I love simple systems. I think somewhere in the process of building a 300-amp motor controller from scratch, we forgot how remarkably simple the electric go-kart really is. It took a while to get there, but in what I think is a good example of engineering system design, the individually complex pieces (most of which I would have preferred to buy off-the-shelf if they actually existed) together make something so easy to understand that it must work.
When the kart engages its regenerative braking circuit, there are only really two places for energy to be stored: in the moving mass, or in the capacitor. The batteries are out of the equation, since the main controller, in an act of almost absurd simplicity, cuts them off and shorts the motor terminals across the capacitor. Of course there is friction, but let's say we idealize by using a 54lb steel flywheel instead. ("That is one of the scariest things you've ever built." -Matt R.)
The only way to move energy from the mass to the capacitor is by energizing the motor's magnetic field, forcing current to flow as it acts like a generator. The flywheel slows down, the capacitor fills up, and along the way a sizable portion of the energy is dissipated in the resistance of the motor. Since we can measure current the whole time, the energy dissipated can be calculated. And guess what? It almost exactly matches the energy difference between the final value of the capacitor and the initial value of the flywheel, every time. The data is up on the site.
I'm not sure why I was surprised by this. There's a huge spinning disk, a huge capacitor, and a huge copper motor winding. There's no where else for 10kJ to go in a matter of seconds without destroy something, so that must be all there is to it. I'm pretty sure I thought this all through and came to a similar conclusion before we even started building. But after months of fiddling with power electronics nuances, it was wonderful to see the conceptual simplicity come to life and to see the data confirm that physics does indeed work.
Take-away: If I draw a resistor and a capacitor and ask people if that's a simple thing, they say yes. If I draw a go-kart with regenerative braking, they say no. Hopefully I've taught a few people to say they're the same.