replace the electric motor of an air compressor with modified bicycle drivetrain. pedal. keep pedalling...
Step 1: Get rid of the motor.
use the existing motor mount to install an axle with one fixed cog and a belt pulley. You'll need bearings. try a 1/2" axle with radial ball bearing cartridges. make the bearing housing out of a slotted tube that has a drilled out nut and a regular nut to allow a bolt to tighten the tube around the bearing. (should i include more desciption?)
Step 3: Weld on the rear triangle of a bicycle, to allow the use of the dropouts and bottom bracket. horizontal dropouts are best.
Step 4: Attach a heavy flywheel to the compressor to smooth out the resistance.
Step 5: Make handlebars, rig up a chair, add gears if you want, and go for it.
Step 6: Toe clips really help.
Step 7: This one takes 15 minutes or so to reach 50 psi, and i think it's a 30 gallon tank.