Thermal heat storage

I purchased a small 6x8 greenhouse a few months back and have been doing extensive research on storing heat in the ground underneath.
I have done many bench tests using different sand and stones and have found that 1 part silica sand (pool filter sand) with 1.5 parts masonry fine sand and granite stone mixed together held heat the longest time.
I’m at the point where I’m about to fill my 6’x8’x2’ ft hole with stones and sand but I’m wondering if I should throw in some large stainless steel 14 gauge metal on the bottom to help conduct the heat from a heating cable to the bigger stones or if the silica sand will be enough.
To my understanding the higher the thermal conductivity is the better it can transfer heat so I put together a small list of the thermal conductivity of each item (in W/mK) that I will be using….. Stainless steel 15-20, silica sand 7.2-13.6, mason sand 0.15-0.25, granite stone 2.9-3.6, granite gravel 1-2.6.
I only want to use regular fine mason sand mixed with silica sand to fill the gaps between the course silica sand and to cut the cost since silica sand is much more expensive than masonry sand. I’m also thinking of using granite gravel between the larger granite rocks so if the granite gravel tends to lessen the amount of sand needed which I may be able to use more silica than masonry sand.
I didn’t want to use concrete because of the expense, because of its low conductivity of 1.3-1.9 and the ease of being able to replace the heating cord if needed with sand and gravel.
I do intend to connect solar panels to the heat cord and make a aluminum pipe manifold to transfer the heat up in the greenhouse when needed.

Does anyone have any suggestions for improving  my thermal heat battery?

Lots of people playing with thermal storage now a days including me. lol Isn't it funny how such similar materials can have such different properties. You might want to try granite dust and or crush and run as a filler that conducts heat as well as mass for holding heat. Also cleaning those rocks of moss, mold or any foreign material as that is a barrier to good conductivity.  

larry kidd wrote:Lots of people playing with thermal storage now a days including me. lol Isn't it funny how such similar materials can have such different properties. You might want to try granite dust and or crush and run as a filler that conducts heat as well as mass for holding heat. Also cleaning those rocks of moss, mold or any foreign material as that is a barrier to good conductivity.  

Yes good point on cleaning the rocks! 👍 It was late after work when I gathered and placed them in the hole but most likely I’ll be pulling them out to place the stainless steel on the bottom.
I have thought about stone dust but couldn’t find any thermal info on it as well have read that it absorbs and holds a lot of moisture.
I might try a bench test on some. 🤔

Jim Griff wrote:

larry kidd wrote:Lots of people playing with thermal storage now a days including me. lol Isn't it funny how such similar materials can have such different properties. You might want to try granite dust and or crush and run as a filler that conducts heat as well as mass for holding heat. Also cleaning those rocks of moss, mold or any foreign material as that is a barrier to good conductivity.  

Yes good point on cleaning the rocks! 👍 It was late after work when I gathered and placed them in the hole but most likely I’ll be pulling them out to place the stainless steel on the bottom.
I have thought about stone dust but couldn’t find any thermal info on it as well have read that it absorbs and holds a lot of moisture.
I might try a bench test on some. 🤔

I would imagine fastening the heat cables to the stainless would make heat transfer easier which would make transfer to the granite and sand easier. If the electric cable transfers heat easier then it reaches high temperature less so would last longer.

Do you realize that stainless steel is one of the worst metal thermal conductors there is?

larry kidd wrote:Do you realize that stainless steel is one of the worst metal thermal conductors there is?

 I’ve read conflicting articles on that. Some say it’s good, some say it’s bad but for the price of only $10 for 2 sheets of 3’x5’ it’s worth trying even if it only does a little improvement.
I ran across this article how scrap aluminum chips from CNC machines mixed with sand makes an improvement. I’ll have to call my local shops and see how much they are selling there scrap. If it’s cheap enough it’s worth a try. 😉

https://www.sciencedirect.com/science/article/pii/S2352152X24009356

Hi Jim,
Sounds like a neat project!
I'll venture a comment on your stainless sheets - they will be useful to spread heat across themselves, and although stainless is a poor conductor compared to copper (~390W/m/K) or aluminum (~160W/m/K), it will be dramatically better than the sand grains. Thermal conduction in a continuous solid, like the sheet or a rock, is worlds better than conduction through a granular media. The contact from grain-to-grain impedes heat flow much more substantially than the conductivity within a grain. Solid sheets will perform better than equivalent-mass chips of the same material. If you're looking at values for "silica" that will likely be a bulk number, whereas "conduction through sand" is going to be much less.

It sounds like you're applying heat to the plates, presumably electrically? It's a good idea to smear those watts over the large area, or you will get hotspots, The sheets would also be a very good place to monitor the temperature of your bed for the sake of controlling the heaters - locate the sense point on the metal, but fairly far from a heater element to avoid dithering on-off too rapidly. The price sounds pretty good for stainless anyway, so even if you don't use them for this project, it might be a deal.

Last thought on shop swarf chips - those will almost all have gone through a machine spraying them with a chlorinated coolant. I'd think twice about putting them (uncleaned) in my greenhouse, though you'd want to do your own due-diligence. Personally, I hate aluminum swarf - the edges are nasty, your magnet can't grab it, and it hurts worse than steel to get a sliver (but not so bad as copper & brass). But maybe you're burying it for good and that's no issue.

Have fun!
Mark

So... why stainless steel sheets? do you already have them? Aluminum would be a far better choice. A scrap dealer would probably give you an even swap for them with maybe a little money thrown in. You could also pour just the heating wire in a thin layer of concrete to act as a heat transfer medium rocks and sand for the balance.

Mark Miner wrote:Hi Jim,
Sounds like a neat project!
I'll venture a comment on your stainless sheets - they will be useful to spread heat across themselves, and although stainless is a poor conductor compared to copper (~390W/m/K) or aluminum (~160W/m/K), it will be dramatically better than the sand grains. Thermal conduction in a continuous solid, like the sheet or a rock, is worlds better than conduction through a granular media. The contact from grain-to-grain impedes heat flow much more substantially than the conductivity within a grain. Solid sheets will perform better than equivalent-mass chips of the same material. If you're looking at values for "silica" that will likely be a bulk number, whereas "conduction through sand" is going to be much less.

It sounds like you're applying heat to the plates, presumably electrically? It's a good idea to smear those watts over the large area, or you will get hotspots, The sheets would also be a very good place to monitor the temperature of your bed for the sake of controlling the heaters - locate the sense point on the metal, but fairly far from a heater element to avoid dithering on-off too rapidly. The price sounds pretty good for stainless anyway, so even if you don't use them for this project, it might be a deal.

Last thought on shop swarf chips - those will almost all have gone through a machine spraying them with a chlorinated coolant. I'd think twice about putting them (uncleaned) in my greenhouse, though you'd want to do your own due-diligence. Personally, I hate aluminum swarf - the edges are nasty, your magnet can't grab it, and it hurts worse than steel to get a sliver (but not so bad as copper & brass). But maybe you're burying it for good and that's no issue.

Have fun!
Mark

Thank you for your sensible comment!🙏
To clarify yes I was thinking of zigzagging the 50 foot heater cord between the larger granite rocks that are laying on the stainless steel sheets so it would heat the steel and rocks and the same time. Since the heat cord is much smaller diameter of the rocks themselves I was thinking of filling the spaces between them with more smaller gravel and hopefully more silica sand than regular sand since it has more conductivity.
I was thinking the same thing about cleaning cnc aluminum waste chips before adding it into the sand. I’m sure it could have either oil or coolant residue from the machining process. I have to also find out if it would be cost is low enough to do the extra work of cleaning it all.🤞
Edit: Yes adding heat to the plates and silica sand using 30v DC current form 2- 395 watt solar panels. My solar panels may be on the low side but I didn’t want to exceed the amperage limit of my heating cable.

David Baillie wrote:So... why stainless steel sheets? do you already have them? Aluminum would be a far better choice. A scrap dealer would probably give you an even swap for them with maybe a little money thrown in. You could also pour just the heating wire in a thin layer of concrete to act as a heat transfer medium rocks and sand for the balance.

My friend has the stainless sheets that he got at a scrap yard for $5 each but he said he didn’t see any large aluminum sheets and most of the aluminum was small, thin and mangled up. We thought the stainless was the best choice at the time because it was larger and a thicker 14 gauge or more since we can’t bend it.
The pipes I have to make a manifold to move air through the heated media is aluminum which is a good thing for transferring heat topside when needed. I do intend to insulate the top of the bed so heat doesn’t radiate out into the greenhouse when the sun is out already heating the greenhouse.
I thought about 3” concrete on the bottom but the price was a bit high and from what I read it has a much lower thermal conductivity than stainless steel. Concrete was 1.4-2.0 W/mK while stainless steel was 15-20 W/mK

I forgot to mention that my heating cord is a 50ft long 30v (32v max) which will be connected to 2-30v 395 watt solar panels.
Yes the solar wattage is on the lower side reason being is because the heat cord is rated up to 32 amps and both panels could put out 27.4 amps. From what the heat cord manufacturer told me is that the cord will heat up to 200°-250° at 27-32 amps. Of course solar panels won’t produce these amounts all the time so plan B is to take hot air from the top of the greenhouse during the day and push it down to the manifold in the media to help the heating up process and cool the greenhouse. 🤞

I finally had some time with some good weather to get some stuff done.☀️ I picked up 1/2 yard of mason sand, 1/2 yard of wash 3/4” gravel, 1/8 yard of 2”-4” crushed granite, 18 bags of pool filter sand (silica sand) and 4 - 4’x5’ 16 gauge steel sheets.
 I pulled out the granite boulders, laid 2” of mason sand on top of the bottom foam board, laid the steel sheets double layered, put the granite boulders back in, snaked the elastic heat cord between the boulders, mixed the pool sand with gravel and started filling, laid the 2”-4” stone between the larger stone and filled more pool sand/ gravel mix to the top of the boulders.
 Next step is to build my aluminum pipe manifold on top of the boulders and fill more pool sand gravel mix between the pipe and top with mason sand, foam board and plywood. 😅
I purchased a 24v 600w 25amp transformer that I purchased on Amazon (late delivery ☹️) so I can plug up the heat cord and start heating up the bed before I top it off and mount my solar panels. I also hope to cook out any moisture because a few bags were damp before adding more. I figured the transformer would give me a backup option in case of several days of no sun. I also purchased a digital thermocoupler which I have already buried the probe at the bottom on the steel sheet.
I just have to hope for more good weather but unfortunately more rain is forecasted. 🤞

Are you trying to make a 3 or 4 season greenhouse?  

What are you growing?  Some plants like spinach and arugula can freeze solid and thaw without dying.

In my opinion the most economical passive solar greenhouse is a “Chinese” greenhouse.  Insulated, opaque north wall with thermal mass, and insulating nighttime blankets over all the glazing.  

As far as thermal mass, often the bottle-neck in heat transfer is convection.  Typical passive convection (solid to air heat transfer) is only about 20 watts per square meter per degree C temperature difference.  For this reason, open gaps in loose gravel might allow air to naturally convect easier over more surface area, even if the total mass is a bit less.

If you want to get a feeling for how far heat will conduct through a solid in a given amount of time, look up heat “diffusivity”.  By the way, with diffusion, time is proportional to distance squared. So, in a dynamic sense (diurnal temperature change) for heat to transfer twice as far takes 4 times as long. In other words, heat transfers much faster over short distances than over long distances.  

Conductivity is a useful parameter if the temperature is not changing.

However, in dealing with thermal mass, the whole point is that the temperature *is* changing over time.  Here’s the reason why this matters. Before heat can be conducted it over some longer distance, it first has to raise the temperature of the material in-between. That material in-between has a heat capacity. Heat is required to raise its temperature. Heat cannot be conducted further along down the line until the temperature of the in-between material is first raised.  That’s why (in the case of dynamic temperature changes) the time it takes to transfer heat depends on the distance squared.  

Mike Philips wrote:Are you trying to make a 3 or 4 season greenhouse?  

What are you growing?  Some plants like spinach and arugula can freeze solid and thaw without dying.

In my opinion the most economical passive solar greenhouse is a “Chinese” greenhouse.  Insulated, opaque north wall with thermal mass, and insulating nighttime blankets over all the glazing.  

As far as thermal mass, often the bottle-neck in heat transfer is convection.  Typical passive convection (solid to air heat transfer) is only about 20 watts per square meter per degree C temperature difference.  For this reason, open gaps in loose gravel might allow air to naturally convect easier over more surface area, even if the total mass is a bit less.

If you want to get a feeling for how far heat will conduct through a solid in a given amount of time, look up heat “diffusivity”.  By the way, with diffusion, time is proportional to distance squared. So, in a dynamic sense (diurnal temperature change) for heat to transfer twice as far takes 4 times as long. In other words, heat transfers much faster over short distances than over long distances.  

It would be nice to get 3-4 seasons but I’m not holding my breath that it will happen. 😆
I have seen many deep winter greenhouses and my cheap Harbor freight greenhouse is no match to them. I could possibly insulate my north and east walls and maybe put another layer of greenhouse plastic sheet on the outside so snow slides off better which “possibly” could get me 10 months of growing cold weather crops in my 6b growing zone. Worth a try though. 🤷‍♂️🤞
Your information is very interesting but I believe it’s beyond my math skills and I’m beyond the point of being able to change things if I have not used the best media. I’ve done lots of research and bench testing different sand, gravel and sand mixed with gravel so I opted to go with the mix that held heat the longest. I’ll just hope for the best that it will work good enough to get 10 months a year use with it and that the solar panels will provide the majority of electricity for the heat.🤞

Mike Philips wrote:Conductivity is a useful parameter if the temperature is not changing.

However, in dealing with thermal mass, the whole point is that the temperature *is* changing over time.  Here’s the reason why this matters. Before heat can be conducted it over some longer distance, it first has to raise the temperature of the material in-between. That material in-between has a heat capacity. Heat is required to raise its temperature. Heat cannot be conducted further along down the line until the temperature of the in-between material is first raised.  That’s why (in the case of dynamic temperature changes) the time it takes to transfer heat depends on the distance squared.  

On a good note I came home from work today and decided to check the temperature on the bottom of the media and it was 7°F higher than the outside air temperature. So just from the sun hitting the sand and with the greenhouse fully open with 1 side with no wall panels the bottom heated up. Currently it’s only 1 foot deep but after I make the aluminum manifold and top it off it will be 2 feet deep.
I believe that’s a good sign! 👍🤞

Definitely insulate the north wall.  If the roof ridgeline runs east west also probably insulate the north half of the roof as your sun in winter should reach around it.  On the insulated north wall add a bunch of mass.  Since your building is small guessing you don't want water barrels which would be the common answer so the other 2 common one are shelves with partially water filled 2 liter bottles painted black or the tire tubes cut in long hanging arches filled with water tied off at both ends.

Beyond that most of the stuff suggests you get roughly 1 zone for each layer of plastic.  As best I can tell it is actually closer to 1 1/2 layers per zone and of course a law of diminishing returns applies.  Now you might be able to add cold frames inside all of this for another layer.  And I would set up for the chinese greenhouse inside sort of so you can add an insulating blanket over this too.  So if you put your greenhouse under a greenhouse then added and inner greenhouse and a cold frame that would give you 4 layers so probably 2 zones worth.

Swedish skirt insulation around the building on the ground outside would probably help too.

If you look up the Mike Oehler greenhouse book another one of his tricks was a raised bed to the cold had room to fall off around the plant giving another tiny buffer.
greenhouse book

Beyond that one in my dreaming is to add an active foam insulation system.  My mid size greenhouse dream would have one side as glass and inside that I want to build a giant plastic bag of greenhouse plastic set in an insulated trough with a heat tape under it so it can't freeze.  Run a fish tank stone into that liquid.  The liquid will be weed sprayer marker foam(sodium laurel sulfate) and water  Turn the air pump on when the sun goes down and hopefully fill the bag full of what amounts to dish foam.  It is supposed to be good for a R per inch roughly so a foot of foam would give me an R-12 window at night.  When the sun comes up turn it off.  The foam will die down and let the light in during the day.  Now there is a bunch of thinking that goes into keeping ice from damaging the bag that if you want just ask and I will share.

some links.

soap bubbles

foam 2

soap bubbles canada

There are a number of others.  Best one is out of the UK from about a decade ago but I can't find it at the minute.

C. Letellier wrote:Definitely insulate the north wall.  If the roof ridgeline runs east west also probably insulate the north half of the roof as your sun in winter should reach around it.  On the insulated north wall add a bunch of mass.  Since your building is small guessing you don't want water barrels which would be the common answer so the other 2 common one are shelves with partially water filled 2 liter bottles painted black or the tire tubes cut in long hanging arches filled with water tied off at both ends.

Beyond that most of the stuff suggests you get roughly 1 zone for each layer of plastic.  As best I can tell it is actually closer to 1 1/2 layers per zone and of course a law of diminishing returns applies.  Now you might be able to add cold frames inside all of this for another layer.  And I would set up for the chinese greenhouse inside sort of so you can add an insulating blanket over this too.  So if you put your greenhouse under a greenhouse then added and inner greenhouse and a cold frame that would give you 4 layers so probably 2 zones worth.

Swedish skirt insulation around the building on the ground outside would probably help too.

If you look up the Mike Oehler greenhouse book another one of his tricks was a raised bed to the cold had room to fall off around the plant giving another tiny buffer.
greenhouse book

Beyond that one in my dreaming is to add an active foam insulation system.  My mid size greenhouse dream would have one side as glass and inside that I want to build a giant plastic bag of greenhouse plastic set in an insulated trough with a heat tape under it so it can't freeze.  Run a fish tank stone into that liquid.  The liquid will be weed sprayer marker foam(sodium laurel sulfate) and water  Turn the air pump on when the sun goes down and hopefully fill the bag full of what amounts to dish foam.  It is supposed to be good for a R per inch roughly so a foot of foam would give me an R-12 window at night.  When the sun comes up turn it off.  The foam will die down and let the light in during the day.  Now there is a bunch of thinking that goes into keeping ice from damaging the bag that if you want just ask and I will share.

some links.

soap bubbles

foam 2

soap bubbles canada

There are a number of others.  Best one is out of the UK from about a decade ago but I can't find it at the minute.

I will definitely insulate the north and west walls next fall and will probably do a full layer of greenhouse plastic sheet over everything outside since snow slides off more easily. When it gets closer to fall and I get a better look on how much sun comes through the north side roof I’ll make a determination to insulate that roof as well.
I wanted to stay away from water as heat storage because of the potential of freezing and taking up valuable space inside my small greenhouse.
The ground heat battery I’m making is looking promising at only half way filled which it has stored heat for a full 24 hours after heating it for only a couple hours but I hope it will improve when it gets completely filled up and insulate the top of it.🤞When that gets done I can start doing some long 4-6 hours of heating to see what temperatures it can get up to and how long it will last.
Yeah this is a different style of thermal mass that I believe will work and will save a great amount of heat electricity by incorporating solar pv panels and a 600 watt transformer as backup when several cloudy days come around. But of course the true test won’t be until fall and winter comes again.

4/22 Update:
I finally had a decent day to get some stuff done yesterday.
I was able to complete my aluminum air pipe manifold but unfortunately I found that 1 of the boulders underneath was sticking up to high which is making the manifold higher than I wanted. In turn this will also require me to purchase more pool sand since what I have put in so far hasn’t reached the bottom of the manifold yet. 🤷‍♂️
Hopefully next weekend weather will cooperate so I can finish the fill. 🤞

I also forgot to mention, one day before I built the manifold I had a party sunny day so I plugged in my digital thermo coupler. I was seeing big swings in the temperature which I believe was from the sun going in and out from behind clouds. When the sun was out the temperature would jump up to nearly 90° but when a cloud came it would drop to high 70° to low 80°.  I believe that is a good sign that the heat is getting transferred through the 1 foot of sand gravel mix. 🤔 I think I may increase my gravel to sand on my next 1 foot of fill around the manifold.

I’m fascinated by soap bubble insulation. It is such a perfect idea conceptually.  It seems so simple in theory, yet I have to wonder, how good is it in practice?  Is it reliable?  Is it messy?  How much cleaning and maintenance does it require. How does it compare to a rolled blanket insulation or even foam-bead blown insulation?

So I finally finished filling up the bed, insulated and floored!
As soon as I was done I plugged in the transformer for the heat cable to see how long it could hold heat. It started at 63° then got up to 90° after 16 hours. I unplugged the heat cord and checked it 20 hours later and it dropped down to 77°. 9 hours later I went to check it again expecting to see the temperature to be close to the temperature before heating it up but to my surprise it actually raised up to 79°. 🤷‍♂️
I’m speculating that during the last 9 hours the sun finally came out and heated the greenhouse really hot and maybe the manifold inlet and outlet pipes sticking up from the floor may have radiated heat back down into the media.
After seeing this I decided to try taking a black painted pipe and place it near the top of the greenhouse and connect it to the inlet side of the manifold and T off the outlet side with valves so one side goes out of the greenhouse and the other stays in. I’ll also put a fan at the beginning of the black inlet pipe.
I’m hoping this will do 2 things. 🤞 Hopefully it will help heat up the bed and help cool the greenhouse when the sun is out then switch the valves at night so it will blow heat up into the greenhouse when it gets colder. If I find that it does work in this manner I’ll replace the manual valves with 12v automatic dampers on thermostats.
For now I have to wait till parts to be delivered.

Update: Since it was to hot in the greenhouse this summer I decided to change my plans so I ran AC current to the greenhouse for lighting, ventilation and to run the heater cord on a transformer.
I was also working on getting another 6kw more solar for the house and figured it would be best to run my greenhouse on more consistent AC current. Unfortunately while replacing the roof that the solar was to go on I ran across a lot of wood rot and structural deficiencies which required much more work than expected. 🤷‍♂️
Before this roof problem occurred I started putting a 2nd layer of polycarbonate on the greenhouse roof on southern walls and a radiant insulation on the northern walls which should make a difference with retaining heat when the colder months come.
Hopefully soon I can get back to working on the greenhouse and wire up an Inkbird and relay to control the heating of the sand and rock bed below the greenhouse so I’m not wasting electricity .🤞