using cooling tubes to for a green house in Texas. 2 ft down.

I live in Austin Tx. I want to put in a greenhouse then cool it with the cooling tubes under the grass beside it. I could fessibly dig 2 feet down. Is this doable, or a stupid idea?

If the digging is easy I'd say do it. I excavated to have my greenhouse sit on a perimeter foundation with the floor sunk down about 70 cm. Digging trenches probably would have broken me, because I'm on a river terrace and there is rock everywhere.

What is the temperature of the ground two feet down?

Where I live in the Hill country of Texas the ground is not cool enough for cooling tubes to work.  Plus my dirt is much too rocky.

I ran the question "what is the average temperature of the soil at a depth of 2ft in Austin, TX."  through an AI program.  Answer back was 70 to 75F.  It should be a 'go', if your soil has no obstructions.  Give it a try.  

While researching this for other locations, I found you need a lot of tube, usually in coils to cool the amount of air you need to cool your space.  This requires loops of tube buried in a trench.  Calculate the cubic footage of your greenhouse.  Pick a fan like a house fan to draw the air.  It will have a cubic feet per minute rating.  Guess or test how long it take to transfer heat from the soil to the tube to the air.  It is a slow transfer.  Then you will have a better idea of how much tubing to use.  It may help to put something in the tube that is porous like a glass beads or steel wool to increase the surface area in contact with the airflow to speed cooling.

You may want to use a 75% shade cloth on the structure rather than a traditional plastic or poly cover.  

https://www.hotspotenergy.com/ground-water-temperature-map.jpg

Andrew Welser wrote:6 foot down the temperature of the earth will vary seasonally, around 20' down it will stabilize (close) to the average annual air temperature of your area, normally very similar to the ground water temperature:

About a year ago this was posted on Permies by Andrew...

The idea of heat exchange/storage for a dwelling (therefore possibly greenhouse) was pretty well laid out more than 40 years ago by John Hait in his book "Passive Annual Heat Storage....". Explores heat(cool) transfer/storage through soil, and the 6 month cycle of earth temperature fluctuations according to outside air temps. Worth reading.
The house he frequently references was built in Missoula around 1980. The idea of using the earth/earth tubes to exchange/store heat in this fashion goes back into the early 70's at least.
Note that the charts/ideas posted say "average" temperature. Summer dirt two feet down in Texas will be nice and warm, my bet.
But cooler than your greenhouse maybe. Horny toads hide in the sand during the heat of the day.

george auckland wrote:I live in Austin Tx. I want to put in a greenhouse then cool it with the cooling tubes under the grass beside it. I could fessibly dig 2 feet down. Is this doable, or a stupid idea?

If you can only dig 2 feet down, could you build a hugel on top of them to increase the distance from the air and the Sun's heat?

I am just south of Austin. I installed some cooling tubes about five feet deep for an earth sheltered structure. My results were not great. In the beginning, the air temperature in the tubes was pretty consistently in the mid 70s but once I started pulling air through they warmed up significantly. As summer wore on the base temperature in the tubes crept up into the mid 80s. Also, my run wasn't long (~25 feet) so I'm sure that was an issue as well.

If you're going to dig, I think you would be better served by building a walipini-like sunken greenhouse.

Fore reference, here's an explanation of how I built my cooling tubes: