A device that can simultaneously harvest energy from both the sun and wind might one day help generate power for "smart cities," researchers say.

Cities are growing smarter as networks of electronics help them monitor and control infrastructure and services. Ideally, these devices would be powered by renewable energy sources such as the sun and wind. Solar energy can come from rooftops and into even windows. However, large amounts of wind energy often gets wasted in cities—conventional wind turbines are usually not suited to urban areas because of their size.

Now scientists at the Beijing Institute of Nanoenergy and Nanosystems and the Georgia Institute of Technology have developed a flat device that can harvest energy from both the sun and wind at the same time. Instead of relying on wind to spin a rotor, the device instead makes use of the triboelectric effect, the same effect behind everyday static electricity. When two different materials repeatedly touch and then separate, the surface of one material can steal electrons from the surface of the other, building up charge.

The researchers coupled a triboelectric nanogenerator with silicon-based solar cells. The triboelectric nanogenerator consists of thin sheets of plastic and Teflon separated by air. When wind blows on the hybrid device, the plastic film vibrates toward and away from the Teflon, generating triboelectricity.

The device the researchers created is about 120 millimeters long and 22 mm wide, making it about as long and wide as a candy bar. However, at 4 mm deep, it is only about as thick as a windowpane. "The device could be extensively installed on the roofs of city buildings," says study co-author Ya Yang at the Beijing Institute of Nanoenergy and Nanosystems.

In experiments, the generator could deliver up to about 8 milliwatts of solar power and up to 26 mw of power from the wind. It could charge a lithium-ion battery from 0.2 to 2.1 volts in 10 minutes, and could also power the kind of temperature and humidity sensors one might find in a smart house, the researchers say.

Future research will seek to get a constant voltage of about 5 volts from the device, Yang says. The scientists detailed their findings in the online5 May issue of the journal ACS Nano.