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Human Generated Power for Mobile Electronics
GVU Center, College of Computing Georgia Tech
Atlanta, GA 30332-0280
Joseph A. Paradiso
Responsive Environments Group, Media Laboratory MIT
Cambridge, MA 02139
Since the 1990’s, mobile computing has transformed its penetration from niche markets and early prototypes to ubiquity. Personal Digital Assistants (PDAs) evolved from GRiD’s PalmPad and Apple’s Newton in 1993 to the Palm, Handspring, and Microsoft-based models that support the multi-billion dollar industry today. While BellSouth/IBM’s Simon may have been the only mobile phone to offer e-mail connectivity in 1994, almost every modern mobile phone provides data services today. Portable digital music players have replaced cassette and CD-based systems, and these “MP3 players” are evolving into portable repositories for music videos, movies, photos, and personal information such as e-mail. Laptops, which were massive and inconvenient briefcase devices in the late 1980’s, now outsell desktops. Yet all these devices still have a common, difficult problem to overcome: power.
This chapter will review trends in mobile computing over the past decade and describe how batteries affect design tradeoffs for mobile device manufacturers. This analysis leads to an interesting question: is there an alternative to batteries? Although the answer has many components that range from power management through energy storage , the bulk of this chapter will overview the history and state-of-the-art in harvesting power from the user to support body-worn mobile electronics.
2 Technology Trends in Mobile Computing
Mobile phone companies often sell more batteries than phones to consumers. The phones sold to users include a rechargeable battery so that the device is immediately useful, but a certain number of consumers are expected to own more than one battery during the life of their phone. The same can probably be said for laptops and camcorders. Yet, there is little incentive for consumers to buy new batteries except for when they fail or when the consumer feels the need for a larger battery. Unlike other areas of mobile computing that benefit from exponential improvements in performance, battery energy density (as measured by joules per kilogram or joules per cubic centimeter) changes slowly so that there is little pressure for consumers to upgrade.
2.1 Battery Energy Density as a Lagging Trend
As Figure 1 shows, battery energy is one of the most laggard trends in mobile computing. Figure 1 shows the progression of technology in the last 13 years for laptop computers, a technology now mostly mature. In general, the laptop technology represented in the graphs would, if repackaged in a body worn device, weigh seven pounds or less and could be used while standing on a street corner in a major United States city. While some mobile computers existed prior to 1990, most weighed over 10 pounds or did not include hard drives. In addition, commercial wireless data networks in the United States were not openly available before 1990 or required amateur radio licenses to operate.
The graph depicts increases in performance as multiples of the state of the technology from 1990 (e.g. the amount of RAM available in a laptop increased by 256X from 1990 to 2003). Due to the exponential nature of the improvements, the y-axis in Figure 1 is on a logarithmic scale.
to appear in Piguet, C. (ed), Low Power Electronics Design, CRC Press, Fall 2004. 1
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