Powering the revolution: tiny gadgets pick up energy for free

Science News,  June 2, 2007  by M. Mitchell Waldrop

• E-mail
• Print
• Link

A few years ago, just to show that they could do it, Paul Wright and his mechanical engineering students mounted a temperature sensor under a staircase at the University of California, Berkeley and fed its readings into the stairwell's thermostat. It was not an especially difficult exercise--except that this sensor, which was about the size of a quarter, had no power cord or batteries. Instead, the device extracted the energy it needed from the vibrations that shook the wooden staircase as students clomped up and down between classes.

In Wright's view, this kind of energy scavenging--with sensors and other electronic devices living off the land, so to speak--could open a new realm for technology.

"The 1990s marked this very interesting period in which devices for computing, communication, and sensing all became much cheaper and much, much smaller," explains Wright. He's an engineer who has worked in robotics and computer science and is currently chief scientist at the Center for Information Technology Research in the Interests of Society, a multicampus program supported by the state of California.

The most obvious result of the miniaturization was a wild proliferation of cell phones, personal digital assistants, MP3 players, and other portable gadgets. But in parallel, Wright says, "researchers were led to this picture of wireless sensor networks everywhere"--in effect, an electronic nervous system that reports on both the built environment and the natural landscape.

In the not-so-distant future, for example, bridges could tell us whether they had been damaged in an earthquake. Helicopter rotors and other high-stress machine parts could warn about developing metal fatigue. Office buildings could track the locations of their occupants, automatically adjusting the lights and air conditioning for maximum comfort and minimum energy use. Automobiles could talk to each other--and to the road--in an effort to avoid both accidents and traffic jams. Implantable sensors could continuously monitor blood-glucose levels and a host of other medical conditions. And webs of environmental sensors could monitor the health of remote ecosystems, tracking moisture, temperature, micronutrients, pollutants, and many other variables. All these developments would rely on networks of minuscule sensors (SN: 5/5/07, p. 282).

These applications and more are under active investigation, says Wright. Indeed, a wide variety of wireless sensors is already available commercially. But one of the biggest challenges continues to be power. Most of the potential applications call for so many sensors scattered so widely through a target area that it would be impractical to wire up each sensor individually--and ludicrous to run around changing batteries.

Thus the need for devices that can draw energy from their surroundings, says Wright. "The 1990s brought us computing, communication, and sensing. And now, I want to add the fourth thing--energy--scavenging devices--and make them as cheap as dirt"

ENERGY EVERYWHERE Energy scavenging is not a new idea. Self-winding wristwatches, in which a tiny mechanical oscillator extracts energy from the wearer's arm movements, first appeared in the 1920s. And, of course, windmills and water wheels have been harvesting natural energy for thousands of years. But the current wave of interest in energy scavenging for microelectronics began in the late 1990s--initially because researchers were looking for a better way to power the newly devised portable devices.

In 1998, for example, Joseph Paradiso and his team at the Massachusetts Institute of Technology's Media Lab demonstrated an energy scavenger embedded in the sole of a running shoe. It relied on the piezoelectric effect, in which crystals of certain materials produce a voltage in response to stress--in this case, from the impact of the wearer's heel on the ground. And it worked, sort of.

A typical adult expends several hundred watts of power while walking, and 1,000 watts or more during strenuous exercise. But our bodies are also remarkably efficient. The MIT team found that a shoe that taps more than a tiny fraction of that energy flow gives the wearer the sensation of walking through mud. In the end, the Media Lab shoe generated only about 60 milliwatts--not enough to power an iPod, much less recharge a cell phone.

As a result of this experiment and others like it, energy-scavenging researchers soon shifted their focus from relatively power-hungry portable electronic devices to a new generation of far-more-thrifty gadgets made with microelectromechanical-systems (MEMS) technology.

The basic idea of MEMS, which dates to the 1970s, is to carve microscopic mechanical structures into the surface of a silicon wafer by using the techniques devised to create microprocessors. By the 1990s, MEMS researchers had produced all manner of gears, springs, cantilevers, channels, and the like, and a first generation of commercial MEMS devices was reaching the market. Applications included the microscopic nozzles of inkjet printers, the chip-size accelerometers that trigger the deployment of an automobile's airbags in a collision, the micromirrors of a digital light-processing display, and the microchannels that move fluid around for analysis in an integrated lab on a chip.