Ben_bai said it pretty well. water is good but it can cause rust, pressure, freeze, leak, etc. sand avoids most of this. the big thing is that sand can be heated much much hotter and this helps make up for its shortcomings relative to water. they likely charge and discharge with air blowing thru stainless steel network of pipes thru the sand. this would require special tuning. as someone else mentioned, the thing with water is that the temp is uniform everywhere because water transfers heat within its volume easily. sand doesn’t, so you need pipes spaced out right.
i’ve wondered about insulation but, i think you can step it out with refractory bricks first then a mineral insulation, then something like perlite or fiberglass. the hot air would interface with heat exchanger to transfer the heat to whichever fluid that would go on to heat the home/s. since the sand can be heated to 1700F or more, a kiln or quartz heating element that can exceed this temp is needed. last, a thermally isolated heat temp tolerance fan would be required. i guess a slew of sensors in the sand and at the exchanger unit and output, i can imagine wanting sensors everywhere for tuning sake.
This is called a Storage heater (https://en.wikipedia.org/wiki/Storage_heater), usually referred to in the UK as a Night Storage Heater as it took advantage of cheap electricity at night to provide heat for a house.
Fitting Night Storage radiators was a much cheaper way of fitting central heating than the normal boiler and plumbing approach, and when electricity was cheaper it made more sense. Now, it makes no sense and if you buy a house with it the first thing you have to do is put in "proper" central heating. But maybe with excess renewables that will swing back.
Any system that involves an electrical solar panel connected to a low-temperature electrical heater is likely to be better served with a solar hot water panel, by about a 10:1 ratio.
MGA Thermal uses tiny aluminium droplets in carbon matrix material to do heat storage with 1: constant temperature storage and 2: at much higher temperatures and energy density that allows for steam generation etc.
I get that sand is inexpensive and simple and I like the idea - especially since it scales to an entire town (!)
But I can't help but think of one "technology" that could make a scheme like this MUCH more effective.
a phase change.
for example, it takes 1 calorie for water to go from 30 to 31 degrees F, but but it take 80 calories to go from solid at 32 to liquid at 32 F.
A project I remember reading about that was really interesting was a house constructed with logs that had a resin that phase changed around room temperature. The idea was that the logs would be heated by sunlight, and the resin in the logs would absorb lots of energy without easily going above room temperature. Then at night, they would slowly solidify and give off heat at room temperature. This would be a cool way to stabilize house temperatures without needing equipment.
EDIT: it was called the enertia house
I'm uncertain of the history, it seems there are a lot of different "enertia house" search hits.
Phase change heat storage is well established techology. It's in the product you buy when you want to have building scale latent heat storage. I've even seen a company trying their luck at the business model of selling heat in twenty foot equivalents, e.g. for building sites. So it's safe to assume that they knew about this option. But there's also the square-cube-law, at some point it will be cheaper to insulate a larger volume of the cheaper material than a smaller volume of a material that requires less volume per unit of energy at the heat range you aim at. It's well possible that this line is between building-scale and grid-scale storage.
The Venera probes used this idea in the 1970s. [0]
> One new idea, for additional thermal protection, was the addition of phase-change material. Lithium nitrate trihydrate melts at 30° C, absorbing a large amount of heat, due to its high latent heat of fusion.
> The sand itself will also be sustainably sourced – it’ll consist of crushed soapstone, which is a manufacturing byproduct of another local industry.
The requirement is not really construction grade sand but any substance with a largish thermal mass. Sand/dirt/crushed rock, etc. whatever you have right on your doorstep. There have been plenty of prototypes using such various materials as thermal mass.
All you need is a big container, some insulation to keep the heat in and lots of mass in whatever form. And some pipes to get heat in and out via e.g. water. The bigger the container, the smaller its surface area is relative to its volume. So these things can be quite efficient. If you make them large enough, you can store enough energy in the summer to last for months during the winter.
Phase changes will probably work well for structures that would otherwise be used for insulation anyway - in the case of this particular idea though, I think heat is moved in and out using another medium, air in this case, so a phase change would complicate things greatly (hard to bubble air through lava). But yeah, something like a phase changing fluid (gas <-> liquid) will probably work well, as it does in air conditioners right now.
Now what would be suitable material with such phase change around 80 to 110 degree temperature? As that is what effective heating needs. Water with the safe change at 0, is clearly too low as then you could just run ground source or air source heatpumps...
> But I can't help but think of one "technology" that could make a scheme like this MUCH more effective.
I think this is one of those things that drive home the point that there are fundamental differences between physics and engineering.
The article states that the thermal silos are heated with excess energy from the power grid. This alone tells you right from the start that efficiency is not the primary requirement.
Sand is inert, doesn't decompose or degrade, is readily available, is easy to work with, and has no moving parts. You can make it work in a silo, or digging a well to fill it with sand. In fact, geothermal heat pumps are already used extensively in residential buildings to regulate temperature. You just have to drill a hole in the ground that's deep enough, run a water pipe through it to heat/cool the water, and run that water through your building to heat/cool the environment. The nifty trick of Polar Night Energy is that they introduce the extra step of actively heating the thermal source with cheap energy supplied by the electrical power grid.
This sort of argument is like complaining that a Formula 1 car is far more efficient than a Volkswagen Golf. Yes it is,but that's a mute point.
I can't endorse this perspective enough. The amount of energy storage we need is staggering and ever-growing. We've someone convinced ourselves that the 'baseline' is consuming with abandon millennia worth of stored energy and anything even slightly less responsive than that is too inconvenient. Given those parameters, we need any and all energy storage options and efficiency is not a priority. Tesla powerwalls were never going to power the world, but giant caverns full of sand might.
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.".
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.
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.
> -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.
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.
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?
> 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.
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.
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.
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.
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.
Here's a smaller scale water battery that heats an entire house year round. The water tank itself is placed in the center of the house across all 3 floors and actually makes for a quite nice design element as well. The energy is generated in the warm seasons and the volume of water is large enough to last through the entire winter. It's a German vid but subtitles are quite accurate.
Doesn't this suffer the problem that you're then putting heat into the device, which is in your house, during the warm season when you want your house to stay cool?
Think of the water tank as being in but insulated from the house.
In the hot summer the hot air within the house is cooled by heat pumping the heat into the water tank which is slowly over weeks bought up from cold to mean summer tempreture.
In the winter the heat is extracted from the water and transfered to the house.
The house and the water mass are out of phase by six months.
It's an interesting concept. Cheap. Doesn't take up a lot of space. Could be driven by a solar furnace.
My concern is whether the cyclone particle separator will do a good enough job - gas turbine blades don't like being blasted with grit. I expect they'd need a heat exchanger to be sure to get only clean gas driving the turbines.
This is great technology, simple and effective. I’ve spent many hours reverse engineering to see how effective and expensive it might be. I’ve found it’s very cost effective but heat can be hard to calculate. i like that they did a prototype. i think i’ll do one at some point. for an individual house it makes more sense to improve my insulation but i think ill still build a small version for fun.
How long ago was this? The idea seems simple enough and would really benefit from the increasing glut of renewables. Even better, unlike a sand battery, you can potentially cycle it for several months of summer, by offsetting consumption by just a few hours feels like it could be economically viable.
Then again, the competition is general purpose batteries which can be used all year and probably require less maintenance.
Since you burn the wood really fast, you need to delay that heat release. That's where sand comes in as a cheap and good enough material. There are also systems that heat water and store it in a tank, which also make sense if you want hot water in your house too.
Readit News
Take a large metal bucket or barrel.
Put a heating element and a simple oven thermostat on the bottom.
Fill with sand.
Connect to a solar panel or other enrgy source.
The air between the sand particles seem to actually provide a bit of convection and insulation.
The thermostat turns the circuit off before getting too hot for the heating element.
The heat accumulated during the day is slowly released over time directly through the metal.
Fitting Night Storage radiators was a much cheaper way of fitting central heating than the normal boiler and plumbing approach, and when electricity was cheaper it made more sense. Now, it makes no sense and if you buy a house with it the first thing you have to do is put in "proper" central heating. But maybe with excess renewables that will swing back.
But I can't help but think of one "technology" that could make a scheme like this MUCH more effective.
a phase change.
for example, it takes 1 calorie for water to go from 30 to 31 degrees F, but but it take 80 calories to go from solid at 32 to liquid at 32 F.
A project I remember reading about that was really interesting was a house constructed with logs that had a resin that phase changed around room temperature. The idea was that the logs would be heated by sunlight, and the resin in the logs would absorb lots of energy without easily going above room temperature. Then at night, they would slowly solidify and give off heat at room temperature. This would be a cool way to stabilize house temperatures without needing equipment.
EDIT: it was called the enertia house
I'm uncertain of the history, it seems there are a lot of different "enertia house" search hits.
Here's one explaining the idea:
https://enertia.net/howitworks.html
https://joeveo.com/pages/the-temperfect-mug
By adjusting the amount of water in the mixture, you can fine-tune the phase change temperature to whatever the customers like for their coffee.
It also boils above 100C so when the customer puts it in the microwave it won't burst unless they boil the cup dry.
And when the customer does break it, it's non toxic and food safe.
https://www.baristamagazine.com/coffee-joulies-engineered-so...
> One new idea, for additional thermal protection, was the addition of phase-change material. Lithium nitrate trihydrate melts at 30° C, absorbing a large amount of heat, due to its high latent heat of fusion.
[0] http://mentallandscape.com/V_Lavochkin1.htm
It depends where you are. Sand thieves are a thing in many places around the world.
Here's a story from 2015 about how residents in one area have lost 25 hectares of land to sand thieves.
https://tuoitrenews.vn/news/features/20150524/residents-losi...
Or a police captain whose legs were severed while trying to apprehend sand thieves.
https://www.vietnam.vn/en/dien-bien-moi-vu-dai-uy-cong-an-bi...
And here's a longer from article on the phenomenon:
https://www.mekongeye.com/2023/05/01/mekong-delta-sand-minin...
tl;dr sand is used in construction
The requirement is not really construction grade sand but any substance with a largish thermal mass. Sand/dirt/crushed rock, etc. whatever you have right on your doorstep. There have been plenty of prototypes using such various materials as thermal mass.
All you need is a big container, some insulation to keep the heat in and lots of mass in whatever form. And some pipes to get heat in and out via e.g. water. The bigger the container, the smaller its surface area is relative to its volume. So these things can be quite efficient. If you make them large enough, you can store enough energy in the summer to last for months during the winter.
Here's a story about a Dutch retiree who built a prototype using basalt as the thermal mass in his backyard: https://deepresource.wordpress.com/2020/03/08/cesar-seasonal...
"The sand itself will also be sustainably sourced – it’ll consist of crushed soapstone, which is a manufacturing byproduct of another local industry."
For energy storage, any sand is sufficient.
I think this is one of those things that drive home the point that there are fundamental differences between physics and engineering.
The article states that the thermal silos are heated with excess energy from the power grid. This alone tells you right from the start that efficiency is not the primary requirement.
Sand is inert, doesn't decompose or degrade, is readily available, is easy to work with, and has no moving parts. You can make it work in a silo, or digging a well to fill it with sand. In fact, geothermal heat pumps are already used extensively in residential buildings to regulate temperature. You just have to drill a hole in the ground that's deep enough, run a water pipe through it to heat/cool the water, and run that water through your building to heat/cool the environment. The nifty trick of Polar Night Energy is that they introduce the extra step of actively heating the thermal source with cheap energy supplied by the electrical power grid.
This sort of argument is like complaining that a Formula 1 car is far more efficient than a Volkswagen Golf. Yes it is,but that's a mute point.
Otherwise, the heat to be stored comes as a byproduct of local industry. Something that would otherwise be vented out and not used.
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.
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.".
Cheaper than water?!
-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.
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.
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.
At a guess, and I confess I'm not capable of running the numbers, this offsets the much higher temperature delta of sand.
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.
Sand? If some spills just grab a skid loader and stuff it back in.
Sand is also a really good insulator if I'm not mistaken. That could also be a factor somehow.
The higher density of rock probably plays a big role.
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.
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.
https://www.youtube.com/watch?v=MBLiNHsod8Y
In the hot summer the hot air within the house is cooled by heat pumping the heat into the water tank which is slowly over weeks bought up from cold to mean summer tempreture.
In the winter the heat is extracted from the water and transfered to the house.
The house and the water mass are out of phase by six months.
https://news.ycombinator.com/item?id=32006791
2 years ago | 287 comments
A sand battery could transform clean energy
https://news.ycombinator.com/item?id=33465272
1 year ago | 143 comments
https://arpa-e.energy.gov/sites/default/files/2021-03/07%20D...
The IP has been licensed by Babcock and Wilcox, who are the source of the nifty compact fluidized bed heat exchanger.
My concern is whether the cyclone particle separator will do a good enough job - gas turbine blades don't like being blasted with grit. I expect they'd need a heat exchanger to be sure to get only clean gas driving the turbines.
Perhaps more interestingly, it could be driven by combustion of an e-fuel as a backup in case the sand becomes depleted.
Until patents are involved
Cheap electricity can make cold, and then a fan can convert the cold water into cold air during peak hours when electricity was costlier.
Fans still took electricity to run, but it'd only be a few hundred watts to run air-conditioning rather than kilowatts of power.
Then again, the competition is general purpose batteries which can be used all year and probably require less maintenance.
It's a tech for time shifting cheap overnight energy to the expensive daytime.
Installing PV gives you cheap power directly and correlates with air-con demand.
Since you burn the wood really fast, you need to delay that heat release. That's where sand comes in as a cheap and good enough material. There are also systems that heat water and store it in a tank, which also make sense if you want hot water in your house too.
Sand is really only useful here because that much clay is usually significantly more expensive and would take forever to dry out.
Source: researched the shit out of and have built a rocket stove.
It's specifically intended to store surplus energy from renewables for later use.