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Trillions of sensors are in our future, and they will need energy. Batteries are routinely used to power tiny devices, but there are other options. Piezoelectricity, the technology that converts mechanical energy into electricity, is gaining attention these days because it can scavenge energy from movement or vibrations.
For this reason, Carnegie Mellon University researchers are exploring the use of piezoelectricity for smart city applications. Smart cities of the future will rely on massive sensor networks, and the sensors in these systems need energy. Continually replacing sensor batteries would be extremely time consuming and produce waste materials that would be difficult to dispose.
"It would be a lot more efficient if you could just live off of scavenged energy. You eliminate batteries and their problems, and instead you harvest energy," said Gianluca Piazza, a professor of electrical and computer engineering and the director of the John and Claire Bertucci Nanotechnology Laboratory.
While other researchers extract energy from solar, heat, and mechanical vibrations, Piazza's team focuses on powering devices with ultrasound. They launch sound waves that transfer over relatively long distances and are captured by tiny piezoelectric devices co-located with sensors, and hence, remotely powering the sensors.
"So you have a power source somewhere, and you have all the sensors. Whenever you need to power them or interrogate them, you just send this blast of sound waves to them. They receive it, and they turn on," Piazza said.
Because these sound waves are a bit more than 40 kilohertz — right above the audible range — they do not bother humans or animals. They can efficiently transmit over 10-30 meters, which is around 30-100 feet.
Piazza's research currently is designed for indoor applications. Take a conference room as an example. A large speaker would send out sound waves to sensors distributed in the room. These sensors, which are about the size of a grain of sand, have membranes that vibrate and generate a charge when they receive the waves.
"It's like the same way when you're moving your foot in your shoe, you're actually stimulating the piezoelectric material and generating a charge," Piazza said.
Piazza's system generates enough electricity to power small radio devices that send and receive signals. Currently, the power source that launches the sound waves needs to be plugged in. Piazza's team would like to further develop the system so they can launch sound waves without the need of plugged-in units. To this end, researchers at Carnegie Mellon and elsewhere are exploring novel piezoelectric materials that can be used to harvest energy, which could be beneficial for indoor communications, smart infrastructure, and implantable or wearable devices.