Geothermal Series – Part 4
Introduction
I will be the first to admit that this system will likely not be as efficient as it could be. I have made concessions in my design in exchange for cost savings. One of those concessions is the use of PEX tubing instead of HDPE. Also, using a methanol blend as the geothermal fluid, reduces the heat capacity and increases viscosity (compared to plain water). This is needed to keep the water from freezing in the components that are above ground. So the goal is to boost the efficiency in other areas. One of those ways is with mud!
If you have not read the previous posts in this series, please consider doing so to be up to date.
Part 1: DIY Geothermal System Design
Part 2: Geothermal Trenches – Digging for Energy
Part 3: Ground Loops
Not Any Ole’ Mud
The type of mud we are referring to is geothermal mud. This is similar to drilling mud commonly used in vertical well installations. Drilling mud keeps the bit cool during the drilling, as well as provide hydraulic pressure to the sides of the bore hole to keep it from collapsing. The YouTube Channel Tech Ingredients has as great video about modifying a window A/C unit, and he talks about the geothermal mud.
This link to this video is in the text above (and encourage you to watch it), but I’ll give a brief summary here. The idea of using this mud is to improve the thermal conductivity of the soil. The mud mixture he referred to had sand, bentonite clay, and water. He also added graphite, which supposedly doubled the thermal conductivity of the mixture. He used the mixture in his own, small scale geothermal project and it seemed to work great.
As a pretend engineer, I wanted to know if I could make this better? The problem with this mud is that the high thermal conductivity is largely based on two things; the first is graphite, which is very expensive in the quantities I will need, and the second is moisture content. The sand in the mixture has high thermal conductivity when it is wet, but when it is dry, the conductivity drops significantly. So I’ve been on a quest to find another solution with the limitation that it had to be readily available and cheap.
A New Mud
I got my inspiration from one of my other jobs, which is being a farmer. While increasing soil thermal conductivity is not a priority for a farmer, increasing the pH is. What do those two have in common? Agricultural lime.
Agricultural lime, or Aglime, is the powdered version of limestone. Limestone’s thermal conductivity is high even when it is dry. Limestone has a dry thermal conductivity of between 1.26 and 1.33 W/m*K. Just to put this into perspective, dry sand has a thermal conductivity of 0.15-0.25 W/m*K. Wet sand is higher but so is anything else that is wet. I am just comparing dry values because, with recent droughts in my area, the likelihood of the soil moisture dropping is pretty high. I want this to function well even if it is low in moisture.
Calcium Carbonate
I will mention that the thermal conductivity value for limestone is for the rock, not the powdered version of the rock. Limestone is about 50% calcium carbonate. Calcium carbonate has a thermal conductivity of 2.7 W/m*k. So it would make sense that a limestone rock has close to half the thermal conductivity of pure calcium carbonate.
Agricultural lime, on the other hand, has a composition of around 80% calcium carbonate. More exact numbers have been difficult to find considering the main number associated with agricultural lime is the Calcium Carbonate Equivalent (CCE). CCE is the lime’s ability to neutralize acids when compared to pure calcium carbonate. It is not a measure of the actual quantity of calcium carbonate as this number can be in excess of 100%.
An additional benefit that this mud will provide is the increase in density. I plan on putting the ground loops in and then pouring this mud in the trench, filling it about 8 inches thick, with the loops in the middle. This can help keep the pipes protected when the trench is refilled. This also increases density around the pipes, filling in all voids. Adding density increases the heat capacity, or the amount of energy needed to raise the temperature of the soil. In theory, this means that more energy is needed to raise the temperature of the mud. This will also allow a greater surface area to contact the less efficient soil, which will improve overall conductivity.
Installation
My original plan when adding the mud to the trenches, was to use a Mud Mixer. A Mud Mixer is a machine, mainly used for making concrete, that takes dry mix (Aglime in our case), and continuously mixes it with water through an auger that deposits the mixed material out of a chute. This plan was going to take upwards of twenty hours of continuous running (at 1 yard of material per hour), but I felt that it would give the best results.
Plan B
Plans change. Unfortunately, the digging of the trenches and preparation and placement of the ground loops took much longer than anticipated. We also had a bad spell of weather, with thunderstorms and heavy rain one night, followed by a day and a half of sub-freezing temperatures. That’s Tennessee for you. When it was time to start pouring mud, the Aglime was wet and would not feed through the mixer. Another problem, but also a comforting realization, was that when the Aglime got wet, it was incredibly hard. So hard that my tractor could not break it up, so I had to spend about an hour with the rented excavator breaking up the largest chunks of Aglime.
From there, I used the tractor to scoop up the Aglime and deposit it in the trenches around the coils. It is important to break up the Aglime, because large chunks of it falling 10+ feet could cause damage to the coils or the sensors. I dropped between 26 and 27 tons of Aglime into the trenches. Then I used a garden hose and a sprayer to spray water down into the trench to mix up the Aglime into a mud. This was not the ideal method as I was sure there were still dry spots in the mix, but it was the best we could do at the time and I feel like it turned out well.
Thank You
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