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Billy Crook wrote:
DC is also great at: Corosion Explosions Arc welding Electrocution
:)
Your comments are generally correct, but don't take into account current technology. Comments inline.
I wonder if the cost savings take into account the price of all that thick copper needed to transmit DC throughout the datacenter. I've seen firsthand, 2 inch copper cables. There's a datacenter downtown that already has dc infrastructure in place, and a big battery, power stepping/switching, and UPS room. It might be more energy efficient, but I wouldn't bother with it, unless you could deliver +-12, =-5, and +3.3 to the rack. DC-to-DC conversion is notoriously inefficient. If you totally ruled that out, you might actually *see* some of the power savings.
Not any more. DC-to-DC conversion can now be *VERY* efficient, thanks to low RDs-On MOSFETs, high switching speeds, and a variety of interesting switching topologies (ie: the multi-phase DC-DC switcher that's likely sitting on the MoBo of your computer, feeding 50-100 Amps of low-voltage into your CPU with tight regulation).
The other problem I'd see is with DC, the voltage (noticeably) decreases over distance.
No more so than AC. Voltage drop is a function of the resistance of the wire and the current you're passing through it. The reason AC works better for long distances (ie: from your house to the power plant) is they run the transmission lines at *VERY* high voltages. High voltage and low current = lots of power with low resistive losses. The voltage is reduced at power substations, and again at the transformer behind your house. DC is actually slightly *MORE* efficient than AC, as you only have resistive losses in the cable. AC also includes losses due to the impedance of the transmission line (parasitic inductance & capacitance making a low-pass filter that increases the effective resistance of the line).
Back in the day, the technology didn't exist to (easily) convert DC power from one voltage to another, but transformers did the trick for AC. With modern power circuits, however, you can convert pretty much anything to anything with very high (90+ percent) efficiency.
If that's noticeable in a datacenter, you'd have to have same-length runs to each rack. Might be especially nice if you didn't have to cool the large AC-to-DC substation as much as the datacenter, or if you could just put it on the roof. It will also probably be less safe, by the nature of DC not pulsing to release two shorted connections. DC circuit breakers are more expensive, and most folks are less experienced with DC.
It sounds like you're referring to arc quenching, which is mostly of concern for things like relays and mechanical switches. For active circuit protection (think GFCI outlets for AC vs. a mechanical breaker) the costs should be basically identical between AC and DC, although obviously demand and production volumes play a roll.
As far as safety, generally, AC won't make you any less dead than DC if you grab onto something you shouldn't. High frequency AC will create a 'skin-effect' where the bulk of the current will run along the surface of a conductor (ie: fry your skin which can recover and not your internal organs which won't), but 60 Hz power is too low a frequency to exhibit any meaningful additional safety margin vs. DC due to skin effect. Even much higher frequencies are still very dangerous:
http://en.wikipedia.org/wiki/Tesla_coil#The_skin_effect_myth
- -- Charles Steinkuehler [email protected]