One of the alternate sources of power considered by OLPC for the XO laptop was Potenco’s Pull-Cord Generator.
After evaluating a number of prototypes, it became apparent that the pullcord (Yo-Yo) design had several serious inherent problems.
 Duty Cycle
The duty cycle of the basic device (shown above) was 50% at best. It only supplied power to the laptop for half the time (the PULL part of the cycle). This means that since we limit the battery charge current to a value that requires 1.75 hours to charge a battery, it would take 3.5 hours to charge a laptop pulling on the yo-yo.
At least one prototype attempted to correct this by providing a bank of super-capacitors, which were also charged during the PULL cycle, and then continued to charge the laptop during the REWIND part of the cycle. While this solved the problem, the required bank of supercaps doubled the price and size of the Yo-Yo, and introduced concerns about its ability to provide 2000 charge cycles before failing.
Another prototype provided two handles, such that a user was constantly pulling on one or the other, eliminating the non-charging REWIND phase. While this was a better solution than adding supercaps, it required using both hands/arms to operate.
It is reported in the original data sheet (Internet Archive Wayback Machine link; the file is no longer on the Potenco site.) that user fatigue time is about 10 minutes. Those of us who have tested the Yo-Yo agree that pulling for more than ten minutes is painful.
Given that it takes at least 1.75 hours to charge an XO, further development of the pullcord generator has not been encouraged by OLPC.