'Sand battery' could solve green energy's big problem

Robert Zwilling

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Using common materials and room temperature semiconductors for cheap green power is slowly creeping forward.
Finnish researchers have installed the world's first fully working "sand battery" which can store green power for months at a time.
The developers say this could solve the problem of year-round supply, a major issue created by the intermittency of green energy.
Using low-grade sand, the device is charged up with heat made by running wind generated electricity through resistive wiring.
The sand is contained in a silo where it is heated up to 500 degrees C. The sand holds the heat for a couple of months. The heat is transferred out of the silo by creating warm water for the district heating system which supplies local homes and businesses.
 

hitmouse

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So it is a big night-storage heater, essentially.

More seriously, a friend of mine built a 4x4x4m version of this in the late 1970s to keep his home warm in the winter. It is not a new idea, but it is nice to see that it can be scaled up to an industrial size.
 

Montero

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Good. Sand is pretty inert and you're not going to have the level of safety or disposal problems that you get with actual electric storage batteries. Won't help with producing electricity, and needs a district heating system in place, but there could be a domestic version created to help each household keep warm. At that level a water based heat store might be easier, but it is a good principle. I know someone with a couple of small wind turbines - as in the sort you put on your roof - and they run electric through underfloor heating for them.
Dick Strawbridge had a greenhouse heater based on a trench of recycled glass beads under floor, with hot air from the greenhouse during the day pumped through it to provide warmth overnight.

And on the Island of Eigg where they have a local distribution network of renewable energy...


"

More than we need​

Sometimes Eigg Electric produces more electricity than we can use, so we use the excess to heat community buildings. If you see a fan heater on in the community hall or the waiting room, we’re not wasting electricity, we’re making too much!
 

Brian G Turner

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The only trouble with using sand is that it's already one of the world's most over-exploited resources:
 

Robert Zwilling

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Probably everything is running low because we are now serving 8 billion people. River rocks, roundish shaped rocks, can also be used to store the heat. Probably have to improve the insulation when using rocks. The cement industry is one of the top polluters, while their processes might be changed by using discarded industrial waste products , the sheer bulk of the industry might keep it at the top of the heap. Still waiting for a good sci-fy solution substitute for cement.
 

Christine Wheelwright

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This is one of those ideas that sounds crazy at first. But then it starts to become more interesting when you think about it. Like @Montero I also wonder about using water which has a higher heat capacity than sand (takes more energy to raise the temperature of 1kg by 1K and therefore surrenders more energy as it cools). I find the idea of a large scale project way more interesting than small domestic versions though. Heat loss will largely depend upon surface area vs volume ratio (a much better story for a centralized system: a large volume of water or sand in one place). Here is an interesting idea: make the container a double wall like a thermos flask and use vacuum to help keep heat internalized. Impractical for small domestic versions (which already exist) but perhaps doable for a larger scale implementation?

Something more solid, like sand, might work better than water in another sense though. Although water has good heat capacity, being a liquid it conducts heat out of the center of the mass to the edges very easily (Brownian Motion etc). Using sand - or something more solid - might help to keep the heat within the core and away from the edges (where it is slowly lost).

Another positive is that this solution can be decentralized (kept off grid). The wind turbine can literally stand next to the thermal mass. Nearby buildings can pump water through pipes within the thermal mass to extract heat. Or put it under the building and allow heat to escape from it by varying the strength of the vacuum barrier (lower vacuum to extract more heat).

No doubt difficult to implement, but it sounds way more feasible than 95% of the stupid ideas out there these days.
 

hitmouse

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The plan with this installation is to heat the sand to 500C. Quite difficult to do that with water, so the argument about relative heat capacity is moot.
 

Montero

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The only trouble with using sand is that it's already one of the world's most over-exploited resources:

Of course. :(

Every suddenly trumpeted bright idea for saving the future seems to come without joined up thinking. I shouldn't have got so excited. :(

Mind you, Dick Strawbridge's chunks of recycled glass would work..... and not take any new sand.....unless of course without the glass recycling, more sand is taken to make glass...... sigh.
 

Christine Wheelwright

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The plan with this installation is to heat the sand to 500C. Quite difficult to do that with water, so the argument about relative heat capacity is moot.
Right. I didn't spot the 500C at first. Actually I think this is the wrong approach as losses at the boundary will be proportional to the temperature differential. Way better to work with more material at lower temperatures! Especially when the material is cheap (both sand and water are very cheap). Also, I wouldn't discount water quite yet. It has a thermal capacity of 4200 J per kg per K compared to 840 for sand. So water at 0C (273K) actually contains more energy than sand at 500C (773K). Strange but true. Hard to extract energy at 0C of course (though not impossible). In general I like the sand because heat can more easily be concentrated in a core (thermal conductivity is relied upon because there is no movement of the medium itself). But working at 500C is nuts. Better to aim for, say, 60C with a far greater mass. Surface area to volume ratio only improves with larger mass and heat loss at boundary improves with lower temperature differential. Win win!
 

Bramandin

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Probably everything is running low because we are now serving 8 billion people. River rocks, roundish shaped rocks, can also be used to store the heat. Probably have to improve the insulation when using rocks. The cement industry is one of the top polluters, while their processes might be changed by using discarded industrial waste products , the sheer bulk of the industry might keep it at the top of the heap. Still waiting for a good sci-fy solution substitute for cement.

The world is running out of rough sand. If tumbled sand works, that's great.
 

Robert Zwilling

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The round rocks were used in the 70s to store heat in the basement of a house. Water was heated up on the roof in solar collectors, then it was run down to the rock pile where it heated up the rocks. After the sun went down, air was blown through the rock pile to generate warm air that was circulated through the house.

One of the problems of the original solar collectors heating up water was the growth of algae inside the clear plastic pipes. Plastic was cheap back then. The designs of today typically have the liquid in a black tube or pipe. Because the cost of photocells has dropped over the years, many solar collectors now bypass the collection of heat and instead just use photocells to generate electricity.

Any action that involves 8 billion people, no matter how good it seems, has immediate and serious consequences that can not be over looked, and might be unavoidable.
 

CupofJoe

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I think McLaren have something similar at the HQ.
 
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