Capacitors used for energy storage
Capacitors are devices which store electrical energy in the form of electrical charge accumulated on their plates. When a capacitor is connected to a power source, it accumulates energy which can be released when the capacitor is disconnected from the charging source, and in this respect they are similar to batteries. The difference is that a battery uses electrochemical processes to store energy, while a capacitor simply stores charge. As such, capacitors are able to release the stored energy at a much higher rate than batteries, since chemical processes need more time to take place.
The amount of stored energy depends on the amount of charge that is stored on the capacitor’s plates. Alternatively, the amount of energy stored can also be defined in regards to the voltage across the capacitor. The formula that describes this relationship is:
where W is the energy stored on the capacitor, measured in joules, Q is the amount of charge stored on the capacitor, C is the capacitance and V is the voltage across the capacitor. As seen from the above equation, the maximum amount of energy that can be stored on a capacitor depends on the capacitance, as well as the maximum rated voltage of a capacitor.
The stored energy can be quickly released from the capacitor due to the fact that capacitors have low internal resistance. This property is often used in systems that generate large load spikes. In such cases, batteries cannot provide enough current and capacitors are used to supplement batteries. During off-peak working conditions, the capacitor is again recharged to a nominal voltage.
There are many applications which use capacitors as energy sources. They are used in audio equipment, uninterruptible power supplies, camera flashes, pulsed loads such as magnetic coils and lasers and so on. Recently, there have been breakthroughs with ultracapacitors, also called double-layer capacitors or supercapacitors, which have extremely high capacitances, reaching in excess of 2kF. Such capacitors can store large amounts of energy and offer new technological possibilities, especially in areas such as electric cars, regenerative braking in automotive industry and industrial electrical motors, computer memory backup during power loss and many others.
Electronic camera flashes mostly use xenon flash tubes. Before a shot is taken, an internal capacitor is charged to a few hundred volts using special circuitry inside the camera. This charging period is occasionally accompanied by a characteristic high pitched noise. After the capacitor is charged, the flash is ready for use. When the shutter button is pressed on the camera, the capacitor is nearly instantly discharged through the tube, creating a very short current pulse. The energy from the capacitor is converted to a flash of light, in a process that lasts only about 1 millisecond. After the flash is used, it takes some time for the capacitor to recharge again before the next shot can be taken.
Bug zappers use diodes and capacitors in a circuit called the cascade voltage multiplier, which increases the supply voltage to about 2kV. The energy is almost instantly released once the insect creates a short between two terminals of the cascade. The capacitors in the circuit recharge during “zaps”.
A car in motion carries a significant amount of kinetic energy, which is normally converted to heat under braking. Since electric and hybrid cars use electricity to power the wheels under acceleration, it became clear that the energy lost to braking could be instead stored and reused when needed. The only problem with this technology was that car batteries could not recharge at a sufficient rate in order to absorb the energy created by braking. With the modern advances in capacitor technology, more specifically supercapacitors, it is now possible to convert and store a portion of kinetic energy as electrical energy. This way, driving a car downhill and using regenerative braking actually recharges the battery, and increases the efficiency of the vehicle. In the case of hybrid cars, this translates to better mileage per gallon, while in the case of electric cars this means more miles per single charge.
Safety and hazards
Capacitors, as well as other capacitors used for other purposes in circuits, can store charge long after they have been disconnected from the circuit, or after the power was disconnected from the device. High voltage capacitors can accumulate charge even if they have never been used due to electrostatic charge buildup. As such, they pose an electrical shock hazard. Because of this, it is necessary to discharge capacitors before handling them to prevent injury. High voltage and high energy capacitors should be stored with their terminals shorted to prevent charge buildup over time.