Harvesting ambient radio waves for low power sensing and computing
Radio frequency (RF) signals provide a near ubiquitous energy source due to the large number of TV, radio, cellular, and Wi-Fi transmitters that proliferate our urban environments. While the traditional use of RF transmission is for data transfer, it is possible to harvest, convert, and store this energy for use in a variety of applications. In our first experimental demonstration of this technology we show a commercially available temperature and humidity meter (including LCD display) that is exclusively powered from ambient RF signals transmitted by the TV tower seen in the background, at a distance of 4.1 km.
TV stations transmit power radially in all directions which allows RF energy harvesters to operate over a large region, on the order of hundreds of square miles. Since these signals disperse radially, the resulting amount of power available to the receiver decrease as the distance from the TV tower increases. This is described by the Friss transmission equation.
As a result RF harvesting application need to adapter thier computational and sensing works loads to accommodate the available power. One major benefit is that ambient RF power is independent of weather condition.
Over the last few decades innovations in integrated circuit manufacturing has dramatically decreased the amount of power consumed by electronic devices. Often referred to as Moore's law (or Koomey's Law), if the number of instructions per second is held constant, the amount of power required to execute the same number of instructions decreases by half approximately every 2 years.
Using the Friis transmission equation and Moore’s Law it can be seen that since the required power to execute a fixed amount of computation is halved every two years, then the distance that same computational work can be powered will double every four years. This means that the operating range of RF powered devices will steadily increase over time.
The WARP node is a wireless sensor node that is completely powered off of harvested RF energy. This is accomplished by extending the RF energy harvester by adding the capability to sense temperature and ambient light levels, perform computation with an ultra low power microcontroller, and communicate wirelessly with a 2.4 GHz radio. The first target application is a weather-monitoring node, which receives RF power from a TV transmitter and wirelessly reports that data back to a host computer. Improvements to the RF energy harvesting have increased the operating range of the node to 10km from the TV tower.
The New York Times: "Bye-Bye Batteries: Radio Waves as a Low-Power Source" - June 18, 2010.
The Economist: "Power From Thin Air" -June 10, 2010.
New York Times: "Smart Dust? Not Quite, But We're Getting There" - Jan 30, 2010.
This project encompasses a number of efforts in developing energy harvesting, battery free sensing systems that can be easily embedded into everyday objects and thus allowing for near perpetual operation. Topics include ambient energy harvesting techniques, platform architecture and power management, and debugging tools that deal with intermittent power.
The WISP is a programmable, battery-free sensing and computing platform designed to explore sensor-enhanced UHF RFID applications. This open-source platform communicates with and harvests all its power from commercially available UHF RFID readers. As part of Intel Research’s WISP Challenge 500 WISPs have been donated to over 50 universities worldwide.
This project originated at Intel Research, Seattle (which has since closed) and the research has continued in Sensor Systems Lab led by Joshua Smith at the University of Washington.