When I started researching about building additional garden beds for food growing I came across the idea of a geothermal air greenhouse while surfing youtube. I had heard of homes being heated/cooled with geothermal and understood it’s efficiency. I started researching the methods used. What struck me as odd is that geothermal homes use liquids, not air like the greenhouses I saw on youtube. Coming from a computer background, I understood the best way to cool a computer, was liquid, not air. Why then use air as the energy transport? I had to do more research.
I ended up watching a thermodynamics lecture series on youtube. This gave me a foundation for more work. Turns out that different substances have different heat properties. For example, to raise the temperature of water by one degree, you’d need about 4 times more energy than what you’d need to raise the temperature of air by one degree. This makes air quicker to heat up, but also quick to cool. Water on the other-hand takes much more energy to heat it up, but retains that energy longer.
I looked at how water is used in radiant heating applications. They use PEX tubing in 9″-spaced serpentine loops in the flooring. Heated water is pumped and the energy transfers bottom-up to the surrounding air. I decided to combine these principles in my greenhouse build. I would dig a hole, lay some PEX tubes through the earth and directly into my garden bed. The garden bed would then “radiant” heat the rest of the greenhouse.
That was the theory then. I was wrong.
Temperatures in my area are now getting really cold at night. Much colder than the 15C that my tomatoes need to be happy. The outside temperatures have gotten as low as 1C. What about inside my greenhouse? Inside, I’ve seen it get to as low as 6C. The radiant heating effect is minimal. So was my experiment a failure? No. Not yet at least.
What I didn’t understand then, but understand more now, is the process of transpiration. Plants move water from the soil up to through the plant to the leaves. I thought I’d be keeping the roots warm, but I’m actually keeping the entire plant warm as it moves the warmer water from the soil up through the plant and out the leaves.
This is why when you want to cool the plant on a hot summer day, you water the soil, not the leaves!
This discovery lead me to the next: greenhouses that use geo air are actually ultimately using water. The plant will store the heat energy in the water inside the plant. It will get it from the soil, which is warmed by air, and it will get it from the ambient air around the plant but ultimately air is cooling/heating water. The above image illustrates how energy is passed around in both situations. There’s not much difference except this: water holds more energy. That means less movement, less digging and lower cost.
There’s still a lot of work to be done before I call the experiment a success. So far the results are positive, but I’ve got several more months of winter to go through.