Ease of transport.

I’ve got a few trucks in which I could transport tons and tons of and but I’d struggle to move more than a few hundred gallons of water at a time. I’d guess most transportation outfits are similar and costs in accordance. Point: sand.

> -Order of magnitude smaller coefficient of thermal expansion.

On the other hand the thermal expansion of water does not matter because water, you know, is a liquid -- it conforms to whatever vessel you put it in.

> -No real risk of phase change - freezing or boiling.

In a vessel that is meant to store thousands of tonnes of water that is hot and that is very well insulated to store energy for months, there is no real danger that the water will freeze. On the boiling side, water make it easy to monitor the temperature and you just stop adding energy if it starts boiling. Your kettle can do that reliably, we can do this for a huge battery.

> No problems with corrosion/scale in high temperature

It is hard to put 5 thousand tonnes of sand in a container and ensure it is dry. There is always going to be water that will be evaporating when you heat the sand in the middle and condensing on the sides that are cold.

There is no possibility of scale when you do not heat water to boiling.

> Sand is great because it's largely inert in a huge range of temperatures.

Crushed rock will release water when heated up.

> Your kettle can do that reliably, we can do this for a huge battery.

Scale is important here, as you'd need an array of sensors immersed in the water so that there's no local pocket of close to boiling temperature.

> There is always going to be water that will be evaporating when you heat the sand in the middle and condensing on the sides that are cold.

That's a slow process, but even if - as long as the temperature is considerably above ambient the water will condense elsewhere.

> There is no possibility of scale when you do not heat water to boiling.

Actually it forms at temperatures as low as 28 °C.