Author Topic: Sand Batteries  (Read 689 times)

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SparWeb

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Sand Batteries
« on: February 20, 2024, 11:29:37 PM »
Anybody heard of them?  Tried to build one?

I just heard about this, and saw a video about building one.  This "battery" is meant to store heat, not electricity.  There are a few examples in commercial use or experiments, and I found a DIY video on Youtube.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
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DamonHD

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Re: Sand Batteries
« Reply #1 on: February 21, 2024, 03:16:32 AM »
Heard of them.  I have a small non-sand heat battery at home.

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Bruce S

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Re: Sand Batteries
« Reply #2 on: February 21, 2024, 09:45:48 AM »
Yes I have  ;D.
I used an old toaster NiChr to build one using a couple tins and cheap play sand.
Worked pretty well to heat the sand, but getting the heat back out of the sand is a lot harder than even I expected.
I'm currently working on redoing it by using a tube-in-tube setup like I did for my bio-diesel heating system.
Below is a link of the vid that I used to model mine from.

https://www.youtube.com/watch?v=Icai6OOIh2M

Hope this helps
Bruce S
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machinemaker

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Re: Sand Batteries
« Reply #3 on: February 21, 2024, 10:55:10 AM »
Back in the 60s or 70s I used to see articles and ads for plans for sand heat storage. This was before outdoor wood furnaces became as popular. The idea was that you had a wood-fired stove in a small building, the exhaust from the stove went through a system of stove pipe above the stove. All of this was enclosed in a well-insulated small building. The space around the stove and the walls of the building was filled with sand. In the sand, tubing was laid out to heat water or ductwork to heat air. The idea was that the wood stove would burn hot and fast, and the heat transferred to the sand for storage.  I have considered doing something similar to heat a greenhouse, but surround the wood stove with old and patched water heater tanks, plumbed in series or two series. Then fill this well-insulated building,( 2 x 6 walls with 2” bead board on the interior) filled with sand. I figure I can get 8 forty gallon tanks around the wood stove. All the tanks would be non-pressurized and vented.

Bruce S

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Re: Sand Batteries
« Reply #4 on: February 21, 2024, 11:47:42 AM »
machinemaker;
The little "test" units I built convinced me that going the route of tube-in-tube . Might be called heat exchangers.
My old Mercedes had two copper tubes, one inside the other, I plumbed the outside pipe through my coolant system that in-turn warmed up the bio-diesel.
The guy who bought the car was schooled on how it all worked and why I had two separate fuel tanks.
 I'll dig around and see if I still have a pic of the setup.

I found that heating the sand is actually quite easy, but getting the sand to give up the heat was much harder.
So I'm going to rebuild it and use the tube-in-tube heat exchange setup to see if it's a bit better.
I'm thinking of building a heliostat to use the sun directly to heat the sand area rather than rely on diversion dump loads to connected to a NiChr bank heater.

Cheers
Bruce S
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Mary B

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Re: Sand Batteries
« Reply #5 on: February 21, 2024, 07:54:01 PM »
Saturate the sand with a liquid that works well to transfer heat... if it is a closed system you could even use waste oil from changing it on the car/truck/SUV...

SparWeb

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Re: Sand Batteries
« Reply #6 on: February 22, 2024, 11:29:51 PM »
Hey, that's the same video I just saw, Bruce.  The Youtube algorithm knows we know each other now.

I've been mulling over the usefulness of water, just like Mary suggested.  I think the ability to heat the sand up above 300F, or higher than boiling water is an advantage that you can't use if you add water.  If it's a closed loop, some glycol in the water would help use higher temperatures.  The mixture of sand and water might also be a problem if it needs filtering, silt traps drains, etc., to work for a long period of time.

Tube-in-tube heat exchange is the standard, taught to every mechanical engineer in thermodynamics class.  If you build one, remember to include temperature sensors on inlets and outlets.  A simple feedback control makes a big difference in the efficiency, worth the effort.

To keep it as KISS as possible, what can be done with a simple tank of sand with an electric heater in it?  One thing is to just use it like an old-fashioned cottage stove - it heats the room by radiating its heat through the sides.  Careful not to touch it though.  For a small insulated building, this could make a big difference.

Another way is to pass the ductwork in your house through a sand tank.  This sounds like it would work, as long as the sand makes lots of surface contact with the duct, or else the air will blow by without picking up much heat.  Think of a basement air duct about 8 inches diameter and you fit a 12" duct around it, filled with sand and some lengths of nichrome wire encapsulated.  Wrap the outside of the 12" duct with insulation so that the sand doesn't just heat the basement.  When the central heating blows air from the furnace, it will be heated a second time by the sand heater.  The central thermostat will detect the house warming up faster, and will shut off the furnace sooner.  The furnace would not run as much, saving on heating.  If the sand heater is driven by PV, the electricity is free.  It would take some math to figure out if the amount of energy collected this way and the heat-exchange process would add up to a noticeable effect.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
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Bruce S

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Re: Sand Batteries
« Reply #7 on: February 23, 2024, 10:24:32 AM »
SparWeb;
He's pretty good at details and generally answers emails and posts.
Having played around with this tin can and using heat from a few different sources, I think I'm going to stay away from any liquid addition to the sand.
In fact I'm working on  cooking the sand as dry as possible, since I'm leaning towards as close to a closed loop system as possible, along with a pressure gauge & relief valve of some sort.

Back when I had my Mercedes 5cyl 300TD I got around to "remembering" how to build a tube-in-tube system and found that the cooling system worked great  to pre-heat the Bio-D. I of course added a few extra temp probes (Both digital and analog) Analog cause I'm nostalgic for swing needles  ;D as I was curious about the returning fuel's temps.
I also remember the disaster here when a steam boiler failed and launched itself killing two onsite and two people who had just started a new job 60+ meters away.
The video liked below gives a very good recreation of what happens.
https://www.youtube.com/watch?v=3Bs6W19cLcY
I know from reading a bunch of your posts and answers to other posts, you understand THE DANGERS when doing these sort of tests and I know JW is a steam guru, BUT anyone else reading these needs to watch the video so they are aware of how quickly things can get deadly  :-[.

I'm also remembering surface contact is vital to extracting the stored energy in an efficient way. OF course all this Mechanical engineering math gets my brain to hurting again and I need to take libation breaks often  8). I still have an old 1M satellite dish that I may setup as a sorta of heliostat to gain a few extra temps.
I've also done some of handed testing of capturing waste heat from our gas dryer by using a few soldered together tins to pump a little warmer air into the house , I borrowed a few surplussed air monitors to make sure there wasn't any nasty un-burned hydrocarbons trying to give me a headache.

I do like the use of tin cans, as they are free, can take a pretty high heat and solder together nicely too. Using the surface area of them I can do these trails pretty cheaply. Also when I ran out of solder I tested a few connections using the Flex-Tape, that worked surprisingly well for a short term test.

Sorry for the disjointed reply;
It's been a busy week  :o

Bruce S





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Mary B

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Re: Sand Batteries
« Reply #8 on: February 23, 2024, 11:02:06 AM »
Tube in tube I would use oil as the heated medium... minimal expansion that could be handled by an expansion chamber keeping working pressure low to non existent...

Just a thought. Could even recycle used motor oil as the heated medium, filtered to get rid of particulates. Oil is a pretty decent heat exchange medium, better than air./sand would be. Downside is any leaks would make a mess...

Bruce S

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Re: Sand Batteries
« Reply #9 on: February 23, 2024, 04:00:29 PM »
Mary B
With the flash point of new motor oil being above 200C , I'm now inclined to give it a try :-) too.
I think I might take the dog out for a walk and give that an extra thought.
Any leak will certainly help in during the discovery part  ;D.

I'll keep moving forward on the sand stuff since I already have a little bit of working knowledge with it. AND I was smart enough to grab a decent pair of FF gloves!!
My other concern is insulating the external tin can. 

Bruce S





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JW

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Re: Sand Batteries
« Reply #10 on: March 01, 2024, 02:45:20 AM »
 
Quote from: Bruce S
Bruce- Reply #7 on: February 23, 2024, 10:24:32 AM »] I know from reading a bunch of your posts and answers to other posts, you understand THE DANGERS when doing these sort of tests and I know JW is a steam guru, BUT anyone else reading these needs to watch the video so they are aware of how quickly things can get deadly.

ACB R&D misc 2004 5

I actually had an old hit and miss engine explode on me, not really it snapped the connecting rod and the piston landed of the roof of the building next to us, from the event we figured out how to 100% prevent that from happening ever again. The design was observed with the 4 cycle steam engine. As long as the intake and exhaust valves are forced open with a camshaft lobe, your good. the engine can run both backwards and forwards and not pressure lock.

JW
« Last Edit: March 01, 2024, 03:15:03 AM by JW »

bigrockcandymountain

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Re: Sand Batteries
« Reply #11 on: March 01, 2024, 06:29:23 AM »
I kinda skipped this thread and then got curious and read a bit if it.  I maybe missed some things. 

The question is, why sand and not water? Quick research says water is 4200J/kg C

Sand is 830J/kg C

For the same weight of battery you would need the sand to be 5x hotter for equal storage.

Say water storage with a temp rise of 70 degrees above ambient, (20C) so 90C max.  Sand would need to be 370C for the same storage.  To me, that is a fire hazard.  It is also way harder to heat because it is a solid so can't have thermosiphon currents circulating to mix in the heat.  Heating elements are cheap and available. It doesn't need to be hauled in.  Just pump it in, drain it out.  Works good with simple heat exchangers.

Am I missing something? 

SparWeb

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Re: Sand Batteries
« Reply #12 on: March 02, 2024, 09:49:40 AM »
The boiling point of water is 100C.  The melting point of sand is about 1500C.
So yes, the battery holds less heat per kilogram per degree but it holds more heat per kilogram.
you have a much larger temperature envelope to work with.
(4200)(80-20C)= 252 MJ/kg
(830)(323C-20C)= 252 MJ/kg

Push the sand battery hotter...

(830)(650C-20C) > 500 MJ/kg

The interior of the natural gas furnace in my house is 2000C and I like it that way.  So high temperatures are acceptable under certain circumstances.
Of course... the furnace in my house isn't a DIY project and there's a different heat-transfer system at play than a sand battery would have.  There are plenty of cautions to be minded when experimenting with high temperatures, especially when bringing them into one's living space.

But here's the base case for comparison:
An air heating system designed with water as the heat-transfer fluid will usually be designed to transport the water through manifolds to maximize heat transfer to the air.  Hence the common radiators we see in apartment buildings.  Buildings usually need a boiler to make this work efficiently and the typical system is circulating steam, not liquid water.  Pressures of several bars and temperatures well above 100C.  There's a lot that can go wrong and the cost of the system reflects that.  None of that is DIY territory either.

Water heat storage and transfer systems that are in DIY territory like yours must keep the water temperature below boiling.  They also rely on thermal switches and controls to prevent overheating.  They also have a minimum temperature, where bacteria like Legionnaire's can thrive.  Additives like glycol improve the thermal efficiency but they also make the water toxic.  Now the heat-storage water can't come in contact with potable water.  Living in the north we must also protect the system from freezing, so it needs to be in a heated space all the time.  You beat all of those problems (and more) in the system you built, but it takes a lot of know-how.  Long term it seems to need care and feeding every year or every season.

So I find it interesting to think about sand as a way of storing heat.  Because sand can get hotter than boiling water, it's probably a bad idea to DIY a system where sand stores heat and transfers the heat with water.  A loop of pipes in a sand bank would need to keep the water constantly flowing through the pipes.  The water could boil if it isn't flowing fast enough.

That's where Mary's suggestion of oil becomes relevant.  Oil as a heat-transfer fluid in its own tubing system may be what Mary meant at first, but I didn't understand right away (her second comment clarified it, but I was still thinking of saturating oil in the sand, which is not something I would do).  I've heard of oil as a heat-transfer fluid in other contexts, and of course oil is a heat-transfer fluid in automobile engines dealing with several hundred degrees there.  It's actually starting to make sense because the working temperature ranges match up relatively well.

That means this MUST have been tried before...  I should do more research and find out, before reinventing the wheel.
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Bossrox

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Re: Sand Batteries
« Reply #13 on: March 09, 2024, 04:25:42 PM »
I built a sand heater in a 6 gallon bucket & put it up on YT. It has 2 large stove burners stacked at different levels. Draws 1,100 amps at 120v & gets the temp up around 6 to 800 degrees. It's perfect for a small well insulated building but mine is in a 10x20 shed poorly insulated but it does a decent job & still has about 125 degree heat in the morning after shutting it down at 4pm the previous day.

MattM

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Re: Sand Batteries
« Reply #14 on: March 10, 2024, 06:40:26 AM »
That could be a perfectly decent way to keep outbuildings heated during winters, especially where natural groundwater intrusion is not an issue.  What happens when water does intrude into one of these systems?