[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
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
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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