Outdoor Thermal Storage

desert_sasquatch

What are your favorite brands or strategies for outdoor thermal storage?

Why I Am Asking Specifically About An "Outdoor Heat Bank"

As far as I can tell each square foot of empty indoor space will cost me about $95. If I had a tank with a radius of four feet--which is about what I'd need for this house--then I'd need 81 square feet for the square area it and 6" of insulation would fit in. If I want the tank to be at all accessible--perhaps mice got in there or something--then it would be more like 12'*10.5'=126 square feet. That's nearly $12,000 just for the indoor space to put the tank. The tank itself, the heat exchangers, and the insulation would cost more, of course.

Of course there are situations where you buy a home that has a basement and it's a perfect place to put a water tank, but that won't be my situation, I'm further south than that and we dig our foundations maybe 4 feet deep.

Also yes, I could do a thermal sandbed (or gravel bed, as it has a somewhat higher thermal conductivity) under the concrete slab of the home. If I put insulation between the gravel bed and the slab I do think it might work. However I'm currently thinking I'll save more money by having a half-basement and putting the heat bank outdoors.

What I Mean By "Outdoor Heat Bank"

I'm in climate zone 6, albeit a southerly part of it. So while I'm open to hearing about uninsulated tanks, just keep in mind that I would need to be able to insulate it, and that will be part of the overall cost. And any finished product would also need to be fairly mouseproof. I'm also not willing to use spray foam--I don't love the huge amount of fire retardants that I understand are included in the formulations.

@hot_rod shared a good example which was, until today as far as I'm aware, viewable here [https://www.latitude51solar.ca/commercial-solar-water-heating/2-uncategorised/148-cocoon-large-solar-storage-tanks]. However when I called the seller they said they no longer sold them and I was unable to find the company that manufactured them. But a styrofoam tank covered in sheet metal would be of interest, if companies were still doing that. Is anyone still doing that?

Or maybe someone thinks that a "normal" poly tank would work well? I have some ideas on how one of those might be insulated, but I'm open to suggestions.

I have some other ideas about an outdoor gravel bed heat bank (it would be insulated). But I'll get to that in another thread. Right now I want to know what I should compare it to.

Thanks!

hot_rod

HDPE and MDPE are usually 140F max. temperature. I've tried pallet tanks and also one of the plastic direct bury septic tanks. They get very soft at and above 140F.
I wanted to use it with a wood boiler, 200F possible.
Price per gallon is reasonable, on PE tanks however.

Looks like fiberglass tanks are a bit higher at 150F, probably $$

I think you would want 10- 12" of foam, that would also be $$.

The Cocoon tanks were extruded not far from me here in Utah. I know the brothers involved in that product.

I know of only one installation, in Wisconsin for a ground mount thermal array.

ncalhvac

How about an IBC tote bladder, rigid insulation panels, exterior siding and pressure treated lumber to store water at atmosphere or low pressure? All square with round holes for plumbing.---
Here is one with air/water heat exchanger https://www.builditsolar.com/Projects/Sunspace/GreenhouseHX/GreenhouseHX.htm. ---
You could use a water source heat pump to transfer the stored energy to higher temperature exchanger.