An entrepreneurial buddy of mine once pitched to me an idea: put generators on exercise bikes so people make electricity while they work out. My philosophy toward alternative energy was then as it is today. Retrofits only make sense if the economics are competitive with conventional technology.
A simple calculation shows that power-generating exercise bikes have no traction. I started with the generous assumption that a human can sustain 100 watts of power output for an hour. I assumed that generated power would offset electricity cost at the 2008 average US residential retail rate, 11.35 cents per kilowatt-hour. I furthermore assumed that the retrofitted bike would be utilized 12 hours per day 365 days per year by exercisers in a gym. Under these munificent assumptions, the retrofitted bike offsets $49.71 in electricity costs per year.
To keep bike retrofit costs down, a 150-watt motor ($30, 2-year warranty) was specified as a generator. Storing energy in batteries would be prohibitive. A grid-tied Enphase Micro-Inverter ($200, 10-year warranty) was a cheaper option. Finally, I assumed $150 in initial costs for component shipping, electrical wire, incidental parts, and associated labor. Using these values, I calculated simple payback with hardware failure and replacement coincident with warranty periods.
When the inverter is replaced 10 years after initial system installation, the retrofitted exercise bike falls about $3 short of the breakeven point. In other words, unless the bike is utilized more than 12 hours per day or is situated in Hawaii or New England (where power costs far exceed the national average), this idea is not economically viable.
Nonetheless, Windstream Power, LLC, of Ferrisburg, Vermont runs a thriving business based upon their $595 bike power generator; a device which never achieves payback. Moreover, the so-called human powered gym is an emerging exercise trend that capitalizes upon the Green obsession: if one is going to work out anyway, it is best to use the workout to make electricity. The article “Make Electricity While You Exercise” provides a nice overview of the synergy between the health and Green crazes.
It is difficult to know whether the human powered gym concept, which makes no sense from an economic standpoint, is going to be successful. It is tough to place a dollar value on providing the illusion to gym users that their workout is helping the environment.