Geothermal Water Reservoir to Soil Energy Transfer

It was cold today.  It’s been cold all week.  The high for today was 7C (about 44F, yes, Tidder, I love you) and the low was 6C.   This caused my soil temperature to drop to 11C (52F).  This is bad.  In addition, it looks like it was very dark most of the day.  My artificial lighting accounted for 85% of the total light energy in my greenhouse (how my light algorithm works).  The sunlight was only able to warm the greenhouse to 15C.

In a conversation on IRC, a friend, wondered if PEX tubing was very good at thermal conduction (ability to transfer heat energy to another substance).  After a bit of searching, I found that PEX has very poor thermal conduction relative to, say, copper.  If PEX is 0.4W/(mK), copper is like 28.  Big difference.  All is not terrible, however, because water is only 0.6W/(mK).

Looking at today’s data, there was a couple degree discrepancy between the reservoir temperature and the soil temperature.  I would have thought it would be almost the same.  Could thermal conductivity explain the difference?

I conducted an experiment.  I boiled some water and poured it into the reservoir.  This brought the temperature to about 21C (from about 14C).  Over the space of a couple hours, I logged the temperatures from both over the space of 3 hours.  Here’s a scatter plot of the reservoir and soil temperatures:


The chart shows a negative correlation between dropping reservoir temperatures falling and rising soil temperatures.  We have energy transfer!  It also shows about how much pumping time is required to raise the temperature of the soil by a few degrees.  This could certainly explain how there could be a difference between the reservoir temperatures and the soil.  If the reservoir heats up quickly during peak temperature, it could be several hours before the soil will rise.

During this test, the control soil bed (unregulated) remained around 13.11C and eventually fell to 12.9C after 3hrs.  The air temperature dropped from 10.1C to 9.7C.  We lost some energy to the air, but not enough to change the air temperature upward.  We also likely lost a significant amount of energy to the geothermal bank which as about 4 or 5 time the contact area as the soil bed.

3 thoughts on “Geothermal Water Reservoir to Soil Energy Transfer”

  1. PEX piping is a pretty good insulator. Most of the time in our climate it’s not required to be insulated separately in a warm attic construction. Perhaps on a future update a poorer insulating piping could be used.

    1. Yeh, this came as a surprise to me, but I think it makes some sense. PEX is used in radiant heating in homes… but it’s also used as you said as cheaper pipe and more insulated pipe than cope just for transferring liquids from point A to point B.

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