Energy-harvesting conference covers both cutting-edge and established technologies
By Margery Conner, Technical Editor -- EDN, June 13, 2007
The NanoPower Forum, which took place last month in San Jose, CA, was the first industry conference to focus on energy harvesting, the ability of a device to acquire and use energy from its surrounding and to eke out power from its environment. Topics included RF-power transmission and harvesting and storing vibrational, thermal, and piezoelectric energy. Designers look to energy harvesting to deliver a reliable source of power in extreme conditions for as long as 20 years—a capability that seems beyond the scope of small, price-competitive batteries. However, Lou Adams, regional manager for Tadiran Batteries, pointed out at the conference that industrial- and military-grade lithium-thionyl-chloride primary batteries have lifetimes of more than 20 years, and they supply approximately 80 mAhr/year with a failure rate of only one per million cells. As a comparison, a microgenerator that generates power from machine vibrations might cost $200. Adams suggested that system designers compare that price with the price of a $2 Tadiran battery that might last for 20 years. The poster-child application for lithium-thionyl-chloride batteries is AMR (automatic meter reading), which requires infrequent, predictable transmission. Batteries are worth considering for applications with the same energy-usage profile of AMR.
Health issues arise as possible concerns about using RF-power transmitters as sources of wireless power. Regan Zane, PhD, of the University of Colorado—Boulder presented a paper, “Efficient low-power RF energy harvesting and power management,” which included a comparison of radiation levels for common consumer appliances with the level of radiation you’d see in an RF-power transmitter. It found that a microwave oven or a cell phone emits 50 mW/cm2 of leakage energy, a radar antenna emits 20 mW/cm2, and a TV-station transmitter emits 10 mW/cm2. On the other hand, an RF-power transmitter like the one that researchers at the University of Colorado built emits only 20 μW/cm2, or about an order of magnitude less than the radiated energy levels you face in everyday life.
The power output and environmental hardiness of the technologies vary considerably, and the audience, which ranged from engineers to business executives to analysts, commented several times about the need for standards to measure the techniques. However, standards usually evolve after a silver-bullet application emerges to drive an industry, and the applications for energy harvesting are still too varied. Possible applications include wireless-sensor networks for industrial applications and patient monitoring. As health-care costs and an aging population make in-home monitoring an economical alternative to hospital stays, the ability to untether monitoring devices from the ac power mains makes energy harvesting an attractive feature.
