[Olpc-open] Foot power revisited...

[Bert Freudenberg]

On 14.01.2009, at 17:39, Mike Dawson wrote:

> Hi All,
>
> Unfortuantely I am just stuck in Jalalabad at the moment as bad
> weather cancelled our flight back to Kabul but I met the folks who are
> doing the Fab Lab here (if anyone has heard of that).
>
> So we did a quick back of the envelope calculation along these lines
> that refreshed my high school physics as soon as they reminded me that
> W = FD
>
> 1. Assuming someone is normally doing about 60 rpm on a bike
> (apparently the figure is 60-90)
>
> 2. Take the distance that a foot pedal gets pushed as being 15 cm
>
> 3. Work done = Force x Distance
>
> 4. Dynamo as being 50% efficient (apparently an alternator can be up
> to 60% efficient http://en.wikipedia.org/wiki/Electrical_generator)
>
> 5.
>
> P = 0.15F / 2
>
> P = Power in Watts out
> F = Force in Newtons
>
> (as at 60rpm the output in Joules will be for 1 second, which is
> therefor the power rating in Watts)
>
> So the force required to generate 9W (e.g. for charging a little and
> running the laptop):
>
> 9 = 0.15F / 2
> F = 120N = 12.23kg (given gravity = 9.81N per kg)
>
> Which does not seem to be an unreasonable amount...  Our idea is to
> see if it is possible to generate continuous power during usage in a
> manner that is both quiet enough and reliable enough.  We can get
> Dynamos from the local market for about $0.80 each.
>
> I know that calculation does not include the required loss to a bridge
> rectifier... but it all just seems a little too simple - can anyone
> spot the mistake here?
>
> If that's correct then when the chopper takes us back to Kabul we can
> try to connect up the rectifier to the machine and see how we might be
> able to charge the XO...
>
> Would also be interested to calculate if one assumes that most of or
> all of the calories burned by the human body come from a certain
> source, then how much will this add to the food bill?  Given that I am
> told a human normally produces 60W of just heat... hopefully not too
> much.
>
> Regards,
>
> -Mike


Producing just 10 watts should be very easy given proper equipment.  
See the table at the bottom of this page:

http://www.los-gatos.ca.us/davidbu/pedgen/stats.html


- Bert -