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Intel demos RF energy harvester

Posted: 29 Jan 2009     Print Version  Bookmark and Share

Keywords:energy-harvesting  RFID reader  RF energy  RF scavenging 

RF energy harvester

Researchers at the Intel Research Seattle Lab have disclosed details of an ambient RF energy-harvesting scheme that scavenged 60µW, or enough energy to drive a thermometer/hygrometer and its associated LCD, from a TV tower at a distance of 4.1km.

The demonstration of RF scavenging, called Wireless Ambient Radio Power (WARP), broadens the range of potential ambient energy sources, which already includes vibration, solar and heat. It was shown as another application of the Wireless Identification and Sensing Platform (WISP), a platform for sensing and computation that is powered and read by a commercial off-the-shelf UHF RFID reader. Each WISP consumes 2µW to 2mW and can be operated at distances of up to several meters from the reader.

According to Joshua Smith, the presenter and co-author of the Rawcon paper, "Experimental Results with Two Wireless Power Transfer Systems," a WISP is essentially an RFID tag with a microcontroller on it. In this case, a Texas Instruments MSP430.

As advanced as it may sound, the key enabler behind the WISP is not so much nifty circuit design or an extraordinary breakthrough in component physics. Instead, said Smith, it's directly attributable to Moore's Law, whereby the increased integration and falling power consumption of digital circuitry has enabled improved functionality per microwatt of scavenged energy.

The improvements are advancing at such a rate that, "The range at which you can power a device [with a given amount of ambient RF energy] should double every four years," said Smith, who may well have just stated his own new Law, in the context of RF energy harvesting.

Each WISP comprises an antenna, impedance-matching components, RF power harvester, demodulator to extract reader-to-WISP data, backscatter modulator for WISP-to-reader data, voltage regulator, programmable MCU (the MSP430) and optional external sensors. The harvester itself comprises a four-stage charge pump.

Powering TV towers
To date, the WISP has been used for a variety of sensing and other applications, including accelerometers, temperature, strain gage, capacitance and a custom neural amplifier. However, the Rawcon discussion was the first to focus on TV broadcast RF energy.

From a balcony at the Intel Research Seattle lab the researchers harvested RF power from the KING-TV tower 4.1 km away, which broadcasts 960kW ERP on channel 48, at 674- to 680MHz. Energy was collected using a manually oriented broadband log periodic antenna (5dBi) designed for TV applications and a 4 stage power harvesting circuit of the same design as WISP, but with a front end tuned to the desired channel. Across an 8kΩ load the team measured 0.7V, corresponding to 60 microwatts of power harvested. According to Smith, that's sufficient to drive many of the WISP's sensing applications. In this case, it drove a thermometer/hygrometer and its LCD display.

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