Communications of the ACM
Energy Harvesting Extends Life of Small Devices
The types of energy typically available to harvest for low-power applications.
Credit: MaximIntegrated.com
No matter how regal your mobile phone, poor battery life makes peasants of us all. One problem is power supply: who hasn't been stuck with a dead phone and no place to plug it in? The maturity of high-efficiency solar power has partly solved this problem, but on-board solar still cannot charge a pocketed phone.
Enter energy harvesting, which collects energy from motion and pressure ("piezoelectric"), ambient heat ("thermoelectric"), and electromagnetic waves, in addition to solar power. The amount of energy captured by small harvesting devices is typically fairly small, on the order of milliwatts or even microwatts, but that is sufficient for certain applications, such as wireless sensor networks, radio-frequency identification (RFID), and implanted medical devices. Some larger-scale harvesting methods may find applications for portable devices.
Tiny Charges, Big Solutions
Will we ever have cellphones that recharge themselves from being jangled in your pocket? Harry Zervos, a principal analyst at market research firm IDTechEx, says no. "That would only happen a very long time away from now, which is practically never."
However, the convergence of three trends gives him hope. "First, energy storage technologies are becoming more competitive and better in terms of power density," he said at the company's recent Energy Harvesting & Storage USA conference in Santa Clara, CA. "Electronics are also becoming lower-powered, and at the same time, energy harvesting technologies are increasing in power that they can harvest. So you could eventually have a smartphone with lower power requirements, better storage in the battery, and energy harvesting incorporated in it."
Tim Shannon, senior field application engineer at embedded-systems company Spansion, echoes Zervos' analysis. Shannon has greater optimism for the undying-phone scenario. "We're increasing our ability to get energy from our environment," he says, adding that he is enthusiastic about new developments in fractal antennas to capture energy from radio waves. "And although cost is what drives lithography shrinks [to get more chips out of a wafer], that's actually inadvertently changing the energy profile."
Powering Implants
Others beg off the phone question for now, preferring to focus on goals that are smaller, if no less important. Tom Snyder, industrial program liaison at North Carolina Status University for the National Science Foundation (NSF) Nanosystems Engineering Research Center (NERC) for Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST), sees medical uses as applications that are both useful now, and important for energy harvesting's long-term development. "Health monitoring-type instruments are going to be a nice stepping stone towards powering a mobile phone," he says, "because you can develop some of these sensors at much lower power levels."
John Huggins, executive director of the university/industry cooperative Berkeley Sensor & Actuator Center (BSAC), points to another advantage of energy harvesting for medical purposes. "A device that's ingested or implanted will need a small quantity of power," he says. "If you can't get in to replace the battery, you might look at energy harvesting, perhaps using electrochemical power."
Challenges
Harvested energy generally comes from sources that are far less stable and predictable than the electrical grid, so devices that use it need some kind of power storage to hold it. Traditional batteries are ill-suited, for a variety of reasons. As ASSIST center director Veena Misra points out, "The challenge is to provide both high energy density and high power density. Typical batteries are really good at providing high voltage, but not a lot of current."
Fortunately, supercapacitors provide high current, although typically with less voltage than batteries. Charles Greene, chief technical officer at PowerCast Corp., a company dedicated to bringing "remote, wireless power capability to micro-power devices such as wireless sensors, data loggers, active RFID and RTLS tags, and more," also "particularly likes" supercapacitors for energy harvesting applications "because you can take them to zero volts. If you take a battery to zero volts, you've ruined the battery." In addition, supercapacitors can tolerate the frequent charge-discharge cycles imposed by energy harvesting's erratic energy sources.
Is that vision of the self-powering mobile phone doomed? Not necessarily, says Misra. "Certain types of body harvesting of such things as large-range motion can actually generate watts of power. But they require a lot of effort on the part of the human being."
The harvesting potential of portable devices goes way up when you can offload it to something else that people carry with them every day: their clothes. Spansion's Shannon, who is enthusiastic about using fractal antennas to capture energy from ambient radio-frequency waves, points out that such antennas are flexible, and could be woven into clothing. Misra says a torso-sized panel could harvest "quite a bit of power with that size, if you assume it's got thermoelectric and piezoelectric [harvesting capabilities}, and a solar panel."
Even without wearables, "If you can get a harvesting device on the market that gives you 75 microwatts, then you'll have a group of engineers who'll design the load size to work at that available level," says Misra. "I think energy harvesting can drive a lot of the innovation on the consumption side."
Tom Geller is an Oberlin, Ohio-based writer and documentary producer.
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