People voluntarily expend quantities of energy working out on cardiovascular machines, many of which plug into the wall.
The Notions of Expenditure project, initiated by A. Laurie Palmer, is looking for speculative proposals to re-design exercise equipment and make them generate (rather than consume) and store energy; and/or to retrofit gyms to function as local power sources linked to the grid.
Notions of Expenditure is not a competition but will result in a public exhibition of all the submissions (only 2 so far).
Each time two kids rocking on the seesaw touch the ground, air is compressed. A generator converts the air and then the generator is hooked up to mechanical devices. According to Dr. Raj Pandian who devised the system two year ago, one to two minutes of playtime can power a 20-watt light bulb for two to three minutes.
South Africa has devised another way to harness the energy generated by kids playing on a merry-go-round.