University Switches To DC Workstations 468
An anonymous reader writes "Researchers at the University of Bath, UK are undertaking an in-depth study of energy consumption within the new network, with the aim of demonstrating that running a large network of devices on DC rather than AC is both more secure and more energy efficient. AC electric power from the grid is converted to DC and runs 50 specially adapted computers in the University Library. Students using the system have noticed that the new computers are more compact and much quieter than the previous systems. The immediate advantages of the new system are not only for the user but for the energy bill payer and the environment."
Con Edison (Score:4, Informative)
I guess Con Edison should have waited [nytimes.com]just a few more years. Apparently 125 was not quite enough.
Re:AC vs DC (Score:4, Informative)
I'm not sure how you came to that conclusion. AC suffers from several effects that make it less efficient and/or more expensive over long distances.
For DC, the power delivered is V*I. For AC, it's similar except the V is really Vrms - you must insulate for Vpeak, but you only get Vrms * I power. For sinusoidal AC, the difference is a factor of 1.414.
With AC circuits that have non-zero reactance, you must choose a conductor that can carry Imax, but the power delivered to the load is only Vrms * Imax * cos(phi), phi being the phase angle between the voltage and current.
AC circuits suffer from the skin effect [wikipedia.org] where the power travels more on the surface of the conductor rather than equally throughout its cross-section. This requires a larger solid or stranded conductor than would be required for DC.
AC has a few things going for it - the ease with which voltage can be transformed, the ease of generation with rotating generators, and ability to drive large, multiphase motors efficiently.
Re:DC-DC conversion? (Score:5, Informative)
Today if you blow a power supply (one of the most common computer failures) you lose one computer. If you blow the power supply for the office floor you might lose 100 or 200 computers.
For reasons which are a long story, I have had several servers up and running on 12V for many years now. The powerstream guys are pretty much the gold standard of ATX 12 volt power supplies, as far as I know:
http://www.powerstream.com/DC-PC-12V.htm [powerstream.com]
Note that these are "honest wattages" not the "marketing wattages" seen in the AC power industry. The price of a 300 watt DC supply seems high compared to a 100 watt AC supply from China that has a sticker claiming 300 watts. However its not too bad compared to a AC supply that actually only provides 300 watts despite having a sticker labeled 800 watts or a million watts or whatever marketing felt necessary. Also the powerstream supplies, to the best of my knowledge, are some of the few computer power supplies you can buy that do not have forged FCC and UL registries, which is worth something to me. In summary, expensive, but strongly recommend based on years of experience.
Anyway, what happens when the primary rectifier goes down, is my battery bank will run the asterisk PBX and friends for something like half a day, during which time I can source a generator and charger, or perhaps casually purchase a new supply, etc. Also I have multiple supplies any of which could theoretically power the whole works (at a cost of high heat and much shorter capacitor lifetimes, etc). So you Y-cable them to run multiple plants off one supply. Guess what, the same Y cable can be used to run multiple plants off one battery, if one fails. Etc.
Theoretically, I could run the entire phone system off an idling car, assuming you have enough gas in the tank. Unfortunately my entire plant draws just a little too much for the cigarette lighter plug, probably 15 amps total. If I could invest in new phones / new servers / etc and get total plant draw down to 5 amps, not only would my batteries be 1/3 cheaper or last 3 times longer in an outage, but I could also run the works conveniently off a car cig lighter port.
Obviously if you have zero battery capacity then you are instantly in deep doo doo, but given three or so figures of amp-hours you're good to go for a very long time.
Wire everything in Amphenol power poles, exactly like the ham radio guys so you can use their DC products, and keep a stock of extension cords and Y cables and other gadgets. Use fuses, and as a subset of that rule, only use automotive fuses because they are infinitely available. Use 12 volts as your standard because you probably own a mobile 12 volt generator (aka your car). Perhaps if you're in the .mil and have a 24 volt humvee, do 24v instead, whatever.
A DC powered system is frankly pretty straightforward and simple.
Re:So... what? (Score:5, Informative)
Tesla was the one who advocated for AC power. Edison was the one who argued for DC as the safer choice, and publicly electrocuted horses and other animals using AC to show how unsafe it was.
Re:AC vs DC (Score:2, Informative)
AC circuits suffer from the skin effect [wikipedia.org] where the power travels more on the surface of the conductor rather than equally throughout its cross-section. This requires a larger solid or stranded conductor than would be required for DC.
Did you READ the article you linked on this? "At 60 Hz in copper, the skin depth is about 8.5 mm." If you've got wires with 17+mm diameter solid conductors in them, you're DOING IT WRONG...
Re:secure? (Score:4, Informative)
Erm, wasted this month: 45W * (24 * 31) hours = 33.5kWh = $2.08.
Also, that kWh rate is fairly cheap...