I was thinking it would'nt need to be liquid to store energy.
When matter changes state from solid to liquid, or liquid to gas, and the reverse also, there is a thing called latent heat. Boiling water for example will stay right at 100C (212F) for a very long time as a lot more energy is absorbed into the water. Once it is turned to gas (steam), it has to release all that energy again to turn back into liquid water. With the molten salt, the solid to liquid change happens at around 220C (over 420F). So when they pump the water through the bath of molten salt, it can transfer a huge mount of energy into boiling the water to make steam, as the molten salt has to release all of it's latent ha energy before the temp will fall below that 220C. This is the same physics property that makes ice stay cold, and those blue packs hold even colder.
I found this article about Molten Salt Energy Storage
Storing energy without latent heat is called "Sensible Heat". That is what most home hot water or hot sand systems try to use. Sensible heat is the heat stored without a phase change, like water at just 80C instead of boiling it. With "sensible heat" the temperature of the storage media changes directly with the amount of energy in it. To get energy in or out of the system, the temperature has to change quite a bit.
Latent heat is the heat needed to perform a phase change. It takes far more energy to move the temperature as the phase changes. When using the phase change to steam and back, water latent heat can store/release 80 times as much energy for just a 1C temp change in the same mass of water. Molten salt works similar to boiling water, but at a higher temperature.
With a carful design, maybe it could be scaled down. Instead of the huge field of mirrors, could PV panels run electric heat to make a material change phase efficiently? The big issue is the 20% efficiency of PV panels. Concentrated solar puts far more of the sun's energy into heat. To the point where using the heat to boil water to spin turbines is still as efficient, or even more efficient for a given solar collecting area to make electricity.
One thing the electric utilities really like about the Ivanpah power plant is the power output easily coasts through passing clouds, unlike PV panels that dip instantly. And even at sun set, when the PV array immediately stops making power, the molten salt keep boiling water to keep the power flowing for a few hours after dark. It gives them plenty of time to get a natural gas plant up to speed to take over the load. And one of the big controversies at Ivanpah, is they made it "Dual Fuel". It can just burn natural gas to keep the molten salt hot and keep the steam and electricity flowing.
www.science.org.au
