[BULK] Re: harvesting energy

Richard A. Smith richard at laptop.org
Fri Oct 28 23:36:03 EDT 2011

On 10/27/2011 11:45 PM, Richard A. Smith wrote:

> In this specific case the question was about human power so its
> reasonable to assume that the setup would use a direct DC input.
> Otherwise you are losing 30% or more of your power in conversion losses.
> The number I use for the DC input is 25 watt-hours. XO powered off. Its
> reasonably constant between XO versions and at various DC voltages.
> Reflecting that power draw to the actual amount of human work required
> needs some data on how good the human->electrical conversion in various
> setups is. I don't have any good data on those setups.

After a bit of Googling I think I have enough info to take a reasonable 
stab at the original question of how much work would it take to charge 
the battery.

I'm assuming this means human work and the closest thing I can think of 
to map that to is fitness Calories (kcals).

The number I found for the eff% of the average DC permanent magnet 
generator is 85%.  A chain drive system is in the 98% range.  Thats 83%. 
  Lets round down to 80% to account for various other bits of friction 
and losses.  Lets also assume that the generator output curves match the 
mechanicals and the output voltage generated is <=25V so it can connect 
to the XO (1.5) directly without any sort of regulator.

25Wh is 90kJ so after the 80% conversion thats 112.5kJ at the human 

Wikipedia claims:

The efficiency of human muscle has been measured (in the context of 
rowing and cycling) at 18% to 26%.

And then goes on to stay that some rowing machine uses as 20%.

So taking 20% as the human eff%, factoring that in, and then converting 
the whole mess to Calories comes up with 134 Calories.  That seems 
pretty reasonable for an average person to do.  Spreading it across 2 
hours though will be pretty boring.

If you time shift the power (aka a battery) so you can do it all in a 
shorter time then let that charge the XO then thats another 75% hit. 
That would put us up in to the 180 Calorie range.

I'm in pretty good shape and I can do over 200 Calories in .5h when I do 
a cardio session at the end of my workout.  Thats a fast walk on a 
treadmill with an incline that increases as I go.  Its a solid workout 
though.  I'm breathing heavy and sweating at the end.

I wonder what numbers Mike was using where he came up with his "hard for 
an adult athlete" calc.  Was he perhaps talking about a conventional 
laptop?  If you were trying to charge the 94Wh battery I have in my 
Thinkpad then thats a different story.

Richard A. Smith  <richard at laptop.org>
One Laptop per Child

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