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 penods;the main application forthis 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 blowingor 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.
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