Analysis of Organic Matter and Pipe Depth in a Geothermal Climate Battery High Tunnel

Final report for FNE22-026

Project Type: Farmer
Funds awarded in 2022: $14,955.00
Projected End Date: 02/28/2023
Grant Recipient: T. L. Fruits and Vegetables LLC
Region: Northeast
State: West Virginia
Project Leader:
Tommye Rafes
T. L. Fruits and Vegetables LLC
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Project Information


The purpose of this project is to determine the relative positive and negative outcomes in terms of performance, cost and design in installing a geothermal piping system in gravel vs. soil and to determine if the depth of the piping system will significantly effect the temperatures emanating from the system.  The methods of this project evolved due to several unexpected challenges during the testing. Challenges included the soil piping air velocity, soil pipe clogging, selecting an accurate temperature measuring device and selecting the specific soil and gravel pipes to analyze. Each of these challenges were overcome by modifying temperature devices, improving procedures and repeated testing (total of 3,371 tests).  The refined procedures included measuring the air velocity and depth of each of the 90 pipes, selecting an accurate temperature measuring device, selecting pipes for analysis, recording measurements and drawing conclusions from the data. As will be explained in much more detail in this report, many of the results of the project were unexpected. First, 34 of the 45 pipes in soil air velocities were under 300 ft per minute while all the pipes in gravel had air velocities at or exceeding 300 ft per minute and most well above that amount of air velocity. Through experimentation we found that the difference in air velocity had little to no effect on the temperature of the air exiting the pipes, but we know from our previous SARE grant research that air velocity does have a significant effect on the overall temperature of air in the high tunnel itself, i.e. the more warmed conditioned air being circulated from the pipes, the warmer the high tunnel in the cold weather months.  Second, several pipes in soil were either partially or totally clogged. Of the 45 pipes running through soil, 24 were clogged (defined as air velocity less than 200 ft per minute) and in most of the pipes the air velocity was not as strong as the pipes in gravel. Third, during cold weather testing, there was no significant temperature difference between the the air emanating from the pipes in gravel versus the air emanating from the pipes in soil with the approximately the same air velocity and pipe depth  Fourth, analysis of the effect of pipe depth in the winter for both pipes in gravel and pipes in soil indicated no heat advantage to either organic matter. Similarly, the soil and gravel tests for the summer testing period demonstrated unremarkable readings between soil and gravel at similar depths and air velocities and at different depths.  It should be noted that air temperatures exiting the pipes for warm weather months play very little to no part in the overall temperature in the high tunnel because the sides of the high tunnel are open and the temperature of the high tunnel is base primarily on the outside air temperature and the heating effect of the sun as it radiates through the high tunnel enclosure.  Stated another way, the primary benefit of the air emanating from the pipes during warm weather months is air circulation and in that regard air velocity emanating from the pipes is of paramount importance. As stated above, the pipes in gravel have higher air velocity because the pipes in soil have a tendency to partially or totally clog.

Results of the testing might lead to the conclusion that geothermal high tunnel pipes can be installed either in gravel or soil and at any depth as long as there is adequate separation (1-2 feet) between the pipes and adequate depth (2 foot and 3-4 feet).  This would be correct if not for the unexpected finding of clogging and reduced air velocity of the pipes in soil. Specifically, it appears pipes placed in clay based soil with the pipes having any arch or dip in the line can seal in such a way as to retain water and thereby clog or reduce the air velocity through the pipes. Note that it is difficult to lay underground pipes with no arch or dip because they normally connect perpendicularly to a rounded manifold and are installed at different levels. Air flow is critically important as the high tunnel heating and ventilation is based on sufficient air flow. Without this air flow the high tunnel will not warm enough during cold weather months to prevent plant damage and will not ventilate as well during the warm weather months. Proper care in design as explained below should be taken to make sure there is no way that water can lodge in the small pipes before entering the manifold and that non-clay based soil (such as sand) or other soil with high permeability is utilized.  

Based on this project, farmers should ascertain the costs of gravel vs non-clay based soil to be placed around the pipes, understanding that with soil a fiber sleeve will need to be added to each 4 inch perforated pipes to prevent the soil from entering the pipes and with gravel a permeable cover will need to be added between the gravel and the top soil. Factors such as inflation, size of the high tunnel, number of pipes, cost and access to gravel vs permeable soil, the local availability of the gravel vs soil products and cost of transporting materials, make it impossible to provide actual costs.  Farmers wishing to install a geothermal high tunnel should seek pricing and availability prior to making a decision as to whether to use permeable soil or gravel.

Outreach included farmers visiting our farm and a presentation at the Small Farmers Conference in Charleston, West Virginia in February 2023. Other outreach should hopefully follow from Farm Conference and the posting of this report by SARE.

Project Objectives:

There are four principal objectives of this project. This project sought to:

  1. Determine which organic matter (gravel vs soil) better absorbs and releases energy to heat and cool/ventilate a high tunnel
  2. Determine whether the depth of underground pipes in the ground within a climate battery geothermal system affects the air temperatures emanating from the pipes. 
  3. Enhance knowledge of optimum geothermal climate battery high tunnel design.
  4. Detail the positive and negative outcomes in terms of design and performance of installing the piping system in soil vs gravel at various depths.

With this information farmers will be able to utilize a more effective and cost efficient geothermal high tunnel design.


High tunnel temperature control is critical to year-round and specialty crop growth; yet is challenging and expensive. In our last SARE grant we found that heating a high tunnel with a geothermal climate battery system was an excellent alternative for farmers wishing to heat their high tunnels during the winter months and that utilizing self sustaining solar and wind energy was successful in powering the fans needed to support a geothermal system. Indeed, further research since the close of the grant has found that by running the geothermal system 24/7 results in even better outcomes, although it also taxes the batteries and requires electrical backup support.  

Now that we have established the viability of geothermal high tunnel systems, farmers will want to know if the design of and materials in the geothermal system can be improved to enhance the efficiency of geothermal energy. Specifically, two areas of inquiry that have not previously been widely studied may be helpful in designing a cost and energy efficient geothermal system. These two areas are the organic matter utilized to surround the underground pipes (gravel vs soil) and the depth of pipe placement.  In our previous grant, we found that it was easier to install pipes in gravel than soil and that perforated pipes installed in gravel did not require sleeving whereas perforated pipes in soil do require sleeving, costing additional time and money. However, gravel may be more expensive than permeable soil based on location and other factors and with gravel there will need to be a permeable fabric between the gravel and the top soil. In addition to organic matter selection, farmers will want to know the optimum depths to place pipes in the ground.  While there is extensive research into underground temperatures at various depths outside a climate battery, we could find no research on how this applies to a climate battery system. And while there are a number of geothermal high tunnels with gravel or soil surrounding the underground pipes, we could find no research comparing the effect of gravel vs soil in transmitting and absorbing heat in a geothermal climate battery system.

Our high tunnel was uniquely designed for researching this project.  Unlike other geothermal high tunnels, ours used both a gravel piping system and a separate soil piping system.  Specially, 45 pipes are surrounded by gravel and 45 pipes are surrounded by soil.  Secondly, unlike other geothermal high tunnels, we did not use manifolds on both sides of the piping system thus allowing us to know the depth and composition (soil vs gravel) of each of our 90 pipes.  Third, our pipes are at various average depths from about 2 ft to 7.5 feet so that we can test where depth has an impact on temperature in an insulated perimeter climate battery system.  Unfortunately, our soil contains some clay and therefore is not as permeable as sand.  This as will be shown, made the experiment much more difficult to perform.

We hope this research will help to improve the understanding of geothermal climate battery systems and help make them more viable cost effective alternatives for farmers.

Description of farm operation:

Our farm, T. L. Fruits and Vegetables, produces over 35 different fruits and vegetables sold through local farmer markets, onsite sales, and year-round CSA customers. Gross sales this year are about $30,000. We have four high tunnels where we grow produce year-round and additional acreage under production. We began our farm in West Virginia in 2011, working with many local growers, volunteers, interns, year-round workers and collaborated with Dr. Lewis Jett on research projects with WVU. In 2016, our farm was awarded the Greenbrier County Conservation Farm award. We have hosted numerous farm tours with the USDA/NRCS and WVU Extension department. In 2017, we converted to solar energy for most of our farm operations and household needs. We have also received three SARE grants.
Tommye is the owner of T. L. Fruits and Vegetables. Since 2012, Tommye has engaged, among others, in the following farm related activities: presents at numerous farm conferences around West Virginia; oversees the Let’s Grow Together Demonstration Garden in Lewisburg, WV; provides farm tours for West Virginia School of Osteopathic Medicine culinary course; grows produce for the local FARMacy program, the local senior feeding programs and USDA kid pop-up markets; serves on the WVSOM Green Fund Advisory Committee; developed and maintains the MARVEL Learning Center school garden; offers 1-on-1 consultations with local home growers; and provides farm job training to disabled individuals through Gateway Industries.
Richard, Tommye’s spouse, has served as president of Sprouting Farms Board and Vice President of Monroe Farm Market Board. His experience includes high tunnel construction, installation of solar driven side wall curtains as part of FNE 18-907 SARE grant, tractor work, installation of a self-sustaining geothermal high tunnel, water recycling systems, pond construction, cold storage unit construction, barn and seed growing area renovations, electrical, solar, plumbing work, and general maintenance.


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  • Dr. Richard Rafes (Researcher)


Materials and methods:

The project methods evolved due to unexpected and unforeseen challenges in the data gathering process.  First, we planned on identifying pipes in soil and pipes in gravel at approximately the same depth and with the same air velocity going through the pipes.  We found that many of the soil pipes had become clogged or partially clogged, but that for the summer testing we had enough unclogged soil pipes to perform the tests as described below under Summer Testing. For the Winter Testing, there were many more soil based pipes that were clogged or partially clogged so we redesigned the tests to address each of the test units as described below under Winter Testing.

We also had a challenge trying to find an accurate temperature measurement device.  The following process for selecting the optimum temperature measuring device was followed with results listed:

  1. Tried to use an infrared thermometer which was aimed at the inside of the pipe about two feet down. Proved to be very inaccurate as the thermometer would give vastly different readings each time we took a measurement at the same pipe.
  2. Tried a second infrared thermometer, thinking the first one was inaccurate. We again received inconsistent results.
  3. We attached an analog metal thermometer to a metal wire made out of a coat hanger and placed it in the pipe hole.  This provided us with measurements that were useable for the summer, but started to malfunction in the colder months as determined by placing a second thermometer in a pipe, comparing the temperature to the original thermometer measurement and getting different readings.
  4.  Tried to use the metal thermometers as a temperature tool in conjunction with the infrared thermometer by pulling the analog thermometer  out of the pipe and immediately taking the temperature of the metal on the thermometer with an infrared thermometer.  This also gave inaccurate readings.
  5. Tried individual digital mini thermometers that could be attached to the wire with electrical tape and placed in each testing pipe. This again gave us inaccurate temperature readings as proven by putting two of the thermometers next to each other and observing different results.
  6. Finally, we purchased a single digital psychrometer thermo-hygrometer that measured the temperature of the air emanating from the pipes. The same meter was used in each pipe to assure consistency.

Summer Testing:

Test one: We identified in our existing geothermal climate high tunnel three sets of two pipes, each set was approximately the same depth in the ground and similar air velocity through the pipes as measured with an anemometer. In each set of two pipes, one had gravel organic matter surrounding the pipe and one had soil. 

Test two: We identified in our existing geothermal climate battery high tunnel three sets of two pipes in soil, each of the two pipes were at different depths (ranging from two to six feet) with the same air velocity going through the pipes.

Test three: We identified in our existing high tunnel three sets of two pipes in gravel, each of the two pipes were at different depths (ranging for four to seven and one half feet) with the same air velocity going through the pipes.

We tested and recorded temperatures morning, noon and night during the hot summer days 30 times per set. We averaged the differential between the readings.

Winter Testing

Test One:  We tested ten sets of two pipes, one pipe in soil and one pipe in gravel with each set having similar velocity and depth and tested and recorded temperatures 5 times when the inside temperature in the high tunnel was below 42 degrees (outside temperatures would be several degrees lower).

Test Two: We tested 18 pipes in soil, all on the same manifold and ranging in depth from 4.5 to 6.5 feet and tested and recorded temperatures 5 times on cold winter days when the inside temperature in the high tunnel was below 42 degrees (outside temperatures would be several degrees colder).  All pipes air velocity exceeded 200 ft per minute and all velocities were recorded.

Test Three: We tested all 45 pipes in gravel in two separate manifolds, and grouped in three sets based on air velocity and tested and recorded 13 times during morning, afternoon and evening on cold winter days. Pipes ranged in depth 4.3 feet to 7.5 feet. All pipe velocities exceeded 300 ft per minute and all velocities were recorded.

The challenges with the pipe blockage and the temperature gauges resulted in many additional tests.  In all, we did 3,371 tests and utilized 705 data points.   

We analyzed the data and then determined conclusions based on data analysis.

Research results and discussion:

The purpose of this project is to determine the relative positive and negative outcomes in terms of performance, cost efficiencies and design in installing a geothermal piping system in two types of organic matter (gravel and soil) and to determine if the depth of the piping system will significantly effect the air temperatures emanating from the system.  We measured the relative temperatures of the air exiting geothermal pipes buried in soil and gravel to determine if there is any advantage to utilizing gravel vs. soil.  See Data Charts 1a, 1b, 1c, and 4. We also tested whether the depth of a geothermal pipe in soil and in gravel can have an effect on the temperature of the air exiting the pipe. See Data Chart 2a, 2b, 2c, 3a, 3b, 3c, and Graphs 1-3. In addition, we observed the effect of velocity of the air going through the pipes on the temperatures of the air existing the pipes. See Graphs 1-3. We found that organic matter (soil vs. gravel), depth of the pipes (4.3 to 7 ft) and velocity of the air emanating from the pipes (in range of 200 to 500 ft per minute) had a de minimus effect on the temperatures emanating from the air in the pipe (less than 1 degree difference on average).

data charts
Graphs 1-3

Previous experimentation has shown in normal situations, the ground is warmer in the winter and colder in the summer than the outside air.  This is the basis for geothermal energy within homes with geothermal systems. However, high tunnel geothermal systems are not designed the same as geothermal housing systems.  Geothermal high tunnels are designed with underground climate batteries. The climate battery is simply a section of ground under the high tunnel that is insulated on its perimeter and in the winter collects the heat during the day and stores it in the ground to be used to heat the high tunnel during the cold nights. During the summer the process is reversed but the air emanating from the pipes is used primarily as a ventilation device as all the cooling is quickly lost due to the sides of the high tunnel being in the open position. There are no deep liquid filled small sized pipelines in the ground to collect the energy as with normal geothermal systems for homes and businesses.  Instead, there are a series of 4 inch air filled pipes about two feet underground and a second series about four feet underground. Some climate batteries contain a third series of pipes as well.  These pipes have air instead of fluid running through them.  Because they often collect air with moisture that condensates, they are perforated so as to drain the water out of the pipes and into the ground.  The perimeter of the high tunnel is insulated (usually with foam boards) from the outside cold ground during the winter and heated ground during the summer and the top of the ground is covered with the high tunnel enclosure so pipes do not need to be deep in the ground. Indeed, we found in this project that the depth of the pipes did not appear to affect the pipe temperature during the winter nor the summer.  We also found that there is no significant temperature benefit to using gravel vs soil, the temperatures remaining close to the same with no clear winner. We believe that to recommend one organic matter over another we should see consistently at least three degrees difference.  That did not occur and in fact in our testing there was minimal difference (less than one degree on average) well within the margin of error for the temperature gauge we used.  The only caveat is that soil had some minimal amount of clay that may have lessened the transmission of heat and definitely hindered air velocity as noted below.

So based on our testing, a farmer should be confident that they do not need to bury their pipe system deep into the ground to take advantage of the geothermal effects of a climate battery. And based on the testing results, soil (at least soil with a small amount or no clay) and gravel both appear to release heat during the winter and absorb heat during the summer at similar temperatures.

There is however a major unexpected factor we observed that should be considered when determining whether to use gravel or soil with any amount of clay. To understand this factor it is important to note that the total volume of air going into a high tunnel during the winter must be enough to counterbalance the decrease in temperature cause by the outside air around the high tunnel. Air velocity through every pipe needs to be high in all pipes and may vary depending on the number of pipes. For our high tunnel we used 90 four inch pipes and our fans pushed the air to an average velocity of about 350 to 400 ft per minute range.  Our geothermal high tunnel piping is now about three years old.  During that time, the air velocity going through all but a few of the pipes surrounded by soil has diminished by at lease half and in many cases the air flow has stopped altogether.  At the same time, the air flow through the pipes surrounded by gravel has increased substantially. This situation has exacerbated since the fall and winter when the ground moisture is higher than in the summer. We believe what has happened to our pipes surrounded by soil is water from condensation in the pipes and from the surrounding soil around the pipes is hindering the full air flow. To test this theory we were able to insert a hose into a ground pipe and slide it up near the connection with the manifold. When we inspected the hose we found water at the end of the hose near the manifold. Although we took precautions to minimize the clay content in the soil, we believe the soil has over time compacted and sealed to an almost water tight barrier causing water to clog or partially clog many of the piping surrounded by soil. We should note that all the pipes are perforated and the soil pipes were sleeved to prevent soil from entering the pipes.  The gravel pipes were not sleeved, but had no air flow restrictions. Given this development, we believe gravel or highly permeable soil (sand) are the preferred materials if financially feasible for the farmer.  If the farmer chooses soil, they should make sure it is permeable with little and no clay content. In addition, it is important that the lateral piping proceeds from one manifold to other manifold without dipping or arching so as to allow water to build up before entering the manifold. Any dip or arch in the line could act like the plumbing underneath a sink, collecting water to stop back flow of air. 

For these reasons, we recommend using gravel if it is available and not too costly or using soil with little or no clay content and with proper piping design as described above. Gravel costs have risen significantly during the current inflation. A dump truck load of gravel, once $250, now could cost around $700.   Depending on the size of the high tunnel, at least one dump truck load would be necessary, likely several more. Permeable soil can also be very expensive or inexpensive depending on the location. Sand is a permeable soil and in some parts of the country is very readily available while in others is quite expensive. Farmers should research the cost in their areas before making any decision as to the best alternative material to surround the pipes.


Research conclusions:

This project sought to provide new information to farmers considering the installation of a geothermal high tunnel. Specifically, we wanted to provide the farmer with an analysis of relative performance of two commonly used organic matters (gravel and soil) that surround the underground pipes in a geothermal high tunnel. We also sought to determine if the depth of pipes in a geothermal high tunnel affects the temperatures of the air exiting the pipes.   We found that from a performance standpoint there was no evidence in the data that the organic matter (soil vs gravel) or depth of the pipes had a material effect on the amount of heat released or absorbed through the pipes to the high tunnel. Stated another way, the organic matter nor depth of the pipes did not significantly change the temperature of the exiting air from the pipes. As an aside, the data indicated that velocity of the air emanating from the pipe in the range of 200 to 500 ft per minute had no effect of the temperatures of the air emanating from the pipe.  These results met our objectives and would have provided the farmer with important information when deciding which organic matter to install had we not also discovered a factor that was not anticipated.  This factor was the relatively poor performance of soil with some clay on air velocity.  We discovered several pipes surrounded by soil that had clogged or partially clogged.  After some investigation and experimentation, we deduced that these pipes had partially or fully filled with water.  We believe that this is because the soil contained some clay that would not absorb the water and that the pipes must have had either dips or arches that caught the water before it passed through to the manifold.  The easy fix for the dips and arches is to make sure the water can run down hill into each manifold and there are no dips or arches in the lines.  There is no easy fix for the soil except to use clay free sand or soil with very little clay.  It is possible that over time most soil based geothermal high tunnels will depreciate the amount of heat the ground will absorb or release.  It would be interesting to discover whether the current soil based geothermal high tunnels that did not use sand have over the years experienced less heat being absorbed in the ground and released back to the high tunnel, that is, are they keeping the high tunnels as warm now during the winter as they did when originally built. Obviously to know that information the farmer would have to keep track of the data for many years. Our research did not include specific costs for permeable soil and gravel installation.  This is because of several factors including inflation, location of the closest gravel company, transportation costs, the costs of  permeable soil, and the access to machinery to excavate and spread the gravel or soil. If costs are comparable or tolerable, we recommend using gravel or sand as the organic matter around the pipes. 





Participation Summary
2 Farmers participating in research

Education & Outreach Activities and Participation Summary

6 Consultations
1 Curricula, factsheets or educational tools
12 On-farm demonstrations
6 Tours
2 Webinars / talks / presentations
3 Workshop field days

Participation Summary:

26 Farmers participated
2 Number of agricultural educator or service providers reached through education and outreach activities
Education/outreach description:

During the past year we have hosted medical students from the West Virginia Osteopathic Medical School during their culinary science course at our farm. While we tour the farm and talk about edible plants we also discuss our use of high tunnels to extend our growing season. The geothermal climate battery high tunnel has the biggest draw and we spend the majority of the visit discussing geothermal energy, solar energy and wind turbines. We have had students revisit our farm to gather more information on our geothermal high tunnel and they bring their parents from out of state when they are visiting.

On Saturday February 25, 2023 Richard and Tommye presented at the West Virginia Small Farm Conference sharing our research, experimental design, along with our results with farmers in attendance.
Improving Geothermal High Tunnel Design Through SARE Grant Research (1)

Learning Outcomes

2 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
Key areas in which farmers reported changes in knowledge, attitude, skills and/or awareness:

During multiple farm tours during 2021-2022 we have discussed our Geothermal high tunnel design and finding. Many found the design interesting and asked basic questions. One participant brought their family from Idaho to discuss how to implement a similar design in their own farm located in Idaho. Most people want healthy produce available all year long and being able to extend the season has been one of the biggest draws to building and operating a high tunnel. Still there is a period of December through February when most crops will stall, or not create any new growth, and it has been discouraging to most farmers in our area. The demand for fresh greens during the winter months is what encouraged us to seek out alternative designs for maintaining an optimal growing environment during the winter months. This seems to be where our Geothermal high tunnel system really shines; the growth of greens is substantially different than the same products grown in our other three high tunnels.  For the 2022-2023 winter growing season we have been able to maintain a steady growth of healthy greens throughout the coldest months of the year. This seems to be what other farmers are also looking for when they tour and visit our farm.

Project Outcomes

2 Farmers changed or adopted a practice
Project outcomes:

In the future should we decide to build another geothermal high tunnel, we will use gravel or sand around all the pipes.  We hope that farmers reading this grant will also consider their options as to whether to use gravel or soil around the underground pipes based on the clay content of their soil, the cost of bringing sand (or soil with a very low content of clay) vs the cost of gravel.  In addition, should we build another geothermal high tunnel we will make sure the piping has no dips or arches that could trap condensed water.  We hope other farmers will be cognizant of this caveat as well.  Finally, should we install another geothermal high tunnel we will place the last row of pipes no more than two feet below ground level and prior rows will be at one foot intervals below the last row.  In other words, we will minimize the depth of the pipes to save excavation and piping costs.  We hope that other farmers consider the same cost savings.

Assessment of Project Approach and Areas of Further Study:

While our methods were reasonable when first conceived, we found that once we began measuring the velocity of the air through the underground pipes, there was a wide discrepancy between the velocity of the air going through the pipes in soil and the air going through the pipes in gravel. While this information is valuable for this research and important for farmers to know, it made the research design much more difficult.  It turned out that selecting soil and gravel based pipes for the research with the the same air velocity and same depth in the ground was difficult. Scientific curiosity lead us to test whether the velocity of the air going through the pipes made any difference in temperature of the air exiting the pipes.  We found that it did not. Note this is very different from the ultimate effect of the amount of air actually transmitted to the high tunnel.  The more warm air transmitted to the high tunnel, the warmer the high tunnel and so velocity matters greatly in the overall temperature in the high tunnel, but it appears that does not matter as to the temperature of the air exiting the pipes, provided there is sufficient time in the pipes to warm the air during the winter months. Specifically, we found no measurable difference between the temperature of air exiting the pipes at 200 feet per minute and at 400 feet per minute, but obviously for heating the high tunnel we would want the higher feet per minute air exiting the pipes.  Nevertheless, we sought to identify pipes with similar velocities and depth and were successful in doing so during the summer when we had less clogged and partially clogged pipes in soil. During the winter we altered the methods to select pipes in groupings rather than pairs.  So for example, we measured all the soil based pipes that exceeded 200 ft. per minute air velocity along the long manifold, noting the velocities of air flow of each and looked for any increases or decrease in temperature. We found that for that test the temperature outcomes were randomly similar in all tested pipes and could therefore conclude that depth and air flow did not matter. A second challenge was utilizing an accurate temperature gauge. We started using an infrared gauge and progressed to analog metal gauges attached to a wire, then to the infrared gauges to measure temperature on the metal of the metal thermometer when removed from the pipe hole, then we tried small digital thermometers attached to a wire and placed in the pipe hole. We found these gauges were also inaccurate. We ultimately used a psychrometer thermo-hygrometer to measure the the temperature of the air emanating from the pipes. We found this method to be by far the most accurate and consistent. These challenges and changes in methodology required more tests than originally anticipated.  In all, we did over 3371 tests.  

Despite these challenges, we do believe we accomplished our goal of analyzing the benefits and detriments of soil vs gravel as an organic matter to be placed around the pipes and our goal to determine if placing the pipes lower in the ground would be advantageous. We did have a valuable unexpected finding regarding soil based pipes that contained some clay.  We found that pipes surround by soil with some clay are susceptible to clogging and that care will need to be given to avoid any dips and arches in the elevation of pipe layout, especially when it joins at the manifolds.  In addition, the soil used should be very permeable. Clay based soil should be avoided or greatly minimized.  We strongly suggest using sand or gravel.

Given the above, we will use gravel or sand should we install another geothermal high tunnel.  If we use sand or gravel we will separate the gravel or sand from the soil above it with a permeable barrier. We did this for the gravel in our current geothermal high tunnel.

We would like to learn from other farmers who have had soil based piping whether they had similar problems. We would also like to learn if any farmers using gravel had any decrease in velocity of the air exiting the manifolds. Unfortunately, they might not know whether any specific pipe is clogged because the common design is to connect all pipes to two large underground manifolds, one on each side of the pipes. As long as some of the pipes are not clogged they will pick up the extra flow of air. However, this will lessen the ultimate heating and cooling effect. We purposely designed our geothermal system for research purposes so that we could measure each pipe individually and discover if any specific pipe out of our 90 pipes is clogged or partially clogged, the actual velocity of air in the pipe and depth of each pipe.

We believe all farmers wishing to install a geothermal high tunnel could greatly benefit from this research. 


Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.