During its brief flight, the outer skin of a missile can reach 300° Fahrenheit, creating an ideal source for thermophotovoltaic conversion cells that can be used to power guidance electronics. Piezoelectric materials can be built into smart ammo shells, so that recoil forces become a power source. And electronics in a foot soldier's clothing can be powered by "heel strike" electromechanical systems built into the boot.
Those are some of the far-out concepts military engineers are exploring in the emerging field of energy harvesting, in which extreme environments are bent to the needs of electronics. The commercial sector, too, is beginning to eye energy harvesting for many of the same reasons. The principal power source for untethered electronics is batteries, which tend to be bulky and have a limited lifetime. The idea behind energy harvesting is to make electronic systems self-sustaining.
The environment provides ambient energy in a wide variety of forms: vibration, strain and inertial forces, heat and light, wind and magnetic fields. And there are a variety of systems that can tap those energy sources. Piezoeletric materials can convert strain and vibration directly into electric current. Magnets and inductive coils can tap inertial forces; thermo- and photovoltaic cells can harvest the energy in light and heat.
It would be a significant breakthrough to find some way to use these ubiquitous energy sources to eliminate the battery from mobile computing devices and wireless sensors, but doing so is proving to be a formidable task. One problem is economic: Batteries are simply better at providing reliable power at the lowest cost, and it will be difficult to dislodge them. But even when the economic factor is removed, as in military systems, it is still difficult to get electronic systems to run on ambient-energy sources exclusively.
"About a decade ago, it seemed clear to me that there was a technological path to integrating sensing, computation, communications and power into a millimeter-scale package. All of the technology drivers were going in the right direction, following Moore's Law-type exponentials down to zero size, power and cost," said Kris Pister, founder and CTO of Dust Networks Inc. (Hayward, Calif.), a wireless-sensor network company. "I coined the term 'smart dust' to describe where all of that was headed."
While at the University of California, Berkeley, Pister began to push down the power consumption of circuits and has continued to set records. At the recent International Solid-State Circuits Conference, his group at Berkeley presented a paper on a 2.4-GHz radio receiver that burns only 200 microwatts.
The university work explored the lower limits of power consumption in terms of the circuits themselves. The objective of research and development at Dust Networks was to build wireless-sensor networks, and reducing the power used at the individual nodes was just part of the overall energy problem.
"The university work was all about low-power hardware; the company work has been about low-power software," Pister said. "We came up with communication protocols that keep the radios off 99.5 percent of the time. Low-power software running on low-power hardware--at that point the power consumption goes down into the single-digit microamp kind of range."
However, even with the ultralow-power architecture, the nodes still need batteries and Pister doesn't see that changing anytime soon. "Whatever you are going to replace batteries with is going to be more costly," he said, "and most energy-harvesting systems are still in the development stage." Low-power operation greatly extends battery life, and energy harvesting could further amplify that. But one would still have to come up with an economic rationale for adding an energy-harvesting component to extend battery life.