Gerry's link describes a storage system that I think has great potential except for a fatal flaw; if it is scaled up to become a significant part of the electric supply there will not be enough saltpeter available.
I think this same flaw exists for nearly all the proposed solar energy storage systems, there will not be enough of the raw materials available to scale them up to a significant level.
There is really only one solar energy storage system for which the raw material is in adequate supply and that is rain. However, even this one is limited because to be useful the rain has to fall on the top of high ground in a location where a dam can be built and this combination of weather and geography is in short supply.
P.S. Nitrates are also fertilizers so this salt solution storage idea has some similarities to bio-fuels in that it would probably have a serious effect on food crops if implemented on a large scale.
Skippy; There are two ways to pronounce *c*y*n*i*c*a*l*; one is "cynical", the other is "realistic".
An open mind is a virtue...so long as all the common sense has not leaked out.