Smartphones and other portable gadgets push the limits of handheld computing, their need for the electricity to run has only increased from time to time with no end in sight. A new technology has developed which aims to address this problem,not by seeking bigger and better batteries but by looking instead to the shoes on our feet.
When a person is walking, the human body is capable to create up to 40 watts of mechanical power as heat when feet hit the ground. A special electricity-generating cushion placed under the soles of a regular pair of shoes will help to transform some of that generated power into several watts of electricity. Over the time , the energy generated , which will get stored in a small battery under the sole placed near to the electricity-generating cushion , provides enough electricity for a pedestrian to extend his/her smartphone’s battery life, and for a soldier to augment his portable power needs in the field, or for someone in a developing nation without an electrical grid to power a night’s worth of LED home light use.
InStep NanoPower concept
By contrast, says Tom Krupenkin, associate professor of mechanical engineering at the University of Wisconsin–Madison, recent breakthroughs in microfluidics can fulfill or even exceed Starner’s power projections.
The idea behind harvesting body energy for portable electronics is certainly not new, although some of this technology is. In 1996, Thad Starner at the MIT Media Lab came up with piezoelectric generators—solids that generate tiny currents when stretched or stressed—could theoretically generate up to 5 W of electricity at a brisk walking pace.
"When you analyze the cellphone power budget," Krupenkin says, "you discover that the lion’s share goes into the high-power RF signal. This is 1 to 2 W. The cellphone by itself often uses only tens of milliwatts."
Starner’s forecasts have proved optimistic. Today’s best known piezoelectric footwear—Nike+ running shoes—aren’t really harvesting energy at all. A 2007 teardown by SparkFun Electronics of a Nike+ piezoelectric pedometer, for instance, reveals that even though the pedometer’s chips consume only tens of milliwatts of power, they still run on a separate battery. The piezoelectric part of the device is used only as a sensor and not to produce power.
The key involves the properties of liquid metals such as mercury and Galinstan, a gallium indium tin alloy. When set on a dielectric-coated conductive substrate with a voltage applied across it, a droplet of liquid metal deforms and spreads across the substrate. When the process is reversed, and the liquid metal in a microfluid device is moved, it induces a voltage.
Raziel Riemer, a lecturer in industrial engineering at Ben-Gurion University of the Negev, in Israel, says he has run the numbers and remains skeptical that InStep’s technology could harvest anything close to 20 W of power. But, Riemer adds, "even if they don’t achieve [this]…it’s not such a big deal compared to the innovation of their new method."
In Step is developing a shoe sole that would store the energy from each footfall in an embedded battery. InStep says it would provide up to 10 W from each foot—enough to power a mini Wi-Fi hot spot that communicates with your smartphone via Bluetooth and handles the phone’s biggest battery-draining function: long-range communication with cellphone towers.