Absolute Power


Solar smartphones


This blog originally posted on the Low Power Engineering Community 2/9/12.


One of the more intriguing smartphone features that hasn’t yet reached the final product is the possibility of a solar cell embedded in the screen that would produce enough energy to power the device indefinitely (at least in the sun).

Rumored via a patent application by Apple several years ago, with a recent prototype by French company Wysips, and even by reclaiming “wasted” light (http://www.phonescoop.com/articles/article.php?a=9703 ) on OLED displays, the days where your smartphone battery needs to be plugged in to be charged may finally be coming to an end. It’s certainly not an unfamiliar scenario. Calculators are just smartphones without the phone and a bunch of other stuff, and they’ve had this mode of operation for more than 30 years.

Current technology is targeting about 30 minutes of talk time in 1 hour of charging, which seems pretty reasonable. Of course, today’s smartphone functions go way beyond just talking on the phone. “Talk time” is dominated by the energy cost of cellular transmission, but the display is typically off and other data functions are inactive. That one hour of charging might translate into 10 minutes of streaming a video, or 5 minutes of interactive online gaming. On today’s iPhone 4S, most usage reports suggest around 9 hours of talk time on the 5300 mWh battery, working out to around 600 mW of power required to operate the phone function. That would put the target at around 300mW for a hypothetical iPhone 4S solar cell, or with the 2”x3” screen size, about 50 mW per square inch.

On a larger scale, I’ve recently installed solar panels on my house, composed of 65”x39” Yingli Solar panels of 235W each, which works out about 105 mW per square inch. I’m a little surprised that the efficiency of the large (inexpensive) panels seems to be twice as high as the target for a compact solar smartphone charger (probably because the panels on my roof are more than two inches thick, or five times as thick as the whole phone), but at least the target doesn’t seem out of reach. In another generation or two, you’ll be able to talk on the phone all day without draining the battery appreciably, assuming you’re not also using the on-the-fly foreign language translation option or grammar correction module at the same time.

So while it seems like we’re still a little ways away from having smartphone-class devices that will power themselves indefinitely, it’s not too early for the entrepreneur’s tip of the month: CLEAR smartphone cases…!

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