Capacitors: the next 'super' batteries

If you read greencarsales.com.au and other green car pages a bit, you'll have come across mentions of supercapacitors and ultracapacitors in increasing numbers. And chances are you'll come across more in the future, as ultracapacitors gain increasing attention in the auto industry as the future of energy storage.

Capacitors, aka condensors, do not store energy electrochemically in the manner of conventional batteries. Instead, they store it in an electric field -- that is, the surrounding area over which a charged particle or electrode exerts influence.

Where batteries use an electrolytic medium (a substance that conducts electricity) between their two electrodes, capacitors use a dielectric -- aka an insulating substance.

A battery cell functions by the exchange of electrons through the electrolyte between negative and positive electrodes. Connecting the electrodes to an external load -- the powered device -- stimulates a flow of electrons through the load from the negative electrode to its positive counterpart. Removing the external load interrupts the circuit and brings the process to a halt.

In a supercapacitor, the presence of a potential voltage difference between the conducting plates creates an electric field in the dielectric, in which it becomes possible to store energy. The wider and flatter the conductors and the narrower the gap separating them, the better conditions become to achieve this.

With the application of voltage, an electric charge accumulates on each plate.

Capacitors offer numerous benefits over conventional battery technologies. They absorb and release charge more efficiently and they last longer, with a lifespan of ten years or more; they're less vulnerable to fluctuations in temperature and climatic extremes, impact and overcharging and they discharge their energy more efficiently. They're also low-maintenance and because they're not made of toxic chemicals, they're kinder to Mother Earth.

Their primary weakness lies in energy storage -- lithium-ion batteries store up to 25 times more energy per unit of weight than high-end ultracapacitors.

There's plenty of R&D budget and effort going into closing that gap. At this point the actuals are overshadowed by hype and counterhype. The technology is nowhere near ready to replace the battery.

In fact it's proving much more useful as a complementary component in high-efficiency powertrain models. They're potentially useful, for example, in maximising efficiency in the absorption of energy from regenerative braking systems and providing useful bursts of power for auto stop-start systems.

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