http://www.centralchronicle.com/20080201/0102303.htm
Watch Tower: Advances in energy storage
There is a need to design and develop supercapacitors which can outperform today's designs by a wide margin at prices that make them attractive for mainstream applications- Dr SS Verma
Electrical energy is the life line of modern civilization, thus, not only its generation but also its storage in large quantities is the most important requirement of present day lifestyle to make successful use of technological advances like electric vehicles from busses to trains. Capacitor is a well known energy storing device. A capacitor consists of two electrodes, or plates, separated by a thin insulator. When a voltage is applied to the electrodes, an electric field builds up between the plates. A capacitor's energy is stored in such an electric field (given as 1/2 CV2 where C is the capacitance of the capacitor and V is the voltage applied),without requiring any sort of chemical reaction. The energy stored in the capacitor is reversibly reconvertible into some other form if the capacitor is discharged. The capacitance of a simple capacitor is given as C= eoA/d. These three main factors determine how much electrical energy a capacitor can store: the surface area of the electrodes (A), their distance from each other (d), and the dielectric constant (eo) of the material separating them. Thus a capacitor has an almost unlimited lifetime. It's also fast. Depending on its physical structure, typical charge and discharge times are on the order of a microsecond; sometimes they are as quick as a picosecond. The common capacitor stores a very small 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. However, with the development of technology, common capacitor energy storage capacity could not be enhanced due to limitations associated with conventional capacitor designs.
But the need is to have ultracapacitors, which are souped-up versions of that tried-and-true workhorse of electrical engineering, the capacitor to store large amount of energy. Because no chemical reaction is involved, ultracapacitors-also known as supercapacitors and double-layer capacitors-are much more effective at rapid, regenerative energy storage than chemical batteries are. What's more, rechargeable batteries usually degrade within a few thousand charge-discharge cycles. The synergy between batteries and capacitors-two of the sturdiest and oldest components of electrical engineering-has been growing, to the point where ultracapacitors may soon be almost as indispensable to portable electricity as batteries are now. Ultracapacitors are already all over the place. Millions of them provide backup power for the memory used in microcomputers and cellphones. They also supply brief bursts of energy to numerous consumer products containing batteries. Perhaps most exciting is what ultracapacitors could do for electric cars. They're being explored as replacements for the batteries in hybrid cars. In ordinary cars, they could help level the load on the battery by powering acceleration and recovering energy during braking. Most deadly to the life of a battery are the moments when it is subjected to high-current pulses and charged or discharged too quickly. Conveniently, delivering or accepting power during short-duration events is the ultracapacitor's strongest suit. And because capacitors function well in temperatures as low as -40 ºC, they can give electric cars a boost in cold weather, when batteries are at their worst.
Commercially available ultracapacitors already address those needs to some extent and can provide many times the power of batteries of the same weight or size. But in terms of the amount of energy they can hold, ultracapacitors lag far behind. The major difference is that batteries store energy in the bulk of their material, whereas all forms of capacitors store energy only on the surface of a material. What the Standard Oil (USA) 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. However, the amount of surface area in ultracapacitor designs has so far been constrained by the limitations in the porosity of the activated carbon.
To increase the porosity of the activated carbon, researchers 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. This approach allowed the engineers at Standard Oil to build a multifarad device. At the time, even large capacitors had nowhere near a farad of capacitance. Today, ultracapacitors can store 5 percent as much energy as a modern lithium-ion battery. Ultracapacitors with a capacitance of up to 5000 farads measure about 5 centimeters by 5 cm by 15 cm, which is an amazingly high capacitance relative to its volume.
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. Recent innovation made researchers at MIT (USA) is to replace the activated carbon with a dense, microscopic forest of carbon nanotubes that is grown directly on the surface of the current collector. By virtue of their dimensions, nanotubes hold the promise of even higher porosity than the activated carbon used in commercial ultracapacitors. Together the nanotubes have an enormous surface area, and their dimensions are more uniform than those of the activated-carbon pores, making them more like a paintbrush than a sponge. Another advantage of nanotubes over activated carbon is that their structure makes them less chemically reactive, so they can operate at a higher voltage. And certain types of nanotubes, depending on their geometry, can be excellent conductors-which means they can supply more power than ultracapacitors outfitted with activated carbon.
Researchers claim that by doing so, they can create a device that can hold up to 50 percent as much electrical energy as a comparably sized battery. To get that much improvement, however, they had 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. Even better, this nanotube-enhanced ultracapacitor would retain all the advantages ordinary ultracapacitors have over batteries: they would deliver energy in quick bursts, they would perform well in cold weather, and they would have much longer life spans. If this ultracapacitor could be developed, it would be revolutionary.
This feat would allow ultracapacitors to supplant batteries in a number of mainstream applications. Hence, there is a need to design and develop supercapacitors which can outperform today's designs by a wide margin at prices that make them attractive for mainstream applications. Development of such supercapacitors will constitute one big step towards making automobiles more convenient and attractive to consumers. Such devices (supercapacitors) are by no means limited to vehicles. Society is in the midst of an energy crisis, and many sources of green energy would benefit from regenerative energy storage. Electric power grids could be 10 percent more efficient if there could be simple, inexpensive ways to store energy locally at the point of use. And if renewable energy is ever to displace fossil fuels, engineers will need to devise better ways to store wind power when the wind is not blowing and solar power when the sun is not shining. Improving substantially on the means to store electrical energy would be a welcome development, and high-density capacitive storage is one promising avenue of research.
Dept of Physics, S LIET Longowall
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