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18 PERVASIVE computing Published by the IEEE CS and IEEE ComSoc 1536-1268/05/$20.00 © 2005 IEEE Energy Scavenging for Mobile and Wireless Electronics A fter Alessandro Volta invented the battery in 1799, predating Michael Faraday’s dynamo by 32 years, bat- teries provided the world’s first prac- tical electricity source until the wiring of cities in the late 1800s relegated bat- teries to mobile applications. Despite vacuum- tube electronics’ weight and large associated bat- tery, 1 people living in the early 1900s lugged such enormous “portable” radios to picnics and other events off the power grid. As electronics be- came smaller and required less power, batteries could grow smaller, enabling today’s wire- less and mobile applications explosion. Although economical batteries are a prime agent behind this expansion, they also limit its penetration; ubiquitous computing’s dream of wireless sensors everywhere is accompanied by the nightmare of battery replacement and disposal. Figure 1 depicts increases in the performance of laptop computers (a mature mobile technology) on a logarithmic scale relative to a laptop from 1990. As the graph indicates, battery energy is the slowest trend in mobile computing. Although new materials are revolutionizing the battery’s form factor, its energy density doesn’t scale expo- nentially; rather, battery capacity proceeds along flattening S-curves. Overcoming this trend requires moving to another energy source. New fabrication technologies have recently resulted in micro fuel cells aimed at recharging handhelds with power plants the size of a candy bar, and they promise fuel cells on a chip for powering wireless sensor nodes. Although research proto- types exist, laptop-sized plants (30–50 watt- hours) tend to be too big to directly power with microcells and too small for standard fuel cells, because their associated chemistry requires sig- nificant overhead in mass. More exotic emerging power technologies exhibit characteristics that force them into niche applications. For example, radioactive decay can power batteries that last for decades, but they provide low current and involve complicated disposal. Furthermore, devices that burn fuel, such as microturbines and microengines, potentially pose issues with exhaust, heat, noise, thrust, or safety. Ongoing power management developments enable battery-powered electronics to live longer. Such advances include dynamic optimization of voltage and clock rate, hybrid analog-digital designs, and clever wake-up procedures that keep the electronics mostly inactive. Exploiting renew- able energy resources in the device’s environment, however, offers a power source limited by the device’s physical survival rather than an adjunct energy store. Energy harvesting’s true legacy dates Energy harvesting has grown from long-established concepts into devices for powering ubiquitously deployed sensor networks and mobile
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