Backyard Aquaponics
A place to discuss aquaponics

I hope this is not too far off topic. I am in the process of designing a geodesic greenhouse for aquaponics and using the tank as the main thermal mass. Growing Domes does this and their greenhouses look very nice. In their model, the water tank absorbs and releases heat to moderate the temperature of the greenhouse. Hot days are made cooler, cold days are made warmer.

My question has to do with 1: the material of the tank and 2:the height of the tank. I would be interested in hearing ideas from anyone who understands these issues better than I do.

1: I am guessing that the water will retain and release the heat but that the tank wall would have to transfer the heat from the light or air to the water. How much does the material of the tank wall have to do with the heat transfer? Would a metal wall be better than a plexiglass wall? How about wood or stone?

2: Would the height of the tank be more important than the width? Does deeper water retain its temperature better? Would the surface of the water absorb and release more heat than the sides? Again, the idea of the tank would be to even out the temperature in the greenhouse.

Thanks for your ideas on this.

Not too sure but...
the larger the mass the more stable things will be!
I guess Stainless S would transfer cold/heat better than plastic...
I will be interested to see what others have to say!

In this sort of scenario, tank construction does not matter, because most of the heat loss and gain will be through the growbeds. Being in a dome, nothing will really matter - its all a closed environment.

I've seen them use a wall of blue drums on there side with glass on either side. Vents in the roof of the cavity for summer and shutters on the outside. Are the tanks going to hold usable water or treated to keep it stable.

One Large tank in the wall hooked up to a solar heat exchange would be handy but it does depend on the use of the water if it is going to be more than for heat storage.

I agree with OBO, that the growbeds will dominate as far as heat transfer is concerned.
A low profile geodesic with a large footprint would provide a nice air volume to mass ratio with minimal surface area and perimeter length

what about if the tank was buried?

then it would be ~15c all year round!

Hi Jaymie
Burying the tank would couple it to the ground, adding to the mass

C1
If you insulate the perimeter and enclose it from above (with a greenhouse) the mass will find a new temperature balance based on the annual input and loss. Over a few seasons it will average higher than the rest of the garden thats open to the elements.
Infinite mass is a perfect insulator and there`s no shortage of soil

OBO, I was interested in your comment that the greenhouse would be a closed system and that the heat loss/gain would not vary. (Did I understand this correctly?) It stikes me that that would be true by and large in terms of air and water flow but that one would still get huge heat gains in the summer days and huge heat losses in the winter nights. I am assuming one will need to release a lot of heat in the summer afternoons and absorb a lot in the winters.

I too was wondering about burying the tank (or part of it) in the ground. How far down must one go to get a stable year round temperature?

I asked the owner of Growing Domes about burying the tank and he felt it needed to be above the ground. I think you all are right however, that the growbeds would be doing the bulk of the absorption and release of heat in this system. This is different than their system.

Oh, and the water tank would be used for tilapia, not only for storing heat.

Thanks for your ideas.

How far down must one go to get a stable year round temperature?


I am led to believe at natural ground height 800mm of depth is required.
But then this opens the ground with a water body for heat loss/gain.
The dome will accumulate heat in both summer and winter.
So shading in summer and venting would be necessary to reduce the thermal build up!
IMHO

Because it is all inside a dome, to control the temperatures your only option is to have a large amount of water volume, this will smooth out the peaks and troughs. When it is too hot, chuck some shade cloth over the top.

Hi rpondpa
The growing domes rely on low angle direct sunlight to heat the tank in winter so they advise it to be above ground. In summer as the tank is located to the rear of the dome it is somewhat shaded by the northwall reflectix from higher angle direct sun which enhances the cooling.

In an AP situation, tank placement is less important as the heat gained in the gravel growbeds heats the water directly so it doesnt rely on the sun hitting it.

I might be wrong but I would think the ground will be working against you if you want the water above 70 degrees for tilapia. In colder weather the ground will be sucking heat away from you as long as the tank is above ground temps (50 degrees), so you'll be heating constantly to keep it 70. (I'm thinking about how my feet start to ache as they're sitting on the uninsulated concrete floor here in the basement. Think also camping in Spring or Fall without an insulating pad under the sleeping bag - brrr!) I would insulate it, and would think about burying it for space reasons in the dome, but that would depend on the configuration - for CHIFT PIST you might want the tank level high. Tilapia wouldn't mind the water getting very warm in the Summer. If the insulation were removable somehow then in the Summer you could remove it to keep the water from getting too too hot. Again this is assuming a warm water fish not a cold water fish. For a cold water fish I would definitely think about burying it uninsulated.

that is the truth DD.
I am hopeful the ground can keep the water cool enough to grow trout all year round! and 21c (70f) is as hot as I want it!

I'm still at 16 deg c