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Was Thomas Edison Right about DC Power?
Declan McCullagh writes "Everyone knows the alternating vs. direct current wars ended with Thomas Edison and Nikola Tesla. But now DC power is being seriously considered for data centers. DC advocates say that plugging servers into AC power is inefficient, and switching to DC cuts down on waste heat and component failure. The University of Florida has even bought 200 DC servers."
I heard of this new power system. Seems like a mix of AC and DC, to create the ultimate power form. AC *lightningbolt* DC was the name, and with a lightning bolt in the name, it has to strike you like thunder.
Tesla and Edison were both right...and wrong. Like many Slashdotters do when debating which operating system is best for any given job, Tesla and Edison wanted to apply one power system to every job. Its like having a toolbox with only a screwdriver in it. Ever try to drive a nail with a screwdriver?
For moving power over long distances, AC is king. But for short distances with most modern electronics, DC would win. The first thing a desktop system or server does with AC is converts it to DC. So if you have a number of machines all in the same room, why not do the conversion in one spot, and eliminate the redundancy in every machine.
Would it benefit the average user with one or two machines? Not at all. But for a major center with many machines in the same room, I can see quite a bit of benefit with going with DC.
I want a new quote. One that won't spill. One that don't cost too much. Or come in a pill.
Wrap the casket in copper, replace the headstone with a magnet, and expose corpse to this article. As Tesla turns in grave, free power.
As long as there is a Second Amendment, there will always be a First Amendment.
The origin of the 48 volt number is that it was convenient, and now it just sneaks under the 50-volt "low voltage" cutoff in the NEC, which I think was written with telcos in mind. The glorious thing about this is that you don't need licensed electricians to do power wiring in a central office.
And the reason it's negative with respect to ground goes all the way back to the telegraph system: Western Union initially ran bipolar lines and noticed that the positive ones corroded much faster. Sodium ions (from dissolved salt) are negative, and thus repelled from lines that're also negative. The whole phone system was built with positive ground because of this, and it's saved incalculable maintenance costs. It does tend to mess with people's heads the first time, if they're used to negative ground systems, but you get over it quickly. (A number of traditions use blue for "hot" and black for ground/return, to help escape your "red equals positive" association.)
DC power as used by telcos is also always redundant. There's an A-side and a B-side for everything, and the cables are sized so that the entire load can run from just one side. This leads to some very fat copper, which is cheap compared to downtime. You don't achieve five-nines reliability with a system that contains single points of failure!
Now, about rack-mounting: This was also invented by the telcos, originally in a very wide (40-inch?) format, for the panelboards and Strowger switches. Some of the old crossbar equipment is still in those huge racks, but the 23-inch width is infinitely more common now. All telco equipment is mid-mounted, with the ears approximately in the center of gravity on the shelf, so the force on the screws is shear. There's no torsion on the mounting flange unless you step on the front or back of the shelf. Cooling is always convective bottom-to-top, or occasionally front-to-back with fans. This leads to a "cool" front aisle and a "warm" back aisle between alternating rows of equipment.
Now, the pro audio industry borrowed the rackmount idea fairly early on, but they were mostly mounting control panels and mixers, which are very shallow, so flush-mounting made sense. They also changed the every-inch Western Electric mounting holes to an alternating-spaces "EIA" standard, and narrowed the rack from 23 to 19 inches.
Somewhere along the line, an absolute idiot decided that computers should be rackmounted, but they should be 19 inches wide, flush-mounted, and use EIA hole patterns. I'm sure this has something to do with mainframe legacy getting perverted by peecee people. The current mishmosh of mounting standards (19" vs 23", two-post versus four-post, flush versus mid, inch versus RU, front-cable versus rear-cable) is what every datacenter tech deals with on a daily basis. Throw overhead racks versus raised-floor cabling into the mix, and you've got a recipe for frustration!
If you're familiar with the concept of "blade servers", where common components are separate from processor resources in the shelf, congratulations. Telco hardware has been built like this since the invention of the circuit board. Actually, the concept of replacable plug-in units goes back before that, but it got vastly easier with printed wiring boards and card-edge connectors in the sixties. Most of the "good ideas" in serious computing circles are actually century-old ideas in the telco industry. Spend a week shadowing a central office tech before you design a datacenter, please!
Also consider: If your datacenter is already built for DC, throw some solar photovoltaic panels on the roof. Inverters are a large part of most PV systems' expense, and you can skip that part. Why not start offsetting your grid demand now?
Also also: Edison was flat-out wrong about DC. The modern switching power supplies that make DC transmission lines practical didn't exist in his day. Besides, long-distance power transmission is an entirely other discussion.
The article conflates several things.
First off: Digital electronics generally requires several voltages. And they're all low, requiring high currents, massive conductors, and local filtering and regulation. So even if you're providing DC power from outside the room, you'll have a switching power supply (or several) in each piece of equipment to convert whatever the rough DC power is to whatever you need, smooth it, and regulate it.
But while some electronic devices use a common switcher to generate all the voltages with one conversion step, others use a "roughing" supply and a bunch of local supplies. Part of that is to get better regulation - part is because the roughing supply must run from 60 (or 50 or whatever) Hz and thus requires big caps to tide you over the low part of the cycles - caps you don't want taking up space near the components.
If you're going to do it in two stages anyhow, you can put your roughing supply OUTSIDE the room and only have the final supplies inside. The roughing supply has a lot of heat dissipation so you save a bunch on your cooling.
Second: There are two standards for power distribution in electronics rooms:
- Your local power line stuff. (120/240/480/208-3-phase in the US)
- The telco standard: x2-redunant 48V DC.
A lot of equipment - especially networking equipment - is manufactured for sale to tellcos and other operations that use the standard. They might have initially used it because some of their equipment was co-located in tellco sites, where only 2x48VDC is available - and they got a quantity discount for buying a bunch of the same stuff and went to 48V for their own sites. Or they might use it because it's MUCH simpler to do backup power with floating batteries and century-old technology than with a building-sized UPS. (Note that a UPS CAUSES at least one outage when first installed and on the averate at least one more within the first year of operation from some malfunction. And a UPS dissipates more power than a roughing power supply or a battery charger.)
But the standard for 48VDC is REDUNDANT 48VDC supplies, with the equipment only requiring one (and typically doing "cutover" with diodes B-) ). With the equipment already set up for redundant supplies it's not a lot of cost or work to wire both sides and put in two 48V feeds to the equipment room. (Four diodes are a LOT cheaper than a pair of 120V roughing power supplies at each box, too.) So of course the users of such equipment normally give it dual supplies. (Even if it's a single rack and so they just put two roughing supplies in the rack fed from two different 120V feeds.)
The result is that all the equipment has redundant power supply, and keeps operating glitch-free through a number of kinds of partial outages - AND power supply repair and replacement. This is what's responsible for much of the claimed increase in reliability.
The whole Edison/Tesla DC/AC war had to do with the economics of CROSS-COUNTRY power transmission. AC beat DC there because a century or more ago it was virtually impossible to jack DC voltages up to levels suitable for long-distance transmission and back down to levels safe for distribution within houses, while AC could do that easily and efficiently. So Westinghouse/Tesla could ship cheap power from Niagra Falls to New York City while Edison had to build fuel-burning power plants IN the city. It has essentially nothing to do with shipping the power around within a single building.
Bantam Dominique roosters crow a four-note song. Once you've heard it as "Happy BIRTHday" you can't NOT hear it that way
Tesla originally worked for Edison, but they had a bit of a falling out, which is possibly why the AC/DC competition was so heated. Edison embarked on a pretty ruthless and gruesome campaign to discredit AC power, at least by modern standards. He electrocuted stray dogs and cats with AC current in public demonstrations intended to show how dangerous AC power was.
In one instance, he even electrocuted an elephant...
During the construction of Luna Park on Coney island, an elephant used as a beast of burden went out of control and killed a couple of people. Topsy, as she was called, was condemned to death. However, there was a wee bit of a problem. Elephants aren't the easiest critters to kill. What happens if you walk up and fire a shotgun at it's head, only just to piss if off? They do have rather thick hides, and we are talking about a homicidal elephant the size of a couple SUV's here. There weren't any cliffs handy to stampede poor Topsy off of, and I doubt dynamite was ever seriously considered. Edison, being the generous person he was, gladly volunteered to execute the elephant with AC current and filmed the whole thing. He showed the resulting film, "Electrocuting an Elephant" (1903) publically on many occasions. It is quite probable that many a cat and dog escaped a crispy fate thanks to this film. If you decide to track down a copy of "Electrocuting an Elephant" today, please be warned that it's a rather gruesome little piece of history, and is not for the faint of heart, or SPCA members.
I don't understand this whole issue with AC and DC. Both require massive investments in overhead wiring, which despoils the beauty of our suburbia, causes copper shortages, introduces losses from line transmission, requires odd things to be done to trees, and gives birds a comfortable place to sit directly above your car.
Why not just deliver the electricity off a truck like everything else in life? This country has gotten so "addicted" to current electricity that we've forgotten that static electricity even exists. A single charged capacitor can supply enough power to run a modern datacenter; the only limitation is capacitance. Say your datacenter runs on 10 kilowatts (that's just a guess) so you need 240 kWh of power a day, or 864,000,000 Joules of energy. Can a capacitor deliver that amount of power?
Sure it can, if it has enough capacitance. Energy storage is 0.5*C*V^2. Say the cap is 1 Farad, and we choose a reasonable charge voltage of 500 kV. How much energy is that? 125,000,000,000 Joules! WOW! That will keep you all set for 144.675926 days of continuous uptime! Every couple months, the electricity truck arrives and delivers your charged cap, and you give your spent cap back to the electricity man to be recharged at some high-sulfur coal plant in another state. (That means recycling which will help get the "greens" on board.)
Of course then, you have the nitpickers. "But what about the gasoline for the truck? Isn't that a wasteful means of electricity transmission?" Just use the energy in the caps to run the truck! It's like hydrogen! Hydrogen has already been shown to be politically viable.
Considering that it would be storing the energy equivalent of 30 tons of TNT, if you were to notice the lid starting to bulge on that cap, it would be wise to run like hell.
Meanwhile back in the real world ...
In a standard PC power supply the incoming AC is rectified and stored in a capacitor. Energy only flows into the capacitor when the voltage after the rectifier exceeds that stored in the capacitor. This results in a waveform which departs considerably from a sine wave - no current flows for most of the time while much higher currents than expected flow at the peaks of the half cycles. Electricians interpret this as a bad "power factor" from their experience driving inductive loads where the current lags the voltage by as much as 90 degrees.
Standard PC power supplies are nothing like 90% efficient largely because of this crude rectification of the mains. Compare the rating of your supply in watts with the input voltage multiplied by the input current. These values should all be marked on the case.
Power Factor Corrected (PFC) supplies are available. The better ones use a switch mode circuit to charge the reservoir capacitor through most of the main power cycle, while the less good ones incorporate a capacitor across the mains to buffer the large peaks of current when the input voltage exceeds that stored in the reservoir capacitor.
One advantage of AC is the ease of transforming it to other voltages using transformers and the ease of using it to drive motors especially with multiple phases. In the modern age where switch mode power supplies are cheaper than those using transformers operating at mains frequency this advantage no longer exists. One disadvantage of using DC is the difficulty in switching the stuff off - inductance in the load drives the current straight through an opening switch or fuse creating a nice sustaining arc which is not quenched by the current dropping to zero twice each cycle.
57.6 volts? Under heavy charge rate and fully charged maybe. Thats so high the electrolyte in the batteries will be history in 3-5 days regardless of the formulation of the individual battery.
I at one time had an older NCR ups, a huge old 150 pound honk rated at 1.2 kva, but it could output that 1.2kva for quite a length of time, running these two machines and one of the monitors for about 2 hours one day before I go nervous and did a gracefull shutdown till Allegheny Power managed to roust out a crew into our neighborhood 3 damned days later.
It originally came with a 4 pack of 12 volt gelcell batteries in it of about 12ah each, but when it came into my posession they were toasted.
On checking the float voltage I found it about 2 volts above what I would have called a good float voltage when divided down to a per cell rating, so I knew they'd been overcharged and dried out. I put in 4 18ah motorcycle batteries after setting it down to 52 volts, and boiled them dry in 9 months. I dropped it another volt and replaced them again, this time they lasted about a year before they were bone dry. As I'd rigged the overflow tubes to dump into a small jar of soda, I checked to see if the soda was affected, but it was still as pristine and white as the day I set it up. As that was about $120 a year for batteries, I said to hell with it, stuck a 2 wheeler under it and parked it on the back porch, replaceing it with the same size Belkin, which turned out not to be anywhere near big enough, shutting itself off rather unceremoniously at about 60% of its rated load. I yelled at Belkin and they sent me a much larger unit thats worked for about 4 years now with one battery replacement about 8 months ago.
Idealy I should have been able to run the wet batteries in a stationary environment for 5 to 7 years, and possibly could have if I'd figured out the right float voltage for those batteries.
Perhaps even a fixed trickle of about a milliamp once charged would have worked, but thanks to NCR's habit of burning old docs, I had none on that unit.
I once ran a set of 225ah big truck batteries for 8 years on a standby generator after reducing the trickle charge till there was no more gassing, which was a current of about 5 ma. At the end of 8 years, they would still turn that Cummins 335 hard enough the first cylinder comeing up fired. And the next, second cylinder firing spun it on up enough to kick out the starter, a total elapsed time of maybe 1/4 second from hitting the button and it was only another second to make 1500 rpm and energize the alternator, for a total power outage to on generator elapsed time of about 3.5 seconds. Those 2 batteries would check at about 27.1 volts anytime.
When you've lived where car batteries can freeze and split overnight if not fully charged, one tends to finetune the voltage regulators in the vehicles that must just start, for each battery. I've had batteries that were happy at 15.8 volts without gassing excessively, but in that home made regulator I had strong negative temperature comp too, slopeing down to about 13.8 at 70F, and the next one boiled like crazy at 14 sloping down to 12.4. Each battery has its own 'personality' I guess. Go figure. Yeah, its the old fart again, pontificating a bit about that which he's played with.
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Cheers, Gene
Considering that it would be storing the energy equivalent of 30 tons of TNT, if you were to notice the lid starting to bulge on that cap, it would be wise to run like hell.
Actually I remember doing some physics problem where I had to calculate the energy density of a simple high voltage paper/oil capacitor charged to near its breakdown voltage. I got an energy density for the cap that was 3% that of gasoline. Chemical fuels are just amazing.
You could run your datacenter off a huge current in a superconducting ring kept near its superconducting transition temperature. As the magnetic field slowly collapses, a circular electric field forms around it. You stick a coil in that field and connect it to your datacenter. Cold magnetic rings can be delivered by (refrigerated nonmagnetic) truck. This scheme is only limited by the current in the ring when it comes off the truck.
You could use a spinning disk. I'm guessing a steel disk spinning almost fast enough to structurally fail and fly apart might have an energy density similar to that of a fully-charged cap. Maybe it's possible to create an ultra-spinnable disk using carbon nanotubes. Then you could spin the disk much faster, and keep your datacenter running longer. I'm too lazy to figure out how fast you can spin a disk like that. But the edge can't go faster than c, or weird relativistic things start happening to the disk. Carbon nanotubes can only get you so far. They could spin the disks up in China, and send them here. But they would have to be careful. If every person in China spun up such a disk at the same time, it might affect the position of the North Star or change the length of the day.
You could just run your datacenter off the 30 tons of TNT.
It turns out Edison was not completely wrong: HVDC
In particular, "Increased stability of power systems" is certainly something that individuals in the Northeastern US and London may be interested in.
Of course, AC still has its uses, but the chart is now thought to be:
really long distance -> HVDC
long distance -> AC
short distance -> DC