Radio Frequency Energy Harvesting

By: Bryn Dixon


What is radio frequency energy harvesting?

Radio frequency energy is emitted by sources that generate high electromagnetic fields such as TV signals, wireless radio networks and cell phone towers, but through using a power generating circuit linked to a receiving antenna this free flowing energy can be captured and converted into usable DC voltage. Most commonly used as an application for radio frequency identification tags in which the sensing device wirelessly sends a radio frequency to a harvesting device which supplies just enough power to send back identification information specific to the item of interest. The circuit systems which receive the detected radio frequency from the antenna are made on a fraction of a micrometer scale but can convert the propagated electromagnetic waves to low voltage DC power at distances up to 100 meters. Depending on concentration levels which can differ through the day, the power conversion circuit may be attached to a capacitor which can disperse a constant required voltage for the sensor and circuit when there isn’t a sufficient supply of incoming energy. Most circuits use a floating gate transistor as the diode which converts the received signal into generated power but in some cases which require a higher generated voltage for their specific application, a capacitor is linked to the drain of the transistor and a second floating gate transistor linked to a second capacitor can enable a higher output voltage once the capacitors reach full potential.


What is the power production potential?

The main technological advancement that has allowed these harvesting devices to generate sufficient power is through the development of receivers which can sense broad ranges of frequencies, not just limited to television UHF signals, while capturing the highest concentration of generated waves. Since the propagation of wave energy dissipates with distance from source, sensors located at far distances must extract energy at low power density which is related to the distance through 1/d2.  A typical television antenna linked to the correct conversion circuit if located approximately 4 meters from the source of radio frequency generation of 677 mega Hertz and 960 kilowatts of effective radiation power is able to produce a detected voltage of 0.7 across an 8 kilo ohm load which is approximately 60 microwatts of harvested power and enough to power an LCD display thermometer. Most circuits require a generated voltage more than 0.3 volts to sufficiently convert all incoming electromagnetic waves meaning the required incoming power from the source of generation must be higher than 1 milliwatt, a requirement that Nokia researchers have easily reached with their radio frequency converting cell phone that currently harvests 5 milliwatts of power from radio waves in the air. They potentially could increase the power harvested to around 20 milliwatts in utilizing higher quality transistors and circuits. The energy harvested is enough to slowly recharge the supplied lithium ion battery when not in use and with a close relation of the antenna to the source of energy generation, the amount of harvested energy can dramatically rise.  


What fields can this be implemented?

Most fields using this upcoming technology rely on sensors located in remote, dangerous or sensitive areas that require maintenance free power at low voltages depending on field of application. Many sensors which require these radio frequency harvesting devices are located within a structure in which battery replacement would create structural failure or in a manufacturing process in which a mass quantity of sensors are utilized and individual battery servicing becomes impractical. Many sensors which use these radio frequency harvesters also link to a rechargeable battery in the case that electromagnetic energy generation ceases but sensor function is imperative. In the biomedical industry, implants use this source of energy generation since it can prolong the life and maintenance of the implant while diminishing the possibility of contamination and instability associated with implanted batteries. Already used as a type of barcode information system with the use of radio frequency identification devices, a few companies have created harvesting systems which use a basic TV antenna linked to power conversion circuitry. Specifically, Intel has created a harvesting device which uses a UHF antenna linked to a conversion circuit similar to that found in radio frequency identifying tags and can generate power close to that of a 1.5 volt AAA. Nokia has also developed an integrated ambient electromagnetic energy harvesting device within a cell phone that can charge the standard battery during standby mode as long as there is a suitable amount of energy generated from the nearby source.