Harvesting of ambient energy can power small devices such as sensors and perhaps even wearable computing devices. This concept is just a miniscule application of what Nikola Tesla had in mind over a century ago, namely, harvesting energy from space and transmitting energy without wires over long distances. I am posting the following article here with the hope that perhaps it may inspire a reader to come up with a novel application for this concept.
Georgia Tech electrical engineering professor Manos Tentzeris and his team of graduate students have hit upon a technique to produce inkjet printable antennae to capture energy from ambient radio waves.
They have discovered a way to capture and harness energy transmitted by such sources as radio and television transmitters, cell phone networks and satellite communications systems. By scavenging this ambient energy from the air around us, the technique could provide a new way to power networks of wireless sensors, microprocessors and communications chips…Tentzeris and his team are using inkjet printers to combine sensors, antennas and energy-scavenging capabilities on paper or flexible polymers. The resulting self-powered wireless sensors could be used for chemical, biological, heat and stress sensing for defense and industry; radio-frequency identification (RFID) tagging for manufacturing and shipping, and monitoring tasks in many fields including communications and power usage.
The scavenging technology can take advantage presently of frequencies from FM radio to radar, a range spanning 100 megahertz (MHz) to 15 gigahertz (GHz) or higher…The researchers are utilizing inkjet technology to print these energy scavenging devices on paper or flexible paper-like polymers. The result would be paper-based wireless sensors that are self-powered, low-cost and able to function independently almost anywhere. To print electrical components and circuits, they use a standard materials inkjet printer. However, they add … silver nanoparticles and/or other nanoparticles in an emulsion. This approach enables them to print not only RF components and circuits, but also novel sensing devices based on such nanomaterials as carbon nanotubes.
The technique involves capturing the radio waves and rectifying the ac to dc and storing the power in capacitors or a small battery. Also, passive RFIDs (majority of which operate at 2.4 GHz, 5.8 GHz and some in the 800-900 Mhz range) already work on this principle of rectifying the radio wave power sent from the RFID reader unit. Thus, the real breakthrough here is in the inkjet printable antennae.
The antennae is basically a half wavelength dipole antennae, which is basically just two pieces of straight wires. In order for the antenna to be efficient, the length of the antenna has to be close to half of the wavelength of the radio wave.
The wavelength of a radio wave can be calculated by dividing the speed of light by its frequency. The size of the antenna is half the wavelength times a fudge factor of 0.95 to account for a 5% radiation damping. Thus, for the FM band, 87.8 MHz to 108 MHz, the size of the antenna varies from 1.6 m (64″) to 1.3 m (52″). For the VHF TV band, 30 MHz to 300 MHz, the antenna size ranges from 4.75 m (187″) to 47.5 cm (19″). In the UHF band, 0.3 to 3 GHZ, the antenna sizes range from 47.5 cm (19″) to 4.75 cm (1.9″). Microwaves are radio waves that have frequencies in between 300 MHz and 300 GHz which require dipole lengths from 47.5 cm (19″) down to 0.5 mm (0.02″). Cell phones, cordless phones, Wifis, Bluetooth devices, microwave ovens, etc. operate in the lower microwave band. Radars operate mostly in the 1 to 300 GHz range (dipole length of 14 cm/5.6″ to 0.5 mm/0.02″). It is easy to see that extremely small antennae are required for microwave frequencies, which also explains why RFIDs can be made very small and inconspicuous. Printable antennae would thus be most efficient harvesting the lower microwave frequencies. There is a lot of radio noise from various sources, as mentioned above, that can be safely harvested for powering small electronic devices.
We may soon see antennae printed onto clothing to power sensors and computing devices that we can wear with our clothing.