David said:
My power engineering courses were a long time ago, but I don't believe
you can increase the power transfer into a resistive load (the filament)
by adding a capacitance to a circuit; the cap just acts as an
*additional* load (even though it's reactive load).
Well, I tried it a few years ago. IANA Electrical Engineer but I had
one helping me.
We tested three different generators and found similar results with
all. Here's some data for a Soubitez bottom-bracket generator. I'll
just give the values for 14 mph, 12 ohm load (i.e. standard generator
headlight bulb), with and without some capacitance. The capacitance
was chosen to give max power boost at about 12 mph, IIRC, and was 100
microFarads. (What's the ASCII abbreviation for "micro"? I'll go with
"mu".)
RMS Voltage: With 100 muF: 7.3 V
without: 6.4 V
Current: With 100 muF: 0.6 A
without: 0.53 A
Power: With 100 muF: 4.4 W
without: 3.4 W
Efficiency: With 100 muF: 42%
without: 39%
The efficiency figures are the least reliable, BTW. Our method of
measuring did not take into account the losses resulting from the
interface between the rubber tire and the generator roller surface, and
those can be considerable.
I'll also note, the September 1995 issue of "Electronics World +
Wireless World" (a British magazine) has a letter to the editor, plus
response, on p. 770 that deals with this issue. For their own reasons,
they used a resistance of 24 Ohms instead of 12 Ohms, with and without
a 50 microFarad capacitor. Above 10 mph, they got more power with the
capacitor than without. A graph on that page shows their results.
So you can get more out of a bike generator by adding capacitance. But
it's not much more, it varies quite a bit with speed, and we judged it
not worth the trouble.
- Frank Krygowski