It looks like the writer was trying to use all the current, hot buzzwords....
The Uni of Bath library used to have a small number of Sun X Terminals for checking email. They were great for when all the PCs were in use during the day, as only a small number of us knew how to use them and how to break out of Pine and run Mozilla.
I'm curious what voltage they run their DC circuits at.
When distributing power throughout your home or office, the AC vs DC argument isn't nearly as important as the voltage. The lower your voltage, the more current you need to transfer a watt of power. That means thicker gauge cabling to prevent overheating.
Obviously, the world has standardized on 110-120 and 220-240 for AC. However, medium distance DC is a mess of voltage standards, including 12VDC, 48VDC and others.
The best solution would be to run 240VDC to the wall and then have a step-down transformer in the form of a power brick or the like for your end devices. Since the brick no longer needs to be a rectifier, its construction is a lot simpler.
Could never tell which one is AC and which one DC
Amazingly, technology has advanced in 100 years, and some things that might have been applicable then, aren't now.
http://en.wikipedia.org/wiki/High-voltage_direct_current
When I first had to deal with telephone equipment, I came across the -48 VDC power standard for things like SONET nodes, digital cross connects, channel banks, and telephone switches. I believe this is due to cathodic protection of buried copper cables.
You can find -48 VDC rectifiers, AB fuse panels (think redundant DC power supplies) and lots of telecom gear in racks that is powered with -48 VDC.
"We can't solve problems by using the same kind of thinking we used when we created them." -- Albert Einstein
They can get away with this in Britain since they use funny electrical plugs.
now we need to go OSS in diesel cars
But they could, especially with PoE+. Essentially, they're just rebuilding technology which is already available commercially.
So, does Edison finally get his final victory over Nikola "The Electric Jesus" Tesla?
Sweet - let's warn Haiti, Chile, China, New Zealand, Australia, and Japan!
But they could, especially with PoE+. Essentially, they're just rebuilding technology which is already available commercially.
There's only so much low voltage power you can send over CAT-5/6 cabling, and even the 25 watt of PoE plus isn't enough to run many laptops, let alone a full powered desktop. Granted there are some very low-powered computers and thin clients that can run on PoE, but it doesn't sound like they are using them here. This fanless Atom based nettop has a 40 watt power supply: http://www.provantage.com/shuttle-computer-xs35-701~7SHCO08P.htm
I think it's safe to say that if they installed a 2000 lb AC-DC converter to run 60 desktops (and went through all of the trouble to widen doorways and cut through walls to get it installed), they aren't using PoE since a few PoE ethernet switches would weigh around 20 lbs -- 99% lighter than the huge power supply they used.
Essentially, they're just rebuilding technology which is already available commercially.
They don't say what voltage they are using, but I suspect they are using off-the-shelf 48VDC, so they aren't really rebuilding any technology, just reusing the same 48VDC technology that's been in the data center for decades.
The problem is voltage not DC vs AC. You can only do short runs effeciently via DC as line losses increase as you lower voltage while the amount of copper required skyrockets.
High voltage DC is exceedingly dangerous.
What about power over ethernet?
Hardocp.com has very in-depth power supply reviews where they not only test efficiency at ideal temps, but they also test efficiency in a heated environment -- which may be the case if your PSU is running at high loads. I'm not sure about the 20% usage efficiencies though.
Actually, we've found that there are quite a few servers and storage devices available from HP, IBM and others that offer -48V DC input option. As others have mentioned here, using 48V DC in racks can lead to increased availability and power efficiency by reducing overall conversion stages. Combining 48V input IT equipment with a row-based DC UPS make a 48V DC critical bus a practical alternative for future datacenter builds.
DC is great for short distances only.
The people running electric power utilities would disagree with you.
You have no idea what you are talking about.
1) The breakers do nothing in electrocution scenarios, as the current required to incapacitate you is on the order of milliamps and you don't have nearly enough resistance in your body to trip a 15A breaker. Ground-fault circuit interrupters will help if you touch just one line, and they disconnect at 5mA difference.
2) AC and DC excite the muscles indiscriminately and play no favorites. You'll grab or throw depending on what part of you makes contact, because the muscles will spasm.
The actual AC to DC conversion in a computer's PS is ridiculously efficient, almost 100%.
Where the ~20% loss comes in is the DC-DC conversion, converting 170...350V DC down to 5, 12, 3.3V.
So - if their distribution network is several hundred volts DC, each computer still has all this loss. If on the other hand, the central PS puts out 12V, 5V, etc, multiple computers are going to run insane amounts of current, which means we have very high I2R losses in the distribution network, and need a fuckton of $3/lb copper to distribute it. Even then I don't think they've be able to get it uniform enough across the network, and the PCs will need some board level regulation. (say transmit 7V, which is 7V in some areas, 6V in others, and have a regulator drop it to 5V on the board - which is going to be inefficient, again, even with a switching reg, 10% loss best case...)
Sent from my PDP-11
From my understanding, it's mostly a 'crap-shoot' at this point, but there's a distinct possibility that it would indeed be more efficient to do DC to DC. While you can use a plain old transformer to alter the voltage of AC, their sizing and efficiency depends on the frequency of the AC. 50/60HZ AC doesn't lead to very efficient or compact voltage converters, so they often use a switchmode power supply. The trick? The first thing most switchmode powersupplies do is rectify the AC to DC before using a 'chopper' to turn it back into a sort of high frequency AC - often with a frequency in the Mhz. Given that a rectifier circuit might take ~1-2% of the power going over it, DC to DC might indeed be more efficient.
I don't read AC A human right
In general, DC power is economical only if the wiring between the computers and the DC source is less than 35 feet in length.
Rewiring an office for DC is not necessarily required. Consider our current desks with their power strips and multiple AC converters (built-in or bricks) for the various devices. Now imaging a power strip that has an AC converter and multiple DC outlets where DC powered devices (computer, monitor, printer, 2.1/5.1 speakers, router, external HD, etc) plug in. I think that would be a practical and easily attainable configuration if we could have a standard (like USB smartphone charging).
I live in DC. Trust me, it is neither safer nor more efficient. The road system is a mess, and there are Segways everywhere.
-- IANAL, this isn't legal advice, and definitely isn't legal advice for you. Also, Squee!