The Calculus Diaries - Jennifer Ouellette [70]
How much work can we get out of the system by letting entropy increase? We can determine this by taking an integral of temperature as the entropy increases over time. Gilmore’s body gains potential energy as he chugs up the hill because he increases his distance from the earth’s center of gravity, but he loses potential energy as he descends, while his kinetic energy increases. (Entropy also increases as he radiates away more heat with his exertions.) The two cancel each other out, and the amount of work produced is zero—unless the Wheelmen could find some way to harness the kinetic energy. For instance, Gilmore could carry a small package to the top of the hill and then work would have been done: The package has more energy as a result of his exertions. It has gained potential energy with the increase in altitude, which can be converted into kinetic energy should Gilmore then decide to toss it down the hill.
SPIN CYCLE
In the late 1980s, Henry Works founder Mike Taggett built his first retrofitted exercise bike with a car generator. He described it as a “Gilligan’s Island human-powered blender” and used it to mix margaritas without an electrical outlet. Twenty years later, the same concept underlies his new retrofitted machine that makes use of both the arms and legs to maximize calories burned and watts created. It looks like your typical stationary bike, except there is a hand crank in place of the handlebars that one can spin while pedaling. All that effort turns the eighteen-inch flywheel, which in turn is connected to a generator.
Boesel started with three of Taggett’s retrofitted spin bikes for his Green Microgym, and then collaborated with Henry Works to build a system of four linked Team Dynamo stationary bikes outfitted with a small motor connected to a bank of batteries. As users pedal, the motor charges the batteries, which then power the TV and stereo system. A single exerciser might only generate 50 to 100 watts of electricity—100 watts would power a small TV—but all four bikes working together can generate about four times that much, depending on how hard each person is working.
But are Green Microgyms practical? Just how much energy do all those überfit exercisers produce? Certainly Boesel’s gym members are expending a great deal of effort during their workouts, but when it comes to harnessing that effort for a practical purpose, we must contend with the grim realities of thermodynamics. Boesel found that, as energy passes through the battery, some gets lost in the conversion process (entropy). So in practice, the battery pack option was less efficient in generating useful energy than a machine retrofitted with a grid-tie inverter, which sends the generated energy directly back into the power grid.
Taggett’s company obliged with its FireWheel InterGrid (FIG) system. Boesel plugs the machine into a wall socket as if it were a common household appliance. The inverter is a clever device commonly used in conjunction with solar panels, enabling those who install the panels to literally spin the meter backward and sell extra power back to their local power company. In this case, the system harvests some of the ambient heat normally emitted by exercise machines (due to friction) while in operation. The battery-based Human Dynamo system uses less than 50 percent of the current coming out of the machine, while the new updated version delivers back to the grid about 70 percent of the total watts produced.
Boesel is a personal trainer by profession, and