Geothermal heat sink greenhouse build
We are excited to be starting the planning process on a geothermal heat sink greenhouse build. We will build this geothermal heat sink greenhouse to allow us to grow our own food year round. It would be wonderful to be able to grow frost intolerant crops like tomatoes during the dead of our winters. Without question, we will be able to grow hardy and moderately hardy crops all year inside this greenhouse.
More details about our green house plans
For the heat sink we will use 48 inches of rock that will be placed under 24 inches of topsoil beneath the greenhouse. We will use a 4 inch thick Styrofoam insulator 6 feet tall around the perimeter of the rock. The foam board will prevent the heat held in the rocks from being drawn out into the cooler soil outside the foot print of the greenhouse.
During the day, the warm/hot air from the peak of the greenhouse will be drawn down through duct work and a fan and be pushed out near the bottom of the heat sink. The 4 inch pipe that is used below the ground will be perforated to allow the air to escape. The air will move up through the rock to a second set of pipes, transferring the heat from the air and into my thermal mass. The air will enter the second set of perforated pipes and be pushed into the greenhouse.
During the night when the green house temperature drops below that of the thermal mass heat sink the system will work in the opposite manner. Cool air from the green house will be pushed through the rock and pick up heat. As the air returns to the greenhouse it will be warmer, keeping the greenhouse temperatures warm through the night.
The high tunnel kit we selected
For our high tunnel kit we selected a 20 x 44 foot kit from Zimmerman’s High Tunnels. They use 14 gauge steel that is 2 3/8 inches in diameter. It is a little larger diameter than most kits we found while searching for kits. Zimmerman’s is a few hours drive from our home and we will be able to pick it up to avoid freight charges.
The drainage pipe we are using
For this project we will be using 4 inch corrugated drain pipe, often referred to as drain tile. All of the pipe that will be horizontal will be perforated to allow air to escape and enter the rock. Anything installed vertically will not be perforated to ensure the air makes it to our heat sink. This is what our 4 inch corrugated and perforated pipe will look like. We will buy it in 250 foot rolls for around $98
There will be two layers of this pipe, ten each, the will run the length of the greenhouse. Both will be buried within the rock 3 feet apart. One group will be 6 inches from the bottom, the second 6 inches from the top. The top and bottom layers will not be connected.
The pipe near the bottom of the rock heat sink will be connected to the fan that draws heat from top of the greenhouse. We intend to use a 55 gallon plastic barrel as a manifold to bundle all 10 of the pipes together. With the fan mounted on top of the barrel it will be able to push air through all 10 pipes.
The ten pipes 6 inches from the top of the heat sink are also perforated and will direct the air blown in from the bottom up and back out into the greenhouse. All of the pipe laid horizontally will be perforated.
Airflow pattern through the heat sink
Air will flow down to the bottom of the 10 perforated pipes at the bottom of the heat sink. The air will be pushed through the perforations and up through the rock to the second set of horizontal perforated pipes. The air enters through the perforations and is directed back up and into the greenhouse.
Where each pipe terminates inside of the rock heat sink we will place a cap designed for the the corrugated pipe. When we connect the perforated and non-perforated pipes we will use a an external connector designed specifically for the corrugated pipe. All unions will be wrapped in a tape intended for drain tile and to be buried.
How we came to decide on our greenhouse dimensions
As with all our gardening here, we will be utilizing the Mittleider Gardening Method in our geothermal heat sink greenhouse build. There will be 4 grow boxes inside this greenhouse that are 18 inches wide and 30 feet long. We want plenty of room to move and work in our greenhouse in additional to being able to do all our seedling production in the same place.
We have settled for an overall length of 44 feet. This will allow a 5 foot walkway at both ends of the grow boxes. At the far end of the greenhouse we will do our seedling production. We intend to build a 4 foot bench along one wall for seedling stuff and storage of gardening equipment. Ultimately there will be batteries here and a charge controller for the solar panels we want to install.
For the width we are going with 20 feet. It was tempting to go wider to add a 5th grow box, but that would have increased the final cost significantly. The added cost for the greenhouse itself wouldn’t have been horrible. But the additional gravel requirements combined with the other costs for excavation and materials would have significant. And I can grow a lot of food in just 4 thirty foot beds using the Mittleider gardening method.
Considering snow load and high winds
A twenty foot wide greenhouse is pretty strong and able to stand up well to wind and the weight of our Missouri snow. We don’t get much snow here so this allows us to save a little money because we don’t need W-trusses. If we were to go wider it would be smart to spend the extra money for W-trusses with our kit. W-trusses for the kit we selected would have been another $500.
While the heat sink and geothermal combined makes sense, we haven’t yet seen it work. We don’t yet know how it will truly work during our winters. If it doesn’t work and allow us to grow year round a larger system will have been a waste of money. We will start with a smaller system and see how it performs. If it works well and inexpensive to operate a larger unit that would allow us to sell to our local community might be a good business venture.
Removing trees before building
Before we start this project there will be several trees to remove. Doing this will prevent shading on the greenhouse and the root system from taking over the ground under the greenhouse. Removing trees close to the greenhouse will prevent a tree from falling on the greenhouse and damaging it.
There are about 5 trees that I want removed from the vicinity of the build site. Two of the trees are west of the site and will cast shadow on the greenhouse if left in place. One tree is massive, an old black locust, that could potentially drop limbs on our greenhouse. The roots from this tree could also end up under the greenhouse. We prefer to avoid the possibility of any of those problems and will remove the potential entirely.
There are a few additional trees that are to the north of this geothermal heat sink greenhouse site that I want removed. They don’t pose any immediate threat to the greenhouse, but I’d like to remove them now and ensure no trees will ever fall on the completed greenhouse and damage it. We will remove them now to avoid the possibility of tree fall damage entirely.
Goal to have the greenhouse off grid
After the greenhouse is completed and in operation we would like to start getting the greenhouse “off gird”. We will monitor the energy usage and design a solar system that will meet our needs. On the end of the greenhouse under the seedling tables we will build an enclosure for the charge controller and batteries.
Through rain collection off our nearby home into a cistern we will be able to water our greenhouse. We have enough annual rainfall that we can easily collect enough water for our needs. Ultimately we want to use an automated sprinkler system for a lawn to water the 4 grow boxes inside our greenhouse.
Goals for monitoring the geothermal heat sink greenhouse build
A secondary goal for this geothermal heat sink greenhouse, is the ability to wirelessly monitor the temperatures of the greenhouse,. Additionally, outside the greenhouse and the ground temperatures of the heat sink. Ultimately allowing others to view current temps and historical data would be nice.
I’ve not yet researched the options of products that will allow this. We do plan on placing a piece of PVC during the build that would allow a temperature probe to be dropped into the heat sink material. The PVC will be relatively inexpensive, we may add two to allow temperatures to me monitored at the top and bottom of the heat sink rock.
Almost ready to start excavation
We have all the materials except the styrofoam for the heat sink on hand. As soon as we can schedule the delivery of the 125 yards of rock we can start the dirt work, weather permitting. Our goal is to have the heat sink portion completed by the end of February. The greenhouse portion we hope to have completed and ready for seedlings mid March.
We will be documenting this project with photos and video, be sure to check back for updates of our progress.