|Fig. 1: Manoj Bhargava demonstrates his electricity-generating bicycle, Free Electric.  (Courtesy of National Geographic.)|
In the United States and throughout the world, the number of bicyclists is increasing rapidly as commuters seek a healthy, eco-friendly, and cost-effective mode of transportation. Wilson predicts that over the course of an hour, a healthy adult male can pedal at an average energy output of just under 200W.  With cycling's increasing popularity there is a lot of green energy to be harvested. In this paper I will give an overview of the energy potential of bicycles and the feasibility of the implementation of kinetic energy harnessing mechanisms.
One stipulation inherent in this experiment is that the energy being harnessed from the bike must abide by the principles of conservation of energy. Therefore, any additional energy required to operate the energy-harvesting apparatus would correlate to a proportional increase in the energy intake in humans. Basically, this means one would have to eat more food to sustain the necessary wattage output necessary for peddling a heavier bike. One could extrapolate the additional energy input as having potentially negative economic impacts, but this discussion is beyond the scope of this paper.
Bicycling is a rich source of kinetic energy. There are two major methodologies in the various practical and theoretical attempts to harvest the kinetic energy of a bicycle. The first and perhaps more obvious of the two is collecting the energy from the rotational motion of the wheels. This kind of device would operate on a simple principle: the rotational force of peddling the bike causing the wheels to spin, this spins a rotor that spins a generator, ultimately producing electricity which is then stored in a battery. Already MIT researchers developed the Copenhagen Wheel, a device that uses the wheel's rotation to charge a battery that automatically powers the back wheel, creating a self-sustaining electric bike.
While this is a legitimate usage of the stored energy, I envision the potential of this energy-harnessing capability on a more global humanitarian scale. Conservatively estimating that a device applied to both wheels could harvest 1/3 of the energy output, an average bike ride being 30 minutes long, gives 33.3 Watt-hours of energy. This might not seem like a lot of energy, but approaching this value from a global scale, assuming 400 million bike rides of 30 minutes long bike in a year (a conservative estimate, considering that in recent years bicycle production has climbed to over 100 million per year), that would yield over 13 million kW. Although this value is only a fraction of the World Energy Consumption, bikes are nonetheless a source of wasted potential energy.
The second method is harvesting the kinetic energy from the small-scale motions of bicycling, such as weaving back-and- forth to maintain balance. The Yang et al. study found that using microelectromechanical systems (MEMS) to harvest energy from the natural balancing motion of riding a bike, they could harvest an average of 8mW/10sec or about 3W/hr.  This is evidently a less efficient energy harvesting practice.
As demonstrated in the previous section, cycling as a source of gatherable energy has potential. This begs the question: why has this technology not been developed on the widespread scale that one might expect? As in most gaps between technological potential and reality, the likely answer is cost. The average cost of electricity in the US is 12 cents/kWh.  Using my prior estimates, that means that an American biking for 15 hours would save only 12 cents, hardly a worthwhile endeavor.
Nonetheless, much like normal currency, energy is weighted in its value. Harvesting energy from bicycles might not be practical in terms of powering a metropolitan area or developed country such as the United States, nor would it create massive savings on an individual basis, but the possibility does have humanitarian merit. Manoj Bhargava, the creator of the popular energy drink 5-hour Energy has built a stationary bike that, when peddled for an hour, can provide electricity for 24 hours in rural households. He plans to distribute 10,000 his bikes, called Free Electric, across India, and hopes that it will affect billions of people (Fig. 1) .
There are about 7,000 undergraduates at Stanford. Lets say 80% of them bike about 15 minutes a day. That is 1400 hours of bike riding per day, or 9,800 hours per week. Biking at a casual energy output of 50W yields 490,000W. Using the same harvesting-efficiency coefficient as earlier, Stanford undergraduates could collectively harvest over 160kW each week. Moreover, the popularity of cycling has undoubtedly grown significantly since 2009 and the efficiency of harvesting energy from a bicycle is likely higher than 1/3. Therefore, if the technology is financial feasible, it makes sense to disseminate this technology. It is more likely that the business model of Bhargava's Free Electric becoming the preeminent standard for stationary bikes, especially given the growing popularity of stationary biking as a means of exercise. Regardless of whether the world adopts a venture like Free Electric or if it implements energy harvesting technology in normal bicycles, further experimentation with collecting the kinetic energy from bicycles is a worthwhile endeavor.
© Joshua Lange. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 D. G. Wilson, Bicycling Science (MIT Press, 2004).
 Y. Yang, J. Yeo, and S. Priya, "Harvesting Energy from the Counterbalancing (Weaving) Movement in Bicycle Riding," Sensors 12, 10248 (2012).
 J. Jiang, "The Price Of Electricity In Your State," NPR, 28 Oct 11.
 W. Koch, "Creator of 5-Hour Energy Wants to Power the World's Homes - With Bikes," National Geographic, 6 Oct 15.