For decades, wind turbines have grown ever larger, chasing greater heights and economies of scale. As cheap as this has made wind-produced electricity, some people want to head in the opposite direction, making wind-powered devices so small you can carry them with you.
In Cell Reports Physical Science, the Beijing Institute of Nanoenergy and Nanosystems' Ya Yang has announced a device that can work in wind velocities of just 1.6 meters per second (3.6 mph). On a still day, the motion of a brisk walker produces airflow greater than that. “We once placed our nanogenerator on a person's arm, and a swinging arm's airflow was enough to generate power," Yang said in a statement.
Rather than a turbine, the invention is a triboelectric nanogenerator, meaning the electrification comes from an insulator and another surface separating. The prototype, known as B-TENG, produces just 2.5 milliWatts even in a strong 8 ms-1 wind, a billion times less than many turbines. On the other hand, its face is smaller than a credit card and could still briefly light 100 LED lights in series and charge a powerful capacitor in three minutes.
The B-TENG doesn't operate like a scaled-down version of the horizontal-axis turbines that now produce 5 percent of the world's electricity. Instead, it is made of two plastic strips, each carrying a silver electrode, constrained inside a tube. When air flows through the tube, the strips slap against each other and separate again. The strips of plastic become charged, like the static electricity generated from shoes rubbing on certain carpets. Instead of being released in a short spark, however, the charge is turned into alternating current based on the strips' cycle of coming together to swap charges before parting again.
Small-scale solar collectors have demonstrated the immense benefits of electricity generation in places where the grid connection is difficult. They've given hundreds of millions of people access to safe and economical lighting for the first time, as well as a way to charge mobile phones.
While solar dominates this market, many inventors have wondered if there aren't niches for alternative methods of harvesting energy from the environment, particularly for places where sunlight is scarce in winter. Everything from the kinetic energy of raindrops to evaporation has been given a run.
"Our intention isn't to replace existing wind power generation technology. Our goal is to solve the issues that the traditional wind turbines can't solve," Yang said. Besides the potential to work at an exceptionally small scale, Yang notes the B-TENG can be made largely from cheap plastics, although it currently uses some materials unsuited to commercial-scale versions.
In optimum winds, the B-Teng harvests 3.2 percent of the available wind energy, a far cry from large-scale wind devices, but exceptional by the standards of similarly small devices. Yang hopes to scale it up so fields of them can be positioned on roofs to power whole buildings where solar is unsuitable.