With the temperatures we’ve had this summer here in Washington, D.C., it’s hard to think of anything but the heat. I only got as far as cranking up the AC, but researchers in Georgia have come up with a device that can make electricity from waste heat that car engines, computers, power lines or even the sun can produce.
Even more remarkable, a Greek philosopher’s discovery that he made 2,300 years ago is the basis for the “pyroelectric nanogenerator” that the researchers describe in a paper that appears in Nano Letters.
When machines convert fuel into electrical or mechanical energy, some of that energy goes to waste as heat. For instance, when gasoline ignites in the cylinder of a car’s engine, the explosion produces energy that pushes a piston, which drives a crankshaft, which turns the axles and the wheels. But only about 20 percent of the explosion’s energy helps to move the car; much of the remaining energy is wasted heat – that’s why cars need radiators and coolant.
Zhong Lin Wang and the other authors of the Nano Letters paper report that the U.S. loses more than 50 percent of the energy that it produces each year—much of it becomes wasted heat.
They found a possible solution for that inefficiency in something called the pyroelectric effect, a phenomenon that the Greek philosopher Theophrastus first described in 314 B.C. Theophrastus was a student and successor of Aristotle at his school in Athens, where he studied and wrote about everything from biology to ethics. In “On Stones” he noted that the semi-precious stone tourmaline attracted small bits of straw when heated.
It wasn’t until the late 19th century that scientists began to understand why that happened. When someone heats tourmaline or other pyroelectric materials, the arrangement of the atoms within the material’s crystal structure changes. This rearrangement creates an imbalance of electrons, just like the static charge that builds up when you rub a balloon against your hair – and it attracts stray hairs or straw for the same reasons.
To ease that imbalance, electrons will flow from one part of the material to another. Moving electrons create an electrical current, one which Wang and his colleagues realized they could harness.
To do that, they made a sort of forest of nanowires standing on their ends, each about twice as tall as the pits etched into a CD. The researchers made the wires from zinc oxide, a white compound with pyroelectric properties. It’s also used in paints, plastics, electronics, food and maybe most famously as one of the original sunscreens.
The researchers report that when they heated or cooled the nanogenerator, it produced electricity. It could conceivably take advantage of regular heating and cooling cycles, like when someone powers cars or computers on or off. Wang even suggested the devices could make electricity from daily fluctuations in temperature from day to night.