Sand Battery Experement

[Turnermjt]

Your target is about 3 to 6 decades too small to work with sand insulation over 3-month perioid.
Put it in other way 1000 to 1000000 times too small to work. 100x100x100m cube of sand with 10m outer layer of sand working as insulation would surely work but it sounds kind of big.

Sand Battery’s Efficiency Explained – Polar Night Energy’s ‘Sand Battery’ Has Efficiency Up To 95 Per Cent — Polar Night Energy

On what basis can we make the claim 'Efficiency up to 95%'? Here’s our Lead Scientist’s take on the efficiency of our energy storage system. Resistive heating of sand is essentially 100% efficient, but the efficiency is inevitably lowered by heat loss through the boundaries of the system. However,

polarnightenergy.fi

95% efficient for 1 or 2 week cycle on 1GWh =1000MWh scale and they supposedly would also use other insulation than sand. Scale up to 3 month storage and I'd look start with 10000MWh minimum feasible. Use sand as insulation and I'd start with 100000MWh for 3 month target at reasonable efficiency.

Waste of time to do the actual maths as nobody is going to have 100000MWh sand battery in domestic use.
Might explain why we don't store heat in sand for winter months.

Water would be totally doable if you dont care about any sort of ROI. Built wisely under the house it might? be even economically feasible in some cases. Even low temperature sand or concrete would be doable but you need ALOT of it.
For example 150m2 house foundation with 1m of sand under the foundation, insulated with about 3x typical insulation thickness ie something like 300-600mm PIR foam would probably get you close. Lot easier than 1 cubic kilometer of hot sand.

thanks for the response, even if it puts plan B to bed.

how does polar night get away with a tower 4 mtrs wide by 7 mtrs tall and their heat figures?

do we know how they insulate?? - is it just they heat the center and the sand insulates it?

 

[Turnermjt]

Your target is about 3 to 6 decades too small to work with sand insulation over 3-month perioid.
Put it in other way 1000 to 1000000 times too small to work. 100x100x100m cube of sand with 10m outer layer of sand working as insulation would surely work but it sounds kind of big.

Sand Battery’s Efficiency Explained – Polar Night Energy’s ‘Sand Battery’ Has Efficiency Up To 95 Per Cent — Polar Night Energy

On what basis can we make the claim 'Efficiency up to 95%'? Here’s our Lead Scientist’s take on the efficiency of our energy storage system. Resistive heating of sand is essentially 100% efficient, but the efficiency is inevitably lowered by heat loss through the boundaries of the system. However,

polarnightenergy.fi

95% efficient for 1 or 2 week cycle on 1GWh =1000MWh scale and they supposedly would also use other insulation than sand. Scale up to 3 month storage and I'd look start with 10000MWh minimum feasible. Use sand as insulation and I'd start with 100000MWh for 3 month target at reasonable efficiency.

Waste of time to do the actual maths as nobody is going to have 100000MWh sand battery in domestic use.
Might explain why we don't store heat in sand for winter months.

Water would be totally doable if you dont care about any sort of ROI. Built wisely under the house it might? be even economically feasible in some cases. Even low temperature sand or concrete would be doable but you need ALOT of it.
For example 150m2 house foundation with 1m of sand under the foundation, insulated with about 3x typical insulation thickness ie something like 300-600mm PIR foam would probably get you close. Lot easier than 1 cubic kilometer of hot sand.

Before i bury this idea completely i had already asked a couple of local insulation companies if they could come up with a solution to insulate my container from 750c down to 25c.

One has come back with a 3 wall solution 250mm thick.

This they say will bring the wall temp down to 50c - which i can then spray with foam the exterior - like you would on a warm WATER heater tank ;-)

it may be a rubbish idea still - but im working on it.

Top insulation
*Superwool Plus 128kg 50 mm 698.2 °C
PAROC Pro Wired Mat (WR) 700 AL1 TH1 1) 100 mm 488.0 °C
PAROC Pro Wired Mat (WR) 700 AL1 TH1 100 mm 53.4 °C
Heat loss 342 W/m²
Nominal weight of insulation 32.4 kg/ m²

can you work out my losses from this information?

thanks again and it may still be a rubbish idea - just thinking it through

 

[MattiFin]

Your insulation company values seem roughly correct but actually WORSE than my proposed 500mm mineral wool insulation. (albeit at less thickness.)

342W/m2 heat loss and 38 to 54m2 surface area(depending if you use outer or inner area of the cube) gives you 12,8kW to 18,5kW losses.

BUT you can't slap foam on top of this: the surface of the insulated cube would become so hot that it burns/melts/degrades the foam insulation.
50mm foam would provide more insulation than the whole 3-layer wool structure if it would withstand the temperature.
For example 50mm foam on top of this concoction would increase the theoretical outer wool wall temperature to 620 degrees. Obviously this wont work without catching on fire.

 

[Hedges]

I've thought about layers of different insulation.
You have to engineer it or regulate it by bleeding heat; too much outer insulation such as foam, and inner surface of foam overheats.

Cost effectiveness increases with rate of cycling. Moderating day/night temperatures gives many more cycles than seasonal storage.
Moderating suggests phase-change materials would be ideal. But no great ones (especially considering cost) for room temperature.
High temperature sand storage could be used for heating at night.

However, I let home be cold at night, heat in morning. Maybe a 50 gallon drum of sand would fit the bill.

 

[Turnermjt]

Your insulation company values seem roughly correct but actually WORSE than my proposed 500mm mineral wool insulation. (albeit at less thickness.)

342W/m2 heat loss and 38 to 54m2 surface area(depending if you use outer or inner area of the cube) gives you 12,8kW to 18,5kW losses.

BUT you can't slap foam on top of this: the surface of the insulated cube would become so hot that it burns/melts/degrades the foam insulation.
50mm foam would provide more insulation than the whole 3-layer wool structure if it would withstand the temperature.
For example 50mm foam on top of this concoction would increase the theoretical outer wool wall temperature to 620 degrees. Obviously this wont work without catching on fire.

thank you - appreciated.

i think they are suggesting the external steel wall temperature would be down to 50c. is that incorrect?

that is below my hot water tank temperature - can i not use foam on it externally? therefore saving additional heat losses?

 

[Turnermjt]

I've thought about layers of different insulation.
You have to engineer it or regulate it by bleeding heat; too much outer insulation such as foam, and inner surface of foam overheats.

Cost effectiveness increases with rate of cycling. Moderating day/night temperatures gives many more cycles than seasonal storage.
Moderating suggests phase-change materials would be ideal. But no great ones (especially considering cost) for room temperature.
High temperature sand storage could be used for heating at night.

However, I let home be cold at night, heat in morning. Maybe a 50 gallon drum of sand would fit the bill.

i do not have enough solar in winter - so want to collect the abundance of excess i have in summer/autumn and use it for 2 or 3 winter months - if necessary only 12 hours at night. i need about 3kw at night to heat my greenhouse

 

[MattiFin]

thank you - appreciated.

i think they are suggesting the external steel wall temperature would be down to 50c. is that incorrect?

that is below my hot water tank temperature - can i not use foam on it externally? therefore saving additional heat losses?

Their numbers apply when steel wall is freely cooled to ambient air. If you add layer of foam on top of that the steel wall is going to be lot hotter.
You could add tiny bit of foam to get the wall temperature up to 100c or so but it has minimal effect on total heat loss.

Instead of 750Cel to 50Cel =700c temperature delta over the fiber insulation you would get 750 to 100cel = 650c temperature delta over the fiber insulation, cutting the losses probably by about 5%
No free lunch here unfortunately.

For 3 month storage you would need either water or sand/rock/concrete and LOTS of it at relatively low temperature. Or new breaktrough in high-temperature insulation materials.

 

[MattiFin]

If cost wasn't issue you could insulate your 2.5m container with Aspen Pyrogel HPS.
500mm layer of Pyrogel HPS would be about 3x improvement over the normal high temperature ceramic fiber.

I have actually used some for speciality projects but on this scale the price would be astronomical: IIRC 10mm Pyrogel HPS sheet was around 50 euros/m2 and you would need around 40 square meters times 50 layers = 2000 square meters of 10mm sheet, totaling 100000 euros.
(at this scale you would probably qualify for some bulk discounts but still..)

 

[Rocketman]

Could you build the sand battery at the end of the greenhouse- then enclose the greenhouse around the battery for the winter - thus using the waste heat?

Although if not done correctly- it could turn into a runaway greenhouse- Venus…

 

[MattiFin]

Could you build the sand battery at the end of the greenhouse- then enclose the greenhouse around the battery for the winter - thus using the waste heat?

Although if not done correctly- it could turn into a runaway greenhouse- Venus…

This would certainly make sense. My proposal was along the same lines earlier as a energy storage under the house.

But the problem still remains: too much heat loss with the attempted high temperature sand solution.

 

[OzSolar]

This would certainly make sense. My proposal was along the same lines earlier as a energy storage under the house.

But the problem still remains: too much heat loss with the attempted high temperature sand solution.

Another problem that I don't think has been touched on is how to deliver the heat.

EG: What is a rated to handle those sorts of temps to move the heat to conditioned space? Even if you found something you'd still have to temper it or you would risk the safety of the occupants.

 

[Warpspeed]

How about a "flow" battery plus a heat pump ?

 

[OzSolar]

How about a "flow" battery plus a heat pump ?

Please elaborate. It's new to me.

 

[Warpspeed]

Flow batteries have electrodes and a liquid electrolyte, but the electrolyte volume is not limited by what is contained in each individual cell.

When charging, an electrolyte is pumped through the cell and the "charged" electrolyte can be stored indefinitely in an external larger storage volume.
During discharge the electrolyte is pumped back in the reverse direction and "discharged" recovering the original stored electrical power.

The cell ampere our capacity is limited only by how large your external storage tanks are.
So it would be quite possible to store energy in summer and recover it many months later in winter.
The main disadvantage in doing this is the relatively high cost of the electrolyte.
So possible, but probably not economic.......

Stored electrical power could be used to drive a heat pump, so if the main intent is for wintertime heat storage, a heat pump could theoretically improve system efficiency.

Suggest you do an internet search for "flow batteries" they are not yet common, but I believe they are currently available.
https://arena.gov.au/blog/south-australia-goes-with-the-flow-battery/

 

[Turnermjt]

Another problem that I don't think has been touched on is how to deliver the heat.

EG: What is a rated to handle those sorts of temps to move the heat to conditioned space? Even if you found something you'd still have to temper it or you would risk the safety of the occupants.

HI, I was thinking of having a 2" or 3" tube in a large coil approx 4' in diameter 100' long - 4' high buried in the sand. Both ends of the tubes would terminate in the greenhouse - i was just going to blow air from the greenhouse, down the tube and the air would be returned to the greenhouse - heated.

i hadn't worked out if i need to add heatsinks to the tube in the sand to get the heat back into the tube, or whether it would happen automatically.

i guessed i could adjust the heat coming back into the greenhouse, by adjusting the amount of air movement.

any body got any calculations for this??

 

[OzSolar]

any body got any calculations for this??

@MattiFin has provided some very nice examples of the calculations in his previous posts. In fact, I think they are great examples of how cool the metric system is. Being from the USA I'm struggling to follow them but fully intend to circle back and study them until I do understand them.

Fundamentally, you either have to figure out how much energy you need to store or how much energy you can store.

EG: My car gets 25 mpg and I need to go 250 miles so I need 10 gallons to get to where I'm going.
or
I have 5 gallons in my tank and my car gets 25 mpg. How far can I go?

You need to figure out how many "gallons" your greenhouse can hold. I'd probably use cubic feet. Then I would try understand how many BTU's are in each each of those cubic feet per degree I can add or take away from it.

First, let's figure out how many cubic feet you have. L x W x H = Cubic feet.

 

[Warpspeed]

That is true only if there are no losses in the process.

EG: My car gets 25 mpg and I need to go 250 miles so I need 10 gallons to get to where I'm going.

What if your fuel tank has a leak ?

You might need 11 gallons, or 27 gallons to go that 250 miles.

 

[OzSolar]

That is true only if there are no losses in the process.

I was planning to get there but first wanted to help them figure out how much potential energy they could store.

We could go straight to the formula. I did not derive this. Google helped me find it here to give credit.

heat = mass flow * specific heat capacity * temperature difference

 

[Johneb]

I'm not sire if this helps or not. I have read of a solar farm that heats salt to liquid form, stores it in insulated tanks, then when the sun goes down uses the salt to heat water to steam to run a genetator.

 

[GXMnow]

I'm not sire if this helps or not. I have read of a solar farm that heats salt to liquid form, stores it in insulated tanks, then when the sun goes down uses the salt to heat water to steam to run a genetator.

What you are referring to is a concentrated solar plant where they have the huge array of mirrors to focus the light onto a tower with the tank of molten salt. The plant on the California / Nevada border is still operating like that. I don't think you could scale it down to make a home system.

Check out this page.

IVANPAH

Rising 450 feet above the California Desert, Ivanpah is the world's largest concentrating solar power facility.

www.energy.gov

and this

The world's largest solar thermal power plant

The world's largest solar thermal power plant is now online in California and can provide enough power for 140,000 homes. Video source: CNNMoney / YouTube.

www.science.org.au