Storing heat in sand?

[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

polarnightenergy.fi

 

[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

polarnightenergy.fi

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.)

 

[Ceefiveceefive]

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.)

What would be the best way to heat the sand using solar - heat resistive wires dispersed throughout the storage container?

 

[Hedges]

Need surface area either in contact with sand or air that then circulates. Protected from being nicked by sharp grains.
Ideally I think the wire contacts aluminum strip to fan out as fins.

You can buy electric baseboard heaters which provide that. A 3' one could be vertical in a drum. Horizontal in larger container.

For simple prototype I would suggest electric radiator heater (or baseboard heater) in drum of sand, mesh covering vents. Temperature sensors to monitor various locations. Switches accessible through a cut-out with duct extension, or external variable control (Variac, high wattage light dimmer.) Measurement of electrical power going in.

Or heating mat, electric fry pan, etc.

Often the wires and controls can't take the heat of the radiating surface. Start with electric fry pan, crock-pot, etc. and take care of handling weight etc. issues?

 

[Ceefiveceefive]

Need surface area either in contact with sand or air that then circulates. Protected from being nicked by sharp grains.
Ideally I think the wire contacts aluminum strip to fan out as fins.

You can buy electric baseboard heaters which provide that. A 3' one could be vertical in a drum. Horizontal in larger container.

For simple prototype I would suggest electric radiator heater (or baseboard heater) in drum of sand, mesh covering vents. Temperature sensors to monitor various locations. Switches accessible through a cut-out with duct extension, or external variable control (Variac, high wattage light dimmer.) Measurement of electrical power going in.

Or heating mat, electric fry pan, etc.

Often the wires and controls can't take the heat of the radiating surface. Start with electric fry pan, crock-pot, etc. and take care of handling weight etc. issues?

Thanks for your suggestions! I was concerned about resistive wiring getting too hot.

Is it easy to attach radiator baseboards to solar panels?

 

[Hedges]

Too hot could be a problem. Limit it to the temperature it was designed for. May need to get a different one for 500C tests.

Any resistive element can be powered by DC PV. Suitably rated switches/breakers could let you select voltage from a series PV string.
Thermostat is more difficult. Not likely to find a DC rated one (except low current "pilot" duty.) For experiments, you can manually set voltage to what gives the element temperature you want. Will rise with sand temperature, of course.

I tested panels with portable heaters by putting MC cables on an electric outlet, with a DC rated knife switch.
Baseboard heater likely has spade terminals. Use high-temperature wire (like for heating appliances.)

 

[SeanK]

This chap is having fun with these!

 

[TBoone]

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.

If you have a typical central ducted air handler with wall thermostat, You could feed a duct from the sand reservoir into the air handler and the tstat would turn the fan on/off. Would work just like central heating. Just turn the heat source off and the sand bat will provide the heated air needed.
The sand bat heat would then replace/augment the electric heat strips or the coils from a heat pump.

 

[TBoone]

If you have a typical central ducted air handler with wall thermostat, You could feed a duct from the sand reservoir into the air handler and the tstat would turn the fan on/off. Would work just like central heating. Just turn the heat source off and the sand bat will provide the heated air needed.
The sand bat heat would then replace/augment the electric heat strips or the coils from a heat pump.

Idea....maybe the ideal heat exchanger would be from a natural gas furnace. They are designed to handle the heat from a natural gas flame...
easily 1000F. Just embed it into the sand and connect entrance and exit air ducts to it Even if it leaked a bit, there would be no danger from noxious fumes, etc,,,

 

[upnorthandpersonal]

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.

You could also just use an insulated water tank for this, and couple a monoblock heat pump to it to make it more efficient. Sand batteries are great, but you need to have a certain size to really make it worthwhile compared to a water tank. This is especially true if you have 'water based' heating such as underfloor hydronic, or radiators even. Because of the lower temperature delta with the environment compared to 500C sand, your insulation becomes easier as well.

 

[Supervstech]

Idea....maybe the ideal heat exchanger would be from a natural gas furnace. They are designed to handle the heat from a natural gas flame...
easily 1000F. Just embed it into the sand and connect entrance and exit air ducts to it Even if it leaked a bit, there would be no danger from noxious fumes, etc,,,

A natural gas heat exchanger doesn’t see CLOSE to 1000F the burner tube has a flame dump into it mixed with air, and has air flowing across the outside of it. This airflow keeps max burner temp around 350F. If the blower stops, the air temp will exceed 140F, and trip the limit that kills the flame…

 

[TBoone]

A natural gas heat exchanger doesn’t see CLOSE to 1000F the burner tube has a flame dump into it mixed with air, and has air flowing across the outside of it. This airflow keeps max burner temp around 350F. If the blower stops, the air temp will exceed 140F, and trip the limit that kills the flame…