Researchers at Georgia Tech have found a way to harvest energy from electromagnetic waves in the air. The harvesting devices are produced using an inkjet printer and can collect small amounts of power from a wide band of frequencies--everything from FM radio up to radar.

The technology isn't new—researchers have floated concepts (and a few devices) that can harvest energy from ambient WiFi signals and other small sources, but these are usually able to pull power only from tiny slices of the electromagnetic spectrum (perhaps just a few KHz). The new system can draw energy from much wider electromagnetic swaths: 100MHz to 15GHz.

Even better, the sensors that harvest the energy are simple to make. To print the circuits on paper or paper-like polymers, the researchers use an inkjet printer and add an emulsion of nanoparticles. Circuits printed on polymers are currently less advanced, but the scientists say they have a wider range and can harvest energy from frequencies up to 60GHz.

Gadgets such as cell phones could one day use residual radio signal to supplement their own batteries, but the amounts of energy harvested are small (on the order of 50 milliwatts) and the system won't currently make even small consumer devices self-sustaining.

However, if the energy is allowed to build up in a small capacitor, it could temporarily power low-energy intermittent devices like temperature sensors or could supplement other energy-gathering mechanisms like solar panels. The harvesters could also function as mission-critical stopgaps, allowing a system to maintain essential functions or send out a distress signal until it can be fixed.