What are the Energy Storage Applications of capacitors?

Capacitors, by nature, store energy when a voltage is applied across them, and then retain it till it is drawn or discharged. Capacitors are electrical energy storage elements by nature. They are used for this purpose in rectifiers for smoothening of output voltage. Oscillator circuits work on the phenomenon of sustained continuous stored energy transfer between capacitor and inductor.

There are several applications where capacitors are specifically designed and used for energy storage. Few among them are as follows:

• Flashlights in cameras – Electrolytic capacitors from 30 μF to 2700 μF and voltages from 300 V to 600 V. Stored energy varies from 1.5 Joules to 450 Joules. The capacitors get charged from battery through a charging circuit, and then stored energy is discharged in flashbulb in milliseconds, which causes the flash.
• Pulsed loads like magnetic coils / lasers etc. – Pulses are produced by discharge of capacitor in primary coil of a transformer or autotransformer-like tapped coil.
• Ignition systems in automobiles- Capacitor of 0.1 to 0.33 μF in case of ignition coil is discharged into primary of ignition coil to produce 12 kV to 20 kV. In CDI systems, CDI capacitor discharge is used to get high voltage for ignition in petrol engines.

• Timing circuits, delay timers in relays etc. – Capacitor charged to a known voltage is discharged (or charged) through a resistor, and RC time constant decides the time when voltage reaches a predetermined value to activate / deactivate a relay or some device.
• Bug zappers – Electronic circuit charges a capacitor to high voltage of about 2kV, which is connected between two parallel wire nets. A mosquito trapped between these nets gets killed by the high voltage.
• Short time emergency supply to hold circuit – Large value capacitor can hold a circuit on in case of supply cutoff, for a time decided by design.

• Regenerative braking – This is particularly useful in vehicles, where an ultracapacitor stores energy at rapid rate from regeneration during braking. Stored energy is passed on to battery for charging, or is used when vehicle is accelerating.

• Rectifier outputs invariably have a capacitor on output side, which stores energy, and supplies to circuit when output voltage goes down between peaks. This takes care of all ripples to get smooth steady output voltage. This capacitor is also called Reservoir Capacitor.
• DC Link capacitor

Energy storage levels differ vastly for different applications. For example, 0.22 μF 400 V ignition capacitor stores just 0.02 Joules. Electrolytic capacitor of 2500 μF 450 V DC stores a huge 253 Joules, while Supercapacitor of 5000 F charged at 2.5 V stores 15,625 Joules, or 4.3 Watt-hours (Wh). There are numerous applications using capacitors in circuits, each having its own requirements of energy storage. A 20 nf vacuum capacitor rated at 20 kV will store 4 Joules when fully charged at rated voltage.

Aluminium electrolytic capacitors have among the highest energy storage levels. In camera, capacitors from 15 μF to 600 μF with voltage ratings from 150 V to 600 V have been used. Large banks of Al. electrolytic capacitors are used on ships for energy storage since decades. Capacitors up to 20,000 μF and voltage ratings up to 500 V are available. They store large amount of energy among electrolytic capacitors, and find use for high ripple current applications like PWM inverters, large KVA line UPS, renewable energy power supplies, wind generator drives, railway traction drives etc. 10,000 μF capacitor charged at 450 V stores an energy of about 1000 Joules.

Largest electrostatic capacitor bank is installed at Dresden, Germany High Magnetic Field Facility. It uses a capacitor bank to store a total of 50 MJ 10 KV energy to build extremely high pulsed magnetic field up to 100 Tesla for 10 ms (or 70 Tesla for 100 ms). Metallized plastic film capacitors are used in several electrical and electronic circuits including rectifiers, power supplies etc., as energy storage devices.

Ceramic capacitors (MLCC) are used for energy storage in electronic circuits for PCB mounting, and for smaller energy storage requirements. They have advantage of high temperature operations, and long life.

These are just a few common energy storage applications of capacitors, and there are several more if we look around. Ultracapacitors (or supercapacitors), developed mainly over past two decades, have been specifically designed to store large energy levels of the order of batteries, and are extensively used all over the world for innumerable applications- from mobile phones, toys, meters, vehicles and power supply systems. They also find applications in grid storage and renewable energy systems. Wind turbine pitch control batteries have long been replaced by ultracapacitors all over the world. They also work a stand-alone energy storage for remote locations and high reliability applications.