home socket with DC voltage

Question

I'm wondering why, in a standard house, instead of using many low power AC-DC rectifier, there isn't a big (like 12V 20~50A) AC-DC rectifier which supplies some wall socket with a DC voltage? In the long run this should be less expensive, given the fact that AC to DC in one big apparatus is done better than in many low-quality transformers, right?

Also, modern chip DC voltage regulators have efficiency of >90%, so smaller (or even larger) voltages are still efficient and possible.

So we could use a 12V socket to charge a phone, desktop computer, laptop, radio etc... and still have an AC socket to move washing machine engine and other things that work better with an AC supply.

Why isn't this done?

edit: real question is: is it convenient, given a standard family (2 laptop, 1 desktop, 4 smarthpone, 1 big tv and some minor electonic) to do so?

Answer 1

I think you are focusing on a small fact about power conversion efficiency, and ignoring all the more important factors:

• extra cost of copper in house wiring to carry significant DC power
  (this is partly why Tesla's AC beat Edison's DC system),
• the lack of a single standard DC voltage so there is still conversion required,
• the requirement for higher-voltage AC anyway for existing and likely future home devices,
• the reliability, availability and economy of low-power AC-DC converters,
• the flexibility of AC-DC conversion techniques
• the flexibility and scalability of converting from a single AC standard to the specific requirement of the device
• the extra complexity of house wiring if two 'standards' have to be routed, e.g. the need for new plug standards, extra cables routed through building
• there will still be a period of transition when both systems must be supported, and IMHO no sane democratic government would mandate the change-over,
• I also believe as power increases the cost of components for DC-DC increases quite quickly above modest power conversion requirements

Edit:
Copper is expensive, London Metal Exchange - copper lists one tonne of copper (cash) is $7050, and that hasn't been processed into cable.
By comparison, London Metal Exchange - steel billet lists one tonne as $430, i.e. copper is 16x more expensive than steel.

So how thick must the 50A 12V cable be so as not to waste more energy as heat than AC? (Remember most of the benefit sited is efficiency, so it seems reasonable to ensure the losses due to heat in the DC cable are not worse than 230V AC.)

Lets compare AC mains cable rated at say 15A (Europe 230V), to 12V 50A DC cable.

Power loss = I^2 R

15^2 Rac = 50^2 Rdc

Rdc = Rac x 15^2/50^2 = Rac x 0.09,
i.e. Rdc must be ~11x lower resistance than Rac to achieve a similar power loss

DC cable would need to have over 11x greater cross sectional area than AC cable to reach the same power loss in heat. Put another way, DC house wiring would need 12x more copper so that the losses in the DC cables were no worse than AC mains.

Further, while the 230V 15A AC cable could carry 3.6kW, the 12V 50A DC cable carries 600W for the same losses.

Yes, for 11x more copper (to maintain comparable energy losses to AC) the DC cable carries 1/6th the power. Even at the scale of a house, the proposed low voltage DC cabling is not viable vs 230V AC.

The significant and important economic, practical, political and transition costs seem to dwarf any notional benefit.

Answer 2

For a given power required by a device, the lower is the voltage the higher should be the current. That would imply much thicker wires, and a possible overheat where the section is not enough.

Even creating your DC rail, what voltage you would choose? If you choose 12VDC, for instance, you should add one or more DC-DC for lower voltages. At that point, where is the convenience?

Answer 3

I've thought that if things were done over, knowing what we know now, houses would better be supplied with 3-phase 60Hz or 50Hz power.

It's easily turned into DC with 6 diodes (no capacitor required for moderate ripple), easily transformed into other voltages with a mains-frequency transformer, and will directly run efficient quiet 3-phase motors that do not require 'run' capacitors. If you want a variable frequency 3-phase motor, you don't need a huge capacitor that is the first thing to die.

Presently, in North America 3-phase power is used in virtually all commercial and industrial installations, and many apartment buildings (though not brought to individual units). It is split out on a street-by-street basis in subdivisions.

The savings in billions of gadgets would probably vastly outweigh the added capital cost.

But, it's too late for that, and standards were set well before any thought of electronics on a mass scale. Most consumers were happy with a few primitive incandescent bulbs, a bulky weak motor in the electric ice box and washing machine etc., none of which was all that concerned with single phase vs. three phase.

If you made an outlet with (say) 300VDC out it would run many of the electronic gadgets we have now (the input filter capacitor life would no longer be an issue). They would not be much simpler, but they might be more compact. However, even today, many devices would go up in smoke with DC input (anything with a mains-frequency transformer or motor, and probably some others on top). So, you'd need a different plug.

Answer 4

Because a "Standard House" came into existance a Century (or so) ago, and slowly modified as needed (public safety and standard electrical code), at the speed of buerocracy. People are very resistant to change. Once the IEC was formed, and consolidation of standards happened, anything too different must have wide support to spread. Which is a catch 22. Unless there is demand, there is no supply.

Since everything already works on the current system, with all the negatives invisible to the common user, negatives that are very neutral. Effeciency doesn't matter to someone who just cares on if it works or not.

That said, low voltage DC is a standard used in many homes. Or I should say alongside homes. It is used in landscaping lighting. And new products pushing DC exist. In wall 2+ Amp usb chargers. AC outlets with builtin usb charger ports. I give it a decade before they have become so common that a house without them will be shunned by young first time buyers.