In-ground Cooling for LFP

[FilterGuy]

I live in Florida where the sun heats everything, including the air itself, a bit hotter than I would like for my batteries to stay. I currently have a utility trailer that has AGM batteries in it, but they are dying from the heat. The plan is to remove the trailer, dig a pit three to four feet deep, and build up from there to make a shed that can keep its cool. The panels would shade the roof and south side of the shed. The other possibility is to dig two feet deep for the shed floor, then put a pit in the floor to house the battery box, which would then get its own insulated lid. The inverter and charge controller would then mount on the wall near the batteries. I expect to make the shed about 6X8 or so to allow room to move around in and store a few lawn tools, oars, etc.

I am not locked into any specific design, but I do have a picture in my head that includes cinder block walls. Obviously, the deeper I go the better for cooling, but excavation and building underground are a pain. It is all sand, so the only hard part about excavating is moving it and waterproofing the walls. Adding a drain to the battery pit for some catastrophic flood event would be pretty simple. I have considered just pouring a footer for the block and using a crushed stone floor so drainage is automagic.

A while back, @FilterGuy mentioned that he had some thoughts on using in-ground cooling for LFP banks. I do not use my power at the generation site, so I have 1-1/4" conduit 20 inches underground that run to my loads. One tube is downhill about 120 feet, and the other is uphill about 80 feet. I'm open to discussion of using the underground conduit to draw cool air into the shed to help keep it below about 86*F (30*C) or so.

Please feel free to ask questions or just discuss. I don't get cranky about hijacking, so let me read your thoughts! Thanks in advance.

This is the current setup (before we adjusted the panel angles).
View attachment 91567


And here is one of the conduit runs; the other is not installed yet in this photo, but goes the opposite direction.
View attachment 91568

A few thoughts:

Typically earth tubes are quite a bit larger than 1 1/4 inches. Usually they are more like 4 or 6 inches. The intent is to have enough un-obstructed space to get good convection currents going. You could try to get some passive air flow going, but with the wires and all, you might need a fan.

One idea is to build a heat chimney. This is an un-insulated black metal 'chimney' that is positioned in the sun and allowed to get hot. The hot air will want to rise, creating a slight suction in what ever is connected at the bottom. If the battery box is well sealed, maybe a heat vertical chimney would be enough to pull air in from the small conduits....????

20" is shallower that what most people do for earth tubes. Most folks go 3' to 5'

The fact that you are on a hill and the pipes are not horizontal is very good. This helps with the convection currents. Furthermore, one of the concerns/problems in an earth tube is condensation. By having it at a slant any condensation will drain down to where it can be removed. Of course, you would need the wires to be water proof as well.

 

[FilterGuy]

Burying a small container is handy enough, but I would like to make sure that it won't rust out any time soon. My thought with concrete or block is that I can waterproof it and not have issues.

Careful about containers. I have heard of people burying them for bunkers just to see them collapse. Since we see them stacked real high on ships I have never fully understood this, but maybe they have a structural frame that carries the weight but the walls can't handle the inward pressure.

 

[Bluedog225]

Underground water tank. 3-4 feet down. The ag extension office can tell you year round soil temp at depth.

Circulate the water to an insulated battery container with hoses using a small pump. Much easier to bring the water up and send it back than bury batteries with all the associated issues.

 

[FilterGuy]

Underground water tank. 3-4 feet down. The ag extension office can tell you year round soil temp at depth.

Circulate the water to an insulated battery container with hoses using a small pump. Much easier to bring the water up and send it back than bury batteries with all the associated issues.

I like that idea.

However.... think through the possible condensation issues. With the high humidity, a cool pipe will sweat a lot. This can be delt with, but you have to think it through so it is dripping into a drain and not on your BMS.

 

[Bluedog225]

I also like the chest freezer with a johnson control to keep it at 75. And keep the drain valve open. Maybe that would help keep condensate at bay.

 

[Clifton]

Peltier is perhaps the least efficient way to cool. I would use a liquid refrigerant system first, but either way it would use energy I would rather waste other ways.

I have read that too but if you need to reduce temps only say 10 degrees on a 3 cubic foot container and it is well insulated using 45 watts non constant. It shouldn't be too bad. I'm not trying to talk you into it though.

 

[justgary]

Underground water tank. 3-4 feet down. The ag extension office can tell you year round soil temp at depth.

Circulate the water to an insulated battery container with hoses using a small pump. Much easier to bring the water up and send it back than bury batteries with all the associated issues.

I have tried to work through the logistics of this in the recent past. It would be fairly pesky to build a cold plate that doesn't leak, but I could make one that works. I have a 55 gallon plastic drum that I could bury and fill with water. Pumping it is not a passive solution, but it would probably be the lowest power active solution.

I also like the chest freezer with a johnson control to keep it at 75. And keep the drain valve open. Maybe that would help keep condensate at bay.

I would expect that condensation is not a problem at all if the box is not opened often. The nice thing about a smart BMS is the ability to keep an eye on the cells without putting your eyes on them. I plan to visually inspect every few months, but condensation in the interim would be limited to only the water that is trapped in the box when it is closed.

I have read that too but if you need to reduce temps only say 10 degrees on a 3 cubic foot container and it is well insulated using 45 watts non constant. It shouldn't be too bad. I'm not trying to talk you into it though.

I think the well-insulated box is the key here. The current situation puts the interior of the trailer easily over 120*F every day for months, cooling to maybe 85*F at night. I have to get out of the trailer and on to a concrete floor at least in order to gain some thermal contact with the cooler ground. Sticking with the trailer means I have to deal with a 50*F temperature differential across any insulation in a box. That's a lot.

Since I really need to put down concrete to get mass and ground contact, I figured the lower the better. Please keep the ideas coming. I have thought of a lot of ideas, but the all have drawbacks. Simple is good. If I have to do some work to get a simple solution, that's OK.

 

[The Dish Doc]

Those containers are very solid indeed, made to stack maybe ten high on a heaving rolling ship, with all containers above fully loaded.
Also made to survive for years in a very wet salty environment.
I doubt if a buried one is going to rust away all that quickly, especially if its embedded in coarse gravel with an agricultural drainage system.

They are made to carry the weight of the trailers above on the corners and will collapse if buried unless internally braced.

 

[The Dish Doc]

Careful about containers. I have heard of people burying them for bunkers just to see them collapse. Since we see them stacked real high on ships I have never fully understood this, but maybe they have a structural frame that carries the weight but the walls can't handle the inward pressure.

They are made to carry the weight of the trailers above on the corners and will collapse if buried unless internally braced.

 

[FilterGuy]

They are made to carry the weight of the trailers above on the corners and will collapse if buried unless internally braced.

I figured it was something like that

 

[curiouscarbon]

cool idea to keep cells cool ?

buried water tank circulating to insulated battery box w/ pump might be easier for maintenance, that idea was mentioned and appeals to me.

for the box interface, cooling the floor of the box and insulating the rest may be viable.

this is my source of aluminum water blocks; the 80x240mm one is pretty big and does not leak. keeping the water block below the batteries and with a drain outlet can reduce the chance of liquid damaging the batteries. https://coldandcolder.com/products/aluminum-water-block

with enough water in an tank 5 feet down, active cooling eg refrigerant or thermoelectric may be not needed to keep the cells below ~35 C

interesting project!

 

[Warpspeed]

It may not need a pump.
Convective thermosiphon might be all that is needed if the battery is inside a very well insulated box.
A bit of experimentation with a small scale working model might be in order.

I would thing that the buried tank would need to be higher than the insulated battery box, hot water rises...

 

[Bluedog225]

yes. Hard to put the buried water above.

 

[Daddy Tanuki]

One idea is to build a heat chimney. This is an un-insulated black metal 'chimney' that is positioned in the sun and allowed to get hot. The hot air will want to rise, creating a slight suction in what ever is connected at the bottom. If the battery box is well sealed, maybe a heat vertical chimney would be enough to pull air in from ...????

I like this idea and its simple and cheap, i think I could put up a 15'chimeny on the side of my battery box and use it to just suck hot air out. heck you could use 4-6" PVC pipe painted black for that matter with just a hat on the top to prevent water bugs and animals getting in.

I might try this over golden week and get back with everyone on my data.

 

[justgary]

I like this idea and its simple and cheap, i think I could put up a 15'chimeny on the side of my battery box and use it to just suck hot air out. heck you could use 4-6" PVC pipe painted black for that matter with just a hat on the top to prevent water bugs and animals getting in.

I might try this over golden week and get back with everyone on my data.

I like that idea as well. The plan now is to put some sort of range hood over the inverter and charger, and duct that up to a black chimney.

 

[Daddy Tanuki]

I like that idea as well. The plan now is to put some sort of range hood over the inverter and charger, and duct that up to a black chimney.

thats even better in its own way. idea's... they be a perkin in my grape.

 

[justgary]

I got some data yesterday. IR temp gun looking at bare soil in the sun was 115F (46C). I dug a hole 15" deep and measured 85F (29C) at the bottom. I dug to 24" and got 80F (27C). I dug to 36" (~1M) and got 77F (25C) at the bottom. After two hours, the bottom of the hole had come up 0.9F (0.5C).

 

[FilterGuy]

I got some data yesterday. IR temp gun looking at bare soil in the sun was 115F (46C). I dug a hole 15" deep and measured 85F (29C) at the bottom. I dug to 24" and got 80F (27C). I dug to 36" (~1M) and got 77F (25C) at the bottom. After two hours, the bottom of the hole had come up 0.9F (0.5C).

Thanks for the data. Keep us posted on what you end up doing. (With Pics)

 

[Delmar]

@justgary before you get too deep into this, maybe ask and answer the question: why don't floridians have basements/cellars as is common up north in higher elevation geography? I don't know the answer, but I'm sure there's a good reason...

The reason southern states don’t have basements:
- We don’t need them. In freeze climates you have to dig the foundation below the frost line. Its easy to dig a few feet more and make a basement.
- When you are near sea level there is no “downhill” for the water to drain away, so basements can flood in heavy rains.
- The soil is clay that swells when wet and shrinks when dry. This places massive stresses on basement walls and makes them cost-prohibitive to build.

I like the idea of the earth tube with the inlet well above flood level, and a fan drawing the air into the battery box. Corrugated drainage pipe would work well.

 

[DerpsyDoodler]

The reason southern states don’t have basements:
- We don’t need them. In freeze climates you have to dig the foundation below the frost line. Its easy to dig a few feet more and make a basement.
- When you are near sea level there is no “downhill” for the water to drain away, so basements can flood in heavy rains.
- The soil is clay that swells when wet and shrinks when dry. This places massive stresses on basement walls and makes them cost-prohibitive to build.

I like the idea of the earth tube with the inlet well above flood level, and a fan drawing the air into the battery box. Corrugated drainage pipe would work well.

I surmised it had something to do with drainage and the potential for flooding.