One concept I love is recovering waste energy. Every time I walk somewhere, some of the energy I put into walking gets wasted. When buildings and bridges sway, that is energy ripe for harvesting. But getting that energy into a useful form turns out to be difficult

The big reason is scale. Buildings, bridges, and people all get rid of excess energy with characteristic mechanical motions. If you want to collect that energy, you need an oscillator with a similar characteristic motion—the same frequency and a similar amplitude. That means you need a pendulum the size of a building to collect waste energy from the building. Which doesn't seem like such a cool idea. A group of researchers have recently shown how a child's toy—the yoyo—can turn these low frequency vibrations into fast rotational motion, allowing for efficient energy conversion without the enormous size.

Let's imagine that we want to get power from a swaying building. The basic device consists of a weight hanging from a cable that is wrapped around a spindle. The spindle itself is attached to a coiled spring. The whole spindle and spring assembly is attached to the building and a generator for power generation. As a building sways, the spindle moves relative to the mass, allowing the spindle to wind and unwind. The unwinding motion winds the coil of the spring, while the winding motion turns the generator.

Still, you can't choose just any mass, spring, and spindle. If you recall your days playing with a yoyo, you will remember that you had to move with just the right frequency to get the yoyo into a large amplitude motion. That frequency depends on the mass of the yoyo and the diameter of the spindle. In the case of this power conversion system, the spring also changes the resonant frequency. By choosing the right ratio of mass, spring constant, and spindle diameter, a specific resonant frequency can be chosen.

The researchers demonstrated this by putting the device on a vibrating table and showed that they could tune the resonant frequency to optimize energy extraction. Their results show that they could get about 40W of power out of the generator, but this number is hard to interpret for a number of reasons. First, we don't know how much power was put into the vibrating table. We don't know how much of that power was actually in the vibrational motion of the table—the rest being dissipated as heat and noise. Finally, even if most of the energy was in vibrational motion, we don't know how it was distributed in the frequency spectrum, meaning what percentage was available for harvesting.

Nevertheless, this is a very clever idea, and I can imagine hanging these things off of bridges and buildings quite nicely. I can't, however, really see them being attached to humans.

Applied Physics Letters, 2011, DOI: 10.1063/1.3644130