On 4/3/2021 1:45 AM, Jeff Liebermann wrote:
On Fri, 2 Apr 2021 20:47:00 -0400, Frank Krygowski
wrote:

The efficiency of bike dynos varies pretty greatly. I could check
numbers, but IIRC top quality hub dynos can hit 65% efficiency. Cheap
bottles can be as low as 40%.

http://www.myra-simon.com/bike/dynotest.html
See "Normal Speed Efficiency" bar graph. Looks like efficiency varies
from 22% to 61%.

OK, my memory was being too optimistic about the low end dynamos.


The first thing to do is throw out the "voltage regulator" that
reduces output (and decreases efficiency) at high RPM (high
frequencies). Replace it with an electronic voltage regulator.

It's not clear to me that the built-in voltage regulation represents a
loss in efficiency. That is, I don't think the power is being taken from
the rider's input by the coils and magnets, then converted to heat by
the inductive reactance. My understanding is that what would be
increased power at higher speed is simply never produced.

I know from private correspondence that David Gordon Wilson (of MIT and
_Bicycling Science_) said he was once working on a scheme to produce a
constant rpm bike dynamo, but once he understood how they worked, he
stopped thinking about his scheme.

If there was some way to get the dynamo RPM up high enough, an
alternator would be a further efficiency improvement. Extra points
for a fixed RPM mechanism, where the efficiency is optimized for a
narrow range of RPM's.

The rotational speed of a bottle dynamo is pretty high! Over 4000 rpm at
12 mph.

The average cyclist has to deliver about 200 watts to sustain 15 mph
on a level roadway. A 3 watt lighting system with 50% dynamo
efficiency will require 6 watts from the cyclist. That's only 3% more
power needed to maintain 15 mph with the lights on.

However, if you want 800 lumens output using 125 lumens/watt LED's
(including optical losses), it will take:
800lum / 125lum/watt = 6.4 watts
to power the light. If someone produced an oversized 50% efficient
bottle dynamo that delivered 6.4 watts, it would require 12.8 watts
from the cyclist or
12.8 / 200 = 6.4%
additional cyclist power to run the lamps. Try that on an exercise
machine or with a cycling computer on a bicycle. It's possible to do
it for short intervals, but not continuously.

What's needed is a dramatic increase in efficiency. If the dynamo
were 100% efficient, then an 800 lumen light would require only a
little more power from the cyclist than the original bottle generator.
100% efficiency is impossible, but there's plenty of room for
improvement between 50% and 100% efficiency.

I strongly, strongly dispute the need for 800 lumens for riding on the
road at night. To me that's very similar to saying bicycles should all
have motorcycle-duty drive chains and spokes.

--
- Frank Krygowski