In the US there are 39 states that now require utilities to allow net-metering, meaning they have to let companies put electricity back into the grid and pay them for it. Not long ago I was taking a tour through one of the our company's factories and got into a discussion with one of the supervisors about the huge amount of noise out on the plant floor. We looked into possible ways to take advantage of it and thought some system based on a dynamic microphone might work (dynamic microphones convert sound into electrical energy when the sound waves push against a diaphragm, moving a coil back and forth over a magnet.) Passive amplifiers could have even been added to boost the decibel level and resulting power. Unfortunately the plant was shut down before we could do any tests.

There are other places where this line of thinking might turn out to be profitable though. Planes for instance. 747's have a big auxilliary power unit (APU) in the rear that produces electricity for various purposes. If the engine noise was converted into electricity it might be possible to have a smaller that could handle the load, thereby decreasing the overall weight of the plane and giving it better fuel mileage.

Another place would be windmill parks. Having never been to one I had to do a little research, but it turns out that the windmills make a lot of noise. Modifying them so they converted the noise into electricity would make them even more of a bargain.