(Phys.org)—One of the greatest challenges in generating energy from renewable sources is finding a way to store the continuously fluctuating energy being produced. Batteries, supercapacitors, and most other energy-storage technologies typically can’t respond quickly enough to the second-by-second fluctuations inherent in wind and solar energy sources. One device that does have a sufficiently fast response is electrostatic capacitors, but their drawback is their low energy density—they simply cannot store very much energy in a given volume.