About four years ago, researchers in Michael Strano’s chemical engineering lab at MIT coated a short piece of yarn made of carbon nanotubes with TNT and lit one end with a laser. It sparkled and burned like a fuse, demonstrating a new way to generate electricity that produces phenomenal amounts of power.
At the time, no one understood how it worked, and it was so inefficient that it was little more than a “laboratory curiosity,” Strano says.
Now, Strano has figured out the underlying physics, which has helped his team improve efficiencies dramatically—by 10,000 times—and charted a path for continued rapid improvements. One day, generators that use the phenomenon could make portable electronics last longer, and make electric cars as convenient as conventional ones, both extending their range and allowing fast refueling in minutes.
The efficiencies of the lab devices made so far are still low compared to conventional generators. Strano’s latest device is a little over 0.1 percent efficiency, whereas conventional generators are 25 to 60 percent efficient.
But Strano says they could be useful in some niche applications, where a sudden burst of power is needed. And Strano says that the further improvements in efficiency mean broader applications could soon be feasible.
The new generators exploit a phenomenon that Strano calls a thermopower wave. The conventional way to generate electricity by burning a fuel is to use heat to cause expanding gases to drive a turbine or a piston. In Strano’s system, as the fuel burns along the length of his nanotubes, the wave of combustion drives electrons ahead of it, creating an electrical current. It’s a much more direct and efficient way to generate electricity, since no turbines or conventional generators are required.
Since the nanogenerator runs on liquid fuels—which store far more energy than batteries—there’s hope that they could allow electric cars to go much farther than they do now.
It’s a setup not unlike the one in an internal combustion engine, in which bursts of fuel are sprayed into combustion chambers to drive pistons. Power electronic circuits could take the bursts of power from several nanotube generators and smooth it out, using it to drive electric motors in a car, for example. The fuel tank could be refilled like one in a conventional car. And because the carbon nanotubes aren’t consumed in the process, they can be used over and over again.
Recently, Strano discovered that switching from nanotubes to flat sheets of nanomaterials—such as single-atom-thick graphene—improves efficiency. Shaping the sheets to direct the energy of the thermopower wave also boosts performance.