Traditional Capacitors: What is a Capacitor? [top]
Following information from electrochem.cwru.edu/encycl/art-c03-elchem-cap
Capacitors in their simplest form are devices that store energy electrostatically by physically maintaining a separation of charges. This is most easily illustrated in the parallel-plate capacitor seen below, made up of two horizontal conducting plates, each with a vertical wire attached to it.
Ultracapacitors vs. Capacitors: What is an Ultracapacitor? [top]
Following information from electrochem.cwru.edu/encycl/art-c03-elchem-cap
Since the most commonly used capacitors are very small, they don't really compare to the actual farad in terms of capacitance, but rather are on the order of picofarads or nanofarads. Some industrial applications may have ranged all the way up to one farad where the physical size of the capacitor wasn't an issue, but in general, the farad was an extremely large unit of measure. That is, until the advent of the ultracapacitor.
Ultracapacitor technology is based on experiments done by
Hermann von Helmholtz in the mid-19th century, though this
research was not capitalized upon until nearly a hundred years later. Von Helmholtz
was experimenting with various electrodes placed in electrolytic solutions (solutions
with suspended charged particles). Unlike
Alessandro Volta's battery, in which the solution carried a
current between the electrodes, Helmholtz used materials that would not allow any
electron transfer between electrodes and electrolyte. This meant that when the
electrodes were charged, cations would build up around the negative plate, and anions
would build up around the positive plate. These layers of charged particles are the
key to ultracapacitors, and have come to be known as "Helmholtz layers" as shown in
the diagram below.
Ultracapacitors vs. Batteries: How Do Ultracapacitors Compare? [top]
Following information from ultracapacitor.net/whatareultracapacitors.php and electronicdesign.com/Articles/Print.cfm?ArticleID=17465(Chart)
All things considered, a kilofarad ultracapacitor sounds pretty impressive, but how
does it compare with the weapon of choice in independent power supply, the battery?
To answer this, it is important to first understand the major differences in
functionality between batteries and capacitors.
The energy contained in a capacitor comes from the separated charges and the electrical force trying to move them back, all the way to their original state, discharging the capacitor completely in a very short amount of time. In a battery, the energy comes from a chemical reaction, which can be sustained for a much longer time. This means that batteries can hold much more energy. However, capacitors can discharge extremely quickly, and since power is the amount of energy supplied per second, this gives them extremely high power densities. The chart at right shows these differences, as well as where ultracapacitors fall.
Ultracapacitors have power densities similar to traditional capacitors, but they have dramatically higher energy densities. However, their energy densities are still lower than batteries' by a factor of about 10, or more for top-of-the-line rechargeable batteries. This is one of the main reasons why it is difficult to use ultracapacitors as a direct substitute for batteries. Also, as an ultracapacitor discharges, the voltage across its terminals decays exponentially. In order to simulate the constant voltage supplied by the chemical reaction of a battery, the discharging process must be modified using some voltage-limiting circuitry. The reverse process offers one of the huge advantages of using ultracapacitors: they can charge fully in less than 1% of the time it takes rechargeable batteries. They are also much more physically adaptable than batteries. They can function at a wider temperature range, and will not degrade if left unused for long periods of time. Additionally, ultracapacitors can be recharged as many as a million times without any significant loss of functionality.
Applications: So What Can Ultracapacitors Be Used For? [top]
One of the most remarkable potential uses for ultracapacitors is a line of buses being developed by the Chinese company Shanghai Aowei Technology Development Company and its American partner Sinautec Automobile Technologies. These buses are powered solely by ultracapacitors, which are able to provide the same power as the standard option, the diesel engine. However, the ultracapacitors run out of charge quickly, giving the bus a range of a few city blocks per charge. Charging stations integrated into every few bus stops will provide the energy for the next leg of the bus route, and the recharge is quick, allowing the bus to continue in a couple of minutes. To increase the range of the buses, regenerative braking mechanisms are in place, which use the kinetic energy lost in reducing the speed of the bus to partially recharge the ultracapacitors. Overall, it is estimated that these buses have only one tenth the energy cost of their diesel cousins!
A typical application of capacitors is to provide small-scale short-term power to circuits that are otherwise powered, in case the power supply lags. Stepping up this function to the next level, RAM Technologies' PFC600PCX 600-watt power supply offers this optional back-up UPS (uninterruptable power supply) which uses only ultracapacitors. It operates at 12 volts, and contains 4,000 joules of energy.
An especially cool application of ultracapacitors is this flashlight featured by wired.com. It uses only ultracapacitors instead of batteries for power, which gives it an amazing 90-second recharge time. The downside, of course, is that it won't stay charged nearly as long as a battery-powered flashlight. Living up to the capacitor motto, it will last thousands and thousands of charge/discharge cycles with undiminished performance. Not surprisingly, it is a little expensive, but it is a workable application for ultracapacitors to replace batteries.
In the same vein, Coleman has this screwdriver which is powered solely by ultracapacitors. Like the flashlight, it is able to charge fully in about 90 seconds. This charge doesn't last very long, even compared to the flashlight, because the electric motor uses much more power than LED's. But then, it is only about half the price, and Coleman holds that it will work according to specifications for half a million charge cycles.
There are also many DIY applications of ultracapacitors, such as this one involving a bicycle light. It uses a small generator powered by the bicycle to charge an ultracapacitor, which in turn should be able to power a bicycle light for several minutes after the generator stops supplying power, rather than the mere seconds of light which could be afforded by a standard capacitor.
Additional Links for More Information [top]