Anablog

The staid and analytical BBC has an article about a solar-powered airplane that uses lithium-sulfur batteries to stay aloft at 60,000 feet for three days. The video is pretty good, actually informative rather then TV-newscaster fluff. Pretty cool, I had not seen LiS batteries before, but the little factual material I can find on them points out the 60 charge cycles you get out of them before they are junk. The datasheet for the batteries in the plane (pdf) shows that most of the energy comes out a 2.1 volts, which means it will need a buck-boost converter for consumer products, and the –20 C rating means it will have a hard time in cars, that need to go down to –40 C. If they do have twice the energy density of lithium polymer that is really something, at least if they are talking about gravimetric energy density, volumetric energy density is not so important in portable applications. The datasheet shows 350 Wh/kg, which is better than the 150 to 190 that you get from li-cobalt. Of course every thinkgeek type of entry on the web has the typical 3 or 4 clueless fanboyz saying "Dell should adopt this right now". The article mentions Sion Corporation as the battery manufacturer, and I have seen reference to Saft (pdf) although it was not clear if Saft was proposing a primary or secondary (rechargeable) battery. As with most batteries LiSu is supposed to be lousy at cold, but the BBC article points out the plane was operating at –70 C, so that makes this achievement even more remarkable.

What also surprised me was the solar plane's use of amorphous silicon solar cells, since they would have less output that crystalline cells, Sunpower is bragging abut 22% efficient crystalline cells, and cell that are based on the III-V materials like gallium arsinide can do even better since it is a direct bandgap semiconductor, unlike silicon. I suspect that they used amorphous cells since it could be bent around the airfoil of the wing. This is a remarkable engineering achievement, I am one of those guys that think an ounce of trial is worth a pound of press releases. I do know better than to think we will all be running our cars and laptops off LiSu anytime soon, when it comes to technology, I don’t think it is real until I can find it in the Digi-Key catalog.