Ultracapacitors

Copyright Honda Motor Co.

Traditional Capacitors   Ultracapacitors vs. Capacitors   Ultracapacitors vs. Batteries   Applications   Additional Links


Compiled by Josh Pomerenke
PHY 131 Honors Project - Fall 2009



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.


A
      |      
+ + + + + + +
- - - - - - -
|
B

When a source potential difference is applied across points A and B (e.g. a battery is connected), excess charges build up on both plates of the capacitor- positive on one plate, and negative on the other plate. Charges build up until their own potential difference is equal to that of the source, "V". In this state of equilibrium, each plate has its maximum charge of +/- "Q". Capacitance is therefore defined as the amount of charge separated per unit of potential difference needed to separate it, C = Q/V. The unit of capacitance is the farad, after Michael Faraday.

The first capacitor was the Leyden Jar, which came about in late 1745. Named for the University of Leyden, the place of its inception, this device consisted of a jar which was lined with metal foil on the inside and outside, which acted as the plates of the capacitor. A metal rod was inserted through the top of the jar so that it contacted the inner foil, allowing it to be charged. This jar would maintain its charge until the rod was connected to the outer foil by a wire or other conductor, allowing it to discharge. Along with Alessandro Volta's first primitive battery, this device became a tool in understanding the nature of electricity, but practical applications for capacitors were not found until the next century.

This kind of jar capacitor, as well as the parallel-plate one described above, would have a very low capacitance- unless it was extremely large. To increase the capacitance, the charge-carrying plates in modern-day capacitors are shaped into closely-wound spirals. This allows a larger surface area for more charges to reside on within a smaller volume. This design is evidenced by the cylindrical shape of most of the capacitors seen at the top of this page. Though this increases the capacitance, the small kinds of capacitors found in circuit boards still only have small capacitances, and are used in things like memory storage systems rather than power supply.





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.


solution
-|(+) (-)          (+) (-)|+
-|(+)      (+)  (-)    (-)|+
-|(+)   (-)      (+)   (-)|+
negative plate  -|(+)      (-)    (+)  (-)|+  positive plate
-|(+) (+)    (-)       (-)|+
-|(+)   (+)        (-) (-)|+
-|(+) (-)   (+)        (-)|+
  Helmholtz layer-'~~~'                                       

In Helmholtz's ideal model, the layer of charged particles exactly balances the charge on the plate, creating a "double-layer" of separation of charges. Moreover, the distance between a plate and its charged particles is much smaller (<1 nanometer) than the distance between the two plates of a tightly wound traditional capacitor (~50 nanometers). This effect gives ultracapacitors their other, more technically descriptive name, electrochemical double-layer capacitors.

Although real applications don't quite follow this ideal model, the relative proximity of the separated charges greatly increases capacitance. To further increase capacitance, modern day ultracapacitors use an additional trick. The space where the electrolytic solution resides is divided in half by an insulator, and each space is filled with a highly-porous conductive solid. These solids become charged by their respective plates, and opposite charges in the solution build up next to their surfaces. Common choices for these solids are high-surface-area carbon materials, especially carbon aerogels. Experimental ultracapacitors have also been made using carbon nanotubes in an effort to absolutely maximize the surface area contained in the area between the metal plates.

These differences in how ultracapacitors work give them capacitances on the order of 10,000 times greater when compared to traditional capacitors of the same physical size. Commercially available ultracapacitors can reach capacitances in the farad range for IC-sized units. Larger battery-sized ultracapacitors extend into thousands of farads.





Ultracapacitors vs. Batteries: How Do Ultracapacitors Compare?    [top]
        Following information from ultracapacitor.net/whatareultracapacitors.php and electronicdesign.com/Articles/Print.cfm?ArticleID=17465(Chart)

electronicdesign.com (Fig. 2) 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]