[Technology Report]
Get The Lowdown On Ultracapacitors
Now that you're armed with more capacitance than earlier generations of engineers ever dreamed of, what do you do with it?
Call them ultracapacitors. Or supercapacitors. Whatever the name, they exhibit vastly greater capacitance than conventional caps. Singly, you can buy radiallead board-mount devices rated for 5 to 10 F at 2.5 V, flashlight-battery size units rated for 120 to 150 F at 5 V, and larger single-capacitor cans good for 650 to 3000 F at 2.7 V. Note that all of those capacitance values are in farads. Not so long ago, a couple of thousand microfarads were a lot of capacitance.
Need more? You can buy off-the-shelf modules spec’d for 20 to 500 F, with voltage ratings from 15 to 390 V. If you understand how to balance them in series/parallel combinations, you can drive a bus with them—no, not two traces on a circuit board, but a passenger-hauling bus. (Although not very far, as hybrid propulsion systems, chemical batteries, and fuel cells are still in the picture. More on that shortly.)
What happened? In developing ultracaps, nobody discovered new laws of physics. In fact, the theory behind them goes back to Helmholtz. Like all capacitors, ultracaps are still about storing power in the form of an electrical charge between two “plates.” The capacitance is directly related to the area of the plates and the permittivity of the material between the plates, and it’s inversely related to the distance between them. After that, the story gets interesting.
Before we had ultracaps to provide astonishingly high values of capacitance, we had electrolytics. Ultracapacitors aren’t electrolytics, but understanding the older tech is helpful in understanding the new tech.
Electrolytics are so named because one (or both) of the “plates” is a nonmetallic electrolyte on top of a metallic backing. During manufacturing, a voltage drives a current from the anode metal through a conductive bath to the cathode. That produces an insulating metal oxide on the surface of the anode—the dielectric.
One of the phenomena that happens inside electrolytics is the charge accumulation and charge separation that occurs at the interface when any electrode is immersed into an electrolyte solution. An accumulation of oppositely charged ions in the solution compensates for excess charge on the electrode surface. The interface is called the Helmholtz layer.
To understand ultracaps, stop thinking about flat plates (or flat plates rolled up into tubes) with a dielectric between them, much like peanut butter in a sandwich. In an ultracap, charging/discharging takes place on the interfaces between porous carbon materials or porous oxides of certain metals in an electrolyte.
The Helmholtz layers give rise to an effect called doublelayer capacitance. When a dc voltage is applied across the porous carbon electrodes in an ultracap, compensating accumulations of cations or anions develop in the solution around the charged electrodes. If no electron transfer can occur across the interface, a “double layer” of separated charges (electrons or electron deficiency at the metal side and cations or anions at the solution side of the interface boundary) exists across the interface (Fig. 1).
The Helmholtz-region capacitance depends on the area of those porous carbon electrodes and the size of the ions in solution. The capacitance per square centimeter of electrode double layers is on the order of 10,000 times larger than those of ordinary dielectric capacitors. That’s because the separation of charges in double layers is about 0.3 to 0.5 nm, instead of 10 to 100 nm in electrolytics and 1000 nm in mica or polystyrene caps.
There’s a catch to this “double-layer” characteristic, though. The double-layer configuration reduces the potential capacitance of a practical device because the ultracap consists of a pair of electrodes, each with half the total mass. In addition, the ultracapacitor is effectively two capacitors in series. Taken together, that means the ultracap achieves one quarter of the theoretical capacitance based on electrode area and ion size.
If you want to read the theory behind ultracapacitors in more depth, check out an article from the Electrochemistry Encyclopedia called “Electrochemical Capacitors, Their Nature, Function, and Applications” (http://electrochem.cwru.edu/ed/encycl/artc03-elchem-cap.htm) by the late Brian E. Conway of the University of Ottawa’s chemistry department. Conway was an important contributor to ultracapacitor research for several decades.
Batteries and ultracaps The popular press likes to lump batteries and ultracapacitors together, obscuring a number of important differences:
Batteries store watt-hours of energy. Capacitors store watts of power.
Batteries depend on chemical reactions with long time constants. They take a relatively long time to charge, and they’re fussy about the profile of the current that charges them. Conversely, capacitors are charged by applying a voltage across their terminals, and their charge rate depends mostly on external resistance.
Batteries deliver power in the form of a more or less constant voltage over long time periods. Capacitors discharge rapidly, and their output voltage decays exponentially.
Batteries are good for only a limited number of charge/discharge cycles, and the number of cycles depends on how deeply they are discharged. Capacitors, especially ultracapacitors, can be charged and discharged repeatedly for tens of millions of cycles. (This is an important way that ultracaps differ from electrolytics—they aren’t cycle-limited by the electrode plating that accompanies electrolytics’ operation.)
Batteries are big and heavy. Capacitors are small and light.
Many of these differences can be heuristically illustrated in a Ragone plot (Fig. 2). Ragone plots have more analytical uses, but essentially, they’re log-log graphs of energy density (in this case in Wh/kg) on the Y axis versus power density (in W/kg) on the X axis. Because they’re log-log plots, discharge time can be represented as straight-line diagonal parameters.
The Ragone plot helps illustrate the differences among different kinds of battery chemistry, clustered on the left, and capacitors on the right. Taken together as illustrated on the Ragone plot, those characteristics make batteries and ultracapacitors complementary to each other, rather than antagonists. In fact, that’s how they’re often used.
I currently work for a company who I must say has looked after me,so I look after them,However in My spare time I am an avid reader of reference material because I am into all sorts of physics and delight in solving problems.If You have a Problem be it mechanical, electrical,or just plain awkward let me have a think about it.If I Dont come up with a solution then you dont make a contribution towards my next holiday. Put yor trust in Me and we are half way towards putting a smile back on your face.
Vinnie Skelly.co.Elliott -December 08, 2008
I currently work for a company who I must say has looked after me,so I look after them,However in My spare time I am an avid reader of reference material because I am into all sorts of physics and delight in solving problems.If You have a Problem be it mechanical, electrical,or just plain awkward let me have a think about it.If I Dont come up with a solution then you dont make a contribution towards my next holiday. Put yor trust in Me and we are half way towards putting a smile back on your face.
Vinnie Skelly.co.Elliott -December 08, 2008
It would appear that the charge and discharge characteristics of ultracapacitors are inconsistent with most applications where it would be desireable to energise them in seconds or miliseconds, then feed the current back out as required. So why dont we furnish the system with an intermittent series of electronic valves. If i have invented them then i shall call them transodes,which is basically a doide that will allow the current in but not out again until a small cue is given to base as in a transistor which in turn will allow the current to flow back out of the capacitor.With todays technology Iv,e no doubt these devices could easily be fashioned and incorporated into ultracapacitors during manufacture.Perhaps the first base activated could send a signal to the second base and the second to the third,etc,until the final gate completes the discharge.These built in components would by my own reckoning easily stand the outflux of energy since none of them would bare the full potential current discharge under the pre-controlled arrangement.Now Who is going to make me a millionire for telling you how to make a transode. Then i can pack in work and concentrate on solving more problems for you.
Vinnie.Skelly.co.Elliotthire. -December 08, 2008
It would appear that the charge and discharge characteristics of ultracapacitors are inconsistent with most applications where it would be desireable to energise them in seconds or miliseconds, then feed the current back out as required. So why dont we furnish the system with an intermittent series of electronic valves. If i have invented them then i shall call them transodes,which is basically a doide that will allow the current in but not out again until a small cue is given to base as in a transistor which in turn will allow the current to flow back out of the capacitor.With todays technology Iv,e no doubt these devices could easily be fashioned and incorporated into ultracapacitors during manufacture.Perhaps the first base activated could send a signal to the second base and the second to the third,etc,until the final gate completes the discharge.These built in components would by my own reckoning easily stand the outflux of energy since none of them would bare the full potential current discharge under the pre-controlled arrangement.Now Who is going to make me a millionire for telling you how to make a transode. Then i can pack in work and concentrate on solving more problems for you.
Vinnie.Skelly.co.Elliotthire. -December 08, 2008
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