An expert on wireless technology figures out how to power devices without batteries.
The energy demands of wireless devices have held back the spread of cheap sensors that could be monitoring our homes, the environment, and physical infrastructure such as bridges. Shyam Gollakota has an ingenious solution—a way for these wireless devices to operate without batteries.
Gollakota’s prototypes use the fog of radio noise that surrounds us from TV stations, cell towers, and other sources as an energy supply and a means of communicating. By absorbing and reflecting those ambient signals, the devices can send messages to one another and even link to the Internet.
When Gollakota became an assistant professor in the wireless lab at the University of Washington in 2012, he joined a team already working on using ambient radio waves as an energy source. The group had found ways to power simple sensors such as those used for measuring temperature and humidity. But transmitting that data is more challenging. The researchers’ devices stored up the trickle of harvested power and occasionally sent out data using a transmitter.
Gollakota saw that a better solution lay in skipping the conventional, power-hungry transmitter. His battery-free devices—the latest prototype is half the size of a credit card—have antennas that switch between reflecting and absorbing ambient radio signals. In the absorbing mode, they collect enough energy to power chips, sensors, LEDs, and even black-and-white displays. In the reflecting mode, they scatter ambient radio signals in a way that nearby devices can detect. The design makes it possible to deploy battery-free sensors or other devices just about anywhere at low cost, Gollakota says.
His latest prototypes can send and receive signals over 20 meters and between different rooms in a building. They can also connect to the Internet, by communicating up to two meters over Wi-Fi with smartphones or home routers.
Gollakota believes that eventually his energy-scavenging designs will make it possible for ambient radio waves to power stripped-down devices. Many poorer parts of the world lack reliable electricity sources but have strong cellular coverage. Even “a primitive computer or device that can only send e-mails by harvesting these signals,” he says, could be valuable.