Finnish City Inaugurates 1 MW/100 MWh Sand Battery • Hacker News Board

Re: Finnish City Inaugurates 1 MW/100 MWh Sand Battery

#11

Post by fulafel » Wed, Sep 03, 2025, 7:07 AM UTC

tarvaina wrote: post #9

fulafel wrote: post #4

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?…

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 f…

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

Re: Finnish City Inaugurates 1 MW/100 MWh Sand Battery

#12

Post by kitd » Wed, Sep 03, 2025, 7:09 AM UTC

I like these technologies. They may not be as energy efficient as using more exotic materials, but what they do use is simple, cheap and often sourced locally. Such economic factors are often as important to the ROI as the purely scientific ones.

Re: Finnish City Inaugurates 1 MW/100 MWh Sand Battery

#13

Post by petesergeant » Wed, Sep 03, 2025, 7:31 AM UTC

I live in a desert where we have district cooling (and no shortage of sand or solar power), instead of district heating. Wonder if they can pull off the same trick.

Re: Finnish City Inaugurates 1 MW/100 MWh Sand Battery

#14

Post by internet_points » Wed, Sep 03, 2025, 7:42 AM UTC

tarvaina wrote: post #9

fulafel wrote: post #4

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?…

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 f…

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)

Re: Finnish City Inaugurates 1 MW/100 MWh Sand Battery

#15

Post by d--b » Wed, Sep 03, 2025, 7:45 AM UTC

fulafel wrote: post #4

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?…

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

Re: Finnish City Inaugurates 1 MW/100 MWh Sand Battery

#16

Post by Havoc » Wed, Sep 03, 2025, 7:45 AM UTC

Interesting capacity vs discharge ratio. Way slower than battery but presumably sand scales well

Re: Finnish City Inaugurates 1 MW/100 MWh Sand Battery

#17

Post by vintermann » Wed, Sep 03, 2025, 7:47 AM UTC

isoprophlex wrote: post #5

fulafel wrote: post #4

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?…

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.

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

Re: Finnish City Inaugurates 1 MW/100 MWh Sand Battery

#18

Post by grues-dinner » Wed, Sep 03, 2025, 7:51 AM UTC

fulafel wrote: post #4

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?…

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.

Re: Finnish City Inaugurates 1 MW/100 MWh Sand Battery

#19

Post by Fwirt » Wed, Sep 03, 2025, 7:53 AM UTC

Interestingly this looks like the same principle as a rocket mass heater or masonry heater, but on a larger scale and powered by renewable energy. They say the system can retain heat for weeks whereas the smaller thermal battery in a masonry heater is exhausted in a matter of hours. I wonder if this is a function of size or if the tank is heavily insulated? It would be interesting if this same system could work at smaller scales for off-grid heating by harnessing excess solar capacity. There’s a lot of waste involved in going from solar to battery vs directly to thermal energy, as long as it doesn’t bleed off before you need to use it.

Re: Finnish City Inaugurates 1 MW/100 MWh Sand Battery

#20

Post by Cthulhu_ » Wed, Sep 03, 2025, 8:00 AM UTC

decimalenough wrote: post #6

fulafel wrote: post #4

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?…

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).

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...