Conventional capacitors have enormous power but store only tiny amounts of energy. Supercapacitors offer a unique combination of high power and high energy. Supercapacitors are capable of very fast charges and discharges, and are able to go through a large number of cycles without degradation.
Batteries are charged when they undergo an internal chemical reaction. They deliver the absorbed energy, or discharge, when they reverse the chemical reaction. In contrast, when a supercapacitor is charged, there is no chemical reaction. Instead, the energy is stored as a charge or concentration of electrons on the surface of a material. The energy can be released in a micosecond, much faster than from a chemical reaction.
An ultracapacitor or supercapacitor, also known as a double-layer capacitor can be viewed as two nonreactive porous plates, or electrodes, immersed in an electrolyte, with a voltage potential applied across the collectors. In an individual ultracapacitor cell, the applied potential on the positive electrode attracts the negative ions in the electrolyte, while the potential on the negative electrode attracts the positive ions. A porous dielectric separator between the two electrodes prevents the charge from moving between the two electrodes.
The plates hold opposite charges which generates an electric field. Unlike batteries where energy is stored in chemical form, supercapacitors store energy via electrostatic charges on opposite surfaces of the electric double layer, which is formed between each of the electrodes and the electrolyte ions. Because ultracapacitors move electrical charges between solid-state materials rather than through a chemical reaction, they can be cycled tens of thousands of times, more rapidly, and are not affected by deep discharges as are chemical batteries. Discharge times range from fractions of seconds to tens of seconds and in some cases several minutes.
Once the ultracapacitor is charged and energy stored, a load can use this energy. The amount of energy stored is very large compared to a standard capacitor because of the enormous surface area created by the (typically) porous carbon electrodes and the small charge separation (10 angstroms) created by the dielectric separator. However, it stores a much smaller amount of energy than does a battery. Since the rates of charge and discharge are determined solely by its physical properties, the ultracapacitor can release energy much faster (with more power) than a battery that relies on slow chemical reactions. Supercapacitors using metal oxide coated electrodes rather than activated carbon, which increases the energy density and reduces the volume for the same capacity, are now in production by NessCap. Organic electrolytes and carbon nanotube electrodes are now the subject of research to increase supercapacitor performance.
- Can be charged and discharged almost an unlimited number of times
- Can discharge in matters of milliseconds or as long as tens of seconds or several minutes
- Can be charged in seconds to minutes
- High power density
- Do not release any thermal heat during discharge
- There is no danger of overcharging; when fully charged the ultracapacitor simply quits accepting a charge
- Are not affected by deep discharges as are chemical batteries
- Have a long lifetime, which reduces maintenance costs; anecdotal evidence suggests that they lose about 80% of their storage capacity after 10 years, with a lifetime estimated to be 20 years
- The DC-DC round-trip efficiency is 80%-95% in most applications
- Operating temperature range as great as between -50C and 85C, capacity increases as temperature decreases below the rating temperature
- They do not release any hazardous substances that can damage the environment
These characteristics make them very useful devices for energy storage including such applications as load leveling, UPSs or power bridging and pulse power applications where a sudden boost of power is needed for a short (fractions of a second to tens of seconds) period of time.
Supercapacitors are well suited to replace batteries in many applications. This is because their scale is comparable to that of batteries, from small ones used in cellular phones to large ones that can be found in cars. Even though supercapacitors have a lower energy density compared to batteries, they avoid many of the battery's disadvantages. Batteries have a limited number of charge/discharge cycles and take time to charge and discharge because the process involves chemical reactions with non-instantaneous rates. These chemical reactions have parasitic thermal release that causes the battery to heat up. Batteries have a limited life cycle with a degrading performance and acidic batteries are hazardous to the environment.
Ultracapacitors have replaced or are being tested to replace batteries in engine starting applications such as large diesel generators, tank engines, submarine engines and locomotive engines. For these applications ultracapacitors take up less space and weight than conventional batteries, but posses excellent cold weather capabilities for starting, long life, and low maintenance. General Motors has developed a pickup truck with a V8 engine that uses the supercapacitor to replace the battery. The efficiency of the engine rose by 14%. The generator is used to provide provide power for lighting and other electrical accessories and to charge the ultracapacitor. They are used in missiles to replace batteries because of their higher reliability and long storage life.
The high specific energy of ESMA (Russia) asymmetric supercapacitors of so-called traction type makes it possible to use them as the sole power source for electric buses, electric trucks, floor cleaners and vehicles such as electric cars, electric bicycles and scooters. Electric vehicles, with supercapacitors as their power source, are designed for operating over short or fixed routes. Cargo or passenger transportation efficiency in this case is high due to the use of a lighter storage device that can be quickly charged (10-15 min) as in contrast to conventional storage batteries. Buses and delivery trucks using these supercapacitors are being used (presumably on a demonstration basis) in Moscow. A similar bus, the “TOHYCO-Rider,” is under development in Switzerland, it uses supercapacitors supplied by Maxwell SA. The bus will be charged after every transportation cycle within 3-4 minutes.
Ultracapacitors have replaced batteries in some missiles because of their greater reliability, longer shelf life and insensitivity to temperature extremes.
HYBRID ELECTRIC VEHICLES (HEVs)
In general, the advantages of ultracapacitors over batteries in the transportation market include their reduced size and weight which yields both design and cost improvements, their operating temperature range, and their ability to absorb and deliver energy very quickly. Ultracapacitors can be used in HEVs to relieve battery's load during high power times, such as initial acceleration and braking. These are the instances when the batteries see the highest current levels. By load leveling these spikes the batteries will last longer, saving the customer money. Ultracapacitor technology allows the battery to handle the energy requirements while the capacitors handle the high power requirements. In addition, ultracapacitors allow for lower emissions, better fuel-efficiency and advanced electrical drive capabilities. A power system using ultracapacitors allows HEV's to recapture and reuse more of the braking energy.
The Honda FCX V3, a fuel cell hybrid, uses a supercapacitor to provide extra power to the drive system when going up grades, during acceleration and when passing. The primary electrical power for the FCX is produced by a hydrogen fuel cell. The capacitor, connected in parallel to the fuel cell, provides quicker response and more power compared to the battery based energy storage used in most hybrids. The capacitor is also used as part of the regenerative braking system, along with the fuel cell, for storing the energy generated from braking. It discharges its energy only as needed, not all in one burst.
New Flyer of Winnepeg, Manitoba, Canada has a standard line of hybrid buses, gasline or diesel, that can optionally be equipped with Thundervolt hybrid-electric drive systems equipped with ultracapacitors. Long Beach Transit has a large fleet of these buses. The Thunderpack II ultracapacitor system is supplied to the drive manufacturer, ISE, by Maxwell. ISE's ThunderPack II ultracapacitor energy storage system is comprised of 288 of Maxwell's Boostcap 10 ultracapacitors, rated at 2500F each. The ultracapacitor packs have several advantages over batteries, including more efficient capture and release of electrical energy, longer cycle lives, and lighter weight. The largest advantage to the transit industry is that the ThunderPack II ultracapacitor energy storage system comes with an unprecedented five year warranty. It is the only commercially available engine or drive system that meets both EPA and CARB 2007 emissions standards.
Supercapacitors are used in UPS or bridge power applications, without the need for batteries, to maintain power until backup power can be established. One of their first uses was in computers to maintain power while batteries are replaced. Ultracapacitors can also serve to maintain the power quality supplied to computers and other sensitive electronic equipment due to dips, surges and voltage reductions. Load leveling is achieved by absorbing power during surges and discharging power during dips. Supercapacitors can provide tens of seconds of UPS service and almost unlimited service time in power quality service.
BATTERY LIFE and PERFORMANCE
Small supercapacitors are used to extend the battery life and enhance the performance and functionality of hand-held electronic devices, digital cameras, remote transmitting devices and toys.
The leading suppliers of ultracapacitors include Maxwell Technologies, San Diego, CA, USA; ELIT Co, Kursk, Russia; Joint Stock Company ESMA, Moscow, Russia; NessCap, Ltd, Yongin, Kyonggi-Do, Republic of South Korea; Cap-XX pty.Ltd, Lane Cove NSW, Australia.
Ultracapacitors, National Renewable Energy Laboratory, Advanced Vehicles & Fuels Research, Energy Storage, February 2, 2005
Large Scale Energy Storage Systems, Cheung, et al, Imperial College of London, ISE2 2002/2003
ThunderVolt Gasoline Hybrid-Electric Bus; First Volume Use of Ultracapacitors in Buses, Press release, ISE Corp, April 2005