A cell phone that never needs recharging might sound too good to be true, but Nokia says it's developing technology that could draw enough power from ambient radio waves to keep a cell-phone handset topped up.
     Ambient electromagnetic radiation--emitted from Wi-Fi transmitters, cell-phone antennas, TV masts, and other sources--could be converted into enough electrical current to keep a battery topped up, says Markku Rouvala, a researcher from the Nokia Research Centre, in Cambridge, U.K.
      Rouvala says that his group is working towards a prototype that could harvest up to 50 milliwatts of power--enough to slowly recharge a phone that is switched off. He says current prototypes can harvest 3 to 5 milliwatts.
      The Nokia device will work on the same principles as a crystal radio set or radio frequency identification (RFID) tag: by converting electromagnetic waves into an electrical signal. This requires two passive circuits. "Even if you are only getting microwatts, you can still harvest energy, provided your circuit is not using more power than it's receiving," Rouvala says.
     To increase the amount of power that can be harvested and the range at which it works, Nokia is focusing on harvesting many different frequencies. "It needs a wideband receiver," says Rouvala, to capture signals from between 500 megahertz and 10 gigahertz--a range that encompasses many different radio communication signals.
      Historically, energy-harvesting technologies have only been found in niche markets, powering wireless sensors and RFID tags in particular. If Nokia's claims stand up, then it could push energy harvesting into mainstream consumer devices.
      Earlier this year, Joshua Smith at Intel and Alanson Sample at the University of Washington, in Seattle, developed a temperature-and-humidity sensor that draws its power from the signal emitted by a 1.0-megawatt TV antenna 4.1 kilometers away. This only involved generating 60 microwatts, however.
     Smith says that 50 milliwatts could require around 1,000 strong signals and that an antenna capable of picking up such a wide range of frequencies would cause efficiency losses along the way.
     "To get 50 milliwatts seems like a lot," adds Harry Ostaffe, head of marketing for Pittsburgh-based company Powercast, which sells a system for recharging sensors from about 15 meters away with a dedicated radio signal.
     Steve Beeby, an engineer and physicist at the University of Southampton, U.K., who has researched harvesting vibrational energy, adds, "If they can get 50 milliwatts out of ambient RF, that would put me out of business." He says that the potential could be huge because MP3 players typically use only about 100 milliwatts of power and spend most of their time in lower-power mode.
     Nokia is being cagey with the details of the project, but Rouvala is confident about its future: "I would say it is possible to put this into a product within three to four years." Ultimately, though, he says that Nokia plans to use the technology in conjunction with other energy-harvesting approaches, such as solar cells embedded into the outer casing of the handset.