The latest energy-harvesting idea from MIT is a bicycle wheel that acts as a source of electrical energy for power-assisted riding as well as for various electronic gadgets, see "MIT's big wheel in Copenhagen:
New bicycle wheel not only boosts power, but also can keep track of friends, fitness, smog and traffic."
Let's ignore the electrical power it provides for gadgets and connectivity as well as the extra weight it adds, and focus instead on the basic "energy harvest/ride boost" functions. I am sure the design is quite clever and ingenious from an engineering perspective, but I fear it has that "something for (almost) nothing" veneer, which is very misleading and actually counterproductive. Why?
- First, the amount of power you can recover via regenerative braking from a bicycle is quite modest. The kinetic energy (energy of motion) of a bicycle is fairly low (remember, KE = ½ mv2); and most of the energy you put into moving a bike is actually lost due to air resistance. Therefore, it can't be recovered when braking, since it has been dissipated in frictional loss with the air (and some more is dissipated via the bike's mechanical losses, which are quite small). Aggravating the unrecoverable drag-loss factor, keep in mind that the air resistance loss goes up with the square of velocity.
- Second, even if there was reasonable amount of energy of motion available, any half-way decent cyclist doesn't use the brakes very often; again, there will be little opportunity to get it back via regeneration.
- Finally, it is a very inefficient use of input energy. Either you'll have to pedal harder to maintain your target speed, while your "excess" goes to charge the battery; or you'll have to ride more slowly while the difference goes to the battery.
But what you are really doing is using grown and digested food–eaten by a person and converted to muscle power–to generate electricity. That's a very inefficient process when you look at the total energy chain from growing and transporting the food, to converting muscle power to electrical power.
Wait a minute: wasn't a major advance and marker of modern civilization that we transformed "work" and motive power from being based on muscle effort–both human and animal–to being based on various types of engines? If I read this MIT thing right, we are going back to this inefficient, exhausting way to extract energy and provide power that resulted in a subsistence existence for most people. And we're patting ourselves on the back for doing it, as well. That's progress to be lauded? I don't think so. Or is Fred Flintstone's "car" their next planned advance?♦