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."

Comments

Sensationalist, but effectively correct

[AKAImBatman]

Was Thomas Edison Right about DC Power?

Oh, well, nothing sensationalist about that headline. (*rolls eyes*)

DC advocates say that plugging servers into AC power is inefficient, and switching to DC cuts down on waste heat and component failure.

In this case they're right. With that much hardware that close together, it's easier to treat the entire room as a single device. As the article suggests, this cuts down on waste heat produced by inefficiencies in AC->DC conversion. In fact, it significantly cuts down on the amount of equipment needed in the entire room. The concept can be taken as far as to cutting down to a single power supply per rack.

The amusing part about this is that the resulting racks might look a lot like Big Iron servers with pluggable motherboards. :-)

Re:Sensationalist, but effectively correct

[LoRdTAW]

Easy UPS too, if the servers use DC then batteries can easily be hooked right to the power bus that feeds them. No ac-dc dc-ac ups systems. If you have a 48V server you get 4 12v lead acid batteries hook em in series and hook them in parallel with a charging resistor and discharge diode. I know its a little too simple but at least it could be easily rigged up in case of emergency.

Thing is how much more efficent is it to have one large ac-dc converter and then smaller dc-dc converters(http://www.nycsubway.org/tech/power/rot ary.html)? You are still converting ac-dc and then dc-dc again just like a normal power supply in a computer. If they oversize the main ac-dc then over size the dc-dc then I can see how it might be better.

LoL It would be fun to get the DC from old rotary converters for a data center. Big mountain of spinning cast iron with slip rings, commutator, brushes and plenty of copper windings. Put in an old marble switch board with carbon arc breakers , synchro scope, volt/amp meters and knife switches. You then have yourself a turn of the century power system running new millenium computers :). Not efficent and high maintenence but how cool would that be!

Re:Sensationalist, but effectively correct

[njh]

LoL It would be fun to get the DC from old rotary converters for a data center. Big mountain of spinning cast iron with slip rings, commutator, brushes and plenty of copper windings. Put in an old marble switch board with carbon arc breakers , synchro scope, volt/amp meters and knife switches. You then have yourself a turn of the century power system running new millenium computers :). Not efficent and high maintenence but how cool would that be!

I believe this is SOP in EMP hardened bunkers.

And 4 12V lead acid batteries would be 57.6V :) 3 is 43V, which is by complete coincidence the same voltage proposed for new cars.

Re:Sensationalist, but effectively correct

[Almost-Retired]

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.

--
Cheers, Gene

Re:Sensationalist, but effectively correct

[MillionthMonkey]

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.

Electrocuting an Elephant

[Cordath]

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.

Why not have separate DC rails for 5V / 12V ?

[stevenm86]

Is that quite so? Wouldn't there be taps on the transformer for approximately 12V and approximately 5V, and then the potentials finely adjusted using DC-DC regulators? Wouldn't that have less loss?
Taking this a bit further, why not have an entire rack power supply that can deliver a rail of 3.3V, 5V, and 12V to each server, thus eliminating the need for a high-current DC-DC converter on the target board? I am excluding things like the exotic voltages for CPU and RAM, but still it is the 12V and 5V rails that would have to be able to source significantly more current.