Storing heat in sand?

[Guda]

Maybe some of you saw David Poz build a water tank that holds 3 days of hot water . My concept it to use sand instead for a few reasons. I think a sand tank would have no maintenance & last for ever. It would not be a possible source of humidity &/or mold. Sand can get hotter than water. No leaks.

How to plumb all that is where I am going to need some advice. Main question is how do I circulate the water (yes copper coil)? But how do I plumb to use the same water to heat the mass & then get that heat out when I need it. My best guess is some kinda valve system. Maybe I should watch Davids video again lol. But I'd also like to know what ppl think of the sand.

 

[DerpsyDoodler]

Maybe some of you saw David Poz build a water tank that holds 3 days of hot water . My concept it to use sand instead for a few reasons. I think a sand tank would have no maintenance & last for ever. It would not be a possible source of humidity &/or mold. Sand can get hotter than water. No leaks.

How to plumb all that is where I am going to need some advice. Main question is how do I circulate the water (yes copper coil)? But how do I plumb to use the same water to heat the mass & then get that heat out when I need it. My best guess is some kinda valve system. Maybe I should watch Davids video again lol. But I'd also like to know what ppl think of the sand.

Sounds interesting. I guess understanding how well it stores heat and comparing that to how well water stores heat would be the first place to start. I think some people around here understand this information well. Perhaps they will offer their knowledge.

 

[MichaelK]

Well, I can see one reasoning flaw that will limit it's application. First you make the statement that sand can be heated hotter than water. Then you say you'll be heating the sand with hot water plumbing. So, isn't the maximal heat level controlled by how hot you can heat the water? Unless this is a presurized water system, the maximal temperature will never surpass 100C.

 

[jimbob420]

Some immediate thoughts - Sand will have a limited contact surface area to any copper piping/coils/manifold compared to water. It will require more work to put heat into, and extract it from the medium. This will vary with the grade of sand ofc. I'm also unsure how well sand will disperse heat, and how much penetration you'd get into, say, a tank.

Control wise, if you're puming hot water in to add heat and pumping cold water through it to extract, two coils and potentially a few motorised valves would seem to be the way to go, if the heat source also delivers heat to other things like radiators etc.

 

[jimbob420]

Well, I can see one reasoning flaw that will limit it's application. First you make the statement that sand can be heated hotter than water. Then you say you'll be heating the sand with hot water plumbing. So, isn't the maximal heat level controlled by how hot you can heat the water? Unless this is a presurized water system, the maximal temperature will never surpass 100C.

This is actually a very astute point to make

 

[jimbob420]

Ok, so I was sad enough to google the heat capacity of sand

This may or may not be of interest? It was the top match http://www.hk-phy.org/contextual/heat/tep/temch02_e.html

Water holds a lot more actual heat.

Also..you don't have to lug it into your house, worry about silica dust when you fill your reservoir (depending on grade), it's a lot easier to dispose of. Water has a lot going for it over sand if you ask me.

I know you said that a sand tank would be maintenance free, not leak and last forever. This might be a half truth of course, but any piping connecting it to anything else will fail first. I deal with sand filters all the time. Yes, the [generally] grp shell is generally very robust, but all the connecting plastic piping and internal fittings do fail eventually.

 

[Guda]

The thought was that the heat will compound. 200f water going through sand would go higher than 200f, maybe. Maybe it just makes the area thats 200f just larger. Not sure.

I can see how water moving around as its heated would be more efficient as sand wont move.

Cant really seal a water tank thats setup to heat up that much. Too much pressure. Asking for mold with big warm tanks of water indoors.

I'll I'll have to crunch number to see if the sand will hold enough heat

 

[Bob B]

I think there is more involved that how much heat it will hold .... for instance what is the heat transfer rate as compared to water?

 

[kenryan]

Better to use a phase change material.

 

[A.Justice]

Well, I can see one reasoning flaw that will limit it's application. First you make the statement that sand can be heated hotter than water. Then you say you'll be heating the sand with hot water plumbing. So, isn't the maximal heat level controlled by how hot you can heat the water? Unless this is a presurized water system, the maximal temperature will never surpass 100C.

I think that could be solved by using an exchange medium that has a higher boiling point than water. Some type of oil, maybe. A slowly moving oil/sand mixture could potentially hold a LOT of heat.

I picture a well insulated tub, full of sand and / or oil, heated with a solar heat exchanger (or excess PV power). Water would flow through a separate loop heat exchanger in the tub and collect the residual heat.

 

[circus]

Better to use a phase change material.

If you know of any 65° to 75° PCM let me know. I'd love to replace the 1500 soda bottle stacked against my wall to buffer my solar air collectors.
As for the sand thermal mass, information is out there. I sat through an hour demonstration twelve years ago at Midwest renewable energy association MREA. His was big enough for seasons. Insulated subbasement full of sand. Buffers between months not hours like my water bottles.

 

[jimbob420]

The thought was that the heat will compound. 200f water going through sand would go higher than 200f, maybe. Maybe it just makes the area thats 200f just larger. Not sure.

I'm quite sure it's physically impossible to achieve a heater greater than the source..without more work input, say, from a compressor or something. This is a fundamental limit if you're using hot water as the heat source. If it was a steel bucket of sand, you could of course just place it over a fire and it will get hotter than that.

Cant really seal a water tank thats setup to heat up that much.

Hmmm.. if you can seal an engine, there will be sealing materials out there that can withstand very high temperatures. But you are right that heat is a concern and does deserve some engineering attention. You would have to make your vessel out of uniform material, so that it all expands evenly.

Too much pressure. Asking for mold with big warm tanks of water indoors.

Is there any need for a pressurised vessel? It could be vented, unless I'm missing the glaringly obvious? I had in mind something like a steel drum? You can get some that secure the top on with a band clamp, allowing you to remove the lid to add fittings to it easily and place items on the inside.

Regarding mold, you would have the option of treating the water of course - though many cooler towers do just dump the water periodically as far as I know. The addition of chlorine, or keeping a low pH, to prevent stuff growing are options is potentially damaging to process equipment - certainly the mild steel drum as above would not fare well in either of those situations.

 

[Bob B]

Cooling towers do occasionally do "blow down" to keep minerals from building up .... but there is also a very tightly controlled chemical treatment program to prevent damage to equipment and piping and the tower.

 

[jimbob420]

Cooling towers do occasionally do "blow down" to keep minerals from building up .... but there is also a very tightly controlled chemical treatment program to prevent damage to equipment and piping and the tower.

It's outside the scope of my experience if I'm honest, I am sure you are right. I guess my point was, that you can't just chuck a few chlorine tablets in a vessel and expect that to be ok on an ongoing basis. Or just run with a continual low pH and pray that your copperwork doesn't get consumed. In fact, a low level of Cl and a bit of acid to bring the pH to near neutral would be ideal..but the cost of an automatic system to do this would be prohibitive for this sort of application. The chore of manually doing it would be too much work for the payback, if you ask me. Test reagents cost too lol.

 

[Guda]

This is really going well. I had not considered the chemical maintenance of the water tank water. Sounds horrible.

My build is not ideal. I had a fire & I am making do with what I have. I've got 2 20' containers in the center of my structure. I have room on top of the containers to build a large low tank. I own a backhoe & everything is open now. So getting the sand in is not a issue.

If you know of any 65° to 75° PCM let me know. I'd love to replace the 1500 soda bottle stacked against my wall to buffer my solar air collectors.
As for the sand thermal mass, information is out there. I sat through an hour demonstration twelve years ago at Midwest renewable energy association MREA. His was big enough for seasons. Insulated subbasement full of sand. Buffers between months not hours like my water bottle

Bingo!

Now, what size copper tube & how far does the heat saturate? In other words, how tight do the coils need to be? Where in the mass would be best for the coil? Yes, heat rises but its going to drop also.

My tank is going to be like 8'x8'x 1'-2' deep. I'm thinking of a coil like you see in a solar hot water collector. Like a disk. Fancy would be a double disk. Or a disk that traced itself back.

 

[kenryan]

Yes, heat rises but its going to drop also.

Heat does not rise. This is a myth that comes from the fact that hot air rises. Heat goes to cold. I would not expect airflow inside a pile of sand, and so I do not believe that gravity is a factor for your application.

For your consideration:

8' x 8' x 2' = 128 cubic feet = 4.74 cubic yards

1 cubic yard of dry sand weighs 2619 pounds. So you are talking about 12,416 pounds of sand. (4.74 x 2619 = 12,414)

The specific heat of sand is 830.

The specific heat of water is 4182.

So a pound of sand will hold about 20 percent what a pound of water will hold. (830/4182 = .198 = 19.8%)

A btu is defined as the amount of energy required to raise 1 pound of water 1 degree Fahrenheit. This tells us that to store one btu of heat, you would need to raise 5 pounds of sand by 1 degree Fahrenheit.

So, for each degree that you raise your 12,414 pounds of sand, you will be storing 2483 btus. (12,414/5 = 2483)

A single gallon of propane contains 91,500 btus. So, you would need to raise the temperature of your box of sand by 36 degrees to store the equivalent heat energy contained in a gallon of propane.

 

[Bob B]

There is good news and bad news ..... the good news is that sand will hold heat very well .... but only 1/2 or so the same amount of water volume would.

The bad news is that it holds the heat very well because the heat transfer rate is very low ..... which mean you will have trouble getting the heat in and out of the sand.

Sand has a heat transfer rate of 0.06 watts per square meter degree celsius .... Water has about 13.

 

[wiseacre]

Google: rocket stove mass heater
It may give you some additional ideas.

 

[circus]

A thousand years ago in Germania massive stoves were popular. The wood burned hot and fast, heating the mass which released the heat into the room all day.

 

[Supervstech]

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?