Design a personal exercise machine that uses the otherwise dissipated human exercise energy in some productive way. For example, the exercise energy could be converted into mechanical, chemical, or electrical energy, which could then be used in some domestic device (for example, to power a computer, TV set, or kitchen appliance).
Another solution would be to design a system that would feed the exercise energy directly into the local electrical power grid for credit against the user's electrical bill (like wind power devices today).
If 100 million people exercised for 15 min every day of the year at a power level of 100. W each, how many barrels of crude oil (at 5.80 × 106 Btu per barrel) and how many tons of coal (at 26.0 × 106 Btu per ton) could be saved annually?