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Finnish City Inaugurates 1 MW/100 MWh Sand Battery

(cleantechnica.com)
190 points erwinmatijsen | 2 comments | 03 Sep 25 06:00 UTC | HN request time: 0.001s | source
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fulafel ◴[03 Sep 25 06:52 UTC] No.45112956[source]▶
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.

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Ekaros ◴[03 Sep 25 08:05 UTC] No.45113356[source]▶
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.

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fulafel ◴[03 Sep 25 13:22 UTC] No.45115461[source]▶
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.

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1. Ekaros ◴[03 Sep 25 13:36 UTC] No.45115636{3}[source]▶
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.

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2. fulafel ◴[04 Sep 25 03:55 UTC] No.45123380[source]▶
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.