A capacitor is a system that can store and release an electric charge. It consists of two conductor plates separated by a non-conductive insulator (dielectric). When an electric charge is connected to a capacitor the positive plate loses electrons and the negative plate gains electrons. The difference in electrons between the two plates creates a potential electric current, much like static electricity. When a connection is made between the two plates, the extra electrons leave the negative plate and move to the positive plate creating an electrical current. The current exists until both plates have balanced out the difference in electrons. The discharge can be instantaneous (as happens in static electrical discharges) or controlled through a resistor, which determines the rate at which an electrical current can leave the capacitor.
Uses for Capacitors
Capacitors have many uses. Some of these include:
- smoothing out an electrical current for a power supply
- releasing a quick burst of electricity for use in flash photography or ignition systems
- filtering audio tones
- tuning radio systems
- storing electricity
Ultracapacitors are designed to store and release much greater amounts of electricity than traditional capacitors while still remaining relatively small. This is achieved by using enhanced materials that increase the amount of surface area where charge carriers (ions) can be stored. These materials include powdered activated carbon, whose porous nature allows for greater surface area, or carbon nanotubes, a material where nanotubes have been synthetically arranged, replicating porous materials but with much greater consistency, i.e., all the holes can be uniformly arranged.
As the technology for ultracapacitors develops there is consideration that they may be able to replace chemical batteries for certain applications. Some of the current uses include recouping energy from braking, electric bus powering, electric car batteries, consumer electronics powering.
Advantages and Disadvantages of Ultracapacitors (section copied from Wikipedia article)
- The amount of energy stored per unit weight is considerably lower than that of an electrochemical battery (3-5 W·h/kg for an ultracapacitor as of 2010 compared to 30-40 W·h/kg for a lead acid battery), and about 1/10,000th the volumetric energy density of gasoline.
- As with any capacitor, the voltage varies with the energy stored. Effective storage and recovery of energy requires complex electronic control and switching equipment, with consequent losses of energy
- Has the highest dielectric absorption of any type of capacitor.
- High self-discharge - the rate is considerably higher than that of an electrochemical battery.
- Cells have low voltages - serial connections are needed to obtain higher voltages. Voltage balancing is required if more than three capacitors are connected in series.
- Linear discharge voltage prevents use of the full energy spectrum.
- Long life, with little degradation over hundreds of thousands of cycles. Due to the capacitor's high number of charge-discharge cycles (millions or more compared to 200 to 1000 for most commercially available rechargeable batteries) it will last for the entire lifetime of most devices, which makes the device environmentally friendly. Rechargeable batteries wear out typically over a few years, and their highly reactive chemical electrolytes present a disposal and safety hazard. Battery lifetime can be optimised by only charging under favorable conditions, at an ideal rate and, for some chemistries, as infrequently as possible. EDLCs can help in conjunction with batteries by acting as a charge conditioner, storing energy from other sources for load balancing purposes and then using any excess energy to charge the batteries at a suitable time.
- Low cost per cycle
- Good reversibility
- Very high rates of charge and discharge.
- Extremely low internal resistance (ESR) and consequent high cycle efficiency (95% or more) and extremely low heating levels
- High output power
- High specific power. According to ITS (Institute of Transportation Studies, Davis, California) test results, the specific power of electric double-layer capacitors can exceed 6 kW/kg at 95% efficiency
- Improved safety, no corrosive electrolyte and low toxicity of materials.
- Rapid charging — supercapacitors charge in seconds.
- Simple charge methods — no full-charge detection is needed; no danger of overcharge.