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[Dave A]

What is the significance of AC vs DC in terms of power consumption?

I suspect the main issue in play is the inefficiencies of conversion from one form to the other. It makes a lot of sense to power DC technology devices from DC generated power.

[adrianh]

Fair enough, but high current devices then require fat cables.

[Justloadit]

AC or DC, each has its own advantages and disadvantages.

When electricity was first used, it was approximately 120V DC, however the problem that arose was the transmission of power over distance, caused the the voltage to drop down to about 100V as people started switching their lights on. This was what the electricity originally was for. Then cam Tesla, and his invention of AC, solved the problem of the voltage drop at the end of the line with increase of load, as the conversion up and conversion down was extremely simple and efficient with a transformer. Not so with DC. Why increase the voltage and transmit it down power lines? Simple, power is the amount of energy required, irrespective of the voltage. So by increasing the voltage, the current will drop respectfully, as Ohms law states P = I x V (Power = Current x Volts). Current is the amount of force required to do the work, so the lower the voltage the higher the current, and to be able to supply the power, requires the use of very thick cables, which translates to lots of copper which of course means cost. So in an AC system, if you want to supply power 100Km away, you naturally step it up to 132KV, and when you get to the other side, you drop it back to 230V AC. If we take a load of say 100Kw for a round number, means that at 230V will require 435Amps, now to carry this type of current, not to lose voltage at the load will probably require wires which need to be around 100mm diameter. Not practical to send power 100Km away, however if you step the voltage up to 132Kw, then the current becomes 750mA, so a cable of about a 1mm will be able to carry that current comfortably, a huge difference as opposed to the cable required for 230V.

Having 12 or 24V DC in the home to drive most loads is impractical, simply because of the thick cables required, and the switch gear must be designed to break these huge currents making it extremely expensive. Switching lights on is OK for 12V DC, but now your fridge starts becoming am issue. Lets take a standard fridge, its around 300W, so at 230V it needs 1.3A, however to run it off 12V now requires 25Amps. That is now a significant sized cable, and the switching device needs to be rated to disconnect the motor when required, and handle the arc at disconnect. Not so practical. As the loads go up in power, so does the amount of current required at 12V, and the cables required. Just not practical to do everything at 12V.

[gombault]

If you power supply (solar panels) is just on your roof then you don’t need to transmit power over long distances. The problem with DC is that no one is making appliances for them and there is no standardizing on a voltage a voltage of around 120v would be nice. All the products that’s made for DC revolves around vehicles, caravans, and boats all things that’s either 12 or 24 volts. Although this DC market is very small its technology has come very far the products that are available are very power efficient. You can buy 100 -150 liter fridge/ freezers that consume only 3 amps. (Google China DC fridges) Look at vehicles and you will see that they don’t use heavy wiring for things like motors as they are very power efficient. I have to disagree on switches even DC high amperage ones are no bigger than the ones used for AC go to any car electrical shop and ask them to look at their range of switches or look at a car relay that’s up to 40 amp and its contacts is no bigger than that of a 220V AC one. Adrian, give me your mail address and I will send you the globe specs.

[Justloadit]

If you power supply (solar panels) is just on your roof then you don’t need to transmit power over long distances. The problem with DC is that no one is making appliances for them and there is no standardizing on a voltage a voltage of around 120v would be nice. All the products that’s made for DC revolves around vehicles, caravans, and boats all things that’s either 12 or 24 volts. Although this DC market is very small its technology has come very far the products that are available are very power efficient. You can buy 100 -150 liter fridge/ freezers that consume only 3 amps. (Google China DC fridges) Look at vehicles and you will see that they don’t use heavy wiring for things like motors as they are very power efficient. I have to disagree on switches even DC high amperage ones are no bigger than the ones used for AC go to any car electrical shop and ask them to look at their range of switches or look at a car relay that’s up to 40 amp and its contacts is no bigger than that of a 220V AC one. Adrian, give me your mail address and I will send you the globe specs.

The problem with DC at the high voltages, is that when you get shocked, it tends to burn your skin, and simultaneously, it compresses or pulls your muscles tight so that you can not let go. When you get shocked via AC, the 50 Hz, causes your muscles to jerk, and usually causes you to jerk away from it by the time the earth leakage trips.

With respect to switching the DC control, the contacts that I was referring too is when you wish to switch off a 500 or 1000 watt load at 12V, the contact difference at 12V 20Amps as opposed to 12V at 50 or 100Amps is another kettle of fish. As soon as you have an inductive load in DC, which motors and any electrical is, the whole switching environment changes. Yes, there is methods to reduce the arcing, by adding capacitors and magnets and so forth, but this does add cost and physical size.

[adrianh]

If you power supply (solar panels) is just on your roof then you don’t need to transmit power over long distances. The problem with DC is that no one is making appliances for them and there is no standardizing on a voltage a voltage of around 120v would be nice. All the products that’s made for DC revolves around vehicles, caravans, and boats all things that’s either 12 or 24 volts. Although this DC market is very small its technology has come very far the products that are available are very power efficient. You can buy 100 -150 liter fridge/ freezers that consume only 3 amps. (Google China DC fridges) Look at vehicles and you will see that they don’t use heavy wiring for things like motors as they are very power efficient. I have to disagree on switches even DC high amperage ones are no bigger than the ones used for AC go to any car electrical shop and ask them to look at their range of switches or look at a car relay that’s up to 40 amp and its contacts is no bigger than that of a 220V AC one. Adrian, give me your mail address and I will send you the globe specs.

Adrian_hill@mweb.co.za

[AndyD]

The problem with switching DC on and off with an isolator swtch, or any switch for that matter, is the arcing.

When you switch AC the waveform is crossing the zero line 100 times each second, at that zero crossing point no current is flowing in the circuit (excluding PF and harmonics etc) so any arc that occurs at the switch contacts extinguishes itself.

With DC there's no time that the waveform crosses the zero line, the current is constant so switching arcs don't self extinguish which means switch contacts suffer enormous wear and tear with DC.

You do get switches that are rated for AC and DC usage but you'll always find they're derated for DC use. For example a 60Amp AC isolator might be rated for 20 or 30Amps DC use as well.

[gombault]

Car starter solenoids contacts switch very high loads between 300 and 600 amps the contacts are around 8mm, starter solenoid can switch many times per day and they last for many years. I don’t know how dangerous a 120/110V DC system will be I just know it’s very popular, all underground battery locos today are running on 110V DC system. One could reduce the voltage to a safe operating voltage. The question is how many high power appliances do you really have in a house if you exclude the geyser and stove?

[Justloadit]

Washing machine, dryer, kettle, microwave, vacuum cleaner, wife's hair dryer and curler, steam iron, slow cooker, steamer, espresso coffee machine, swimming pool pump, lawn mower, weed eater, 230V electric gate, .......but a few I can think off hand.

[Dave A]

Dishwasher, tumble drier...