In-ground Cooling for LFP

[justgary]

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).



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

 

[Warpspeed]

Beneath a very large tree might be cooler than behind or under a shed.

 

[justgary]

Beneath a very large tree might be cooler than behind or under a shed.

Under ground would be cooler than the shade of a tree, don't you think? The air here in the shade is still hot due to the humidity.

 

[Warpspeed]

Definitely agree underground.
What I was actually thinking was underground beneath a large tree, but what I wrote did not fully convey the whole idea.
It might also help keeping the ground moist, evaporative cooling is a powerful heat removal tool, especially in low humidity.

 

[Daddy Tanuki]

people used root cellars for years to keep produce fresh enough for consumption. I think if you dug down about 3' and then used a post hole digger to dig a grid of holes and then pour concrete you would have a ready made heat sink, add some poured walls a little taller than the batteries and then use the blue foam attached to a plywood cover and just build the rest of the shed up from that. use blue foam in the walls and ceiling and it should stay fairly cool on its own.

I built a small 4' x 12' shed and used the blue foam in the floor, walls ceiling and door and it was enough to keep the battery bank above freezing when I had AGM's but the LiFePo4's are so much more efficient when charging that I was not getting enough battery heating and had to build a heater for them when the temps dip below -17c (1.5 f).

For the AGM's the summer months I had to open the shed to allow heat to escape and that was all that was needed, with the liithium's this will be the first year so I will have to monitor the temps to see whats needed hopefully it will stay below 30c and I will not have to do anything. If it climbs above 30c too many times without opening the doors, i am considering adding a second roof with about 4" of spacing to prevent heat transfer and.

I kinda wished I had thought about semi burying my cells... I probably would not need a heating pad like I do now.

 

[justgary]

I kinda wished I had thought about semi burying my cells... I probably would not need a heating pad like I do now.

Indeed. The plan here is rapidly evolving into more like a root cellar, about five feet underground and with the excavated dirt placed over it. The north facing wall would have wings to hold back the dirt. I'm looking at precast concrete panels to drop in as walls and a ceiling. I would prefer panels that I can lift with my tractor. If I can't lift them with the tractor then I will rent a track hoe to do the digging and wall placing. The panels would then mount on posts over the cellar to help shade the area.

 

[circus]

If I can't lift them with the tractor then I will rent a track hoe to do the digging and wall placing

Getting a little carried away aren't you?. I'm confused, the trail isn't very movable with conduit attached to it.

 

[Warpspeed]

If you are going to go that far, how about burying a small container ?

 

[DerpsyDoodler]

@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...

 

[justgary]

@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 as I see it is that the water table is generally too close to the house. I also has to do with the freeze line in northern climates. At my house, the water table is ten feet down, and I personally verified that. At the other side of my neighborhood, it is four feet. Frozen soil is not an issue.

This particular installation is much higher above the water table, so digging five feet or so and installing a box will not be a problem.

 

[justgary]

Getting a little carried away aren't you?. I'm confused, the trail isn't very movable with conduit attached to it.

In what way am I carried away? The trailer gets very hot during the summer, and helped cook out my AGM batteries. Passive cooling beats active cooling any day, doesn't it?

Don't be confused. The trailer will be disconnected, including the conduit. The conduit will then be free to enter the cellar at 20" below the soil line.

 

[justgary]

If you are going to go that far, how about burying a small container ?

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.

 

[DerpsyDoodler]

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.

Just a thought, how quickly will rain drain down to the water line through the soil in heavy rain? i imagine the soil can get saturated fairly quickly. You mention water-proofing, but even up north (my aunt in PA) they have to have a drain gutter around the floor of their basement with a sump pump to carry the water out. I urge you to do considerable research and see if there are any successful implementations of something like this in your area.

Personally, I think its awesome if you can do it. I would be sad to hear it was a failure for any reason, though. I would especially hate to hear you lost your battery bank due to flooding...

 

[Warpspeed]

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.

 

[Clifton]

Have you looked into a peliter cooler? I plan to box the batteries in foam mini house, dense 1" foam is R6 and use this to keep them under 80*. My well insulated garage will get mid 90's in summer in my state, rarely below 60 in the winter. They make larger ones too.

https://www.amazon.com/gp/product/B079QWMCW3/ref=ox_sc_saved_title_2?smid=A11QHKPQ6UGOMK&psc=1

 

[justgary]

Just a thought, how quickly will rain drain down to the water line through the soil in heavy rain? i imagine the soil can get saturated fairly quickly. You mention water-proofing, but even up north (my aunt in PA) they have to have a drain gutter around the floor of their basement with a sump pump to carry the water out. I urge you to do considerable research and see if there are any successful implementations of something like this in your area.

Personally, I think its awesome if you can do it. I would be sad to hear it was a failure for any reason, though. I would especially hate to hear you lost your battery bank due to flooding...

Basically it is mostly sand with a little clay. The water moves very quickly down, but it does stay damp due to capillary action. I don't expect any issues with the floor flooding, but just to be sure I will probably put in a drain connected to a pipe that emerges to daylight down the hill a bit.

 

[justgary]

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.

I bought a container for the farm, and it is trying to rust already. That is just in the air, not in the ground. I agree that packing gravel around the cellar would help shed water.

 

[justgary]

Have you looked into a peliter cooler? I plan to box the batteries in foam mini house, dense 1" foam is R6 and use this to keep them under 80*. My well insulated garage will get mid 90's in summer in my state, rarely below 60 in the winter. They make larger ones too.

https://www.amazon.com/gp/product/B079QWMCW3/ref=ox_sc_saved_title_2?smid=A11QHKPQ6UGOMK&psc=1

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.

 

[Warpspeed]

Should be possible to bore a hole with a hand auger down a few feet.
A thermometer there should give a pretty good indication of yearly overall average temperature.
The ground has a pretty high thermal mass.

 

[justgary]

Should be possible to bore a hole with a hand auger down a few feet.
A thermometer there should give a pretty good indication of yearly overall average temperature.
The ground has a pretty high thermal mass.

The springs around here tend to flow about 70 degrees F. I'm pretty sure that I can rely on 60 to 80 degrees year round. I will dig a hole and measure the temperature and report back.