Using Wi-Fi to power the IoT
The power-over-Wi-Fi system – or "PoWiFi" for short – is a University of Washington (Seattle, WA) research project aimed at powering the Internet of Things (IoT) using Wi-Fi signals. Its concept is that Wi-Fi receivers could, in addition to retrieving the information being transmitted over Wi-Fi, be designed to harvest the energy in these signals as well.
The biggest hurdle in achieving this is the discontinuous nature of normal Wi-Fi signals – data is often broadcast in bursts, on a single channel. Initial testing with a temperature sensor that had been fitted with a Wi-Fi antenna showed that while voltage across the sensor often came close to the 300 mV needed for the sensor to operate it was not enough.
To get around this, the researchers injected non-intrusive "power traffic" onto multiple 2.4-GHz Wi-Fi channels to increase channel occupancy – while minimally impacting network performance – so as to allow energy harvesting across multiple channels. They also designed a multi-channel harvester to retrieve the energy from the resulting transmissions, from both the artificial and normal traffic across three non-overlapping channels, and to allow for continuous power delivery.
Using this approach, the researchers found that the temperature sensor could operate at distances of about 6 meters from the router – a distance that increased to 9 meters when a rechargeable battery was added. A further test with a Wi-Fi antenna-enabled low-power camera equipped with a low-leakage storage capacitor showed that it could operate at up to about 5 meters without a battery, and about 7 meters with a battery.
Also demonstrated was the remote battery recharging of a Jawbone fitness tracker. Once equipped with the PoWiFi antenna and placed in the vicinity of a PoWiFi router, the device’s coin cell battery was able to be brought to 41% charged from a no-charge state in 2.5 hours.
According to the researchers, the impact of the system on normal Wi-Fi traffic and performance was minimal to users on the networks tested. For more, see "Powering the Next Billion Devices with Wi-Fi" (PDF).