wires are the best!

I’m now hopelessly late with this, so I’ll try (and fail) to make it brief. On Friday Ryan discussed D.C.’s ban on overhead “catenary” wires, which would be necessary for electric streetcars. Apparently you can’t use an electrified sunken rail in cities that have to salt their roads — there are corrosion issues. Unfortunately, changing the law to allow overhead wires would require congressional involvement. Ryan mentions a company that’s pushing a technology providing for the wireless transfer of power from the street to the streetcar, allowing the system to be sealed and immune to salt. It sounds like a pretty clever solution. But that doesn’t make it a good one.

Like a transformer, this technology works through induction, converting electricity to magnetism and back again. In a normal transformer you have a core of some sort — picture an iron ring — and you’ve got two wires, one for input and one for output. These get wrapped around the core like in this picture. Send alternating current into the input wire and its wrappings will generate a magnetic field, which will be conducted along the core, which will excite electrons in the other wire’s coils and generate an output current. If you change the ratio of windings on the input and output coils the voltage will change, too, which is a very useful thing to be able to do. As you might imagine, transforming energy from one form to another in this way isn’t perfectly efficient (although in large, well-designed units it can be very close to it). Some electricity is lost to heat, which is why those heavy old wall-wart adapters — heavy thanks to their iron cores — tend to be warm to the touch after they’ve been plugged in for a while.

Newer “switchmode” adapters use a different technique for changing voltage levels. This method doesn’t require an iron core, which is why they can be so much lighter and smaller. Strictly speaking, transformers don’t need the iron core, either. The problem is that they’re much less efficient without one — which brings us to inductive power transfer and the streetcars. In this case one coil is sitting in the street and one’s sitting in the streetcar. The core, such as it is, is made of air, which is terrible at guiding magnetic fields, and gets ever-more terrible the further apart the coils are — this is not a useful technique for moving power anything but very short distances.

The point of all this is that you’re going to be wasting energy if you try to move it around wirelessly. Worse, it’s going to be more expensive to build the system to do all this than it would be to just make the connection with wires. It’s not as if inductive charging hasn’t been tried in the marketplace: some early electric cars used inductive paddles to charge, and various efforts are intermittently made to provide magical laptop-charging desks. But aside from electric toothbrushes — which, as the previous link notes, can afford to waste some energy if it lets them stay watertight — there just aren’t that many applications where choosing a more expensive, more wasteful way of transmitting energy makes sense.

And I have a feeling that the same will prove to be true of streetcars. Wikipedia cites an 80% efficiency number from an experimental bus system developed in 1990. That was a while ago, but it’s hard for me to imagine that the situation has gotten that much better — or that real-world applications could match the performance of a system in a closed test track. It’s going to cost more money than catenary wires, and you won’t be able to get your coils very close together on hilly streets, so your electricity bill is going to be 10% or 20% more than it would if you used direct conduction. It would be a shame to let an accident of bureaucratic history make this engineering choice.

This entry was posted on Wednesday, January 28th, 2009 at 2:48 pm and is filed under DC, science. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.