District heating using a sand battery as storage for peak demand

https://interestingengineering.com/energy/sand-battery-polar-night-energy

“The net result: Pornainen fulfilled all of its municipal climate targets with a single installation. Oil use dropped 100 percent, emissions fell 70 percent, and woodchip combustion was cut by 60 percent. According to the Mayor of the Municipality of Pornainen, Antti Kuusela, the municipality now heats all its public buildings, including a new sports arena opening in September 2026, entirely through this district heating network.”

Nice.
Necessity is the mother of invention.

But you can't really call it a 'battery' , as it stores heat, not multi-purpose electricity.
Although you could re-purpose the heat above 100^o to heat water into steam and drive generators ( with some losses ), and that last 100^o of water would still have about 5 times the heat capacity of the 100^o sand.

Pretty interesting method of storing energy. At 500-650C, there is plenty of energy to make steam if desired, even enough to turn a turbine. The description does lack somewhat, like, how is the medium heated with electricity? Are there electrodes running through it? Also, is the medium static or does it get hot enough to partially liquify? I would be very interested in seeing blueprints but that is probably out of the question for proprietary reasons or some such.

2 minutes ago, npts2020 said:

Pretty interesting method of storing energy. At 500-650C, there is plenty of energy to make steam if desired, even enough to turn a turbine. The description does lack somewhat, like, how is the medium heated with electricity? Are there electrodes running through it? Also, is the medium static or does it get hot enough to partially liquify? I would be very interested in seeing blueprints but that is probably out of the question for proprietary reasons or some such.

Well it seems to say that the sand is heated by heating air and passing that through large bore ducts within the sand.

It also says that the designers wish to keep it simple which is perhaps why did not use water/steam as MigL suggests since corrosion is a very real problem with steam at those temperatures and special materials ( probab;ly expensive) are required.
Also in the event of a breakdown the pipes will not be damaged by frozen water.

8 minutes ago, studiot said:

Well it seems to say that the sand is heated by heating air and passing that through large bore ducts within the sand.

It also says that the designers wish to keep it simple which is perhaps why did not use water/steam as MigL suggests since corrosion is a very real problem with steam at those temperatures and special materials ( probab;ly expensive) are required.
Also in the event of a breakdown the pipes will not be damaged by frozen water.

Ok, how large is large? 5 cm? 20? 100? What kind of volume/pressure/flow is required?

Also, they say they are building some to make steam. IMO the problems with steam are being overstated, anyway. Every plant in the world that generates power with steam, and there are a lot of them, operates with the same high temperature water constraints.

26 minutes ago, npts2020 said:

Pretty interesting method of storing energy. At 500-650C, there is plenty of energy to make steam if desired, even enough to turn a turbine.

Just for comparison, a primary steam power station will typically produce main steam at 165 barg and 570^oC. This implies 230^o of superheat. The intermediate cycle would be superheated to the same temperature, but at ~60 barg.

Bear in mind that energy content is almost independent of pressure: it's all about the temperature.

19 minutes ago, npts2020 said:

Also, they say they are building some to make steam.

But that's dry saturated steam at 200^oC which just happens to be industry standard for the drying sections of pulp and paper mills, a vital cog in the wheels of the Finnish economy.

No one serious generates power from this quality of steam.

Edited May 20May 20 by sethoflagos
misprunt

12 hours ago, sethoflagos said:

No one serious generates power from this quality of steam.

Which has more to do with the fact that once the steam has been produced, it is relatively easy to add energy to it for more efficient extraction than whether or not it can be done. So the guys doing this must not be serious, because they have plans to do exactly that.

If the goal is heating, why add the inefficiency of creating steam to make electricity?

4 hours ago, npts2020 said:

Which has more to do with the fact that once the steam has been produced, it is relatively easy to add energy to it for more efficient extraction than whether or not it can be done. So the guys doing this must not be serious, because they have plans to do exactly that.

Their target round-trip efficiency is stated as 30-35%.

We've discussed Round-trip Efficiency quite a bit recently in regard to pumped storage etc.

Comparison of Round-trip Efficiency for Energy Storage Methods[4][5][6]

Storage Technology Median Efficiency (%) Efficiency Range (%)

Lead-acid battery. ~75% ~60% – 90%

Li-ion battery ~90% ~85% – 95%[7]

Sodium–sulfur battery ~60% – 90%[8]

Flywheel ~92% ~85% – 95%

Supercapacitor 85% – 95%

Superconductive ~90% ~85% – 97%

Compressed air ~52% ~41% – 90%[9]

Thermal[10] ~40% ~30% – 50%[11]

Pumped hydro ~75% ~65% – 85%

Redox flow 60% – 75%

Green hydrogen ~40%[12] 28 – 52%[13]

Green ammonia 23 – 42%[13]

Please explain where you think thermal storage presents a competitive advantage?

... or in what sense serious steam cycle power generation is 'simple', come to that.

1 hour ago, sethoflagos said:

We've discussed Round-trip Efficiency quite a bit recently in regard to pumped storage etc.

+1 Thanks for the info and link.

1 hour ago, sethoflagos said:

... or in what sense serious steam cycle power generation is 'simple', come to that.

I noted the article said the Finns wanted 'simple'

9 hours ago, sethoflagos said:

Their target round-trip efficiency is stated as 30-35%.

We've discussed Round-trip Efficiency quite a bit recently in regard to pumped storage etc.

Please explain where you think thermal storage presents a competitive advantage?

... or in what sense serious steam cycle power generation is 'simple', come to that.

I don't think efficiency is much of a consideration if they aren't using outside sources for power. The competitive advantage is in cost of the medium, basically waste vs rare earth or other expensive materiel. Steam generation will substantially raise the cost (but has other advantages), which is why they didn't try it to begin with I would imagine.

Please repost where I ever said or even implied that any of this, especially steam power generation, is "simple".

On 5/22/2026 at 3:15 AM, npts2020 said:

I don't think efficiency is much of a consideration if they aren't using outside sources for power.

The primary input power is low tariff rate grid electricity, isn't it?

On 5/22/2026 at 3:15 AM, npts2020 said:

Please repost where I ever said or even implied that any of this, especially steam power generation, is "simple".

On 5/21/2026 at 12:23 PM, npts2020 said:

the fact that once the steam has been produced, it is relatively easy to add energy to it for more efficient extraction

Significant superheat even at this modest pressure makes water quality critically important and mandates a materials upgrade from carbon steel to chrome-moly alloy.

Not to mention, 'ot fog carries a serious safety burden.

15 hours ago, sethoflagos said:

Please explain where you think thermal storage presents a competitive advantage?

Reversible ground source heat pumps do seem to offer advantages, where the gains are independent of a narrower measure of efficiency of the gross heat flows in and out of the underground thermal mass. How energy efficient can total operation get if eg, air source heat pumps are used through Summer to charge up the thermal mass as well as using heat pumps to extract it in Winter and boost the delivered temperatures? A quick search didn't turn up real world examples with numbers and I'm not sure I could do hypothetical calculations. (I note that for large buildings with borehole ground source heating and cooling they often have to include external cooling to prevent underground heat accumulating above ideal working temperature range.)

I've also read of trials (I posted somewhere) of conversion of a gas pipe network to district heating - where those pipes may run close to sewage pipes that shed waste heat into the ground, achieving (if I remember) COP above 5.

I think we will also see more and better heat pumps for direct supply of industrial heat. The best currently manage up to 200 C (but I think those rely on waste heat rather than air or ground source).

Edited May 25May 25 by Ken Fabian

I’ve already read about this in recent weeks, though when it comes to the sand battery, I’d really stick to the concept of a heating network. Please do correct me if I am mistaken on this point, but to me, it sounds more spectacular as an electricity storage solution than it actually is. If you convert electricity into heat, and then want to generate electricity again later, the process of converting it back to electricity will most likely consume a large portion of it. The situation is quite different with pure heat, because you don’t need that detour.

That is precisely why I find the approach interesting. It should definitely not be seen as a battery for all and sundry, but rather as a down-to-earth storage solution for district heating, process heat, or seasonal peak loads. It does, however, have certain advantages: it is a relatively cheap storage medium, has few moving parts, operates at high temperatures, and contains no rare battery metals. But it simply cannot replace a lithium battery if you want to recover electricity quickly and efficiently.

For me, the question is rather where heat is needed directly. In Finland, with its district heating networks and cold winters, this makes more sense than in other latitudes and longitudes without this infrastructure. As soon as you add steam, turbines and electricity generation, the simple sandpit turns back into a proper power station problem.

3 hours ago, aliceinwonderland said:

For me, the question is rather where heat is needed directly. In Finland, with its district heating networks and cold winters, this makes more sense than in other latitudes and longitudes without this infrastructure. As soon as you add steam, turbines and electricity generation, the simple sandpit turns back into a proper power station problem.

Exactly.
In other parts of the world, where you need electricity to run your A/C, this might not be the best solution, but it is certainly brilliant for small communities in Finland.

4 hours ago, aliceinwonderland said:

I’ve already read about this in recent weeks, though when it comes to the sand battery, I’d really stick to the concept of a heating network. Please do correct me if I am mistaken on this point, but to me, it sounds more spectacular as an electricity storage solution than it actually is. If you convert electricity into heat, and then want to generate electricity again later, the process of converting it back to electricity will most likely consume a large portion of it. The situation is quite different with pure heat, because you don’t need that detour.

That is precisely why I find the approach interesting. It should definitely not be seen as a battery for all and sundry, but rather as a down-to-earth storage solution for district heating, process heat, or seasonal peak loads. It does, however, have certain advantages: it is a relatively cheap storage medium, has few moving parts, operates at high temperatures, and contains no rare battery metals. But it simply cannot replace a lithium battery if you want to recover electricity quickly and efficiently.

For me, the question is rather where heat is needed directly. In Finland, with its district heating networks and cold winters, this makes more sense than in other latitudes and longitudes without this infrastructure. As soon as you add steam, turbines and electricity generation, the simple sandpit turns back into a proper power station problem.

It would be a good idea to use understand the term 'battery' correctly, as there is such a thing as a heat battery in engineering thermodynamics.

History

The Term Battery

Historically, the word "battery" was used to describe a "series of similar objects grouped together to perform a function," as in a battery of artillery. In 1749, Benjamin Franklin first used the term to describe a series of capacitors he had linked together for his electricity experiments. Later, the term would be used for any electrochemical cells linked together for the purpose of providing electric power.

https://learn.sparkfun.com/tutorials/what-is-a-battery/history

A heat battery is a device that stores thermal energy rather than electrical energy. It absorbs energy from electricity (such as solar or off-peak grid power) or renewable heat sources, stores it efficiently, and releases it on demand for your home's hot water or central heating. [1, 2, 3, 4]

Google search.

Electrically the 'electrical battery' derived from the grouped collection of individual 'cells' rather than from the latin and old french 'to beat'.

Strictly what in popular parlance is an AA battery is infact a cell.

As tables show water is the best heat soaking option; last two winters I put a 5 gallon cooking pot with water on a single 'hot plate burner' inside the house, powered by 1200Watt / 120VDC spare extra solar panels. By noon was typically boiling, and kept its heat until ~10pm. helping heating the dwelling at no cost to me.

Next winter would like the plan of a container with a heating element buried in ~40Kg? sand instead. Heats up when sunny, releases when when not. The heat soaking is less than water but the temperature can be higher and no boiling. What else should I consider ?

Or heating a junk engine block instead of sand ? I can be your guinea kid, no problem.

5 hours ago, Externet said:

As tables show water is the best heat soaking option; last two winters I put a 5 gallon cooking pot with water on a single 'hot plate burner' inside the house, powered by 1200Watt / 120VDC spare extra solar panels. By noon was typically boiling, and kept its heat until ~10pm. helping heating the dwelling at no cost to me.

Next winter would like the plan of a container with a heating element buried in ~40Kg? sand instead. Heats up when sunny, releases when when not. The heat soaking is less than water but the temperature can be higher and no boiling. What else should I consider ?

Or heating a junk engine block instead of sand ? I can be your guinea kid, no problem.

Ya, water is great because of its high latency during phase change. It kind of depends on what you want. If you want speed, the engine block should be faster at discharging but will not stay charged nearly as long unless heated to a much higher temperature. From the sound of it, you will be adding the same energy regardless and discharge can be regulated as required, so it really should not matter much which method you use. Just do whatever is easiest.