Freevolt Grabs Energy from the Air
Tom Lombardo posted on October 03, 2015 |
The Low-Energy Internet of Things may soon be powered by stray radio signals.

I first learned about energy harvesting as a teenager, when my parents gave me an electronics hobby kit. One of the projects was a “crystal radio” - basically an antenna, a resonant tuning circuit, a diode to demodulate the AM radio signal, and an earphone. I was fascinated by the fact that the circuit could extract energy from thin air, but disappointed that it was such a tiny amount of energy that it only allowed me to listen to local AM stations through an earphone. The geek in me loved the science behind it, but my inner music freak was less than impressed by the limited functionality. A scientific oddity at best, but maybe someday...

Catch the Waves

These days the air is filled with radio frequency (RF) energy from WiFi, GPS, satellite, and cell phone signals, in addition to the AM, FM, and TV waves that drifted around during my childhood. Radio waves that are not picked up by their intended recipients bounce around, are gradually absorbed by walls or other objects, and their energy is eventually dissipated in the form of heat. (It’s a tiny, insignificant amount of heat, but energy doesn’t simply disappear.) Tiny or not, that’s wasted energy, and like surfers, electrical engineers hate to let a good wave go untapped. Case in point:

Low Power Electronics

In order to extend battery life on our numerous gadgets, engineers have developed very low power electronic devices and intelligent circuits that remain in ultra-low power standby mode until awakened by an external event. In fact, some of the circuits use so little power that they don’t require batteries at all. Instead, they harvest “free” energy from a variety of sources, including vibrations, temperature differentials, sunlight, human motion … and now, the multitude of radio waves that fly around us every day. While the concept isn’t exactly new, it’s been somewhat impractical and costly.

Engineers at Drayson Technologies hope to change that with their patented Freevolt RF energy harvesting device. The Freevolt captures ambient RF waves and converts them to DC electricity. Very low power devices can run exclusively on the energy extracted by the Freevolt. Higher power devices may need a battery or supercapacitor as their energy source; in those cases the Freevolt provides enough power to constantly trickle-charge the batteries or supercaps, allowing the products to run almost indefinitely. This is ideal for remote applications like smart sensors on bridges or buildings, and for home safety products such as smoke detectors and security sensors.

Image courtesy of Drayson Technologies

The graph above shows typical power densities from a variety of ambient energy sources. Most of these sources are intermittent and the conversion efficiencies are relatively low. That’s adequate for some applications, but when a steady source of power is needed, RF may offer the most reliability, assuming it can be harvested efficiently. That’s exactly what the Freevolt is designed to do.

Freevolt consists of an antenna, an impedance-matching network to assure maximum power transfer, a rectifier, a filter, and a power management module. The antenna and rectifier can be tuned for a variety of applications and RF conditions. For example, if the end product is in a building with a lot of WiFi traffic, the circuit can be tuned to that range of frequencies.

Image courtesy of Drayson Technologies

Drayson will soon offer a Freevolt development kit for low power devices that run on harvested RF energy. The first commercial product to license Freevolt technology is the CleanSpace Tag, a portable air pollution sensor.

For more information on Freevolt’s underlying technology, please see Drayson’s white paper.

Recommended For You