[dorkbotsea-blabber] Electricity and human power

[Kevin Purcell]

Wrong kind of "calories"

What people who do nutrition call a "calorie" is really a "large  
calorie" or a "kilogram calorie" what an (old) scientist would call a  
kilocalorie (a thousand small calories).

I leave out the bit about calories at different temperatures (as it  
doesn't make a difference here) but you can get all the details in  
Wikipedia.

<http://en.wikipedia.org/wiki/Calorie>

When they start labeling our food in kiloJoules when know we've got  
somewhere with science education.

Onto the other half of the question ... rather than just make stuff  
up I'll use stuff that's already been made up at Wikipedia ...

<http://en.wikipedia.org/wiki/Bicycle_performance>

> A human being traveling on a bicycle at low to medium speeds of  
> around 10-15 mph (16-24 km/h), using only the power required to  
> walk, is the most energy-efficient means of transport generally  
> available. Air drag, which increases roughly with the square of  
> speed,[1] requires increasingly higher power outputs relative to  
> speed. A bicycle in which the rider lies in a supine position is  
> referred to as a recumbent bicycle or, if covered in an aerodynamic  
> fairing to achieve very low air drag, as a streamliner.
>
> On firm, flat, ground, a 70 kg man requires about 100 watts to walk  
> at 5 km/h. That same man on a bicycle, on the same ground, with the  
> same power output, can average 25 km/h, so energy expenditure in  
> terms of kcal/kg/km is roughly one-fifth as much. Generally used  
> figures are
>
>     * 1.62 kJ/(km∙kg) or 0.28 kcal/(mile∙lb) for cycling,
>     * 3.78 kJ/(km∙kg) or 0.653 kcal/(mile∙lb) for walking/running,
>     * 16.96 kJ/(km∙kg) or 2.93 kcal/(mile∙lb) for swimming.
>
> The average "in-shape" person can produce about 3 watts/kg for more  
> than an hour (e.g., around 200 watts for a 70 kg rider), with top  
> amateurs producing 5 watts/kg and elite athletes achieving 6 watts/ 
> kg for similar lengths of time. Elite track sprinters are able to  
> attain an instantaneous maximum output of around 2,000 watts, or in  
> excess of 25 watts/kg; elite road cyclists may produce 1,600 to  
> 1,700 watts as an instantaneous maximum in their burst to the  
> finish line at the end of a five-hour long road race. Even at  
> moderate speeds, most cycling energy is spent in overcoming  
> aerodynamic drag, which increases with the square of speed;  
> therefore, power needs increase approximately with the cube of speed.


So you can put out about 100W for a extended time period. So the  
headlight bulb is perhaps a bit larger than you think it is plus  
there are probably losses in the system.

BTW, the resting energy of most humans in is the 80W to 100W range  
just to keep your body warm you use about 80% of your calorific  
intake for that. It's one of the resons that exercise seems to work  
to loose weight -- it increases you resting metabolism so the time  
your are not exercising you are burning off more calories than the  
amount you use during the vigorous exercise.

Little known fact: When testing house insulation 75W to 100W bulbs  
are often used to simulate the heat output of a person living in the  
house. You can even get them to move around by switching lights on  
and off.

On Jul 29, 2008, at 2:45 PM, Seth! Leary wrote:

> His theory seems wrong to me. You have to pedal like mad to get a  
> headlight
> on a bike generator to light up. That's maybe a 5 watt bulb, right?  
> How can
> the same mechanism (albeit with a possibly larger rotor) illuminate  
> a whole
> bank of 60 watt bulbs? I think that a person would burn about 600  
> calories
> per light bulb to keep it going for ten seconds, if no energy were  
> lost to
> friction. That's a third of the normal daily energy of a person-- 
> for one
> bulb for ten seconds!

--
Kevin Purcell
kevinpurcell at pobox.com