Supercapacitors

What are they?

Supercapacitors (more technically referred to as electrolytic double-layer capacitors) are electrochemical devices that store unusually large amounts of electrical charge, much more than regular capacitors.

In a regular (electrostatic) capacitor, two plates are separated by a dielectric, which prevents charge from moving directly between the plates. Charge can, however, be moved externally by connecting a battery to the terminals. After removing this external influence, the charge on the plates persists. The thinner this dielectric can be made, the more charge that can be stored, i.e., the greater the capacity.

The first major breakthrough in capacitor design was to replace the millimetre (10-3 m) thick dielectric with a micron (10-6 m) thick oxide layer on an aluminium electrode with an electrolyte solution serving as the opposite electrode. In these electrolytic capacitors, charged atoms (or ions) in the electrolyte crowd against the electrode of opposite charge—positive to negative, and vice versa, thereby storing charge.

A supercapacitor (also known as an ultracapacitor) takes the electrolyte capacitor design to new extremes by using two porous graphite electrodes with an electrolyte solution pervading their nanometre (10-9 m) size pores. The charge-separation distance is thus reduced to the dimensions of the ions within the electrolyte, with enormous increases in capacitance.

Why do we need them?

When designing an energy storage system one must consider both the total energy needed and the rate at which it will be delivered—the power. Ordinary capacitors provide very high power, but can store only small amounts of energy. Batteries, in contrast, can store a lot of energy, but only release that energy slowly, that is, they provide low power. Supercapacitors bridge the gap between these two devices, as they store much more energy than ordinary capacitors, and provide much higher power than batteries.

Where can they be used?

Small electrolytic capacitors with a capacitance of a few farads or less are used throughout the electronics industry in devices such as mobile phones, digital cameras, PDAs and so forth. They are used to provide a back-up power source to protect memory during short power interruptions. Supercapacitors have a capacitance of a few thousand farads, and these are thus suitable for large-scale applications such as transport or for supporting renewable energy generation.

Transport

Supercapacitors can be used in electric and hybrid vehicles to provide power boosts to start the engine or to assist the vehicle's motor during acceleration. They can also be used to store energy regenerated from braking. However, they do not have sufficient energy to be used as the sole energy source in a purely electric vehicle. Supercapacitors are instead used to supplement a primary energy source, which may be an internal combustion engine, a battery pack or a fuel cell. Some vehicles use three energy sources—for instance, a fuel cell, battery pack and supercapacitor pack—each working together to provide for the different power levels required.

Renewable energy

Most renewable energy sources do not generate energy continously—they are intermittent. Energy storage using supercapacitors can support renewable energy generation by allowing excess energy to be stored until it is needed.

Supercapacitors can also be used to support renewable energy generators even when they are operating at full load. For example, each time a blade of a wind turbine passes the support tower, there is a momentary dip in the power it supplies. This causes oscillations which can be damaging to the generators, shortening its useful lifetime, while also wasting energy in the form of the physical movement of the turbine rather than it being converted to useful electrical energy. Incorporating a bank of supercapacitors into a wind turbine system can help "smooth out" the energy generated while also protecting the generators.