AC/DC and renewable energy

I would like an episode explaining the difference between alternating and direct current, is one more efficient than the other, what applications are different between the 2 forms, and how do they apply to renewable energy. I saw something that mentioned solar power being DC? I am a medical person and have always been quickly lost by discussions about energy and transference of it. It seems like magic to me. I know we're on AC b/c of the machinations of the elite of the industrial era, but beyond that, I'm lost.


  • ScrappyDooScrappyDoo Annapolis, MDPosts: 1,067 ✭✭✭
    I can drop some knowledge here but I need to know the level I should start at. @Bekabirder do you have an understanding of basic physics? We use AC because it is much easier to change the voltage and can be better transmitted over long distances. DC is only viable over very short distances (perhaps a mile or 2)
  • MichkovMichkov Posts: 104 ✭✭
    edited July 2014
    As their names imply AC uses alternating current. Think electrons moving back and forth in the wire. As ScrappyDoo said its very good way to transmitting power over larger distances. Also once you want to turn that power into motion you are going to need AC. Because you need a changing current to work the electric motor, or a changing magnetic field, but that's really two sides of the same coin.

    DC on the other hand has the current flowing in only one direction. Pretty much all electronic devices are using DC to do their processing. If plugged into the wall there will be an AC/DC converter somewhere between the outlet and the device. Battery operated machines run directly on DC.

    Regarding PV cells, due to their way of generating power you'll only get DC out of them. To feed it into the grid you basically hack the direct current apart to get a AC signal. Other power generation modes provide AC as they convert rotation into electricity in the reverse way a electric motor works
    by Michkov
  • BekabirderBekabirder Posts: 8
    edited August 2014
    Thank you for the explanations. I took and aced physics 5 years ago, but I basically just parroted back the equations without completely understanding the concepts behind them.
    by Bekabirder
  • ScrappyDooScrappyDoo Annapolis, MDPosts: 1,067 ✭✭✭
    So the reason we use AC is that it is easier and more efficient to distribute. No conspiracy involved. Local generation could do DC (no need for efficient distribution in a local service) but bigger plants are also more efficient than small ones. And even if you generate your own power you want to convert it to AC and pump it into the grid so that you can use the grid as your battery at night or in case of cloud cover.
  • MichkovMichkov Posts: 104 ✭✭
    First part yes, second no.

    There is no such thing as using the grid as a battery. You'll have to use electricity as its generated, while you can store low amounts of energy in batteries(phone laptop etc). But those are not significant in the big picture. It's the big problem with solar and wind right now, you have to take the power they provide right now. Germany has to pay other countries to buy their electricity at times because they simply cant use as much as they're solar and wind plants generate. And as soon as you get cloud cover or no wind you'll have to start up quick reacting power plants that aren't that green like gas turbines.
    There is plenty of research going on on how to store the excess energy by pumping water up a damn or lifting a piece of rock hydraulic. So you're exchanging electrical power for gravitational potenial.
  • ScrappyDooScrappyDoo Annapolis, MDPosts: 1,067 ✭✭✭
    Sorry I was speaking metaphorically. As an end user using the grid as a battery is not technically accurate but the differences are transparent to the client.
  • Gara_the_engineerGara_the_engineer In a log house at the edge of the forestPosts: 596 ✭✭✭
    It's actually not DC being DC that's the problem when talking about transmitting it over long distances, but the voltage. When you're transmitting electrical power, you want an as low current as possible because it's the current that causes losses in efficiency. If you want a certain amount of power, and you want to halve the current, you have to double the voltage. The higher voltage you have in the power grid, the lower currents you get and therefore less efficiency losses. It's really easy to change the voltage of AC, transformers have existed for a very long time and have a very simple construction, but until recently there wasn't really any way of changing the voltage of any larger amount of DC. That meant that you had to transmit it low voltage high current, which is very inefficient and you have to have really thick conductors. Nowadays there's a way of changing voltage levels of DC too, although it's a lot more complicated than with AC. There is, however, already a large cable in the Baltic Sea between Sweden and I think it's one of the Baltic countries, that's a high voltage high power DC cable. That's the only one that I know of yet.
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