My 11-year-old son has been really intent upon trying to generate power from muscle energy, and we noodled this up this weekend.
We went to our local "Repairers of the Breech" store this AM, and found an old exercise bike for like $5.62 with tax; then to Wal-Mart for a ~$30.00, 400W inverter.
A drill motor wouldn't spin the little 24V / 250 Watt Chinese motor fast enough to make more than 10V - the inverter wouldn't cut on. Measuring and calculating, we figured the 20" exer-bike wheel with and inch-and-a-half diameter roller would give us enough rpm, so on went a urethane roller, and we just clamped the motor to the frame.
It worked! With no "pre-regulation" of the motor/generator output voltage, it varied directly with rpm. Son figured out quickly that the little exercise bike speedometer needed to stay between 14 and 19 mph, or the inverter would undervolt or overvolt.
Then, with him pedaling smoothly, we plugged in a 100W bulb to the output of the inverter - it would glow for a bit, then the inverter would fault. We found that the exercise bike wheel was wompy (adjusting the spoke tension wouldn't true it), and the roller would slip under load at a low spot. So - then we became a 14th century lathe team - he'd pedal, and I'd run a lathe tool across the solid rubber wheel to true it up.
We welded an old door hinge on as a pivoting mount, then the motor/generator, with a spring to pull it against the tire. Tried it out, and now no skip - he could maintain speed and power output -
Then a long strip of 3/16" X 1" steel; inverter held on with J&B Weld -