Energy Harvesting Comes of

Is energy harvesting ready for prime time? It is certainly getting closer, according to Silicon Laboratories, which develops low-power wireless transmitters, and IDTechEx, a consultant in the field. Energy harvesters are devices that capture or recapture energy—vibration, heat, solar, electrostatic—that is otherwise lost. Most often, harvesters store the energy and reuse it later for power. Common technologies range from small piezoelectric devices that convert machinery vibration into enough electricity to power a small wireless sensor to regenerative braking systems that recharge batteries for use when starting a car. One popular application is wireless sensors. As Silicon Laboratories noted in a white paper, "Running main power to wireless sensors is often neither possible nor convenient, and since wireless sensor nodes are commonly placed in hard-to-reach locations, changing batteries regularly can be costly and inconvenient." Until recently, this was a problem most engineers had to live with. Energy harvesting devices simply could not generate enough electricity to power wireless communications. This has begun to change.

Intel's Claremont chip runs on solar power. Piezoelectric devices, among the most common energy harvesters, have grown increasingly efficient. Four years ago, they broke through the microwatt barrier and into the milliwatt regime. This is the power domain where most microcircuits operate. "It's not just energy harvesters that are getting better though. It's also power consumption requirements that are coming down," IDTechEx technology analyst Harry Zervos noted. Wireless sensors are increasingly integrating functions into single chips to minimize power draw. They sleep between measurements to conserve power. When they do broadcast, they used stripped-down protocols to minimize the amount of information they need to send, and may adjust their range to available power. Intel's prototype Claremont microprocessor actually adjusts its workload when it has less power. When running on solar power alone, it draws less than 10 milliwatts. While the Claremont is a research demonstrator, Intel could adapt the technology for commercial chips, the company's chief technology officer, Justin Rattner, says. The combination of improved harvesters and low-powered electronics has yielded new products. Last year, Germany's Micropelt introduced two sensors based on its thermoelectric technology. The first, developed with MSX Technology, is a sensor for pots and pans that controls kitchen cooktop temperature. It can reduce energy use during cooking by up to 50 percent. The second, qNode, created with Schneider Electric, is a wireless machinery condition monitor. Zervos expects future harvesters to generate more power. Last November, for example, the National Institute of Aerospace demonstrated a multilayer piezoelectric device that can harvest four times more energy than conventional piezoelectric systems. The researchers, led by Tian-Bing Xu, hope to demonstrate harvests of up to 1 watt in 2012. Researchers at Stony Brook University in New York led by Lei Zuo have developed small generators that harvest electricity from the motion of shock absorbers. Zuo estimated that a passenger car traveling down a smooth highway could generate 100 to 400 watts of energy under normal driving conditions. Such energy could power a vehicle's auxiliary electrical systems. Or a system could store electricity in supercapacitors and use the energy to drive an electric motor that assists vehicles accelerating from a full stop. To read the latest issue of Mechanical Engineering, click here