What is an Ultracapacitor?

UltracapacitorsWhat is an ultracapacitor? First and foremost, ultracapacitors are a type of capacitor. A capacitor is an electronic unit that releases an electric charge to power something. Capacitors are designed with varying power levels and can be composed of any of a wide assortment of materials, including glass, ceramics, metal film, and aerogel. Today, virtually all electronics use a capacitor of one type or another. An ultracapacitor is a capacitor with remarkably high power and energy density, giving them much higher efficiency. Ultracapacitors can also be described as mechanical batteries, due to their similarities to chemical batteries, and are very small in size (they are approximately the same size as traditional chemical batteries). Recently, ultracapacitors have stepped up to the forefront of the capacitor field because of their high capacitance and their multiple advantages over traditional capacitors and batteries. The substantial ultracapacitor advancements that have been made within the past 10 years, in both their material science and the construction/manufacturing techniques used to produce them, have only added to their appeal.

The first ultracapacitor was developed by General Electric in 1957. The company used porous carbon electrodes to create the double-layer mechanism that characterizes ultracapacitors. The advantages of this mechanism weren't fully understood until nearly a decade later, when the Standard Oil Company of Cleveland patented their double-layer interface energy storage device in 1966. Ultracapacitors' first general use was as low-power, low-energy, long-life back-up power sources for consumer electronics such as VCRs. Today, ultracapacitors go by several different names (including "Electrochemical Double-Layer Capacitors (EDLCs)" and "supercapacitors") and are used for many different applications.

How do Ultracapacitors Work?

Capacitor WorkingsDielectricTo understand how ultracapacitors work, it is helpful to understand how capacitors in general work. Capacitors are circuit components that store electric charges. They store their energy in electric fields, which are created by the interaction between the capacitor's two conducting surfaces. These surfaces have equal and opposite charges, with one surface collecting positive charges and the other collecting negative charges. These two surfaces, which are usually plates called "electrodes," are always electrically separate from one another. The plates are separated by an insulator (also called a "dielectric"), which helps to give the capacitor its high capacitance because it polarizes the material's molecules. As the plates are conductors, the positive and negatives charges are stored on their surfaces; however, since they have equal and opposite charges, the capacitor's net charge is zero.

What allows the capacitor to hold its charge is a property called "capacitance." Capacitance (C), measured in Farads (F), is the ratio of the magnitude of the charge on the plates (Q) to the magnitude of the voltage between the plates (V). Thus, C = Q/V. Farads are coulombs/volt; one Farad is when one coulomb causes a potential difference of one volt across the plates. Capacitance is proportional to the surface area of the plates (in other words, it is proportional to the size of the electric field) and inversely proportional to the magnitude of the distance between the plates. Capacitance also varies based on the type of insulator. The closer together the plates are without actually touching, the greater the capacitor's capacitance.

The relationship between capacitance and surface area is what makes ultracapacitors different in their design and function from traditional capacitors. The plates in ultracapacitors are not separated by insulators in the same way as those of capacitors. Capacitors use dielectrics to increase their capacitance by allowing their plates to get very close to one another. However, the practical limitations on the surface area of the plates and the distance between the plates reduce capacitors' ability to have the same high levels of capacitance found in ultracapacitors. Ultracapacitors solve this problem by using a new technology that allows them to have enormous surface area relative to the distance between the plates.

When two pieces of activated carbon are immersed into a liquid electrolyte, they form an amazingly effective capacitor. The success of this method is primarily due to the carbon's large surface area-to-volume ratio, a product of the many microscopic nodules that cover its surface. Ultracapacitors are made by coating two metal foil electrodes with this activated carbon, separating them with a thin piece of paper, and immersing the carbon-coated plates (the foil electrodes) into a liquid electrolyte. The carbon on the negative plate collects electrons, which then attract positive ions from the electrolyte into the pores of the carbon. The carbon on the positive plate collects positive charges, which then attract negative ions from the electrolyte. The thin piece of paper keeps the two plates from touching, which keeps the current from flowing between the two plates, allowing the positive and negative ions to move freely within their respective plates. This creates two layers of charge, making the ultracapacitor look like two capacitors in a series; this is the double-layer mechanism that gives ultracapacitors their name of "Electric Double-Layer Capacitors (EDLC)." This design means that the charges on each plate of the ultracapacitor can be incredibly close to one another while still maintaining the carbon's large surface area, giving ultracapacitors their high capacitance.

Ultracapacitors vs. Batteries

Ultracapacitors vs. BatteriesUltracapacitors function by supplying large bursts of energy to power a product and then quickly recharging themselves. Their extraordinarily low internal resistance, or "Equivalent Series Resistance (ESR)," permits them to deliver and absorb these high energy currents, whereas the higher internal resistance of a traditional chemical battery would cause the battery voltage to collapse. Ultracapacitors' "mechanical" charge-carrier mechanisms give them an advantage over the chemical mechanisms found in batteries. Today's ultracapacitors can have power densities of up to 20kW/kg and still maintain an energy level that is miniscule when compared to that of traditional chemical batteries. Also, while a battery generally demands a long recharging period, ultracapacitors can recharge very quickly, usually within a matter of minutes. They also retain their ability to hold a charge, even after multiple rechargings, much longer than do traditional batteries. When combined with a battery, an ultracapacitor can remove the instantaneous energy demands that would normally be placed on the battery, lengthening the battery's running time and slowing down the battery's loss of charging capacity over time.

BatteriesWhereas batteries often require maintenance and can only function well within a small temperature range, ultracapacitors are maintenance-free and perform well over a broad temperature range. Ultracapacitors also have very long lives, as they are built to last until the end of the lifetime of the electronic products they power, while batteries often need to be replaced multiple times over the course of an electronic product's lifetime.

Ultracapacitors have an additional advantage over batteries with their Proton Exchange Membrane (PEM), an ultracapacitor fuel-cell technology. This is high-efficiency energy conversion device is designed to operate perpetually as long as there is hydrogen fuel available to it. Ultracapacitor PEMs are environmentally friendly and help make ultracapacitors a highly reliable source of backup power for applications. Although batteries are also capable of providing backup power, ultracapacitors have a distinct advantage over them as a backup power source because of the limited energy needs of backup power sources and the crucial need for backup power source reliability.

Ultracapacitors vs. Capacitors

Ultracapacitors vs. CapacitorsUltracapacitors are highly power- and energy-dense, particularly compared with electrolytic capacitors. Ultracapacitors are compact in more than just their size; they can store far more energy than traditional capacitors and can release their stored energy either slowly or quickly, depending upon the needs of the application.

CapacitorsOne of the major differences between traditional capacitors and ultracapacitors is the difference in their surface areas. The section of overlap between the two plates within a capacitor constitutes the "active area" of the capacitor. The greater the size of this overlap, the greater the capacitance of the capacitor. For both electrolytic capacitors and ultracapacitors, the two plates have no electrical distance from one another, rendering the entire surface area "active area." Electrolytic capacitors further heighten their capacitance by increasing their surface area through an aluminum foil etching process. Ultracapacitors have taken this technology to an entirely new level. Ultracapacitors do not use aluminum foil; rather, they use a woven carbon fiber textile, which has enormous surface area (in the range of a square kilometer of surface area per gram of carbon fiber). This gives ultracapacitors a capacitance literally millions of times greater than that of traditional capacitors. Their capacitance is usually rated in Farads (F), as opposed to the smaller microfarad (F) units generally used by traditional capacitors, and customarily runs in the 0.5-10F range.

Last Updated: Thursday, December 6, 2012