Journal LehiNephi's Journal: I'm sick of AC power! 3
Historically, AC power has been preferred for a couple reasons. The first is because it has been easier to transmit long distances. Simply run it through a transformer to boost the voltage up to some hundreds-of-kilovolts, string it up on high-tension power lines, and step down the voltage at the other end. The advantage of this method is that the high voltage means that relatively little current is required in order to transmit the same amount of power. That decreases line losses in transmission. It's also easy to step voltages up and down very efficiently for distribution.
The second reason why AC is preferred is because that's how almost all electricity is generated. Some piece of rotating machinery (gas turbine, steam turbine driven by coal or nuclear or gas, windmill, etc) spins a generator, which produces AC. The only exception I can think of is photovoltaics, which at this point are still too expensive from a consumer's point of view.
Then why would you want DC? For bulk transmission of large amounts of power, DC is better for several reasons: You can put more DC current through the same wire, since DC uses the entire cross-section of the wire instead of only the surface, as AC does. With DC, you also do away with (or can ignore) capacitance between phases and capacitance between conductor and environment. It's also easier to control power flow with DC.
So AC is better on the generation side, DC is better on the transmission side, and AC is better for distribution. What a fun world we live in.
So why do I want DC? Simple: I recently took a quick survey of my house, and discovered something startling: Practically nothing in my house needs AC power. Seriously, almost everything in my house either already converts AC to DC, or could be fairly easily redesigned to use DC.
Now, this may seem kind of obvious. Computers naturally run off DC, as does everything associated with them--I lump printers, scanners, and network gear of all sorts in that group. Incandescent lights can run off DC just fine--they're fairly close to a simple resistive element in any case. Our stereo and VCR/DVD player run off DC (thanks to internal conversion), and almost everything inside the TV gets power after it's been converted to some sort of DC. Our fridge runs on AC, but fridges that run off DC exist--it's just a matter of using DC motors for the compressor and blowers instead of AC motors. Even fluorescent lighting, including CFLs, can easily work with DC; the electronics in the ballast that control current flow for AC can easily be designed to do the same for DC.
There's one more reason that's a bit of my personal fetish: power backup. That heavy brick of a UPS you have in the corner? It's converting AC power to DC in order to charge your batteries. When the power goes out, that same UPS then converts that DC back into AC to feed into your computer. Once it's in the computer, it gets converted once again to DC before it powers all the fun stuff inside. That's a fair amount of loss incurred through those three conversions, even if it's only when the power's out and only as long as the battery lasts. With a DC-powered house, however, the outside DC would run directly to a power conditioner in the PC, with very little loss along the way. Your UPS would enjoy much higher efficiencies. And if you had a big enough battery bank, your own house could serve to reduce the effect of startup current when your teenage daughter turns on the hairdryer or when someone turns on the vacuum cleaner.
But what about the costs of the inefficiency of converting AC to DC at the supplier's end? They don't matter. You see, you're paying $.xx/kWh for your electricity, and losing some of that energy through AC-to-DC conversion. In effect, you're paying more for each kWh of electricity that's actually doing something beneficial. Pushing the AC-to-DC conversion upstream towards the energy producers would increase the "sticker price" of the power you get, but you would end up using less of it, balancing out the cost.
The second reason why AC is preferred is because that's how almost all electricity is generated. Some piece of rotating machinery (gas turbine, steam turbine driven by coal or nuclear or gas, windmill, etc) spins a generator, which produces AC. The only exception I can think of is photovoltaics, which at this point are still too expensive from a consumer's point of view.
Then why would you want DC? For bulk transmission of large amounts of power, DC is better for several reasons: You can put more DC current through the same wire, since DC uses the entire cross-section of the wire instead of only the surface, as AC does. With DC, you also do away with (or can ignore) capacitance between phases and capacitance between conductor and environment. It's also easier to control power flow with DC.
So AC is better on the generation side, DC is better on the transmission side, and AC is better for distribution. What a fun world we live in.
So why do I want DC? Simple: I recently took a quick survey of my house, and discovered something startling: Practically nothing in my house needs AC power. Seriously, almost everything in my house either already converts AC to DC, or could be fairly easily redesigned to use DC.
Now, this may seem kind of obvious. Computers naturally run off DC, as does everything associated with them--I lump printers, scanners, and network gear of all sorts in that group. Incandescent lights can run off DC just fine--they're fairly close to a simple resistive element in any case. Our stereo and VCR/DVD player run off DC (thanks to internal conversion), and almost everything inside the TV gets power after it's been converted to some sort of DC. Our fridge runs on AC, but fridges that run off DC exist--it's just a matter of using DC motors for the compressor and blowers instead of AC motors. Even fluorescent lighting, including CFLs, can easily work with DC; the electronics in the ballast that control current flow for AC can easily be designed to do the same for DC.
There's one more reason that's a bit of my personal fetish: power backup. That heavy brick of a UPS you have in the corner? It's converting AC power to DC in order to charge your batteries. When the power goes out, that same UPS then converts that DC back into AC to feed into your computer. Once it's in the computer, it gets converted once again to DC before it powers all the fun stuff inside. That's a fair amount of loss incurred through those three conversions, even if it's only when the power's out and only as long as the battery lasts. With a DC-powered house, however, the outside DC would run directly to a power conditioner in the PC, with very little loss along the way. Your UPS would enjoy much higher efficiencies. And if you had a big enough battery bank, your own house could serve to reduce the effect of startup current when your teenage daughter turns on the hairdryer or when someone turns on the vacuum cleaner.
But what about the costs of the inefficiency of converting AC to DC at the supplier's end? They don't matter. You see, you're paying $.xx/kWh for your electricity, and losing some of that energy through AC-to-DC conversion. In effect, you're paying more for each kWh of electricity that's actually doing something beneficial. Pushing the AC-to-DC conversion upstream towards the energy producers would increase the "sticker price" of the power you get, but you would end up using less of it, balancing out the cost.
illusion? or not? (Score:2)
I don't think you've made a case for high voltage DC distribution, and certainly not for an expensive infrastructure change.
Sam
Re: (Score:2)
And I'll agree that DC is probably not the solution for distribution.
So I didn't think everything all the way through. However, here's an idea: Why not place an appropriately-sized AC-to-DC converter outside each home (by the breaker box would be fine) to supply, say, 240VDC throughout the house? You push the nasty, inefficient, noisy, heat-producing conversion out
Re: (Score:2)
However, you could do some maths to show that losses per metre for the same gauge cable at say - 24V dc are less than the losses for transmission of the same power at 120V 60Hz ac; and also for 240V 50Hz ac;
because if the same internal wiring can be re-used for dc with reduced loss in domestic distribution, capable of delivering