improved high-energy-density capacitors
High-performance Energy Storage, American Physical Society
Science Daily, July 4, 2007
North Carolina State University physicists have recently deduced a way to improve high-energy-density capacitors so that they can store up to seven times as much energy per unit volume than the common capacitor. High performance capacitors would enable hybrid and electric cars with much greater acceleration, better and faster steering of rockets and spacecraft, better regeneration of electricity when using brakes in electric cars, and improved lasers, among many other electrical applications.
A capacitor is an energy storage device. Electrical energy is stored by a difference in charge between two metal surfaces. Unlike a battery, capacitors are designed to release their energy very quickly. They are used in electric power systems, hybrid cars, and all kinds of electronics.
The amount of energy that a capacitor can store depends on the insulating material in between the metal surfaces, called a dielectric. A polymer called PVDF has interested physicists as a possible high-performance dielectric. It exists in two forms, polarized or unpolarized. In either case, its structure is mostly frozen-in and changes only slightly when a capacitor is charged up. Mixing a second polymer called CTFE with PVDF results in a material with regions that can change their structure, enabling it to store and release unprecedented amounts of energy.
The team, led by Vivek Ranjan, concluded that a more ordered arrangement of the material inside the capacitor could further increase the energy storage of new high-performance capacitors, which already store energy four times more densely than capacitors used in industry. Their predictions of higher energy density capacitors are encouraging, but have yet to be experimentally tested.
Physicists have long been interested in the electrical properties of the polymer PVDF, because it is known to be a dielectric material. In its solid state, PVDF can be either polar or non-polar, and it doesn`t change states when an electrical field is applied, leading to small energy storage. The researchers discovered that if they introduced "impurities" in the form of CTFE into a non-polar phase of PVDF, the resulting polymer had the ability to switch phases from non-polar to polar, enabling it to store and release much larger amounts of energy with a smaller electric field.
"Essentially we are moving atoms within the material in order to make the polymer rearrange with the least voltage," Ranjan says. "We believe that we can tailor the atomic structure of the polymer to get the best performance in the presence of different electric fields as well."
Current "State of the Art":
NASA has a research project to use supercapacitors in an electric bus called the Hybrid Electric Transit Bus. The energy used to start the engine and accelerate the bus is regenerated from braking. During test runs, a bus loaded with 30 supercapacitors, each of them weighing 32 kg and releasing energy of 50 kJ at 200 V managed to run for four miles.
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