"Rock, sand and concrete has a heat capacity about one third of water's. On the other hand, concrete can be heated to much higher temperatures (1200 °C) by for example electrical heating and therefore has a much higher overall volumetric capacity."

and

"Polar Night Energy installed a thermal battery in Finland that stores heat in a mass of sand. It was expected to reduce carbon emissions from the local heating network by as much as 70%. It is about 42 ft (13 m) tall and 50 ft (15 m) wide. It can store 100 MWh, with a round trip efficiency of 90%. Temperatures reach 1,112 ºF (600 ºC). The heat transfer medium is air, which can reach temperatures of 752 ºF (400 ºC) – can produce steam for industrial processes, or it can supply district heating using a heat exchanger."

For those who haven't seen it there is a famous Mark Rober video: https://www.youtube.com/watch?v=My4RA5I0FKs

Insulation isn't such an issue with sand because sand itself is fairly good insulator and obviously doesn't convect. 1m of sand is about the same as 10cm of air. 500C through 1m of sand if roughly 125W/m². Which isn't nothing but it's also 7m from the center to the edge, and the efficiencies only improve the bigger you make the silo.

Presumably they have a double-skin gap and other external insulation too. As the Icelandic hot water pipe systems show, which drop only a few degrees C over hundreds of kilometres of pipe (and thus a gigantic surface area to volume ratio), you can have really quite good insulation if you have space to make it thick.

The hassle of handling hot water is also presumably why they use hot air rather than water as a working fluid for heating the sand in the first place. The worst case if you spring a leak in a heat-transfer tube in the tank is that a bit of air escapes. Leaking super-heated high-pressure water or steam into the tank would be a much larger problem.

Even the lower range doesn't leave much delta in best case of boiling water. So you would need some type of heat pumps instead much simpler heat exchangers. So that is also one cost optimization.

Square cube scaling means that insulation becomes trivial in total costs as you scale the installation up. Something that's convenient for a single household would probably be too hard to insulate, but this thing holds 2000t of sand.

There are significant trade-offs with this technology.

It's storing heat, so if you need electricity then you eat a lot of efficiency. I think Vernon said ~45% round trip efficiency. Batteries are 90%+.

The storage is at a high temperature (500-600C) which means that you can't use heat-pumps to produce the heat to be stored. This means that you miss out on ~400% energy gains possible from converting electricity to heat.

So the efficiency is pretty low.

That said, solar PV is really cheap and moving large amounts of earth into a pile is also a very much solved problem so in some cases, notably higher latitudes which have very long days and low heat/electricity demand in the summer and the opposite in the winter, it could still be a very good solution.

yes and given that the energy you put in is practically free, it doesn't matter if it's not as efficient.

Also the point of this plant is to exploit the counter-correlation of cheap electricity and cold. Usually there is a bigger correlation between cheap electricity and heat.

It sounds to me like you're likely an outlier here, for curiosities sake, where do you live?