For their senior design project at Arizona State University, these guys built a regenerative braking system for their bike. As they brake, the system spools up to give them a push back up to speed. They achieved 25% efficiency on one run, which isn’t too shabby.
The site has all kinds of information. You can check out their different plans and ideas from before they decided on this specific layout as well as all their data from the test runs. Be sure to look at the piece by piece breakdown of what changes were made from the initial design.
[thanks Mario Gomes]