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Ambient EM Energy Powers Small Devices

The environment is filled with ambient electromagnetic (EM) energy, which is transmitted from sources like cell-phone networks, satellite-communications systems, and radio and TV transmitters. At Georgia Tech Research Institute, researchers have developed a new way to scavenge this freely available energy. Once collected, it can then be used to power small devices, such as wireless sensors, microprocessors, and communications chips.

This research is being led by Manos Tentzeris, a professor in the Georgia Tech School of Electrical and Computer Engineering and faculty researcher in the Georgia Electronic Design Center. Tentzeris and his team are using standard materials and inkjet printers to fabricate sensors, antennas, and energy-scavenging circuits on paper or flexible polymers (see photo 1 and photo 2). Yet they add what Tentzeris calls "a unique in-house recipe," which contains silver nanoparticles and/or other nanoparticles in an emulsion. In addition to RF components and circuits, this approach allows the Georgia Tech team to print sensing devices based on nanostructures like carbon nanotubes.

The team's scavenging devices can capture energy from communications devices in the environment, convert it from alternating current (AC) to direct current (DC), and store it in capacitors and batteries. Currently, this scavenging technology can take advantage of frequencies ranging from FM radio to radara range spanning 100 MHhz to 15 GHz or higher.

Scavenging experiments utilizing TV bands have already yielded power amounting to hundreds of microwatts. Multiband systems are expected to generate 1 mW or more. That amount of power is enough to operate many small electronic devices, including a variety of sensors and microprocessors. By combining energy-scavenging technology with supercapacitors and cycled operation, the Georgia Tech team expects to power devices requiring above 50 mW.

In the team's approach, energy builds up in a battery-like supercapacitor. the energy is utilized when the required power level is reached. The researchers have already operated a temperature sensor using EM energy captured from a television station that was 0.5 km away. They are currently preparing another demonstration, in which a microprocessor-based microcontroller would be activated simply by holding it in the air.

Once available, these paper-based wireless sensors would be self-powered, low-cost, and able to function independently almost anywhere. As a result, they 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 fields like communications and power usage. Notably, a scavenging device could be used by itself or in tandem with other generating technologies. For example, scavenged energy could assist a solar element to charge a battery during the day. At night, when solar cells don't provide power, scavenged energy would either continue to increase the battery charge or prevent discharging.

Utilizing ambient EM energy also may provide a form of system backup. If a battery or solar-collector/battery package failed, scavenged energy could allow the system to transmit a wireless distress signal while potentially maintaining critical functionalities. Tentzeris notes that the technology exploits a range of EM bands, thereby increasing the dependability of energy-scavenging devices. If one frequency range fades temporarily due to usage variations, the system can still exploit other frequencies.

A presentation on this energy-scavenging technology was given July 6 at the IEEE Antennas and Propagation Symposium in Spokane, WA. This finding is based on research supported by multiple sponsors, including the National Science Foundation, the Federal Highway Administration, and Japan's New Energy and Industrial Technology Development Organization (NEDO).

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