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Comment by NishanStepak | Hacker News Reader

Part of this is about a new way to store energy. In the Netherlands, they have started storing heat energy with sand, in Germany, they are using bricks. This has some advantages. It allows the direct use of heat for industrial processes and heating industrial and commercial facilities. They can either convert energy into electricity or use the heat directly. This technology can be used with a wide variety of energy sources. This includes nuclear because it allows thermal storage of excess heat from nuclear energy. This kind of setup is supposed to be more stable for the grid and more reliable for storing energy.

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I know someone who runs a mineral water company. His biggest expense was the energy cost for washing the glas bottles. He now built a absolutely massive solar farm that feeds directly into heating a well insulated underground water tank. At the time he told me about it (ca two years ago) he said according to his research it is not unlikely to be the biggest such hot water storage in Europe.

The amount of energy needed to test things is just intuitively huge. Boiling one liter of water takes the equivalent amount of energy as lifting 34 metric tons up a meter. Or to translate it to human scale: the same as walking a vertical climb of 1000m with a weight of 34kg.

Was this electric or direct water heating?

I think it was electric, given the scale.

Correction, the sand battery is in Finland: https://ioplus.nl/en/posts/the-sand-battery-that-could-stop-...

You are correct Polar Night is in Finland. Denmark is working on molten salt batteries. https://www.warpnews.org/energy/denmarks-first-molten-salt-b...

I think I have read that heat is hard to convert to useful energy. Can someone confirm? But the, if that's true, why use heat as energy storage?

Use it directly, see my silbling comment.

so all such 'energy' storage are to be used as heat and no other form of energy? The name energy storage is somewhat confusing then, even if not incorrect? Heat storage would be more accurate iiuc.

No of course not. But using the heat directly is of course favourable, since you don't have conversion losses. But that doesn't mean you can't use it also to power turbines as you wish to.

The advantage of that type of storage is that it scales very well. Batteries are cool when space is an issue. But when it is not, digging out a hole and insulating it is surprisingly effective per unit of work stored there.

Heat can't be converted losslessly to useful energy, but converting to heat to other forms of energy is extremely common. What do you think a steam turbine, boiler, internal combustion engine, etc. do?

But all the examples you give have a heat source of high temperature and dense energy, and a lot of energy is lost in it's transformation, as illustrated by the combustion engine. How can heat be a good energy storage if so much is lost when you want to use it?

If it's free to collect (solar) then efficiency matters a lot less.

It isn't just the cost to collect, it is also the cost to store. Heat is easy and safe to store at just above room temperature in rocks - but you need a lot of rocks. Just to heat my house for a few days would need tons of rock at temperatures I'd allow in my house (I have kids and so I worry they will open the rock container and get burnt). Rocks are not free. You can also use water for storage, but again you have the issue needing a tank (and water is a drowning hazard, plus the worry about leaks...)

Square cube law will optimise heat storage. These Finnish sand based heat batteries are a prime small-scale example.

> Just to heat my house for a few days would need tons of rock at temperatures I'd allow in my house (I have kids and so I worry they will open the rock container and get burnt). Rocks are not free

Sand/rock heat storage is for industrial and utility scale application, not your house.

> You can also use water for storage, but again you have the issue needing a tank (and water is a drowning hazard, plus the worry about leaks...)

Tank heat pump water heaters are available and can already do load shifting on a daily basis (using off peak cheaper and cleaner electricity). I have one and it works great, and I guarantee it isn't a drowning hazard.

There are versions of these systems that can store enough heat to heat a house, not just provide hot water at the tap.

> Sand/rock heat storage is for industrial and utility scale application, not your house.

I've seen them for a house as well. (when the utility offers a large discount for power used overnight - 40 years ago my local coop was trying to sell my parents on this, but since we had natural gas heat it wasn't worth it) To hold more than a days worth of heat you need a lot of rocks.

Heap pump water heaters work, but you need a large one to hold a days worth of hot water (40 years ago my family had 200 gallons of hot water storage - the utility gave my parents a discount on water heating because we only heated water at night, we ran out of hot water once that I remember). If you want to store more water than a days worth of use then you need more storage. This can become a drowning hazard if we are talking about a couple weeks worth.

> I've seen them for a house as well. (when the utility offers a large discount for power used overnight - 40 years ago my local coop was trying to sell my parents on this, but since we had natural gas heat it wasn't worth it) To hold more than a days worth of heat you need a lot of rocks.

Can you point at a residential scale rock heat storage system that is available? I saw one startup in the UK trying this several years ago, but can't find them now, and haven't seen anything since.

> Heap pump water heaters work, but you need a large one to hold a days worth of hot water

Mine (for a household of 4) is 45 gallons - small by US standards - but we keep it at a high temperature (along with a mixing valve) to get higher "virtual" hot water capacity during peak usage hours. It serves us fine for daily usage, but we also have efficient flow taps and showers throughout the house so that helps too.

> If you want to store more water than a days worth of use then you need more storage

I don't think it makes sense to store hot water for days if the goal is to minimize energy costs. The standby losses would be pretty significant, even with a very well insulated tank.

If the goal is resilience to a grid outage, it makes more sense to have a solar PV array + batteries that you can use to run the heat pump water heater, or have a backup gas-powered heater.

That depends on what you need energy for. Converting heat energy stored during the day to heat my house at night is easy. Converting heat energy stored into light is much harder (as the other reply stated, it needs high heat and thus is a fire hazard)

Heat is really useful if the energy form you want is heat. If you want electricity or motion it's not the best.

"Low grade heat", less than the boiling point of water (often), can be stored in a sand system and recovered efficiently and distributed later.

But low grade heat cannot efficiently drive a heat engine to produce other, more useful and transferable versions of energy, like electricity. For that you need "high grade" heat.

The reason is, heat engines do not operate on heat. They operate on heat flow. You must have a high temperature side and a low temperature side, and you extract energy by taking heat out of the high side and injecting it to the low side, through various processes. By thermodynamics, this process has hard efficiency limits of around 35%, with that efficiency heavily affected by the delta between the hot and cold side.

The "low" heat side is often a great source of low grade heat, for industrial or communal purposes, but cannot be used to run an efficient heat engine. It's already "used up" essentially.

The "grade" you refer to is simply temperature. To efficiently extract energy from a heat flow you want a really hot hot side and a really cold cold side. On earth we can't get a cold side much colder than outside air temperature (not without wasting more energy than you gain) but luckily that is cold enough when the hot side is thousands of degrees. But you can't get much energy from roomtemp and roomtemp+10. A warm cup of coffee will run a Stirling engine for a party trick, but it's not going to charge your phone, much less your car.

Yes. This is sometimes called "Carnot tax".

The efficiency of any heat engine is bounded by the Carnot cycle. In simplified form, it depends on the temperature differential between the cold and the hot parts. So if your hot part is at 300C (572 Kelvin), and the cold part is at 30C (300 Kelvin) then the absolutely best possible efficiency is around (572-300)/572*100% = 47%.

You see that it quickly becomes inefficient as the differential goes down. E.g. if you use 90C to store the heat, then the maximum efficiency is just 17%.

Nuclear reactors do exactly that

"What, sir, would you make a ship sail against the wind and currents by lighting a bonfire under her deck? I pray you, excuse me, I have not the time to listen to such nonsense." (Napoleon Bonapart ~1800)

A lot of energy generation is just "produce heat, boil water, steam moves a turbine

So in that regard heat is very useful in producing energy. But converting directly from heat to energy? Not so much as far as I know.

It's inefficient, but still better than completely losing out on it. Better yet would be direct reuse or remote heating.

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