Geothermal Air vs Geothermal Water – The Theory

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.

Water vs Air - New Page

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.

4 thoughts on “Geothermal Air vs Geothermal Water – The Theory”

  1. I find this most intriguing. I almost followed the design of LDSPrepper’s geo thermal air greenhouse, but decided to wait another year. I an intrigued with your comparisons between water and air, but am unclear about a few things. First of all, was your tubing with the warm water in the ground or up on raised benches? If all the heat is going to the soil around the plants and needs to radiate enough heat to warm the entire greenhouse, doesn’t the soil get too warm? LDSPrepper is warming the air in the greenhouse which also warms the plants and the soil, but the soil would not get nearly as warm as yours with the tubes in the soil. If you were to build a greenhouse today, would you heat it with water or air? He recommends you determine what 10% of your air space would be and put that many feet of 3 inch drain pipe 8 feet under ground. Do you know how many feet of pex you would need underground to maintain a reasonable temperature during the winter?

    1. “was your tubing with the warm water in the ground or up on raised benches?”

      The tubes run from a reservoir, through the soil in the raised bed, then into the ground (about 6′ deep, and another layer 4.5′ deep)and back up the reservoir.

      “If all the heat is going to the soil around the plants and needs to radiate enough heat to warm the entire greenhouse, doesn’t the soil get too warm?”

      What I’m finding is that the surface area of the soil bed is not enough to radiat enough heat to warm the greenhouse. The soil got warm in the summer, but not nearly as not as the air during the daytime.

      “If you were to build a greenhouse today, would you heat it with water or air?”

      I’ll let you know in 3 months ;). Right now I’m thinking it would be more efficient to dig a large hole and bury a large water tank. I haven’t done any cost/benefit of that method over pex though.

      As for how much tubing… I don’t know how LDSPrepper got the 10% rule, but for this water method you need much less. You are only heating the soil in the raised bed. So its 10% of that area (cubed?) if you use 1″ PEX or 20% for 1/2″ PEX.

      I’d need to actually do the math on that to be sure though.

  2. Working out a design for my greenhouse using horizontals closed loop geothermal and am considering this very same question after watching LDSPrepper’s videos.

    How did your experiment work out?

    1. I’m starting on my second year, but I would call it so far a success. I think I would do a couple things different though. In the very least, i would use about twice the geothermal mass that my tubing goes through, or even bury 200+ gallon totes underground and run water through those. Check out my videos on youtube and I have more on the geothermal water system and I even did a video comparing LDSPrepper’s system to a theoretical water system that he could do (https://www.youtube.com/user/tripzero0). For the same geothermal mass, water is going to be better than air in all cases except in certain situations when you get energy transfer bonuses from condensation or evaporation. You get some of those bonuses with water (ie, I see condensation on my geothermal water heat exchanger when using it for cooling), but it may not be as pronounced.

      I should probably do another air vs water post or video and really compare the costs and performance of each… If you have any questions, let me know.

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