Storing heat in sand?

[glenkleidon]

Still if you can avoid water it would be better. If you have 1800 Celcius sand, then you can easily heat air enough and use that. You could probably also use a Stirling engine to generate electricity - again with no plumbing. Just not sure if there is a small scale Stirling engine commercially available to do that with....

HA! there is https://www.alibaba.com/product-det...offerlist.normal_offer.d_title.463763d8h1KtxE but this one runs on diesel I think - so in principle you could supply the heat from the sand battery instead of burning diesel.

 

[eXodus]

I envisioned a tank, not a pit. An insulated tank as well. A 1000 gallon insulated tank of sand would weigh about 16,000 lbs + the weight of the tank.
Not much to go wrong with a tank of sand, some oil, and some hot water piping inside.

Ok a tank sounds more reasonable.

If you don't use the oil as fluid flowing through the sand should be OK

You'd have to account for about 300+ feet of 1 inch diameter PEX Tubing coiled up in the sand

Do really use the thermal mass of sand you need high temperature, and pex might melt or degrade fast. I would use a cooper coil.
A huge metal tank with fiberglass or mineral wool insulation on the outside -no plastic parts - so you can go to several hundred degrees.

Two primary ways if you're in a vacuum. We don't live in a vacuum, so we also include convection.

I would argue that sand - even with oil between the grains would behave more then a solid object then as a fluid or gas.
So convection in my Physics would be a secondary way of heat transfer - not a primary one. But we are splitting hairs on terminology.

If you have 1800 Celcius sand, then you can easily heat air enough and use that.

yep -corrugated copper pipes and just blowing air through the superheated sand - that's what they are doing in Finland. But not at 1800C - more like 600-800C

 

[techdatt]

Sam - Sand batteries are Awesome - but the Thermal Conductivity co-efficient of Sand is exceedingly low - so you wont get any conduction - you will need to insert Thick metal rods or plates to get the heat out. That is the real advantage of sand. It will stay hot (100s of Degrees) and wontly loose any appreciable heat for weeks (depending on thickness of the walls of the sand). Water will get cold very fast because it will loose it to the sides and to the air. You probably should also store the metal rods/plates INSIDE the battery until needed.

How do i heat the sand with resistive heater? I googled and it says a fax is needed to circulate the heat around the sand, which needs addtional electricity to run the fan. Can the resistive heater embedded inside the sand to heat up the sand OR how can it be done?

 

[Hedges]

If you have resistive heater, you have electricity, so no problem powering a fan.
No energy loss, because all power consumed by fan is converted to heat.

A heat pump (e.g. mini-split) could also serve to put more energy into sand than the electricity you supply.
Maybe sand could also be heat/cold reservoir for mini-split, but might not transfer fast enough.

I think air will give slow convection through sand. A heater at the bottom should work.
Maybe an oil-filled radiator (baseboard or portable)?
The sand would behave something like an insulating blanket, probably transfer heat more slowly than convection causing air circulation in a room.

You'll then want to extract heat. Finned tube (e.g. radiator, either hydronic baseboard or automotive) at the top, with either air or water circulating?
Or just out top of container if room above, with insulating plug?

 

[SeanK]

How do i heat the sand with resistive heater? I googled and it says a fax is needed to circulate the heat around the sand, which needs addtional electricity to run the fan. Can the resistive heater embedded inside the sand to heat up the sand OR how can it be done?

Hmmm, how long will this element last I wonder...

 

[TBoone]

oops

 

[Mart Hale]

Today I decided to test out making a sand battery. I had this old pot with a hole in the bottom, I then took two cans cut out the bottom on those and put inside the pot over the hole then filled the pot with sand fir the first layer. Second layer I cut down the can a bit so it would fit in the pot and I laid in another layer of sand. For this first go around I had temps at the bottom at 325 degrees and at the top at 165 degrees.... After this I brought is inside put a welding blanket inside the cooler and put the pot inside.... I noticed a large amount of moisture building up inside the cooler, which figures as the sand I used was from outside. I am formulating how I could improve the heating of the sand was a fun experiment.

 

[ghostimage]

From what i undestood you are trying to heat the sand to heat the water. (or store heat in the sand medium) Why? just heat the water directly with solar. Grains of sand do not tightly fit together (air in between/ air is an insulator) concrete is better for heat transfer and storage or lead or cooper, better yet use water/oil as a storage medium. Just heat the water directly with solar. Pull you head out of the sand and enjoy the sunshine

 

[Ampster]

. Pull you head out of the sand and enjoy the sunshine

Better yet, the OP should step back and look at the issue from a high level. Insulated water tanks are inexpensive and insulation can be added to make them even more efficient storage mediums. Then the only issue is the source of heat. Generally it is most efficient to transfer heat rather than use resistance to convert electricity to heat. If one is in a temperate climate, a Heat Pump Water Heater is a very efficient source of heat.
Later, after he gets that system working he will be more able to evaluate whether playing with sand has any utility.

 

[Mart Hale]

From what i undestood you are trying to heat the sand to heat the water. (or store heat in the sand medium) Why? just heat the water directly with solar. Grains of sand do not tightly fit together (air in between/ air is an insulator) concrete is better for heat transfer and storage or lead or cooper, better yet use water/oil as a storage medium. Just heat the water directly with solar. Pull you head out of the sand and enjoy the sunshine

No, you don't have the picture.... I heat the sand to heat my feet inside with no smoke for hours...

Or, I heat the sand and use heat retention to cook my food for hours.


I do heat the water directly, but bring steam into the home is something I don't want to do..... I am considering heating the water then running an insulated line inside where it can heat the sand where it can release the heat.....

You never know what ideas you may run into when playing with tin cans ;-)

 

[Mart Hale]

Better yet, the OP should step back and look at the issue from a high level. Insulated water tanks are inexpensive and insulation can be added to make them even more efficient storage mediums. Then the only issue is the source of heat. Generally it is most efficient to transfer heat rather than use resistance to convert electricity to heat. If one is in a temperate climate, a Heat Pump Water Heater is a very efficient source of heat.
Later, after he gets that system working he will be more able to evaluate whether playing with sand has any utility.

I have done this. I have used my vacuum tub solar heater to heat water, then put into a thermos, kept the water hot for over 18 hours, I was amazed.

 

[PVorBust]

From what i undestood you are trying to heat the sand to heat the water. (or store heat in the sand medium) Why? just heat the water directly with solar. Grains of sand do not tightly fit together (air in between/ air is an insulator) concrete is better for heat transfer and storage or lead or cooper, better yet use water/oil as a storage medium. Just heat the water directly with solar. Pull you head out of the sand and enjoy the sunshine

Your argument is valid if you only heat the sand to 100C (same as water).
The disadvantage of sand is as you describe, but ...
The advantage of sand is you can heat it way hotter than you can get water (ignoring pressure vessels).
"way hotter" (a technical term) not only gives you the same heat storage in a much smaller volume.

It's been a while, but from memory, the energy you can extract is defined by: T^4 - t^4

As a retort, after your sunshine has got the sand to 1000C, try putting your head in the sand

 

[ghostimage]

Your argument is valid if you only heat the sand to 100C (same as water).
The disadvantage of sand is as you describe, but ...
The advantage of sand is you can heat it way hotter than you can get water (ignoring pressure vessels).
"way hotter" (a technical term) not only gives you the same heat storage in a much smaller volume.

It's been a while, but from memory, the energy you can extract is defined by: T^4 - t^4

I agree with everything you said.
However how do you heat the sand uniformly? Sand heat transfer per inch is very poor for the reason I've mentioned. In the dessert top layer is scorching hot and 6" deep its significantly cooler. Poor heat transfer. Concrete would be better. I actually like oil idea. You can heat it to a higher degree. Maybe mix sand and oil? Will that work better for storage. ?‍

 

[PVorBust]

I think you'd end up with the properties of the lower performing medium as a limiting value.
For the reasons you list, concrete would take up heat faster than sand, but would have a much lower Tmax
for some light reading
inis.iaea.org/collection/NCLCollectionStore/_Public/28/057/28057515.pdf

most is irrelevent as it is mostly concerned with mechanical strength over time of the concrete, but you can still see how the properties change due to elevated temp.

 

[Hedges]

It's been a while, but from memory, the energy you can extract is defined by: T^4 - t^4

I think energy storage of a solid is for the most part proportional to temperature, no power of 4 or power of 2 involved.
Phase change is where things get interesting.

Capacitors there is a V squared term. Springs too. The power can be recovered in a lossy manner, just delivering current or displacement through a regulator. Or, more efficient mechanism like SMPS or variable transmission.

Stored heat can just warm something by conduction, but high-grade heat can more efficiently be converted into other forms, e.g. with Peltier or a steam engine. Then it could power a heat pump and deliver more total heat, given suitable ambient conditions.

 

[Hedges]

Poor heat transfer.

Layers of aluminum foil or wires or tubes in sand? One or two axis thermal conduction.

 

[PVorBust]

I think energy storage of a solid is for the most part proportional to temperature, no power of 4 or power of 2 involved

Apologies, it's been a while.
The 'power of 4' equation is the radiation flux - which goes a long way to explain why insulation becomes so important as temp rises.

Some light reading for those not afraid of partial differential equations.

https://faculty.utrgv.edu/constantine.tarawneh/Heat%20Transfer/HeatTransferBooklet.pdf

 

[Hedges]

I'm afraid of partial differential equations.
I was top of my class in semiconductor physics and did at least OK in statistics.
I perform calculations assuming rectangular bounding box where possible, use Pythagorean theorem or trigonometry when necessary.
Don't think I've applied partial differential equations outside of college classes.

So maybe power of 4 relates to energy transfer. I figured storage was linear.

 

[Danke]

Technology — Polar Night Energy

Technology

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[TBoone]

At 1Atmosphere of pressure, “air” when heated will expand.

There are a LOT of forces at play on the atmosphere. If we focus on a ventilated room, the forces are simpler.
gravity, and convection. If a furnace pulls in return cool air, heats it and supplies it back to the room, it will, if dumped out high without a diffuser, sit on top of the cooler air, then convection will spread the heat from the air around the room.
If the heated air is dumped out low in the room, again, without a diffuser, it will be forced to the top of the room by the denser air around it, and convection will again distribute the heat.

If you use an electric heater to warm the air in the room, cold air will be the densest, so the heaters are as low as possible, as the air is heated, more cold air pushes out the heated less dense air.
there is no rising… there is gravity pulling denser air below the less dense air.

Remember… there are only two forces. Gravity, and convection. Gravity pulls towards the center of mass of the planet. So… in the room… down. Convection is a circulation motion… radiant energy transfer barely provides any change in the scenario, and radiant certainly does not rise, or fall… it radiates.

Technology — Polar Night Energy

Technology

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Quick calculation....one could store ~341 KBtu in 2000 lbs(~20cuft) of sand at 500C. This would require ~100 Kwh from my panels which I could easily do over a several day period. I could then use this for reserve heating in a grid-down/poor solar scenario from an ice storm, etc.. This would be nice when one sees another ice storm coming like the one we just had in N. Texas.
Could use something like range top heating elements to heat the sand. Cheap, readily available, and should be able to handle it.

-TBoone

 

[Ceefiveceefive]

I've been interested in making my own sand battery for winter heating. I would use a medium sized approach - not as big as the video's and not as small as a bucket. I'm thinking of the equivalent of a 2500 gallon water storage tank.

What would be a simple way to make solar resistive heating for that size?

 

[Hedges]

How many kW?

Oil filled radiator? Think it will need air convection through the sand to transfer at suitable rate.
Maybe buried in a layer of gravel for better airflow, sand on top?
How do you plan to collect the heat?
It's element will run hotter, but thermostat switches based on lower air temperature.
It's design probably assumes certain transfer rate, so sand/gravel packed around it could result in it running too hot. Might have to use lower wattage settings.

Pipes with fluid, like radiant under-floor heating, would be another.

Simple could be tape heating elements, or electric radiant floor mats.

Metal conduit across the bottom would allow inserting different heating wires.

Suggest you build a 5 gallon or 55 gallon prototype and experiment before full scale!

Have an opening bottom of side to "drain" and sweep out gravel, contingency for the future? You don't want to have to shovel out the top. And broom much more gentle on wires/plumbing than a shovel.

 

[MattiFin]

I envisioned a tank, not a pit. An insulated tank as well. A 1000 gallon insulated tank of sand would weigh about 16,000 lbs + the weight of the tank.
Not much to go wrong with a tank of sand, some oil, and some hot water piping inside.

Add about 50 gallons of mineral oil (400lbs?) (same type used in transformers) to increase thermal transfer.

Sand weighs about 100 pounds per cubic foot, and 1000 gallons of volume = about 160 cubic feet.
So... 160 cubic feet x 100 lbs per cubic foot = 16,000 lbs of mass.

16,400 lbs of mass (16,000 sand + 400 oil) at 200°F, with a DeltaT of 100°F would provide about 1.6 million BTU's. Or it could raise the temperature of about 1968 gallons of water (16,400/8.33 lbs per gallon = 1968 gallons) by 100°F, and the sand would still be at 100°F after it was done.

A 1000 gallon tank is about 6 feet in diameter by 6 feet tall.

Of course, my math is a little off. You'd have to account for about 300+ feet of 1 inch diameter PEX Tubing coiled up in the sand, and account for insulation losses.



Two primary ways if you're in a vacuum. We don't live in a vacuum, so we also include convection.

Sounds like you are off by about factor of five.
Specific heat of Water is 1 btu/lbs/F but sand is only 1/5 of that. https://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html

Water is hard to beat as heat storage medium. Easily available, easy to heat and extract the heat from and excellent heat capacity per weight and volume.

 

[Ceefiveceefive]

How many kW?

Oil filled radiator? Think it will need air convection through the sand to transfer at suitable rate.
Maybe buried in a layer of gravel for better airflow, sand on top?
How do you plan to collect the heat?
It's element will run hotter, but thermostat switches based on lower air temperature.
It's design probably assumes certain transfer rate, so sand/gravel packed around it could result in it running too hot. Might have to use lower wattage settings.

Pipes with fluid, like radiant under-floor heating, would be another.

Simple could be tape heating elements, or electric radiant floor mats.

Metal conduit across the bottom would allow inserting different heating wires.

Suggest you build a 5 gallon or 55 gallon prototype and experiment before full scale!

Have an opening bottom of side to "drain" and sweep out gravel, contingency for the future? You don't want to have to shovel out the top. And broom much more gentle on wires/plumbing than a shovel.

For my experiment, I was thinking of using separate pipes inserted into the heated sand and simply recirculate the air into my home using a fan.

I also would try using a water pump, piping, and a radiator in a closed loop.

 

[Hedges]

Water 70C rise; above that need pressure vessel.
Sand maybe 470C rise? Similar total storage per pound, heavier than water.

Circulate air through pipes in sand and through center of a gas water heater, now you've got hot water. Rig millivolt thermocouple to switch circulation fan (which somehow must withstand the heat) or a valve for convection loop.)