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Friday, April 29, 2011

AC/DC

You use two types of "dynamic" electricity every day. One is called AC, and the other is called DC. I'll ignore static electricity in this post because it's not relevant.

AC is what enters your house from the power company. Most of the load in your house -- the heating and air conditioning system, the washer and dryer, the hairdryer, the waffle maker, the iron, the lights, and so forth all use AC directly. One of the reasons why AC is so prevalent is that it's easy to change AC from one level of voltage to another with a simple transformer.

DC, on the other hand, is what comes out of batteries. Practically all of your household electronics -- your TV, your computer, your cellphone -- run on DC. In some cases these devices plug into an AC outlet, but they have an internal converter from AC to DC. No converter between AC and DC is 100% efficient, so energy is lost in every conversion.

For 100 years this was a comfortable arrangement. 99% of the power in a home was used by direct-AC devices, and most of the DC came from disposable batteries. But two things have changed:
  • new DC-hungry devices have grown in power consumption, and
  • photovoltaic solar panels generate DC.
As I look around the room in my house where I'm writing this blog post, I count 18 DC devices in plain view. I'd love to be able to install a few photovoltaic solar panels on the roof, connect them to a storage battery that's augmented by an AC-powered charger, and run these 18 devices directly off DC. It would be much more efficient, and I wouldn't have the ugliness of 13 "bricks" for AC/DC conversion all over the room. But it's not practical to do so:
  • the electrical code requires that the solar panels and the DC distribution system be carefully grounded,
  • there is no standard for a DC power plug -- other than USB, which has a relatively low capacity for current,
  • a storage battery would have to be enclosed in a fire-proof, spill-proof container, and
  • the price of copper wiring to handle the DC amperage is considerable.
Consequently, people find that if they do put photovoltaic solar panels on their roofs, it makes more sense to install a DC/AC converter, leave everything in their house as AC, and push the surplus AC back into the grid. That's alright, but we can do better. I wonder how long it will take for the housing, electrical, and electronics industries to converge on standards and practices for in-home generated and in-home stored DC. Probably 50 years. That's a shame, because we need better efficiency today.