[ben_bai]

It's more effective volume wise. I did the math, some time ago here are the rough numbers.

Sand has way less heat capacity then water per kg (about half).

Water can be heated to 95C with standard unpressurized vessel. Sand in this application is heated to 600C.

Sand is denser then water (kg/m3).

For the same heat energy stored this comes out to about 2.5x more volume of water(95C) compared to sand(600C).

Water and Sand are both dirt-cheap.

Hot water can be managed with standard plumbing equipment.

Sand needs some high temperature piping (hot air to water heat-exchanger, resistive heat tho heat up the sand).

How well both contain the heat is primarily dependent on the isolation. Which favors the smaller footprint of sand, but needs to isolate a higher temperature difference...

[tjmc]

One advantage of heating water over sand is that you can heat it up with high temperature heat pumps which currently have CoPs ranging between 2.4 to 5.8 [1]. So for every kW of electrical energy you put in you get at least 2.4kW of thermal energy out.

So yes, the volume of 95C water would be much greater than that of 600C sand, but if volume wasn't an issue you could do it much more efficiently. Alternatively, you could use battery storage for just the electrical capacity required and not the (much higher) thermal capacity which may be more cost effective when you look at the conversion.

[1] https://www.sciencedirect.com/science/article/abs/pii/S03605...

[ben_bai]

In short: Sand for industry scale heat storage, water for private heating storage.

Using a heat pump will increase the yield. Usable temp range from 95C water all the way to 0C ice in theory (latent heat).

And a modern isolated home helps, but seasonal heat water storage is basically a big tank, with a house build around it.

https://www.energie-experten.ch/de/wissen/detail/waermespeic...

[shellfishgene]

The temperature also matters. If you need at least 50 C water to run district heating, about half of the energy stored in near boiling water cannot be utilized. This is much less with 600 C sand.

[Ekaros]

And 50C is too low. That is minimum temperature of the heated tap water(Legionella and other diseases). And preferably you want some higher. And then as distance increases there are losses and other people using the heat. So temperature you need is actually quite high and in very cold days can be over 100C...