Pister is looking at using photovoltaic cells in that role. Even inside buildings, the ambient-light levels are high enough to gain a distinct benefit from photovoltaics, the most mature energy-harvesting technology, with a long development history. Indeed, photovoltaic technology's struggles over several decades to find a niche in the energy field is a good indicator of the hurdles other, less-mature energy-harvesting approaches will have to overcome.
"When people talk about energy-harvesting systems, they usually overlook photovoltaics," observed Michael Robinson, vice president of marketing at MicroStrain Inc. (Williston, Vt.), which is also developing wireless-sensor networks for industrial applications.
Photovoltaics, while available and relatively low in cost, do not solve some of the problems MicroStrain has encountered, however. The company has developed high-performance piezoelectric materials and is applying them to building strain sensors that can be implanted in structural elements to monitor their integrity. For example, MicroStrain hopes to be able to place such sensor networks in highway bridges, where there is a significant level of vibration. Highway engineers could then read out data on structural integrity using a wireless unit.
The material is also being eyed for harvesting mechanical energy in a variety of applications.
Neither photocells nor batteries are a good match for such applications, but engineers at MicroStrain have found a way to run wireless nets entirely off the energy harvested from vibration. The system uses a cantilevered beam with a weight attached to the end to amplify vibrations. As the beam bends, it generates electric power. This work is still in the research stage.
"At the end of the day, wireless networks will always be hampered by the need to change batteries," said Robinson. "Harvesting energy is the only way to avoid that."
Problem of leakage
Broad-scale energy harvesting can't truly take hold until low-power design becomes entrenched at all levels of electronics, from systems to circuits. And that is beginning to happen, notably in microprocessor design.
One trend in the circuit business that isn't following a Moore's Law curve is current leakage, which has a direct impact on static-power consumption. Increasing circuit density only drives up the leakage. The problem cuts across applications and is particularly troublesome for any system that needs to run off low, intermittent power.
"We are all trending into smaller geometries so we are getting many, many more transistors on a piece of silicon. The bad news is that leakage is growing quite rapidly as a percentage of overall power," said Rick Hetherington, chief architect for Sun Microsystems Inc.'s new Niagara processor, which is optimized for low-power operation. Niagara is going into large servers and data centers, a far remove from the remote applications of energy-harvesting schemes. However, system designers can no longer take an endless power supply for granted.
"Many of the clients we have been talking to have been filling up their data centers--they are running out of space, they are running out of power," Hetherington said. "The next step up for them is quite expensive--it means building new data centers."