In-ground Cooling for LFP

[Delmar]

After rereviewing your pictures it appears your house is on a pier foundation, elevated above the ground. Will be much cooler under the house, and cooler with tempering from the nearby lake. Difficult not to get jealous.

I would wire the panels in series for high voltage, and instead run the PV power through the existing conduit. Under the house install a high-voltage SCC that will output the proper voltage for your batteries & inverter. Example Victron Energy SmartSolar MPPT RS 450/100-Tr

The high-voltage SCC will also allow you to charge direct from an electric car too...

 

[FilterGuy]

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.

One more reason: High water table. A lot of Florida has a water table that is not very deep. If you dig below the water table the hole will fill with water. If a basement is below the water table it will eventually fill with water.... no matter what you do to try to seal it up.

 

[justgary]

After rereviewing your pictures it appears your house is on a pier foundation, elevated above the ground. Will be much cooler under the house, and cooler with tempering from the nearby lake. Difficult not to get jealous.

I would wire the panels in series for high voltage, and instead run the PV power through the existing conduit. Under the house install a high-voltage SCC that will output the proper voltage for your batteries & inverter. Example Victron Energy SmartSolar MPPT RS 450/100-Tr

The high-voltage SCC will also allow you to charge direct from an electric car too...

The little "house" is a separate solar 12V installation, but it does get 240VAC from the trailer. The trailer is 120 feet away, and roughly 40 to 50 feet uphill from the lake. I don't plan to move the main solar down there (not enough room inside or underneath), nor do I plan to pump lake water up. I also don't plan to buy a new inverter or charge controller. I already have a Magna Sine 4448PAE inverter and an Outback 80 MPPT charge controller. A 12V four-cell battery will go under the "house" with its own SCC since everything there is already 12VDC except the air conditioner.

The solar trailer is the one under consideration, and is the one going underground in roughly the same place. I measured the slope this weekend also. I have about 56 inches of rise over about 42 feet of run. I can start the ingress into the hill and get the back edge of the cellar over four feet deep, which should work for me.

A friend gave me a short 48" wide steel insulated door, so the plan is to make an arched roof with around 78" inside clearance at the center, tapering off to the sides. I can cover the entire thing (except the north-facing door and wall) with about a foot of dirt and sod, then put the panels over most of it for shade. I think I can do the whole thing with around 400 blocks, including wing walls to hold the hill in place. A wide door means I can use the tractor to get the battery box into the door, then roll it across the floor to the opposite wall.

The 30 cells arrived Friday, so I'm now behind on this project. I had assumed that the COVID slowdown in China was going to affect my shipment, but I was wrong. Now I'm designing the cellar and building batteries at the same time....

 

[justgary]

- The soil is clay that swells when wet and shrinks when dry.

Luckily, not here. Jackson, MS has a huge problem with "Yazoo Clay." If you ever drive I-20 through that area, you will notice that the highway has huge heaves in it. Crazy.

We are basically on sand here with very little clay. Of course, that means that it is hard to grow some crops.

 

[AE4KR]

I just discovered this interesting read. I've been trying to imagine a way to use the earth as a heat source/sink. My application is a remote transmitter, off-grid solar powered, LiFePO4 storage. I need to both dissipate about 250 watts continuous heat when the transmitter is operating, and keep the batteries above freezing on very cold nights, even when the transmitter is idle.

My first thought was to leverage "cave effect" by digging a hole for a battery vault, to protect the batteries from freezing. But then, I thought, what about using the idea in both directions? Sink a metal box in the ground, and use it to both source heat for the batteries, and sink waste heat from the transmitter. Hmmm...

It seems intuitive that a durable vault could be created which had enough thermal inertia to remain near the characteristic temperature of the ground while moving heat in both directions. But how fast can heat move into and through various soils? It's one thing to have a large thermal mass, but I suspect heat transfer across the vault-to-soil interface may be very inefficient.

Has anyone played with this or found a good source of info?

 

[Maitake]

My brother recently moved to Brasil.
He has no active heating or cooling system, yet his house remains comfortable year round.
I'm not 100% sure on the construction, but it sounds like 5' of crushed stone underground and hollow uninsulated clay block walls.
I think the sun hits the block and draws cool air from the stone.
Might be worth researching.

It gets 20 below zero here where I am. I have a walk out basement where the inverters are located. I am considering mortaring bricks to make a battery bunker on the garage floor. 50f constant at 8' depth here.

 

[FilterGuy]

Has anyone played with this or found a good source of info?

If you search on "Earth tube" you can find a lot of info on what folks have built and some guidelines on how to do it.... but I have seen very little detail on how fast various types of soil will conduct heat.

With the earth tubes, they typically make the tubes really long and have either a fan or convection draw the air through them. The idea is that with a really long tube, there is a lot of contact with a lot of soil so there is plenty of heat transfer overall but no one patch of soil gets very much heat transfer.

 

[JRH]

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.

I have a black 6x4 homesteader trailer With 6 BB lithium batts and all the other solar supporting electrics wall mounted inside.. it is a Mobil solar power trailer with a different trailer holding the array that connects together.
When the temp is 80ish degrees outside it used to be 100 ++ inside . I bought a 70 dollar 6 inch whisper fan that is powered by the trailors battery bank and mounted it near the top inside sucking heat outside through a 6 inch air deflector.. I mounted another vent in the floor as an access to the cooler air in the shade under the trailer. Both intake and out going vent will close if need be. It draws incredibly low power to run 24/7 all summer and stays off in winter .. It has a continuous use rating of about 7 years constant duty. It is super quiet and cycles all the air in the trailer once per min When the doors are closed.
I did insulate the walls and ceiling before this whole prodject and covered the insulation with good grade plywood.. insulation and wood is about 2 inches thick all around total…
THE RESULT… IT always 5 degrees cooler than the outside ambient temp during the day .. the black paint on the outside in the midday is normally about 130 -150 degrees as measured, and inside surface is about 75 ish ….totally solved all heat problems.
It pretty much is shaped like your trailer but is single axle.
I am thrilled with its results. The temp is well within the suggested temp BB recommends.

Its a similar discussion but I fabricated a internal heating system for it this past winter that keeps it about 50 ish + degrees when we’ll below freezing.. A single 50/watt lightbulb and the heat of the SCC and inverter and stuff keep it plenty warm When it’s cold here. A whisper fan keeps the air rorating around the inside of the trailer to distribute the heat . Uses about 10 watts of power ..Works great.
it took a bit of work to get this figured out but it’s now finished .. hope this gives someone some help in someway…
Jim.

 

[justgary]

I have a black 6x4 homesteader trailer With 6 BB lithium batts and all the other solar supporting electrics wall mounted inside.. it is a Mobil solar power trailer with a different trailer holding the array that connects together.
When the temp is 80ish degrees outside it used to be 100 ++ inside . I bought a 70 dollar 6 inch whisper fan that is powered by the trailors battery bank and mounted it near the top inside sucking heat outside through a 6 inch air deflector.. I mounted another vent in the floor as an access to the cooler air in the shade under the trailer. Both intake and out going vent will close if need be. It draws incredibly low power to run 24/7 all summer and stays off in winter .. It has a continuous use rating of about 7 years constant duty. It is super quiet and cycles all the air in the trailer once per min When the doors are closed.
I did insulate the walls and ceiling before this whole prodject and covered the insulation with good grade plywood.. insulation and wood is about 2 inches thick all around total…
THE RESULT… IT always 5 degrees cooler than the outside ambient temp during the day .. the black paint on the outside in the midday is normally about 130 -150 degrees as measured, and inside surface is about 75 ish ….totally solved all heat problems.
It pretty much is shaped like your trailer but is single axle.
I am thrilled with its results. The temp is well within the suggested temp BB recommends.

Its a similar discussion but I fabricated a internal heating system for it this past winter that keeps it about 50 ish + degrees when we’ll below freezing.. A single 50/watt lightbulb and the heat of the SCC and inverter and stuff keep it plenty warm When it’s cold here. A whisper fan keeps the air rorating around the inside of the trailer to distribute the heat . Uses about 10 watts of power ..Works great.
it took a bit of work to get this figured out but it’s now finished .. hope this gives someone some help in someway…
Jim.

Thanks for the information. I thought for a long time about insulation and a fan setup. Our air in the summer is filled with hot water, and tends to run about 95*F for several months. I worry that the batteries would still be over 90 with insulation and fans. That is only 32*C, but I would still prefer to keep them under 30*C if possible, with a goal of 25*C year round.

 

[JRH]

Thanks for the information. I thought for a long time about insulation and a fan setup. Our air in the summer is filled with hot water, and tends to run about 95*F for several months. I worry that the batteries would still be over 90 with insulation and fans. That is only 32*C, but I would still prefer to keep them under 30*C if possible, with a goal of 25*C year round.

It is my understanding after doing some pretty deep research of good sources that there is a minimal, if any , difference between the batts performance at 77 F and 90 F. This is not meant to challenge common thought about heat affecting a batts ability's. I don’t want to open that door and start fights. . , BUT from the graphs I saw the lifepo operated about the same in a range of 75 to about 90+ …I’m not addressing longevity , cycle life and other things ,as that is a huge , lengthy and controversial issue. I am aware of different opinions on this ,but I am just saying what I have arrived at from test it in my gear and what I have researched..

I have no problem at all knowing my batts may get in the 90 F area.. it’s only 13 degrees difference than what your talking , and both temp are in the “fat middle zone “of acceptable for normal performance.
Later this summer I am going to test putting a white simi reflective tarp over the top and sides of the trailer .. It should lower the external surface black paint temp a large amount.. I don’t need to do it but I want to know if that affects the conductive heat coming through the insulation. I don’t know , but I want to to find out.

My goal is to keep the batts and gear in the trailer between 45F and 90F and not OCD over it. So far, for a year, it’s been better than I hoped for.
If that cannot be attained I can always briefly add an extra lightbulb in winter during a very cold snap with no problem. It just doesn’t get over about 85 to 90 here so that isn’t a prob… ALSO if you insulate the trailer ( you better) , study on it , don’t cheap out , do solid and quality work and leave at least some air gap in the sandwich, I’m amazed how well it keeps out heat or cold .
Good luck in your efforts.
Jim.

 

[copec]

I would probably try and over-complicate it: box made from rigid foam insulation with a poly loop around in the interior of the box connected to a geothermal poly loop in the ground, with some low power circulation pump.

 

[JRH]

I would probably try and over-complicate it: box made from rigid foam insulation with a poly loop around in the interior of the box connected to a geothermal poly loop in the ground, with some low power circulation pump.

Or line your trailer wall with cooling coils and fins , pumping liquid nitrogen through the walls .. add several freezer fans to increase dispersion and even temperature. Add visual heat monitoring with blue tooth thermal imagining tv displays to look for hot spots ,and alarms for when asleep… I can relate to overthinking stuff so I try to not to. It cost to much… and your friends whisper among themselves when they see you coming.

 

[curiouscarbon]

paint the outside of the trailer white or high albedo paint.
insulate the internal walls (or external).
pump the heat from inside to outside.

in this order, i suppose.

1) will reduce heat gain from solar radiation
2) will reduce transference of said heat gain into the interior
3) will remove transferred heat

used to live in a building that had the roof painted white to save on cooling costs. the sun is SO powerful.
there's a reason NASA likes to build stuff white colored. to reduce heat gain from sun exposure ?

 

[curiouscarbon]

the majority of the surface of the walls of this structure are dark grey, which will absorb lots of energy from sun into heat energy.

even the "shiny" (stainless steel?) will absorb heat and become very hot.

i have a fridge that has black corners and stainless steel walls.
after covering the black plastic and stainless steel with white material (coroplast, white gaffer tape, and gorilla hot glue, lol..)
the heat gain from sun decreased drastically.
for good measure i added a fiberglass vacuum insulated panel on the top side and reduced the energy requirements.
don't have comparative analysis, but it is simple math. albedo is real

Albedo - Wikipedia

en.wikipedia.org

 

[Geokilroy]

What most of you are talking about is just “Geo-Thermal” tech. Google it and look over the results. Find something that will work for you. I live in Las Vegas, NV. There are many types of this tech to choose from.

What I know about Geo-Thermal is that if you dig down (even in the desert) the temp goes down with depth until getting to a certain point that it stays the same all year round. I have read the temp at about 5 or 6 feet will get to about 60 degrees year round. Even in Death Valley you don’t need to get down over about 10ft to get constant temps.

Check with your Community College to see if they have any “Geo-Thermal” classes. If they do you could probably get some textbooks from the class. Their bookstore should have used books available. Might the instructor give you some tips for your area?

 

[Daddy Tanuki]

@FilterGuy , it works. I put this in in late May and my battery box temp never went above 29°c. In the past it had went as high as 38°c and on average it hovered around 35°c. Now with the chimney stack installed it hovers around 27°c which is an average 6-7°c drop.

in the winter I will use the cleaning plug on the chimney pipe to stuff a foam plug in it to prevent heat loss.

 

[copec]

@FilterGuy , it works. I put this in in late May and my battery box temp never went above 29°c. In the past it had went as high as 38°c and on average it hovered around 35°c. Now with the chimney stack installed it hovers around 27°c which is an average 6-7°c drop.

in the winter I will use the cleaning plug on the chimney pipe to stuff a foam plug in it to prevent heat loss.

Does that have a fan, or is it a solar chimney?

 

[curiouscarbon]

Does that have a fan, or is it a solar chimney?

Solar chimney, no fan

as far as i understand

 

[Daddy Tanuki]

Does that have a fan, or is it a solar chimney?

strictly solar chimney. draws from below the container box which forms the basis for the shop next to the battery box

 

[FilterGuy]

strictly solar chimney. draws from below the container box which forms the basis for the shop next to the battery box

I love it. Simple, low cost, reliable, zero draw from the batteries, easy to build, easy to operate, easy to maintain. and most importantly, it works. What more could you ask for?

As an engineer, I find beauty in simplicity.