In our April cover feature, “Charging into the future”, we looked at developments in battery technology aimed at storing large amounts of charge for long periods; the main application for this is in storing intermittent renewable energy when it is available, so that it can be sent to the grid when the sun is not shining, the wind is not blowing or at periods of slack tide.
However, batteries are not the only game in town even when it comes to electrochemical storage. Capacitors, originally a method for storing electrical charge and, in fact, the forerunners of batteries, have now developed to the point where they can be used alongside – or some developers believe, instead of – batteries. Often called super-capacitors or ultra-capacitors, these devices are capable of storing a great deal of charge, and as materials chemist Prof Claire Gray of Cambridge University explains, they tend to store at high power but with lower energy density than batteries. “We are now looking at systems which are a hybrid between super-capacitors and batteries. They have electrodes which are able to handle very high rates of power and design principles of both a battery and a super-capacitor. The chemistry is almost a mixture between the two,” she said.
In the design of batteries and capacitors, power and energy always trade off against each other. Batteries tend to have low power but high energy density, while capacitors are the opposite. Gray explained that the traditional role for capacitors is an application such as AC to DC rectification, where they store charge as the AC waveform peaks and discharge it as it troughs, changing the electricity supply from a sine wave into a flat line.
But the new generation of super- and ultra-capacitors do not fit into this paradigm. For example, UK start-up ZapGo is positioning itself as an ultrafast charging energy storage solution, but to basic principles it looks far more like a capacitor than a battery.