Problems with ultracapacitor energy storage may just be a storm in a teacup.
The main problem with using ultracapacitors or supercapacitors for storing energy is that they have a relatively low energy density, compared with batteries and fuel cells. They are bulkier and heavier than batteries at the present time, for an equivalent amount of energy being stored by both methods. Fuel cells have the optimal energy density. There is research going on to try to reduce the size of ultracapacitors. The big advantage with ultacapacitors is that they can be charged and discharged very quickly and efficiently.
Another idea which I do not know if it would work, might be to see if you could build an ultracapacitor that could handle higher transient voltages for very short amounts of time than it could store reliably long term. The amount of energy stored on a capacitor is related to the square of the voltage across the capacitor. After a threshold voltage is reached the capacitor can discharge through the plates, catastrophically, rather than store the energy.
If a capacitor could handle a higher transient voltage and a higher amount of energy for a very short amount of time without blowing up or being damaged, it might be possible to create a loop of a number of such capacitors such that the energy being cycled around the capacitors might be more than the individual capacitors could statically store in sum. Such a loop would also have to deal with very large currents, and might have to be significantly cooled. So there would also be a dissipative energy cost to maintain such a loop. There would be tradeoffs to consider.
A capacitor alone is usually an efficient energy storage medium and does not lose energy through heat when it charges and discharges, but it may not store that much energy. The idea is that it might be possible to design ultracapacitor circuits, if you could build components that are reliable for very short amounts of time, which might have a higher energy density comparable to batteries and, maybe eventually, fuel cells. It’s worth a research project to see if it would work. With research some things prove to work, and other things don’t. That’s how it goes.
7 Nov 2006
Going off the last reference on the Wikipedia for Ultracapacitors (on the 7 Nov 06), the direct link is here, perhaps a better way to describe this idea is that while the energy density for individual capacitors is low, a circuit of capacitors might have a very high power density – provided that energy is pulsed around a circuit of capacitors repeatedly. As a unit it may be possible to put energy into and out of such circuits and to consider the energy density of the unit as a whole. Maybe such circuits could improve on the energy density of individual capacitors by themselves.
The graph at the end of the CAP-XX webpage lists the range of “0.25 to 10,000 kW/kg” as the power density for capacitors – for only very short durations though.
Still not sure if it would work. It would also depend on how long a capacitor would need to recover from briefly holding that pulse of energy, and that would determine the size (and weight) of the complete energy storage unit.
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