http://www.spectrum.ieee.org/nov07/5636The Charge of the Ultra - Capacitors
By Joel Schindall
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It's almost engineering heresy to suggest that a capacitor could power a car. Indeed, the common capacitor stores a puny amount of energy. At equivalent voltage, a chemical battery can store at least a million times as much energy as a conventional capacitor of the same size. Put two ordinary capacitors the size of a D-cell battery in your flashlight, each charged to 1.5 volts, and the bulb will go out in less than a second, if it lights at all. An ultracapacitor of the same size, however, has a capacitance of about 350 farads and could light the bulb for about 2 minutes.
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Three main factors determine how much electrical energy a capacitor can store: the surface area of the electrodes, their distance from each other, and the dielectric constant of the material separating them. However, you can push conventional capacitor designs only so far. What the Standard Oil engineers did was to develop a capacitor that functions differently. They coated two aluminum electrodes with a 100-micrometer-thick layer of carbon. The carbon was first chemically etched to produce many holes that extended through the material, as in a sponge, so that the interior surface area was about 100 000 times as large as the outside. (This process is said to “activate” the carbon.)
They filled the interior with an electrolyte and used a porous insulator, one similar to paper, to keep the electrodes from shorting out. When a voltage is applied, the ions are attracted to the electrode with the opposite charge, where they cling electrostatically to the pores in the carbon. At the low voltages used in ultracapacitors, carbon is inert and does not react chemically with the ions attached to it. Nor do the ions become oxidized or reduced, as they do at the higher voltages used in an electrolytic cell.
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There are two major limitations to the conductivity of activated carbon—the high porosity means there isn't much carbon material to carry current, and the material must be “glued” to the aluminum current collector using a binder, which exhibits a somewhat high resistance. If my colleagues and I replaced the activated carbon with billions of nanotubes, we predicted we could make an ultracapacitor that could store at least 25 percent—and perhaps as much as 50 percent—of the energy in a chemical battery of equivalent weight. (To get that much improvement, we'd have to make a number of other changes, as well, such as increasing the number of ions in the electrolyte to reflect that new-found storage space.)
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It's not a straight path from high-density ultracapacitors to practical electric cars, but what my colleagues and I have done may constitute one big step along a tortuous route to making such vehicles more convenient and attractive to consumers. Even if it takes many years before ultracapacitors on their own can power either full battery-electric or hybrid cars, we're already at the point where such devices could easily assist lithium-ion batteries. "How to Ultracap A Car"] When the car's electric motor needs high current for a short time, the ultracapacitor supplies it. After the demand eases, the ultracapacitor recharges from the battery. When the motor, working now as a generator, delivers high current for a brief interval—which is typically what happens with regenerative braking—the same thing happens in reverse. A computer would monitor voltages, the state of charge, load, and demand, and then adjust the current flow accordingly using some additional dc-dc power electronics. The added weight and expense involved might not matter if it improves vehicle performance and makes the battery last longer.
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