We've cut the cord for communication, thanks to Bluetooth and Wi-Fi, but charging our little pocket supercomputers still takes a tether. Judging by the range of wireless charging technologies in the works, that might not be the case for much longer. A team from the University of Washington has demonstrated how lasers could be used to charge a device from across the room.
The team mounted a power cell on the back of a smartphone and hit it with a narrow laser beam in the near-infrared part of the spectrum. From a distance of 4.3 m (14 ft), the laser was able to deliver 2 W of power to a 97-sq cm (15-sq in) area, charging the phone about as quickly as a regular old USB cable.
The laser emitter is designed to automatically sense when a phone is ready to be charged, while the smartphone was programmed to send out high-frequency "chirps" inaudible to the human ear that tells the emitter where it is.
"This acoustic localization system ensures that the emitter can detect when a user has set the smartphone on the charging surface, which can be an ordinary location like a table across the room," says Vikram Iyer, co-author on a study describing the device.
A couple of safety measures were built into the system. To keep people from walking through the invisible beam, four low-power "guard beams" were also emitted surrounding the charging laser. These bounce off retroreflectors around the power cell, and reflect back to photodiodes on the emitter itself. If at any point that signal is interrupted, the system instantly shuts off the charge beam.
"The guard beams are able to act faster than our quickest motions because those beams are reflected back to the emitter at the speed of light," says Shyam Gollakota, co-author on the study. "As a result, when the guard beam is interrupted by the movement of a person, the emitter detects this within a fraction of a second and deploys a shutter to block the charging beam before the person can come in contact with it."
To dissipate the excess heat generated by the charging beam, the researchers used strips of aluminum as a heat sink around the power cell. Not all of that heat was wasted though – a small thermoelectric generator harvested some of the energy and fed it back into the phone's battery.
According to the team, future iterations of the system could deliver power to an area up to 100 sq cm (15.5 sq in) from as far as 12 m (39 ft) away, potentially charging devices placed anywhere on a table.
But we still have to question just how practical this technology would be. Currently, commercial wireless chargers require phones to be placed on a mat (or a special piece of Ikea furniture), which is fine for topping up overnight but not much help if you want to use the device while it's charging. This laser emitter system doesn't seem to solve that problem – especially if the beam shuts off anytime you reach for the phone.
For wireless charging to really reach the realms of ubiquity enjoyed by wireless communication systems like Wi-Fi, it needs to fill a room or a wide area without us thinking about it. Strides are being made in that direction though. The Pi charger emits a short-range bubble around itself, which can wirelessly charge multiple nearby devices while they're in use. Duke University has outlined a wall-mounted panel that achieves something similar, and an ambitious Disney Research project managed to turn a whole room into a wireless charger.
The new study was published in the journal Proceedings of the Association for Computing Machinery on Interactive, Mobile, Wearable & Ubiquitous Technologies.
Source: University of Washington
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