Comment by onetimeuse92304 | Hacker News Reader

Comment by onetimeuse92304 | Hacker News Reader

onetimeuse92304

100MWh = 3.6 * 10^11 joules

sand's heat capacity = 830 J/kg degree C

sand battery size is "13m tall and 15m wide". Assuming most voluminous possible shape that's 13m * 15m * 15m = 2925 cubic meters of sand (100% fill, no account for insulation, etc.)

Dry sand density is about 1600kg/m3

Total weight of sand would be 2925m3 * 1600kg/m3 = 4.7Mkg (4.7kt)

4.7Mkg of sand has a heat capacity of 830J/kg * 4.7Mkg = 3.9 * 10^9 joules / degree C (it takes this much energy to heat up entire battery by one degree C)

So from this, we get that 100MWh of energy would heat up the battery by (3.6 * 10^11 J) / (3.9 * 10^9 J/C) or about 100C.

If we include a different shape (a cylinder), and account for a thick insulation needed, this becomes closer to 200C of temp diff.

I guess it checks out... It is going to be more difficult to estimate heat loss.

But sand is quite expensive so my question is, why sand and not water? Water has 5 times higher specific heat per weight, about 3 times per volume. Water is way cheaper than sand and much easier to find, transport and extract energy from. The only real problem with water is you can only heat it up to 100C.

> But sand is quite expensive so my question is, why sand and not water?

The article says they will use a byproduct from a local industry, perhaps it's available for cheaper.

"The sand itself will also be sustainably sourced – it’ll consist of crushed soapstone, which is a manufacturing byproduct of another local industry. This material can apparently conduct heat even better than regular old sand.".

> The article says they will use a byproduct from a local industry, perhaps it's available for cheaper.

Cheaper than water?!

If that water has to come from underground, yes, it would be cheaper and easier to just grab surface stone and crush it for material to make a heat battery.

Also, water tends to make for a horrible heat battery as it is much more thermal-emissivity than rock particles. You need all sorts of additional insulation to retain the heat, whereas the sand will insulate itself.

It's the not the cost of obtaining the water that makes water less viable, it's the cost of storing superheated water. Water has this pesky ability to expand 1600 times larger when it goes over 212 degrees Fahrenheit. That means that if not handled carefully you get deadly steam explosions.

Hot sand / crushed rock doesn't have the same problem. If you read in the linked article it says

> with the sand heated to somewhere around 500-600 degrees Celsius (932-1112 °F).

That would be extraordinarily hard to do with water as you'd need significant containment and safety measures.

I can think of several reasons I would choose sand:

-Order of magnitude smaller coefficient of thermal expansion.

-No real risk of phase change - freezing or boiling.

-No problems with corrosion/scale in high temperature. On one hand regular water contains minerals which can build up on the heat exchanger element, on the other demineralized water sucks in carbon dioxide and oxygen from the atmosphere, causing corrosion of steel parts. You would need an airtight container to alleviate this.

Sand is great because it's largely inert in a huge range of temperatures.

> -Order of magnitude smaller coefficient of thermal expansion.

On the other hand the thermal expansion of water does not matter because water, you know, is a liquid -- it conforms to whatever vessel you put it in.

> -No real risk of phase change - freezing or boiling.

In a vessel that is meant to store thousands of tonnes of water that is hot and that is very well insulated to store energy for months, there is no real danger that the water will freeze. On the boiling side, water make it easy to monitor the temperature and you just stop adding energy if it starts boiling. Your kettle can do that reliably, we can do this for a huge battery.

> No problems with corrosion/scale in high temperature

It is hard to put 5 thousand tonnes of sand in a container and ensure it is dry. There is always going to be water that will be evaporating when you heat the sand in the middle and condensing on the sides that are cold.

There is no possibility of scale when you do not heat water to boiling.

> Sand is great because it's largely inert in a huge range of temperatures.

Crushed rock will release water when heated up.

> Your kettle can do that reliably, we can do this for a huge battery.

Scale is important here, as you'd need an array of sensors immersed in the water so that there's no local pocket of close to boiling temperature.

> There is always going to be water that will be evaporating when you heat the sand in the middle and condensing on the sides that are cold.

That's a slow process, but even if - as long as the temperature is considerably above ambient the water will condense elsewhere.

> There is no possibility of scale when you do not heat water to boiling.

Actually it forms at temperatures as low as 28 °C.

Ease of transport.

I’ve got a few trucks in which I could transport tons and tons of and but I’d struggle to move more than a few hundred gallons of water at a time. I’d guess most transportation outfits are similar and costs in accordance. Point: sand.

Another point, and I'd love to be corrected here, is that with a container of water you're going to get a ton of convection currents leading to a much sharper heat gradient at the edges, resulting in significant heat losses with the same amount of insulation.

At a guess, and I confess I'm not capable of running the numbers, this offsets the much higher temperature delta of sand.

They heat the sand to 600c. But you could just use more water. I don’t get it also.

Interesting! Wonder if it's also due to ease / lower risk of containment. And sand doesn't expand if you accidentally let it freeze, which is again nice from an "it won't rupture" perspective.

As to cost, the article does note that they're reusing crushed soapstone from a local byproduct, so maybe that helps reduce the cost?

That said, their FAQ says it's about heat capacity: https://polarnightenergy.fi/sand-battery

> Why do you use sand?

> Many solid materials, such as sand, can be heated to temperatures well above the boiling point of water. Sand-based heat storages can store several times the amount of energy that can be stored in a water tank of a similar size; this is thanks to the large temperature range allowed by the sand. So, it saves space and it allows versatile use in many industrial applications.

So perhaps they're also specifically targeting future applications where they would need to supply > 100C heat.

Another concern is losses. Water can evaporate, can leak, and tends to corrode fittings.

Sand? If some spills just grab a skid loader and stuff it back in.

More water takes more space, and perhaps the higher temperatures make it easier to reuse the heat? And you probably don't want your water to get above 100 degrees C, because then you need to deal with pressure.

Sand is also a really good insulator if I'm not mistaken. That could also be a factor somehow.

I cannot explain the physics, but a big rock that has basked in the sun is really warm for a long time after sunset. A bucket of water loses its temperature faster.

The higher density of rock probably plays a big role.

> I cannot explain the physics

Here, this is actually the problem.

Water can store many times the amount of energy per volume or mass, per degree Celcius, than rock.

What you see is that rock has much lower thermal conductivity. It can be hot inside but it is not as good at transferring that heat outside. It means when you have hot rock it will stay hot for longer than equivalent amount of water. That because water emits that energy faster.

But put that rock and water in a well insulated vessel and you will find that the properties of the insulation and the vessel will start dominating the process and what counts now is how much energy you can store in the material inside.

For water evaporative cooling is a big factor

If it’s going to function as an energy source doesn’t it need to run a turbine so it can’t be water? The article said it’s going to just use heat directly so I guess they could use water.

But if ever the need arose it needs to be higher than 100C so it can generate steam for the turbine? Maybe I’m way off, I’m just a guy.

They might get quite a good deal on types of sand which are no good for construction but are highly abundant like sea sand or desert sand.

That’s the question, innit. Is there a sand-ologist around here?

I was about to write just that. Also, desertification is a problem, so they could possibly get free raw material from many places, paying only shipping, and doing something good at the same time.

Also, water is constantly trying to leak out of whatever you put it in.

Other considerations, water grows microorganisms and is a solvent. The upside of sand is (presumably) less maintenance.

Why not water - I wonder if it's because it's in Finland and 6 months a year you have to prevent the water from freezing.

You can heat sand above 100C without the sand turning into a gas.

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