SUPERCHARGED VERSIONS of the capacitors commonly used in electronics are approaching the storage density of conventional batteries and could replace them for many uses.
The big advantage of the devices, called ultracapacitors, is that they can be charged and discharged rapidly, although they can give a steady, longer term, more battery-like flow depending on the load.
One possible long-term application is a laptop battery that can charge in as little a minute – the speed would be dictated less by the ultracapacitor than by the size of the mains adapter required to deliver the charging current.
Capacitors store charge and smooth out current flow in circuits in much the same way as a lake in the course of a river steadies its flow and acts as a reservoir. Except that most capacitors are more like tiny ponds, with a capacitance of billionths or millionths of a farad. A conventional capacitor with a rating of one farad would be massive.
The charge density of ultracapacitors is orders of magnitude higher. One already on the market, about the size of a D-cell battery, has a capacity of 350 Farads. Like other ultracapacitors currently available it is based on activated carbon, which has a structure like a sponge with nanoscale pores providing an enormous surface area to which charge adheres.
Researchers led by Joel Schindall, associate director of the Laboratory for Electromagnetic and Electronic Systems at MIT, are developing ultracapacitors with an even greater surface area using carbon nanotubes rooted on an aluminium substrate, like threads on a think-pile carpet.
“We have already achieved [two times] the energy density of activated carbon and within weeks or months we expect to get [five times],” Schindall said at the Emtech conference.
That would give ultracapacitors 25 per cent of the energy density of a standard Li-ion battery. Schindall said that for some heavy duty applications that is better than it sounds because the batteries used for them operate in the range of only 50 per cent to 60 per cent of their capacity and so actually have far less available energy than their theoretical storage capacity.
The rapid charge and discharge rates of ultracapacitors make them suitable for driving electric vehicles and regenerative braking, whereby kinetic energy is stored during deceleration so that a vehicle wastes less energy.
This task also exploits another advantage of the new ultracapacitors – they can be recharged more than a million times without deteriorating. Effectively that gives them an indefinite working life, says Schindall.
In the shorter term they could be used in rapid-charge auxiliary batteries for laptops and other electronic equipment. But Schindall says these are early days and energy densities could rise still further, which could make them a viable replacement for Li-ion batteries.
He said, “When digital cameras came in I wondered if they would ever match the resolution of film. How many people use a film camera now?”
The hope is that the cost can also be brought down to that of conventional batteries. But Schindall points out that even if they are much more expensive they could work out cheaper in the long run because they would not need replacing. µ