DC Power Saves 15% Energy and Cost @ Data Center 371
Krishna Dagli writes "Engineers at the Lawrence Berkeley National Laboratory and about 20 technology vendors this month will wrap up a demonstration that they said shows DC power distribution in the data center can save up to 15 percent or more on energy consumption and cost. The proof-of-concept program, set up at Sun Microsystems' Newark, Calif., facility, offered a side-by-side comparison of a traditional AC power system and a 380-volt DC distribution system, running on both Intel-based servers and Sun systems."
DC power can be a good thing... (Score:3, Informative)
When they used to talk about DC power systems be less efficient, you have to remember that most of them were talking about 12-48 volt systems.
From the article:Not according to my electronics class, if they're really going to be running at 380 volts. They'll need more insulation instead. I'd also want to be real careful around those wires. DC will kill you much quicker than AC of the same voltage/amperage. Then again, you don't have to worry about shorting yourself to ground with DC.
For now.
switching costs? (Score:3, Informative)
Deadly DC? (Score:5, Informative)
I always thought the opposite was true. Here is a wiki quote that also supports that:
Taken from http://en.wikipedia.org/wiki/War_of_Currents/ [wikipedia.org]
What's new about this? (Score:5, Informative)
You can also store DC whereas you cannot store AC, meaning UPS always need an AC-DC followed by a DC-AC stage. Since we have had large FET power transistors it has been possible to make DC/DC conversion very efficient - especially since, if you were beginning again, you would not choose 50 or 60 Hz for best efficiency.
In fact, already the PC is using a DC bus to power small peripherals (USB) and it works surprisingly well.
I may be wrong about this, but it was Edison who accused DC power of being more dangerous ("Westinghoused") only to have AC adopted for the pleasant US custom of humanely frying criminals.
Re:Safety (Score:5, Informative)
In a properly designed DC system, your no more/less safe than your already are.
(Sorry for the repost - I finally remembered my login)
Re:DC power can be a good thing... (Score:2, Informative)
Re:What's new about this? (Score:2, Informative)
Industroal systems do this all the time (Score:4, Informative)
But as for DC killing you quicker, I would disagree that its the type of system that kills you, it will depend on the type of damage that the shock causes. You can use a 9VDC battery to kill yourself if you apply it in a manner that a small current (mA level) flows to your heart and I would guess that the same level of AC current would also do the trick. On the other hand if you pass a large current through your body that causes physical damage (major burns etc) then it won't matter if its AC or DC if the so much of the body is destroyed as you will die eventually.
As for not worrying about grounding yourself with DC
"Larger power cables" - WRONG (Score:5, Informative)
From the article:
(emphasis mine)
The power lost in the cables varies as the resistance of the cable and the current in the cable.
The power delivered to the equipment varies as the current in the cable and the voltage on the cable.
A 380 volt DC system can deliver as much power per unit current in the cables as a 380 volt AC system (assuming a near-unity power factor).
Ergo, the size of the cables for a 380VDC system will be the same as the size of cables for a 380VAC system.
So, if the comparison is against a 240VAC system, then a 380VDC system will have SMALLER cables, not larger. Only if the system being compared against is a 440VAC system will the cables be larger.
Also - a 380VRMS AC system will have a peak voltage of about 540 volts (two significant digits in, two significant digits out), and thus will require MORE insulation than a 380VDC system.
Also - the first things a switching power supply does is rectify the AC into DC and dump it into a capacitor (and usually do power factor correction): so a power supply designed to run from DC needs neither the power factor correction nor the big capacitor (a smaller cap will still be needed, but not one that can carry the system through the bulk of the AC cycle when the voltage is below peak). This makes the power supply simpler, and removes switching losses from the rectifier (granted, a modern synchronous rectifier based on IGBJTs will have a very low loss - but it still is a loss.)
Also - creating a backup for 380VDC is pretty easy - you use a battery bank floated at the 380VDC level. No need to "switch" from mains power to battery - you are ALWAYS running on battery, and the mains power is just charging the battery. This is how the phone company does it - the central office has a bank of batteries providing 48VDC, which is float charged from the mains. Lose mains power, and the system doesn't even blink.
(Yes, you need to have fusing to prevent those batteries from going nuclear if shorted, but that is a much simpler problem to solve than the issues of switching to backup power for an AC system.).
Yes, you have to design the equipment to run off the 380VDC - so you need different power supply front ends: most power supplies are split into 2 parts - the front end that takes mains power and makes about 300VDC on a cap, and the back end that makes the lower voltages from that - so the back end of the power supply does not need to be redesigned. Moreover, most power supplies use an off-the-shelf front end module, and any "magic" is in the back end - so this is NOT a major issue.
Re:What's new about this? (Score:2, Informative)
Re:Here, here! (Score:2, Informative)
Re:DC power can be a good thing... (Score:2, Informative)
The savings will be somewhat in components and somewhat in power. After your UPS system (in which AC power is converted to DC for battery storage) there will be no need for the DC/AC conversion and then AC/DC in the silver box. This deletes one conversion stage, in theory (in theory communism works, in theory...)
AC supply -> UPS AC/DC -> battery storage -> bus converter -> several servers
rather than
AC supply -> UPS AC/DC -> battery storage -> DC/AC -> rack AC/DC -> several servers
But you'd still need an intermediate bus converter at the server rack to drop from 380 to 12/5/3.3/-5. This could be a nice opportunity for an APC or someone like them.
Also there are efficiency savings. If you optimistically quote 95% efficiency for each conversion in the existing topology, the overall efficiency from supply to server is (0.95)(0.95)(0.95)(0.95) = 81%. For the DC-only system, it would be (0.95)^2 perhaps, or 90%. These are certainly optimistic numbers but are useful for scale. For a 300W system the difference is 27W. For 1000 300W systems, it's 27000W, at
Am I assuming too small a server farm? This doesn't really seem worth it.
As far as safety goes, 380VDC and 380VAC are both gonna hurt you. Remember that the AC voltage is spec'd as RMS, so they're basically equivalent in power delivery, which means they should kill you equally fast. My recollection is that DC voltage will cause your muscles to tense, so if you grab an energized part you cannot let go, which is a pretty scary idea. On the other hand, it ought to cause less interference (no more 60Hz).
On wire thickness... no more skin effect, at least.
Re:Safety (Score:4, Informative)
http://en.wikipedia.org/wiki/Three-phase_power [wikipedia.org]
Re:Safety (Score:5, Informative)
DC is harmless unless it has a path to carry it. You can grab a 380 volt DC line and not feel a thing. now if you then touch a grounded object, or the return path you are dead. But you have to make the connection. AC is lethal at 220v. As others posted it does have the advantage of forcing the mucsles to spasm so you can let go of the wire, But still zaps you every time you touch the cable.
Go look up the history of Edison vs Westinghouse. Edison wanted DC power all around because it is inherently safer. a Broken AC wire can zap you, were as a broken DC wire can be touched with bare hands.
Re:switching costs? (Score:3, Informative)
Most AC power is 80% efficient, which is where the 15% gain is coming from (and remember, it is UP TO 15%, not 15% all of the time). But AC power supplies are becoming more efficient, with IBM claiming its BladeCenter power supplies are 90% efficient. That means that DC will probably only give a 5%-7% gain in efficiency.
A typical high density blade rack could use 24 killowatts of power. That would then require about 78k BTUs of cooling, which would take about 8 killowatts of power. That is 32 kWatts of power per rack, or about $2.56 an hour at $0.08 a killowatt. A saving of 7.5% on average would mean about $0.19 an hour, or $4.56 a day/$1664 a year.
The PDU/UPS for that single rack is going to be thousands of dollars. And if the blade rack has 80 blades, even a $50 per blade (probably very low) switch would be an extra $4000. The entire switch is probably going to be close to $10,000. That would take 6 years to recoup the initial cost.
I do not know what the lifespan of a server rack is, so maybe a six year turnaround would be good enough. I have dealth with enough banks and loan officers to know that American companies rarely look ahead that long for a ROI. Three years would be a little more like it. But maybe my calculations are far off, I have never worked with more than a couple of server racks before so I am not familiar with how the costs will scale for large datacenters.
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Re:Here, here! (Score:3, Informative)
Re:Here, here! (Score:5, Informative)
Conversely, if you halve your current by boosting the voltage, you can reduce your transmission losses by 75%. Thats a pretty good reason to go with higher voltage. And since this is in the datacenter, you can train your people not to pee on the red wire.
Let Go (Score:3, Informative)
Re:Safety (Score:3, Informative)
Which is why my sister-in-law once told me to use the back on ones hand if you aren't sure if a line is still live or dead. Your hand will contract around nothing thus giving you a slightly better chance of survival.
Re:Safety (Score:3, Informative)
On a good day, there is minimal voltage difference between neutral (or common) and ground, but if the site has a poor or floating building ground, you can see some pretty severe voltage swings. Also, if the load on the three phases isn't reasonably well-balanced, that'll nudge neutral away from ground and you'll get current leaking to ground which is wasteful and even dangerous at higher amperages.
I've even seen old wiring in metal conduit where abrasion somewhere had tied the conduit and ground wires to hot...I managed to arc-weld about half the end of my screwdriver to the recepticle finding that out, and the worthless breaker at the site didn't even trip.
Nice shower of electrical sparks and molten bits of the other half of the screwdriver tip, though...
Speaking of conductor sizes.... (Score:5, Informative)
Speaking of conductor sizes, the article said this:
A DC system also would mean having to bring in larger cables than now exist with AC power.
I challenge this notion. Conductor size is not related to whether the power is AC or DC or what frequency of AC it might be; it is related to current.
Larger cables are needed when more current is passed. Traditionally, you need larger cables for DC, because traditionally, DC power systems were lower voltages (12, 24, 48) than AC systems, and these lower voltages required larger currents for same power (e.g. 100W= 830mA at 120V, but 8.3A at 12V). Running at 380V, however, you get to lower the current (excluding the reduced current caused by the 15% power savings) versus a 120V system.
Expanding on that, the reduced conductor size is proportional to the square of the reduced current. Simply by going from 120V to 380V (a factor of 3.17), you change the current flow downward by a factor of .32. This means you can change the cable cross-section area to by a factor of .1; you reduce the cable to one-tenth its original size; one tenth the copper.
Re:Safety (Score:3, Informative)
DC blows you free, AC grabs you plain as that and the parent is spreading misinformation I've seen here 100X before.
Re:Safety (Score:5, Informative)
From http://www.andamooka.org/reader.pl?pgid=liecDCDC_
Note to the wise: Wherever possible, always approach a circuit with the back of your hand. If it is DC, the muscle reaction in case of contact/shock will tend to pull your arm away. If it is AC, same thing will happen. Depending on the voltage present on the conductor, you may even feel the hairs on the back of your hand react to the field produced, i.e., they will 'stand up'.
CPD.
Check out Rackable systems (Score:2, Informative)
Blade enclosures also use a similar trick, the blades all get DC. And many data centers also have DC available already, you just have to ask.
http://www.rackable.com/ [rackable.com]
Re:Here, here! (Score:3, Informative)
Although, you're right that they don't use 22-gauge wire for that purpose; one of the PBXes at a client site has a 15 or 20amp/48VDC power supply, for example, which seemed to be using 14 gauge wiring, for example.
Re:Safety (Score:2, Informative)
Re:Safety (Score:4, Informative)
Re:Here, here! (Score:4, Informative)
The power conductors in central offices are oversized out of paranoia, and because sometimes you have a foot-thick pileup of power cables leaving a fuse bay and you want to make really sure resistive heating is negligible. Also, most equipment has redundant power feeds, A and B, but either feed is large enough to handle the entire load. During normal operation when both sides are sharing the load, the resistive drop in the wires is absurdly low.
The other advantage of 48v is that it's below the 50v "low voltage" standard in the NEC, which means it's easier, legally, to work with. The 300-plus voltage they're using in this study loses that advantage.
Also consider this: AC voltage and power are measured RMS, but the insulation has to withstand P-P voltage. So to deliever the same power on the same conductors, the DC system's insulation has a greater margin of safety.
Re:Safety (Score:2, Informative)
Edison had massively invested in DC and was desesperatly looking for a mean to transport it on "long" distances without big looses associated with DC transportation. When he hired Tesla [wikipedia.org], he dismissed what this young engineer was showing him (Tesla had just invented AC). After Tesla resigned and when on his own (with the finantial support of Westinghouse), Edison went on a personal war against Tesla. Edison had great political influence and tried everything possible to kill AC current, but the technological advantage was on Tesla's side.
The campaign of fear directed by Edison worked for a time, but when Westinghouse won the contract to light 1893 World's Fair, the World's Columbian Exposition in Chicago. This success revealed AC current to the face of the earth as a working technology. Many times, Tesla demonstrated how inoffensive AC was, risking his own life. ;)
As a side note, Tesla experimented high voltages (reaching 1 million volts) with Tesla coils, skin current conduction, he invented radio transmission, AC current, three phase motors, new efficient turbines, hydro-electric dam, energy wireless transmission, the death ray, received the first signal from space (Mars), and many more. He his surely the greatest engineer who ever lived.
Re:Speaking of conductor sizes.... (Score:3, Informative)
Not really correct: as the frequency increases, the current tends to flow in the outer regions ('skin') of the conductors, known as the skin effect [wikipedia.org]. Because the core of the conductor is not used, the effective area is reduced and therefore the resistance increased. For this reason hollow or flat conductors are used for high frequency applications.
Re:where does that DC come from? (Score:3, Informative)
A datacenter takes 3-phase 440VAC in, which goes directly into the backup power system. This converts the the AC into DC to be fed into the batteries, then the batteries are fed into a DC-AC converter to put out 60Hz sine wave AC. The AC from that converter then gets distributed to each computer. Each computer in turn takes that AC and converts it into 12/5/3.3VDC. Unfortunately all those AC-DC converters sitting in each computer are unnecessarily inefficient. By eliminating the DC-AC-DC steps, it's possible to make things much more efficient. Simply take the 380VDC from the batteries, and convert it to lower voltage at the computers. Of course there's loss in the DC-DC converter, but it's much less than the standard DC-AC-DC because it uses a high-frequency square wave instead of low-frequency sine wave AC.
dom
Re:Safety (Score:1, Informative)
Still many kitchentable 'engineers' are happily unscrewing the case of these beasts not hindered by the slightest experience..
Re:Safety (Score:3, Informative)
Ummm....No.
1) 480 3 phase can be 3 wire or 4 wire. 3 wire is called Delta (floating ground or one of the legs can be tied to ground). 4 wire is called Y (typically the 4th wire is the "center" of the Y and is grounded.
2) You get 277 VAC reference to ground with Y. You get nothing stable with Delta floating. And you get 480 VAC or 0 VAC with Delta one leg grounded. I'm not advocating one way or the other, it depends on a lot of things which configuration you choose.
3) Either way, to get 120 VAC, you have to use a transformer to reduce the voltage. Phasing is not adusted. If you have 480 VAC 3 phase, you'll get 120 VAC 3 phase, though that's misleading because you always connect 120 VAC 3 phase in the Y configuration and measure 120 reference to ground, not phase to phase. The actual number phase to phase is some weird number I can never remember like 177 or something.
4) Corrolary to #3, in a home system, you have 240 VAC which is really only two 180 degree phased 120 VAC lines. To get 240 VAC 3 phase you need a specific transformer which will have seperate taps to provide 120 VAC if you so desire (or just use two different transformers to achieve it).
Re:Here, here! (Score:3, Informative)
Stick a voltmeter across tip and ring. If the line is on-hook, you'll see 48V DC. If someone calls, 90V AC will be superimposed on it to run the ringer. Take the line off-hook and the voltage goes down to somewhere around 6V DC.
Tesla (Score:3, Informative)
No, he just had good marketing. People keep repeating a lot of things he never actually invented, just proposed some vague idea, together with other things that proved totally impractical. I wonder why they never quote this:
"The aeroplane is fatally defective. It is merely a toy-a sporting play-thing. It can never become commercially practical. It has fatal defects."
Nikola Tesla
Edison, it's true, had his personality defects. In many cases he was not quite ethical. He was wrong in trying to push DC technology at that time, although with modern electronic components transmission of DC at very high voltages is not only possible but the only practical alternative [wikipedia.org] in many cases.
Now consider Tesla's idea of bladeless turbines for instance, impractical at the time, still impractical today. In order to work with reasonable efficiency, it needed a set of disks with a tenth of a millimiter in thickness separated by less than a half millimeter. Try sending a flow of high pressure steam into such a machine, a fraction of a second later you'll have a big mess of crumpled metal foil. Or how about what is possibly Tesla's most famous experiment, a wireless system for transmitting electric power. It radiated energy away in all directions, to be received here and there by special antennas. With luck this system could use maybe a fraction of a millionth of the total power transmitted. A few cities in the early 20th century had Edison's impractical DC power distribution system, but Tesla's wireless system was never adopted anywhere because it was so extremely inefficient.
The main difference between Edison and Tesla is that Edison made research with practical applications in mind. Tesla proposed many ideas without even building prototypes, Edison made painstaking trials until he got it working. For instance, in his incandescent light bulb, Edison's lab tested more than a thousand different filament types before finding one that worked. Edison's main invention that Tesla ignored was a system for developing applications for new ideas. It's not enough to have brilliant ideas like Tesla did, one must put those ideas to practice, like Edison did.
Re:DC power can be a good thing... (Score:1, Informative)
We treated nothing different when working on the DC or AC systems, although we were trained that DC current was "worse" because it would disrupt the heart more then AC current would and the tendency to NOT be thrown away from a live DC circuit if touched. Operationally, they were treated exactly the same.
Re:DC power can be a good thing... (Score:1, Informative)
The Grand Central Terminal electrification uses 660VDC and the breakers are set to trip at 14,000A - most light rail systems use 600VDC (standard trolley voltage since the mid 1890's) - The Milwaukee used 3,300VDC in Montana, Idaho and Washington, the catenary could supply 4,000A.
As other people have mentioned, a DC bus can be floated on a battery - also can be floated on a capacitor bank (check out Maxwell's ultracap line) - and is the preferred interface for Pentadyne's flywheel cabinets.
Biggest problem with DC buses is designing the circuit breakers - you don't have the zero-crossings as exist with AC.
Re:dc / dc converter (Score:2, Informative)
Would be interesting to know what the efficiency is of a 380 -> 12/5 DC-DC converter, compared to a traditional 110 AC -> 12/5 DC converter.
I would venture to say it's a little bit better, and here's why:
Your average switching computer power supply unit (PSU) (including the commodity consumer one's like the one running your computer used to access ./) converts either the 120Vac or 240Vac into it immediately into about 300-340Vdc with a standard bridge rectifier. That little switch on the back of your PSU changes a connection between the rectifier and the capacitors to double the rectified voltage from the 120Vac so that it's near 300-340Vdc too. That DC supply is then used to feed a switching circuit that changes it back to AC at a frequency significantly higher than 60Hz that is fed into the transformer. Because the (AC) frequency is much higher, the transformer can be much smaller for a given power level.
I've gutted many a PSU, and nearly all of them have a pair of 200V capacitors in series (making a capacitor with a 400V rating and a center tap that's used for the voltage doubler circuit necessary for 120Vac input). The selection of 380Vdc makes sense as the common PSU is easily modified to feed directly from it. On many PSUs (don't try this at home unless you know what you're doing, lest you get a Darwin award) if you switch the input switch to 240Vac you can drive the supply with 300 to 380 Vdc. For a little more efficiency, you take out the bridge rectifier in the front of the supply, and can shrink the input capacitors for a manufacturing cost savings.
The efficiency you save in each supply may be lost in the AC input to DC converstion that you have to do somewhere else in the data center. The advantage of doing the AC to DC conversion in fewer large units is that you can add some complexity to the circuit to improve the power factor of the conversion. Strait up bridge rectifiers on the fronts of most PSU don't lead or lag power factor, but they only draw power during the peaks of the AC wave. This makes them generate noise on the power line. Put enough of them in a room and it becomes something you have to take into consideration.
Putting a UPS on a DC system is easier than on an AC system... just stack up enough batteries to meet the necessary voltage. Well, it a bit more complicated than that, but it's much less complicated than what you need for an AC system.
BTW, IAAEE... I am an electrical engineer. Everything else, IANAx
To the comment somewhere else in this story that you can get 120 from a 277/480 supply by using the neutral, WRONG! Someone please mod up the response that said otherwise. Using the neutral on a 277/480 supply gives you 277.
Re:Safety (Score:1, Informative)
AC will *not* always give you a zap anymore than DC will always give you a zap!
If you've worked on this so much, though, haven't you ever come across an isolation transformer? Ask one of your buddies that's done work for a hospital, they will definitely give you some insight into what I'm saying that AC and DC are equally dangerous, but that the isolation transformer magically makes AC safe (for a single conductor).
120 volts of DC or AC will hurt you, no question.
12 volts of DC or AC will (normally) not hurt you, though. It doesn't matter the waveform, it matters if the voltage is high enough to allow enough current through your body to hurt you. Your body is nothing more than a resistor, so the amount of current will depend on the voltage and resistivity of your body. It's basically Ohm's law, and Ohm's law doesn't care about DC or AC.
>Now for the other part of your idiotic idea, that 0000awg cable would be used overhead or underground to deliver electricity to you, not in your house. We use AC because it is cheaper to run distances of miles and miles.
What????????????
Okay, I don't know the EXACT voltage Edison's DC power ran at, but I know for SURE it was less than 50 volts, since anything above that approaches being dangerous. Considering he suggested it was so safe you couldn't even feel the electricity, I'd probably say it's closer to the 12 volts a car runs at.
For the other slashdotters that don't work with electricity, Ohm's law: I = E/R and P = IE
A normal house will have 100 Amp service at 220 volts. That's 22,000 watts. 100 Amp service requires a minimum 6 AWG cable (4 is preferred).
Now, let's reverse 22,000 watts into what amount of current that is at 50 volts and 12 volts.
At 50 volts it is 440 Amps, requiring OVER a 0000 AWG cable (honestly, look up the charts [powerstream.com] if you can find one for that cable size, they all seem to top off at 0000 AWG though) . At 12 volts it is 1834 Amps (rounded). I can't even imagine the AWG for that much current, but I can only imagine it is about 000000000 AWG.
AC is no more efficient than DC, though, in a general sense. In specific cases, an argument can be made for each. AC motors are easier to build. DC works very well for digital circuits. But in a general sense, 1,000 watts DC is the same as 1,000 watts AC.
The only REAL difference between AC and DC is the waveform. That's it. In all honesty, DC is basically just AC at 0 Hz.
>So Every day you touch DC conductors and don't feel a thing.
And most people touch AC conductors and don't feel a thing every day too! I don't know how many times I've seen the neutral line used (often improperly, double insulated my ASS) as a ground on items. That ground is then usually exposed through the chassis (for example, on an old fridge). Since the neutral is bonded to ground at either the electrical panel in a house (good, done in North America) or at the transformer (bad, done in Europe) the voltage on it is pretty close to ground in the home (in North America I usually read less than 0.7 VAC potential betweeen true ground and neutral). You can touch that neutral and the ground at the same time and not get hurt. Of course, if the outlet is wired backwards or isn't polarized, well, then you touch what is supposed to be a chassis ground (which is actually hooked up to HOT/LIVE now!) and real ground and OOPS! Pow!
But that isn't because it's DC or AC. It's because you are touching the LIVE to GROUND through your BODY! It'd be no different than hooking 10 car batteries in series and touching the body of the car and the positive of the last battery at the same time! POW!