Homebrewed Electricity > Storage

Sand Batteries

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JW:
 
--- 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.
--- End quote ---

ACB R&D misc 2004 5
//www.youtube.com/watch?v=A5O7zYJE-GQ
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

bigrockcandymountain:
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:
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.

Bossrox:
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:
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?

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