Alternative Energy

Recently I've been reading about a lot of alternative energy sources. I read about the new tidal generators, which use buoys in the ocean, which have pistons in them that are driven by waves. I also just read this article:

http://www.engadget.com/2008/12/11/piezoelectrics-installed-in-tokyo-railway-station-floors-generat/

It talks about generators in the floor which generate electricity due to vibrations in the floor I believe. These examples got me to thinking...what other places do we see an up and down motion which we could harness for electricity? My first thought was that of elevators. Obviously you need energy to get the elevator to go up, but what about when it's coming down? It seems to me like you would be able to harness the force of gravity to create some electricity. I could see multiple ways to do this...The most straight forward being to attach a rope to the elevator car, and have the motion of the car either turn a wheel, or move a piston up and down. Along with this type of generator, I was thinking about another alternative energy story I saw about cars that run on compressed air. You could use the motion of the elevator to compress air for use in generating more energy later (in other words, use the elevator as a huge pump). I don't think any of these are breakthrough ideas, but with the issue of alternative energy popping up everywhere, it's got me thinking about ways I could make a difference. Take this as the first installment of a series of Alternative Energy posts I intend to make.

Here's a picture of the generator installed in the UK dance floor, as well as the link to the article:

http://www.geek.com/articles/news/uk-club-gets-dance-floor-that-creates-electricity-20080718/