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
interface.
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 <[email protected]>
One Laptop per Child
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