Slashdot Mirror
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.
Does big iron still use 3-phase power?
Tesla, you're fired. --Thomas Edison
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I've always wondered (from a non-technical point of view) whether there was a benefit in having our homes wired up with two sockets (or maybe a 5 pin mains plug) giving standard AC voltage and a low-current DC voltage as well (12V?). So many devices only need low voltage, wouldn't we all benefit in having a power system in our houses in this way?
Jolyon
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ac goes into data centres, systems run on dc. Either it gets distributed to each computer as ac and converted in a medium-sized box in the back of each system, or it gets converted in one big box and distributed to the systems as dc.
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)
FGD 135
trouble shooting and correcting DC power is simpler than working with linear power supplies. Unfamiliarity is the problem, not the technology.
I wish homes were wired for DC. I think we'd save a lot of power doing the conversion in one place per household/street, rather than using a separate transformer for each device. Plus you'd wouldn't have to waste time trying to find your devices transformer, or waste space when packing the device when you go on holiday.
Any truly serious data centre would already have at least several power engineers on their staff.
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.
Cyric Zndovzny at your service.
There is no standard Telco DC connector. This can cause headaches since you need special connectors and a AC-DC box for every type of DC appliance you have. Most every AC appliance uses one of a few types of industry standard AC connectors.
What we really need are pluggable racks. i.e. Move all the hardware necessary to support the blades (power supply, network switch, cooling, KVM interface, etc.) right into the rack case, then design a common interface to plug the blades into. Then an admin only needs to plug in the new server and run with it. No need to mess with tonnes of wires.
Wait. Did I just reinvent a 64 way Sun server? Imagine that.
Javascript + Nintendo DSi = DSiCade
I don't see what the problem is. Each box would be built exactly the same as before, except it wouldn't have a power supply unit - just a straight wire from the power socket. And the central power supply is just a big AC/DC converter - most EEs should already be familiar with those. There's not really any new technology here, it's just slightly re-arranged.
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.
The conversion from and to the UPS is saved. Every watt that is lost in the conversions is converted into heat. That heat has to be cooled of by cooling systems using also an enormoes amount of energy.
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.
"His department conducted a study that said 80% of those servers were running at 5% to 15% utilization"
Why such low utilization ?
Any other industry would scrap 80% of that equipment to save costs and power.
I started working with IP in a small ISP. We were bought by a loal Telco and over the years have got used to having all our routers and switches running on DC current.
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.
...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.
Sure, but how often do the backup generators connect inside the UPS?
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 cooking your servers while you're under emergency conditions!
The DC step in the middle is so you can chop it into a high frequency square wave. You save power and space because you can vary the duty cycle of the square wave for regulation, and the high frequency allows for smaller components.
I've had enough abrasive sigs. Kittens are cute and fuzzy.
The argument for AC to DC conversion on each device is that individual power supplies provide isolation e.g. no direct current path from DC ground of one device to another device. This is why Ethernet is transformer coupled. Eliminates ground loops and propagation of damage during major hardware meltdowns, nearby lightning strike etc.
As long as you're feeding a site with AC (this is the only efficient way to transmit it from the Hoover dam to some farm in Iowa), then at some point, there is conversion from AC to DC before it gets to the circuits of any computer system.
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 idea how efficient they are).
Now, even if there isn't an efficiency gain, there are two other reasons to do this, which is entirely why telcos do it (not to save power). First, telco equipment has to run when utility power is lost. Every switch is powered off DC that comes straight from BATTERIES. There is no loss in service if AC power is lost, because the batteries are already in use. This explains why you can use your POTS line even during a brief power outage. We have such a box sitting in a machine room in attached building.
Secondly (and perhaps more germane to the concept of DC in non-telco datacenters), telco equipment is often housed in locations that have poor environmental controls. By positioning the not-so-efficient AC-to-DC conversion hardware away from the sensitive electronics, you reduce the heat load in the climate-controled space. This alone makes some sense for the direct DC feed concept.
Charles
There exists no way of exchanging information without making judgments. --Bene Gesserit Axiom
The first step will be in the home office. Have you taken a look at the rat's nest under most desks? Most of it is AC/DC conversion. If the industry could just arrive at a DC power standard, we could start with a single AC/DC "box" under the desk with a standard plug end for all DC peripheraps. Add daisy chaining and wireless USB or Bluetooth, and that nest is largely eliminated.
At this point, we could start to build it into houses and other buildings.
More
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.
Is that DC power is naturally unstable. As loads fluctuate, the conversion and distribution system can change dramatically and result in very unclean power. If you are proposing to ditch AC in the server room and run DC from the UPS hardware directly to the rack, you will need to add in a lot of hardware to guarantee that the servers get exactly the voltage they need. This hardware will probably be less costly and wasteful than the AC systems currently in use, but they will also be more proprietary and (in the short term) more expensive to buy into. This is not the magical solution many envision, but it has a good future since transistor technology is getting a lot better and hence voltage management will be easier and easier as time goes on. The opportunity to move the conversion heat away from the inside of the server allows for better heat management, since you can let a transformer/transistor power system toil away only cooling it from the air duct on the roof and save the crisp AC for the servers.
Wouldn't you still need a power supply to convert to the various voltages required by a computer, which may change over time. Most DC power setups I've seen run at 48v, which still requires conversion to 12v, 5v, and 3.3v. You can buy a 48v power supply for most servers and other equipment today. With a switched power supply, you'd need larger capacitors or a higher switching frequency in order to smooth out the lower powered DC. It's very unlikely that you would eliminate any heat loss. I would assume that telecommunications equipment uses a 48v setup due to legacy issues and that it was a better idea before switching power supplies became cheap and efficient.
.8 or so, meaning 20% of the billable power is effectively wasted and could be recovered for a slight increase in cost.
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
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.
The slashdot story intro implies that the advantage of DC is that you
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.
We're all concentrating on the electronic, switching power supply stuff.
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?
Transmitting DC over long distances doesn't work very well
i um/PACIFIC.htm
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
http://www.transmission.bpa.gov/cigresc14/Compend
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.
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 an electrician for 8 hours to come extend a power feed. You've obviously never had to deal with licensed electricians.
How so? AC-DC switchmode power supplies start by rectifying the AC into a high DC voltage, and then perform internal DC-DC conversion to produce the output voltages. Even the ones that work straight from the AC are no more efficient once you include power factor correction.
Not true. Remember I said the telco power system is -48vDC with respect to ground? All the logic levels (12, 5, and 3.3v) in the cards are positive just like you're used to. The DC-DC converters are isolating; all they stipulate is that there be less than a 300v total differential between the inputs and the outputs. You're free to reference any part to ground, or leave it floating if your heart desires.
Telco grounding is insane anyway. Most places have #6AWG from each rack to a 1/0 aisle ground cable, and all the aisle grounds meet on a 750KCMil that runs the length of the building, over to the "office principal ground point". Track down a copy of TP76200MP and read up.
The article on raised flooring was an interesting question, but stupid solutions.
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
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.)
Hey Johnson, we are running out of power in our datacenter!
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%!
ignorance is bliss. googlefiberatx.com
I wonder how much energy a household could save if we were all using DC outlets instead of AC.
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 consumption. Some things can swing either way, but as a whole, your energy efficiency is much higher with AC than with DC. AC is much better at running motors than DC is. The real issue that AC to DC conversion has with respect to efficiency is that it produces harmonics on the power lines, which are undesirable and require some damping. The more AC to DC we use, the worse the harmonics are, and the more expensive it is to dampen them. But that's a really really small issue, in comparison to switching everything to DC.
I submitted this story last night, and it didn't get posted.
I think this article might be using the term "engineer" a bit too loosely. I doubt any company would hire an engineer - an actual person with a Professional Engineer's License - to work on these systems. A more appropriate term might be "technician," which usually refers to someone who is trained to repair and work with a single type of technology. Engineers, on the other hand, are usually trained to work with a large variety of technologies and usually work on either (A) Research and development, (B) Manufacturing, or (C) Failure analysis and redesign.
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.
There are several approaches to power distribution. One is "telco type" -48VDC distribution. 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, 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. 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. 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 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, 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.
Hi everyone,
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.
The reason that the terminals on your battery get "yucky" is the sulfuric acid leaking from behind the terminals onto the metal. A mixture of baking soda and water is good at cleaning corroded terminals because it neutralizes the acid, which then allows the ions to dissolve in water. If you were to dip the terminal of a corroded battery cable in mixture of baking soda and water, you'll notice that after a while the water turns a greenish-blue -- those are the copper ions that the acid has "liberated" from the metal of the terminal. This effect has nothing to do with AC vs DC and everything to do with leaky acid-cell batteries.
Copper and aluminum bus bars in AC power substations corrode just as much as they would if they were carrying DC; in fact, if you were to ever watch a substation being put together, every electrical connection is slathered with an anti-oxidation compound like "NoOx" (for copper) or "NoAlOx" (for aluminum) to prevent oxidatation that could then lead to hotspots and eventually fire.
Again, it just doesn't make economic sense at all to install all that extra cabling, come up with new standards, etc., just because some people don't like all their wall-warts. Sure, it might be a more elegant solution than wall-warts, but practically it makes no sense at all.
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 were in place) it might very well make sense to have a standard DC voltage or bus which all the equipment runs on. Because of the sheer number of power supplies involved which would be replaced, and because of the close proximity of all the pieces of equipment, this might very well make economic sense. One large power supply could be made highly reliable and redundant, it would have higher conversion efficiency than many small supplies, and the wiring losses due to lower voltage would be minimal because everything is in the same room, although you'd still probably want to stick with something like 48V rather than 3.3V, 5V, and 12V. Also extremely important, it would make the integration of battery-powered UPS equipment very simple and eliminate any losses there. I believe telcos use 48VDC equipment for these reasons.
Now, for your too-many-gadgets office, it might eventually make sense to standardize on a particular DC voltage, and have a large DC power supply which connects to all your devices in a star or daisy-chain topology. But it would have to be optional, because not everyone has that many devices, or maybe someone wants to install one device by itself somewhere, etc., so this arrangement would need to use a standard connector and standard DC voltage. The problem here, however, is stupid consumers, who might pick a larger DC supply that's not large enough for all their devices, and then blame the manufacturers. When dealing with stupid consumers, it's easier to just include a cheap wall-wart instead of letting them use 3rd-party equipment.