I recall from my hobbyist days that high-voltage capacitors can keep a charge for a significant amount of time, especially if they are cut- off from the circuit subsequent to charging.
I'd like to know whether it might be feasible, from a technical and cost-effectiveness standpoint, to use a large array of specially designed capacitors to hold excess electricity from solar plants for, say, periods of a week or more. This would be helpful to bridge gaps caused by overcast weather, and for other purposes.
I know that there are potentially problems with dielectric breakdown and leakage. On the plus side, such "batteries" could be charged and discharged very quickly, as needed, and without the complications involved in conventional batteries using chemical electrolytes.