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Photocapacitors

I’m interested in saving money on batteries. Recently I built a FET instrument preamp that draws only 200 microamps at 3VDC. . . it seems like so little power that a battery is almost overkill. Considering battery alternatives, I picked up a 1.5 Farad capacitor from Mouser for under $4. While they have a lower energy density (amount of charge for a given size) than batteries, these supercapacitors actually have other superior characteristics such as the ability to be charged millions of times without deterioration. Some quick calculations suggest that if this capacitor was used to replace the battery in the FET preamp, it could run for about 3 hours on a full charge. The advantage of using a capacitor is that it should never wear out. So there’s no replacing anything, ever. I plan on charging it with a small solar panel using a zener diode shunt to limit the charging voltage to the capacitor’s 5.5V working limit. In theory, this is a guitar pedal that you can just leave in the sun during the daytime before you take it to a gig. Now I have to dig up some solar cells…

Lets stay on the topic of charging capacitors with solar cells but think more creatively. What’s a solar cell made of, and what is a capacitor made of… can they be combined into one device, basically a flat capacitor that absorbs light and stores it as charge to be used later? Traditional photovoltaic (solar) cells are essentially large area semiconductor diodes based on crystalline silicon, while newer and more efficient photovoltaics are being designed with layered films of material such as titanium dioxide particles and carbon nanotubes. Capacitors are built with layered structure as well. The basic structure of a capacitor is a pair of conductive surfaces separated by a thin insulating material; these are typically wound into a tight spiral roll which is why capacitors are often cylindrical in shape. Recently a special class of capacitors has come on the market, known as electric double-layer capacitors, capable of storing thousands of times more charge than ‘normal’ caps of the same size. Instead of the traditional spiral construction, supercapacitors rely on the huge surface area of activated carbon granules.

So photovoltaics and capacitors are both sandwiches of some sort, right? Can they be combined to make a flat capacitor that charges itself in the sun?

I searched and found an article dated 2004 in a physics journal that reports the creation of such a device, called a photocapacitor. Japanese scientists combined dye-coated titanium dioxide, activated carbon, and a platinum-coated glass electrode into a six-layered structure and observed its successful conversion of light to charge. Hopefully this technology can be developed to the point that it is practical for commercial use. If that becomes possible, the cost of utilizing solar energy could decrease because the need for expensive batteries to maintain power during dark cycles would be obviated. And fewer batteries are always a good thing for safety and the environment.

Maybe somebody can figure out how to assemble a DIY photocapacitor based on the article I cited above? Supercapacitor-grade activated carbon is available, as is high quality titanium dioxide. Transparent conductive glass seems a little trickier to find commerically, but here is one example. Somebody should give it a try. Don’t forget to patent it if it works, b/c you could become stinking rich.

update: in 2005 the same laboratory reported a moderately improved version of their original photocapacitor, using three electrodes instead of two. This design gave a five-fold increase in energy storage density.