When the U.S. Navy decided it needed to monitor the condition of its aging aircrafts' wings, it tried embedding wireless sensors inside them. Each sensor, attached to energy-harvesting circuitry, periodically checks the wings for damaging stress and strain, says Zoya Popovic, a professor of electrical engineering at the University of Colorado, Boulder, who worked on the project. To recharge or activate the sensors, a technician holds a transmitter a meter away from a wing to create a low-energy electric field within range of the sensors' energy-harvesting circuits.
In solving this problem, engineers dusted off a decades-old idea: radio-frequency energy harvesting, be it from strategically placed transmitters or from the ambient energy emitted by cellphone towers and television stations. The concept was once dismissed as unfeasible because of the rapid dissipation of electromagnetic waves as they travel from their source. But even microwatts, if trickled into a battery or supercapacitor, can be enough to power some sensors for more than a decade. The combination of extremely low-power microprocessors, increasingly affordable supercapacitors for energy storage, and budding markets for sensors that make buildings more energy efficient and monitor inventory has enabled a new generation of energy-harvesting devices.
Typically, wireless sensors are designed to observe environments in a more flexible way than wired ones can--tracking cattle in the middle of a field, generating early warnings of impending earthquakes, and assessing the structural health of bridges, for example. But a sensor's power supply is the most confounding problem. Each option has its limitations: a battery alone has a short lifetime, and solar cells, the most common energy-harvesting technique, can't soak up photons from inside an airplane's wing.
The technology for harvesting wireless power is essentially based on radio-frequency identification, or RFID. Atransmitter sends a burst of radio-frequency energy that both carries information to a chip and can be converted to dc electricity to power it. A tag consisting of an antenna and a microchip responds by sending back data about the object it is attached to.
Turning those simple tags into fancier monitoring devices requires more power, so the RF energy would need to be captured and stored or transmitted continuously. Some food companies, for example, have begun tracking their delivery trucks more closely, according to one RFID technology company. Atruck outfitted with a transmitter can both recharge and query RFID-based sensors that periodically check temperatures inside the truck or perform antitheft surveillance. When the sensors detect a change in their environments, the tags relay that data back to the transmitter.
A key development has been a steady growth in the distances over which the tags can communicate. Power companies hoping to install sensors along electric power lines, for example, would like to gather data from those devices without having to check each one individually. Norman D. McCollough Jr., a director of technology at Hendrix Wire & Cable Inc., a power-distribution systems company in Milford, N.H., anticipates that his remotely located energy-harvesting sensors, which now can transmit data across 100 meters, will soon be able to return information to a transmitter more than a kilometer away. McCollough has been using a chip introduced in January that can both pick up lower-power signals and operate at a higher power to send data back over longer distances, thanks to a redesigned power-amplifying circuit.
With significantly more flexible sensor installations come new applications. For example, using energy-harvesting sensors to control thermostats in office buildings could improve the energy efficiency. To assess whether sensors in an office building could be powered ambiently, Marlin Mickle, an electrical engineering professor at the University of Pittsburgh, mapped the radiated power from the radio stations around his city. ”When we talk about ambient energy, the source is not under our control. But it's still out there, and we're showing that we can make it work,” Mickle says.
But even so, one essential problem remains: a reliable source of RF energy isn't always available. In addition to RF harvesting and attaching solar cells to microprocessors, engineers have explored converting temperature changes or mechanical movements--perhaps from vibrations or the flipping of a light switch--into electrical energy. Greg Durgin, an assistant professor of electromagnetics at the Georgia Institute of Technology, in Atlanta, thinks that ultimately the solution will be more complex.
”A really emerging area is getting hybrid power supplies to work on this,” Durgin says. Linking two energy-harvesting techniques on one sensor might finally make the devices truly independent.