[fulafel]

It doesn't list the advantages over water, which seems the most common in https://en.wikipedia.org/wiki/Thermal_energy_storage systems.

You'd think water would be easier to exchange heat with since it can slosh around the heat exchanger elements in the tank more easily. Which should translate to lower costs since you don't need as many exchanger structures in the medium.

Any guesses for the motivation in using sand? Maybe it's that you can heat it over 100C? But then big heat differences to the environment mean high conductive/radiation losses or heavier insulation requirements.

[grues-dinner]

Sand also mostly stays where you put it. While obviously water can be put in tanks easily enough, there's still more maintenance and inspection required and a gigantic watertight tank that will last n decades is substantially more expensive then a steel sand box. Plus it only goes to 100C unless you pressurise it and that really gets hard. Unplanned release of that much water at 100C is also extremely dangerous. Whereas even 500C sand will mostly just sit there. Plus the usual corrosion and scaling effects water systems love to develop at high temperatures.

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 inside the tank is that a bit of air escapes. Leaking super-heated high-pressure water or steam into the (unpressurised) tank would be a much larger problem, and unloading up to 2000 tonnes of hot, damp, sand to plug it would be operationally very annoying if nothing else.

[tarvaina]

The Wikipedia article says:

"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."

[internet_points]

I learnt some new concepts here, specific heat capacity vs overall volumetric, things I kind of understood intuitively, but now much clearer:

If I add some fixed amount heat to some fixed volume of water, it might rise by 1℃, while the same volume of concrete rises by 3℃. And by the same logic, on release, that fixed volume of water dropping by 1℃ releases 3x as much heat as when that fixed volume of concrete drops by 1℃.

So if you can max heat water to 100℃, and max heat concrete to 1200℃, and on release you let it go to 10℃ (probably the range is less in practice), then the water can drop 90℃ and the concrete 1190℃, so even if the water releases 3x the amount of heat per ℃, the water just releases 270 (per volume) while the concrete releases 1190 (per volume)

[privatelypublic]

Also to add some practicals: you can drive a steam turbine with the concrete temps, but not with the water.

Also, looking at how hot water could theoretically get (decomposes between 2200-3300C), it looks like 1200C is an interesting limit. Above that and you get safety(practical) and cost issues with every material I could find (common salts, pure elements).

Sand just makes sense! Though, don't ever youtube sand battery.

[stavros]

Why not YouTube sand battery? I did it, and nothing much happened.

[privatelypublic]

Previously it was a bunch of overunity nutjobs.

[stavros]

Ahh right, I did see some preppers there, to be fair.

[internet_points]

> don't ever youtube sand battery

Huh? I just get stuff related to this article?

[fulafel]

The higher temperature output is a good point, you can't get 400C output for industrial processes from a 100C water based heat battery.

[dinkblam]

a Blast furnace needs closer to 2000° than 400°

in any case, how would you transport high temperatures to the industrial sites? water boils at 100° and few liquids boil above 400°. most liquids will be impractical due to cost or safety (combustibility, toxicity…).

[happosai]

Of course you can't do blast furnace with a sand battery. But there is still a sizable market for industrial heat in between 100c <> 400c.

[rsynnott]

Pump water through, producing steam to drive a turbine, use turbine to generate electricity, use electricity for industrial process.

Now, in practice you _probably_ don't want to do this, because, in this case, you have district heating demand, which is a far more efficient use of the power.

[fulafel]

Steam?

[Tuna-Fish]

> But then big heat differences to the environment mean high conductive/radiation losses or heavier insulation requirements.

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.

[decimalenough]

The article mentions that they heat the sand to 500°C, which is not possible with water (well, at least not without turning into steam along the way).

[Cthulhu_]

To be pedantic, yes you can but you'd need to pressurize it to uuhh... According to this calculator [0], you can get water to 370 degrees C if the pressure is 207 atmospheres, which is about the pressure of the ocean two kilometers deep.

[0] https://www.engineeringtoolbox.com/water-vapor-saturation-pr...

[killingtime74]

Interestingly that's also about the pressure of gas in scuba tanks. Can't imagine how much energy to pressurize water to that

[trhway]

Due to incompressibility of liquids, pressurizing a liquid is very cheap energy-wise - orders of magnitude cheaper than pressurizing a gas. The issue is that pressurized liquid also requires correspondingly strong and expensive vessels and pipes.

[sfn42]

You don't need to pressurize it. You just put it in a tank and apply heat, the water evaporates and creates pressure. Like a pressure cooker.

[nick49488171]

How many kiloton of TNT equivalent?

[grues-dinner]

Well, if you say the energy stored is the 100MWh from the headline figure, and say you can arrange release every joule of all at once by flashing high-pressure water to steam at 1 atm that's about 0.1kT.

So quite a bang - allegedly this is 200lb, so about the same: https://www.youtube.com/watch?v=ZDgvar7ON54

[Ekaros]

District heating tends to operate at 50-70C at lowest. But more often up to 115C and in some case even 180C.

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.

[fulafel]

I think it's more like 90C in (winter), 40C out. The temps have been going down in newer infrastructure as it's more energy efficient. See eg https://www.shcbysweden.se/wp-content/uploads/2019/11/Best-P... p. 7

And of course it's still a win if you can heat the return water half of the way to spec with the battery, it's not necessary to have the battery heat it all the way to the plant outgoing temp.

District heating systems have been happily using ~90C water based heat batteries for a long time.

[Ekaros]

Other limitation that I know is that for tap water minimum is 55C. So the last building in loop should get at least that much. So 40C is only acceptable back at power plant.

In the end it comes to balance with cost, simplicity, capacity and such. Heat pumps do allow extracting heat from colder storage medium. But on other hand electric heating elements and heat exchangers are very much simpler and cheaper.

[fulafel]

In the good systems there's two pipes, return gets its own (like eg the earlier linked PDF describes), so it's fine for buildings to output cooler water to the return pipe.

But yes heat pumps are used in some parts as well in DH.

[d--b]

They probably use the trick where they blow air in the sand to give it "liquid" properties where they need the sand to flow.

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

[isoprophlex]

For this specific use case, you need to heat to far above the boiling point of water to retain some thermal efficiency. Sand/rock is better suited for storing the thermal energy at ~500 celcius.

[vintermann]

I wonder if there are any chemical effects from heating the sand to 500 degrees Celsius. Finely roasted sand.

[grues-dinner]

This is crushed soapstone, so it's mostly talc. Talc is apparently more or less stable up to about 800C, where it starts to break down into enstatite and silica: https://nvlpubs.nist.gov/nistpubs/jres/15/jresv15n5p551_A1b....

If it were pure silica sand, you could presumably get even hotter before anything changes chemically, but at the that point you start having materials issues with metal parts of the system: 500C is about the limit for ordinary steels to lose strength (and many are less than that - heat effects can often start at 300C).

[isoprophlex]

Interesting, thanks for pointing that out, I didnt catch that they're not using actual sand.

[isoprophlex]

None, really. Pure, fine sand being mostly silicon dioxide, it melts at ~2000 and boils at ~3000 C, still without decomposing or reacting. It is really extremely chemically stable.

That said in practice, at scale... before filling up your storage tank you'd probably need to pre-heat it once to remove all moisture and volatile gunk adsorbed onto the sand.

[LtdJorge]

It's as inert as it gets

[chii]

perhaps sand is easier to heat to higher temps, and also it's less thermally conductive, so you'd lose less heat in storage for the same sized container.