There was a thread going on the tropical fruit forum about Geo Thermal heating. It seems that a number of people are mixing up terms for different types of heating. True Geo Thermal is when you tap into existing hot springs, or when you drill really deep and bring the hot water up and use some type of heater exchange blower.
What people where talking about is a climate battery or earth battery design. That is when you dig down 6 to 8 feet. It depends on where you live, but here in Portland the constant temp is around 8 feet down, and it's 55 degrees. You basically build a radiator with 4" pvc drain pipe using 6" hard pvc for the inbound and outbound into the greenhouse. Youtube has many examples, and a few were posted.
I'm going to start building one next week in Portland, OR zone 8b. It will be a 100 x 30 double poly hoophouse. It will have about 10,000 linear feet of pipe underground. and when the 6" fans are running it will circulate the air in the greenhouse 3 to 4 times per hour.
I have been in one built about an hour south of me, and over the last two winters it never went below 40F. This last winter wasn't bad, the winter before had three or four weeks of around freezing or below.
I'm going to plant 25 Yuzu's, and a bunch of Kumquats, and other citrus on the hardier side. No Oranges, or Limes.
Pdxain, I look forward to hearing about your greenhouse. I have a greenhouse similar to the one you are going to build. Mine is 72 X 30 double poly roof and polycarbonate ends and sides. The greenhouse is located at 5,440-ft elevation in Colorado. During the winter nights reach freezing temperatures every night from the middle of November until early March, It can get as low as -15-F. I have 100 black plastic 55-gallon drums filled with warer lining the greenhouse walls, and four benches also made out of the barrels. They do help greatly in reducing the propane heating bill, but cannot supply the total amount of heat needed during the night. If I were to also put in an earth battery, I'm not sure how deep the piping would have to be, as the soil in my location freezes rock solid to a depth of 4 feet. I would appreciate it if you would provide us with a step by step explanation of your progress along with picture if possible. Much the same as how Brian did with his greenhouse. Thanks for your post,
We will start to fill it back in tomorrow. During the day when the greenhouse warms up the hot air will be drawn down into the ground which will heat the ground up, at night the cold air gets drawn down into the ground that heats the air. That is how you "charge" the climate battery. I think in an area that has months of below freezing weather, the ground inside the greenhouse would probably start to cool. Here, we rarely have more than a couple of nights in a row below freezing.
There are a number of reasons that I am doing it. I am in zone 8a/8b. On an average year we get below 32 degrees for nine nights. Our winters are cool and rainy. I am focusing on citrus on the hardier side.
A guy I know build one just like this design about a half an hour from me, and his stays above 40 degrees, and costs less than 5 bucks a month to run.
If you don't have friends who own excavators and bobcats, then ya, it would be really expensive.
So ya, long term cost is a factor, also curiosity.
The geothermal name somehow became what people call these, it is also sort of older terminology, but it is also not correct. Geothermal is using super hot water from a hot spring or from digging hundreds of feet down, and using that water to heat a radiator or other unit.
Awesome project pdxian! I had an earth shelter greenhouse in The Sierra Nevada Mountains. It would get as high as 80 deg F in mid winter, from passive solar + earth heat. Outside temps got down to -5 to -10 deg F some years. Excess humidity was the main problem.
There is a system called a "Coolith Field" That draws air through a maze like yours and then into home living space as passive Air Conditioning.
We have power now, got the fans hooked up and tubes run up to the top of the greenhouse. I installed sensors at the roof, floor and one of the outbound pipes. We will see what happens over the next few days.
I know that it is not going to work as well as it should this winter because it didn't have a chance to run through fall and heat the ground up.
Why are you putting the warm out flow pipe at the top of the greenhouse instead of putting it near the crop at the floor level? Because heat rises, celling fans blowing down put the warm temperature at plant level where it can be used.
The intakes are at the top, the outbound are at plant level. I'll have more data in the next few days about overall temps, but yesterday around noon it was 41 outside, 71 at the top where the intakes are, 64 at ground level, and 58 coming out of the pipes.