All capacitors consist of two metallic electrode plates separated by an insulating medium called the capacitor's dielectric layer. Some types of capacitors also require a solid or liquid electrolyte. There are three main classifications of capacitors: electrostatic, electrolytic, and electrochemical.
Electrostatic capacitors use an insulating material in between the metallic electrode plates to act as the dielectric material. They have low capacitance values, do not use an electrolyte, and are nonpolar. Examples of electrostatic capacitors include ceramic, dc film, motor run, Teflon, mica, and porcelain types.
Electrolytic capacitors use a solid or liquid electrolyte in their construction and have higher capacitance values than electrostatic capacitors. The dielectric layer in an electrolytic capacitor is formed as an oxide on the metal plate surface. They are inherently polar due to their construction, but nonpolar ratings are available in some product classes. Electrolytic capacitors include solid and wet tantalum as well as aluminum types.
Electrochemical capacitors have a dielectric layer that forms naturally in the electrolyte with applied voltage. This dielectric forms in a very thin double layer on the surface of the capacitor's electrodes. Because of this effect, these capacitors are also known as double layer capacitors (DLC). The charge in these capacitors is stored electrostatically and not chemically, as the name “electrochemical” implies.
EC Capacitor Characteristics
By using a very high surface area substance for the capacitor electrode, EC capacitors can reach 50,000+farads (F) in a single cell. Activated carbon is a common electrode material due to its high surface area (1000+m2/g), availability, chemical stability, and relatively low cost.
The voltage rating of an EC capacitor cell is limited by the decomposition potential of its electrolyte. For a capacitor cell using an aqueous electrolyte such as KOH (potassium hydroxide), the usable cell voltage range is 0.8Vdc to 1.6Vdc. The EC capacitor cell voltage using a nonaqueous electrolyte (organic or inorganic) can reach as high as 3Vdc to 4Vdc. However, the use of a nonaqueous electrolyte requires the use of dry rooms, vacuum chambers, and other expensive processing methods that result in a significantly higher product cost.
To gain the higher voltages required by many applications, the cells must be stacked in series to make the final capacitor. For example, an automotive application could specify a maximum voltage of 14Vdc. Using a KOH electrolyte, an EC capacitor manufacturer may determine that it can use multiple EC capacitor cells, each rated at 1.3Vdc. To satisfy the application requirement of 14Vdc, 11 cells connected in series would be needed (11 cells×1.3Vdc = 14.3Vdc). For any particular application, the EC capacitor manufacturer determines the cell voltage and necessary number of cells.
The following two types of cell designs have emerged for the larger EC capacitors:
- Symmetrical cell designs use the same material for both metallic electrode plates, such as activated carbon. The nomenclature for this cell design is carbon/carbon.
- Asymmetrical cell designs use a different material for the two metallic electrode plates such as nickel hydroxide and activated carbon. This cell design is referred to as nickel/carbon.
Symmetrical EC capacitors can be put in the application without regard to polarity. An additional advantage of the symmetrical design is that the EC capacitor can be discharged to 0V like a conventional capacitor.
For an asymmetrical EC capacitor, you must strictly observe the polarity. Also, asymmetrical EC capacitors have a minimum discharge voltage you must observe like a battery. The key advantage of the asymmetrical design is that the EC capacitor will have four to five (or more) times more energy density than a symmetrical EC capacitor. An analysis of the EC capacitor manufacturer's application requirements will show which design will be the most suitable, cost-effective, and easiest to implement.
Units used for final EC capacitors are typically kilofarads (kF) or farads (F), which is a marked difference from conventional capacitors measured in the microfarad (μF), nanofarad (nF), or picofarad (pF) ranges.
Voltage Balancing of Cells
EC capacitors are made up of cells, so voltage balancing is an important issue. When applying voltage across the terminals of the capacitor, voltage splits up among the cells that make up the capacitor. In the 14Vdc capacitor made up of 11 cells (described above), each cell should have 14Vdc/11 = 1.27Vdc per cell when the 14V is applied to the capacitor. This assumes the voltage is split up evenly among the cells. If the cells do not have closely matched impedances or if there is a problem with one cell, the voltage can split unevenly causing excessive voltage on one or more cells. This can cause the capacitor to fail in the application.
Some EC capacitor designs require active or passive voltage balancing by the user. Active voltage balancing uses smart electronics such as ICs or microprocessors to actively keep the voltage balanced across all of the cells. Passive voltage balancing uses simple components such as resistors, conventional capacitors, and inductors to keep the voltage equal across each cell. If the EC capacitor design requires voltage balancing, it will increase the user's cost and complexity. Therefore, voltage balancing is an important issue for EC capacitor designs.
Capacitor Comparisons
EC capacitors have much higher capacitance density than conventional capacitors. This means that EC capacitors can store much more energy in the same size package. Although a conventional capacitor may have an energy density up to 0.15 wh/kg, EC capacitors can reach up to 12 wh/kg. A watt-hour (wh) = 3600 watt-sec. = 3600 Joules. Fig. 1 shows the energy vs. power of state-of-the-art EC capacitors optimized for energy, such as those used for traction or load-leveling applications.
On a per farad basis, EC capacitors have the lowest cost of any capacitor technology. Although an aluminum electrolytic capacitor will cost from $100 to $300 per farad, EC capacitors currently cost anywhere from $20 to $1 (or less) per farad. Replacing a bank of large aluminum electrolytic capacitors being used for energy storage or pulse discharging with one EC capacitor can save the user a substantial amount of money.
EC capacitors also have high shock/vibration survivability. Because its dielectric forms naturally with applied voltage, EC capacitors are not affected by shock or vibration. On the other hand, shock and vibration can have a profound effect on the dielectric of conventional capacitors.
On the negative side, EC capacitors have slightly higher ESR (equivalent series resistance). This is due to the ESR of the individual cells adding together as they are stacked in series to reach the required application voltage level. Because of the higher ESR, EC capacitors have a slightly more sluggish response. This means they can't release their energy as quickly and efficiently as conventional capacitors. However, in most pulse discharge applications, the ESR of the EC capacitors is low enough to satisfy the energy requirements. Fig. 2, on page 70, shows the normalized ESR for state-of-the-art EC capacitors.
In another difference, EC capacitors have a larger capacitance temperature coefficient. A preferred operating range is typically -40°C to 60°C. With some trade-offs in design parameters, these capacitors can be designed to work from -50°C to 85°C. At extreme cold temperatures, the EC capacitor may lose up to 30% of its nominal capacitance. Conventional capacitors are usually rated to work at higher temperatures (85°C+), and will typically have a smaller capacitance change at extreme temperatures.
EC capacitors also have a higher self-discharge rate due to their higher DC-Leakage (DCL). Impurities in the metallic electrodes cause this leakage current. Electrolytic capacitors may measure in the microampere range for DCL, which EC capacitors can measure in the milliampere range. For high power applications, this DCL level is insignificant. However for low-power or low-current applications, this higher DCL level can be an important parameter to consider.
EC Capacitors vs. Primary Batteries
You can also use EC capacitors in applications where batteries are employed. For example, they're used in voltage backup applications where the voltage must be held a certain level for a few seconds. For applications that require batteries, the cost to maintain and replace the batteries can be high and labor intensive, which is especially true for remote locations. Compared with batteries, EC capacitors don't require system maintenance.
Another advantage of EC capacitors is that they are safe and environmentally friendly. They do not contain toxic chemicals or any heavy metals that make operation, charging, and disposal hazardous. Manufacturer's can process the asymmetrical nickel/carbon designed EC Capacitors through standard recycling channels to reclaim the valuable nickel in the unit.
Higher power capability is also an advantage with EC capacitors. Although batteries excel in energy storage, EC capacitors excel in supplying high-power levels. The power density of EC capacitors can reach up to several kW/kg. However, batteries may only reach from 0.1 kW/kg to 0.5 kW/kg. In Fig. 3 is the curve of energy vs. power for state-of-the-art EC capacitors optimized for power, such as those used in engine starting applications.
EC capacitors have a wider operating temperature range than batteries. A battery uses a chemical system, therefore cold and hot temperatures affect a battery's capacity to supply energy. EC capacitors store their energy electrostatically, so they still work efficiently at temperatures as low as -55°C. A battery couldn't supply power at these temperatures. The capacity vs. temperature curve (Fig. 4, on page 71) compares state-of-the-art EC capacitors with lead-acid batteries.
You can recharge EC capacitors quickly and easily. Unlike batteries that require sophisticated charging systems that may take up to 10 hr to recharge, EC capacitors are easily recharged in seconds or minutes with simple charging systems.
EC capacitors have virtually unlimited cycle life. Battery life is measured in the 100s to 1000s of charge-discharge cycles, whereas EC capacitor life is typically measured in the 10,000s to 1,000,000s cycles.
A disadvantage of EC capacitors compared with batteries is a lower energy density. It's nearly impossible to compete with a chemical battery system for energy storage. However, the large EC devices can reach up to 1/10 to ⅓ the energy of batteries.
Applications
For larger EC capacitors, main application areas are engine starting, traction, and pulse discharge/load leveling.
Engine starting applications range from starting small engines in recreational-utility-type vehicles to starting large diesel generator and locomotive engines. You can use EC capacitors in conjunction with batteries or in place of the batteries in special applications. Advantages of EC capacitors in these applications include less space and weight, excellent cold weather starting, improved breakaway torque, increased battery life, maintenance free operation, virtually unlimited cycle life, and a lifetime cost savings.
Traction applications include electric material handling or utility vehicles, such as forklifts, golf-type electric carts, fixed route buses, and other electric vehicle applications. The EC capacitors can replace the batteries in these vehicles resulting in fewer loads on the vehicle (more payload potential), no maintenance, simple on-board charging, quick charge times, and virtually unlimited cycle life.
Because of fast charging times and simple on-board charging circuits, opportunity recharging is possible with EC capacitors because there's no need to return to a complex central recharging station. Unlike batteries, they do not emit hydrogen while charging.
Pulse discharge/load leveling applications include those currently served by batteries (lead-acid, lithium, etc.) or capacitors (aluminum, tantalum, etc.). Examples include audio line stiffeners, UPS systems, and high-power portable electronic equipment. Advantages for using EC capacitors include more capacitance for less cost, potentially less space, no maintenance, and virtually unlimited cycle life.
Because the cost to maintain and periodically replace lead-acid batteries far exceeds their asset value, EC capacitors are ideal for remote applications, such as cell phone towers, beacons, remote sensing sites, or emergency switch-on stations.
An advantage of EC capacitors in load-leveling applications (UPS systems) is you can periodically test them to ensure reliability. There's no way to test to find out how much life a battery has left. Unfortunately, when one battery fails, the whole battery bank fails. That's why systems often have a primary battery bank and one or two backup battery banks. This multiplies the expense and time needed to maintain and replace the batteries.
Future Issues:
Comparing batteries to capacitors, EC capacitors can't compete on price alone. However, if you consider size, weight, performance, maintenance costs, replacement costs, and downtime costs, then EC capacitors become the obvious choice in the right applications. As the production of EC capacitors increases with growing sales, then the economies of scale will bring down the raw material and production costs for these capacitors.
To increase power and energy, EC capacitor manufacturers are continually pushing to increase the capacitance density and cell voltage, and lower the ESR of individual cells. Lowering the ESR is key to raising efficiency and improving response times. Heavy research is continuing by EC capacitor manufacturers to refine materials, production methods, and capacitor design techniques.
References
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Beliakov, A.I., Investigation and Development of Double Layer Capacitors for Starting of Internal Combustion Engines and of Accelerating Systems of Hybrid Electrical Drive, Proceedings from the 6th International Seminar on Double-Layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, Florida, Dec. 1996
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Burke, A.F., Recent Test Results for Advanced Ultracapacitors, Proceedings from the 7th International Seminar on Double-layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, Fla., Dec. 1997
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Zogbi, D., DLC Capacitors and Similar High Energy Storage Devices: World markets and Opportunities, Proceedings from the 7th International Seminar on Double-Layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, Fla., Dec. 1997
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Varakin, N., Klementov, A.D., Litvinenko, S. V., Starodubtsev, N.F., and Stepanov, A.B., New Ultracapacitors Developed for Various Applications, Proceedings from the 8th International Seminar on Double-Layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, Fla., Dec. 1998
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Miller, M. and Burke, A.F., Comparison of the Power characteristics of Ultracapacitors and Batteries, Proceedings from the 8th International Seminar on Double-layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, Fla., Dec. 1998
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Miller, J.R., Engineering Battery-Capacitor Combinations in High Power Applications: Diesel Engine Starting, Proceedings from the 9th International Seminar on Double-Layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, Fla., Dec. 1999
Electrochemical Capacitor History
The theory behind the electrochemical capacitor has been known for more than 100 years, but it was not until 1954 that Becker (GE) developed an energy storage device using the double layer capacitance concept.
In the early 1960s, Sohio developed an energy storage device, but the first successful product on the market was an EC capacitor from NEC Inc., for memory backup applications in the 1970s.
More recently, this industry has recognized Russian EC capacitor manufacturers as leaders in this technology for their large EC capacitors. Their primary motivation for developing the large capacitors rooted from cold weather starting of various engines and generators.
NEC and Pinnacle were two of the first companies in this market, so we often refer to EC capacitors by their trademarked names: “Supercapacitors” or “Ultracapacitors.”