There comes a time in a graduate student’s life when he finds himself thrust headlong into a project he didn’t think he’d be thrust into. Such was the bicycle generator: I had a request from a friend with the Tufts Institute of the Environment (TIE) sometime mid-2012 to build one of these contraptions. So I did. It ended up looking like this:
In principle, it’s pretty simple: put a bicycle on a stand and hook it up to a generator, and voila! Pedal power. The devil, as the say, is in the details.
First off, problems of the mechanical world had to be solved. The bicycle had to be immobilized and coupled to the generator. I opted to design and build a simple custom setup involving angle irons and two by fours. Then there was the issue of coupling the spinning rear wheel to the generator. I opted for replacing the tire with a long v-belt which went to the pulley of the generator–in this case, the alternator from a car. The nice thing about using car parts is there are a million accessories in existence catering to cars and car voltages, so the electrical output of the alternator could simply be fed to an automobile voltage inverter–and voila! Mains-level AC power.
Of course, it wasn’t that easy. The alternator, turns out, needs a control unit to regulate its output voltage (unregulated and unloaded constant current devices, e.g. generators, will produce damagingly high voltages), and even so the balance of power produced by the pedaling and consumed by the alternator teeter-totters all over the place without the introduction of a car audio supercapacitor (one farad!) to smooth things over. Finally, it turns out that car alternators are bootstrappers, themselves! As they have no permanent magnets inside, the initial magnetic field is created by energizing their field windings. In a car, this initial current can come from the battery, but from this setup, well…a battery had to be introduced! I opted for a lantern battery hooked up via a push-button switch accessible to the individual pedaling (should be good for several thousand starts). The startup sequence consisted of pedaling followed by a brief press of the switch, energizing the field windings and initializing power generation. After that initial button press, the electricity generated is used to keep the field windings energized–for as long, of course, as the bicycle is continuously pedaled.
The bicycle itself was the bane of my existence–essentially unfit for road use–so I repurposed it for stationary use and repainted and overhauled it to make it nice and presentable. The alternator, inverter, voltage regular, supercapacitor, and lantern battery were all off-the-shelf purchases (BoM here).
The project itself was a success, but there a slew of improvements I’d make for a v. 2.0. Mechanically, the v-belt was a convenient way of coupling the rear wheel to the alternator pulley, but I should have a) designed in less forwards-backwards slop on the angle-iron supports and b) incorporated a tensioner pulley to address the propensity of the belt to develop slack. Electromechanically, the choice of car alternator led to a somewhat awkward setup; the use of a permanent magnet generator would have eliminated the need for the lantern battery and push-button startup. It is also possible that the commercially available automobile voltage inverter was overkill: it was fussy on the input it received, and the things which were powered with it were typically strings of lights and other simple electronics which did not need tightly controlled AC mains voltages. A simpler, cheaper, and more intuitive solution for the bicycle generator operator could have been made from a few solid-state electronics.