Friday, November 21, 2008

Your Next Battery Is a Capacitor

Benjamin Franklin may have had it right in the first place. When he conducted his famous kite flying experiments, he was trying to capture electricity and store it in a battery. In terms of Colonial technology that meant Leyden Jars. A Leyden Jar is a glass jar with foil conductors wrapped both inside and outside. The glass in-between forms a dielectric insulator that allows electric charge to be stored in the conductors. Charge it up with static from shuffling your feet across the floor or atmospheric charge from a thunderstorm and you have stored electricity that can be useful later. You can even link multiple Leyden Jars together in a "battery" of jars to get a really big charge. That's where the idea of a battery started.

Fast forward from the days of Ben Franklin to this year. The latest iteration of the capacitor is the Graphene Ultracapacitor created by researchers at The University of Texas at Austin. Ultracapacitors are physically small but powerful capacitors. By replacing big glass bottles with nanometer thin dielectrics, capacities on the order of farads can be produced in something the size of your thumb. Compare that to the Leyden Jar which has a capacity measured in picofarads, a million millionth of a farad.

The thing about farad level capacitors is that they start acting like batteries. A Leyden Jar or a capacitor typically found in electronics filtering applications will give you a good jolt, but it won't power a cell phone more than momentarily. In high school electronics class, a popular prank was to charge up a capacitor using a DC power supply and then leave it laying on the desk for some unsuspecting victim to pick up. Oh, the ROFL joy we experienced every time that happened. It was great amusement until the day one of the dimmer students plugged an electrolytic cap into a wall outlet. In seconds the end cap exploded with a loud bang, spewing aluminum oxide powder up his arm. I'm not sure who was more shocked: The student who thought he'd been shot or the teacher who laid down the law for all of us.

Fortunately, this was in the days before capacitors with more than a couple hundred microfarads at a hundred volts or more were too expensive and too large to be used in most applications. Nowadays you might vaporize a piece of wire with a larger ultracapacitor. Or, you could put all that capacity to good use running your electronics. Capacitors and batteries can both be used as power sources. Capacitors have an advantage in that they charge very quickly. But until recently, they've had the disadvantage of being able to pack less power in a given size package. With the development of new material technologies such as graphene, it may not be long before your cell phone, computer, watch and even hybrid vehicle are all powered by high technology versions of Ben Franklin's Leyden Jars.

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