diy solar

diy solar

Storing heat in sand?

Water can achieve any temperature... the thing is, without pressure, higher temp means a change in phase.
Water close to 212F holds a massive quantity of heat before becoming steam. That heat massively transfers into its surroundings when it phases back to water.
Can you design a setup to include low pressure steam?
Err - no. Sand can get to 1800 Celcius at 1 atm, water can only get to 100 Celcius at 1 atm. Yes, you can go higher by increasing pressure but that is the killer - high pressure systems require energy to pressurize, complex plumbing, valves and complex safety systems because pressurised steam is freaking dangerouos. Water looses head at a very high rate, is corrosive (to steel at least) and WILL find a way to leak out and erode the soil around it. Sand just sits there.
 
filling the whole sand pit with oil? Hehe :) sounds like good plan. What could go wrong.

There are two primary ways of transferring heat:
Conductivity
Radiation


the question is - is this an issue? If you got low conductivity you increase surface - people figured out how to cool a space station in vacuum, which is close to zero conductivity, still works.

How? Radiation - when you get your heat exchange hot enough it radiates heat and transfers it that way. Same the other way around - just heat the sand up to multiple hundred degrees and it will radiate into the pipe - no conductivity needed.
Yep absolutely right. All I am saying is that you do need a heat exchanger inside the battery. Ideally you have the heat source buried deep enough in the sand to stop heat radiating to the air, so you need to insert the heat exchanger well inside.
 
filling the whole sand pit with oil? Hehe :) sounds like good plan. What could go wrong.
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.


There are two primary ways of transferring heat:
Conductivity
Radiation
Two primary ways if you're in a vacuum. We don't live in a vacuum, so we also include convection.
 
Err - no. Sand can get to 1800 Celcius at 1 atm, water can only get to 100 Celcius at 1 atm. Yes, you can go higher by increasing pressure but that is the killer - high pressure systems require energy to pressurize, complex plumbing, valves and complex safety systems because pressurised steam is freaking dangerouos. Water looses head at a very high rate, is corrosive (to steel at least) and WILL find a way to leak out and erode the soil around it. Sand just sits there.
Well… liquid water can only achieve 100C at 1atm, but water vapor can achieve any temp. Many homes were successfully heated with <10psi low pressure steam. Certainly steam at any pressure is dangerous. The massive quantity of heat energy stored in it is unbelievable.
 
Well… liquid water can only achieve 100C at 1atm, but water vapor can achieve any temp. Many homes were successfully heated with <10psi low pressure steam. Certainly steam at any pressure is dangerous. The massive quantity of heat energy stored in it is unbelievable.
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....
 
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.
 
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
 
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?
 
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?
 
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...

 
Last edited:
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.
 

Attachments

  • IMG_20221220_124253.jpg
    IMG_20221220_124253.jpg
    182.4 KB · Views: 10
  • IMG_20221220_124101.jpg
    IMG_20221220_124101.jpg
    213.7 KB · Views: 9
  • IMG_20221220_124054.jpg
    IMG_20221220_124054.jpg
    160.7 KB · Views: 8
  • IMG_20221220_124053.jpg
    IMG_20221220_124053.jpg
    187.4 KB · Views: 8
  • IMG_20221220_120808.jpg
    IMG_20221220_120808.jpg
    128.6 KB · Views: 8
  • IMG_20221220_120801.jpg
    IMG_20221220_120801.jpg
    157.3 KB · Views: 8
  • IMG_20221220_120052.jpg
    IMG_20221220_120052.jpg
    232.5 KB · Views: 8
  • IMG_20221220_115404.jpg
    IMG_20221220_115404.jpg
    305.4 KB · Views: 8
  • IMG_20221220_115015.jpg
    IMG_20221220_115015.jpg
    251.5 KB · Views: 8
  • IMG_20221220_114641.jpg
    IMG_20221220_114641.jpg
    307.8 KB · Views: 9
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 ;)
 
. 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.
 
Last edited:
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 ;-)
 
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.
 
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 ;)
 
Last edited:
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. ?‍♂️
 
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.
 
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.
 
Back
Top