Alternating current (AC) versus direct current (DC)
Have you ever wondered how AC/DC, one of the hardest rocking bands in the world, came up with the name? Or what the acronym stands for? It turns out that the name came from the band-members’ sister after she saw “AC/DC” on a sewing machine. I know, it’s kind of a lame story for the namesake of a classic rock and roll band but that’s the truth. And the acronym? It stands for Alternating Current/Direct Current.
Let’s start off with direct current and use the same hill metaphor that we used to introduce solar power in the Solar Energy 101 article. An electrical current is simply moving charge, usually negative charges in the form of electrons. Negative charges want to move toward positive charges. So if we think of an electron as a ball moving down a hill, the bottom of the hill represents the positive terminal that the electron is traveling toward and the top of the hill represents the negative terminal that the electron is repelled by. The more balls you have rolling down the hill, the more current you’re generating and the more current you generate, the more power you produce.
Direct current is simply when the top of the hill and the bottom of the hill always remain negative and positive, respectively. This means that the electrons (and our metaphorical balls) are always rolling in the same direction. Batteries, laptops, fuel cells and solar cells all use direct current.
Our hill metaphor breaks down for alternating current, however. Whereas the terminals of direct current always remain the same and the electrons always travel in the same direction, the terminals of alternating current change constantly, making the current alternate directions. Power plants and the sockets in your home use alternating current, which alternates at a rate of 60 times per second in the United States.
Using alternating current is beneficial to energy companies because it allows them to transmit high voltages over long distances using transformers. The physics behind transformers are beyond the scope of this article; however, the benefit of using high voltages is that it is much cheaper than using high currents. Basically, voltage is the metaphorical equivalent of steepness in our hill analogy. Energy companies have to transmit large amounts of power to people that live far away from its source. This means that energy companies either need to send a lot of balls at once, which would require a very wide hill (i.e., a thick wire with low voltage), or a very steep hill with a small number of balls (i.e., a thin wire with a high voltage). Using the latter method allows energy companies to save on the cost of the expensive metals that make up wires. If you’d like to learn more about the physics behind these principles, I’d suggest picking up an introductory book on electricity and magnetism. However, to enjoy the benefits of solar energy, that won’t be necessary.
The bottom line for solar energy users is that you will be producing direct current with your solar panels, whereas your home or building will be using alternating current. This is a problem with a simple solution called a solar inverter, which converts direct current to alternating current with an efficiency of about 95%. That means that you’ll only lose five percent power in the conversion process. Not a bad deal when considering the fact that up to 40% power is lost by the AC-to-DC converter in computer chargers.