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

Progress 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 3041 tests).  The refined procedures included measuring the air velocity and depth of each of the 90 pipes, selecting accurate temperature measuring devices, selecting pipes for analyses, 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, all but one of the pipes in soil had slower velocities than the pipes in gravel which translates in less moderating air being transferred to the high tunnel. Second, air velocity had little to do the temperature of the pipes except where the pipes were clogged. This conclusion was determined by taking readings of the gravel pipes at full velocity and then reducing the velocity to half speed. The temperatures were the same within a small margin of error. Third, the pipes in gravel were slightly warmer than the pipes in soil during the winter testing with the approximately the same air velocity and pipe depth . This amount only 1.29 degree, which is within the margin of error for the digital thermometers used and is not a rationale for choosing gravel organic matter in and of itself.  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.  

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 as will be explained in more detail later in the report. Specifically, it appears pipes placed in clay based soil can seal in such a way as to retain water and thereby clog or reduce the air velocity through the pipes.  This is critically important as the high tunnel heating is based on sufficient are flow. Without this air flow the high tunnel will not warm enough to prevent plant damage. Proper design to make sure there is no way that water can lodge in the small pipes before entering the manifold and usage of non-clay based soil (such as sand) are critical in successfully heating a high tunnel during the colder months and in ventilating the tunnel during the warmer months.

Based on this project, farmers should ascertain the costs of gravel vs non-clay based soil to be placed around the pipe, understanding that with soil a fiber sleeve will need to be added to each 4 inch perforated pipe to prevent the soil from entering the pipe 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 planned 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 seeks to:

  1. Determine which organic matter (gravel vs soil) better absorbs and releases energy to heat and cool a high tunnel
  2. Determine whether installing piping deeper in the ground increases the air temperatures released from the pipes during the winter and provides cooler air temperatures during the summer.
  3. Enhance knowledge of optimum geothermal climate battery high tunnel design.
  4. Provide a more effective design for geothermal climate battery high tunnel.
  5. Detail the positive and negative outcomes in terms of design and performance of installing the piping system in soil vs gravel.

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 susccessful 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 permeable 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. 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 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 a manifold on one side 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 our 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 WVSOM 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 of 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 and comparing to the original thermometer measurement.
  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. We found on Amazon a digital mini thermometer that could be attached to the wire with electrical tape and placed in the pipe. This gave us very accurate and consistent temperature readings. 

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 the same 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 will be 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 about 60 times per test. 

Winter Testing

Test One:  We identified three sets of a total of eight pipes, half the pipes in soil and half the pipes in gravel with similar velocity and and depth in each set and tested and recorded temperature 13 times during morning, afternoon and evening on cold winter days  (in high tunnel below 46, outside several degrees colder).

Test Two: We identified nine pipes in soil, all on the same manifold and ranging in depth from five feet to six and one half feet and tested and recorded 13 times during morning, afternoon and evening on cold winter days.  

Test Three: We identified 15 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.

The challenges with the pipe blockage and the temperature gauges resulted in many additional tests.  In all, we did 3041 tests.   

We analyzed the data and determine conclusions based on data analysis.

Completed report to SARE and promulgate findings as noted in the outreach section to various constituencies



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 gravel vs. soil and to determine if the depth of the piping system will significantly effect the 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. We also tested whether the depth of a geothermal pipe can have an effect on the temperature of the air exiting the pipe. Research 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 during the cold evening. During the summer the process is reversed. There are no deep pipelines into the ground to collect the energy as with normal geothermal in a home.  Instead, there are a series of pipes about two feet underground and a second series about four feet underground. Some 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 the 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 only one degree difference and that would be well with in the margin of error for the temperature gauges we used.  The only caveat is that soil had some minimal amount of clay that may have lessened to transmission of heat.

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 some 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 and would vary depending on the number of pipes. For our high tunnel we used 90 4 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 blocking 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 partial 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. Given this development, we believe gravel or permeable soil (sand) are the preferred materials if financially feasible for the farmer.  We should add however that gravel and sand very greatly based on the location of the high tunnel.  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 reason 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 costs more than $700.   Depending on the size of the high tunnel, at least one dump truck load would be necessary. 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 strong evidence in the data that the 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 depth of the pipes did not significantly change the temperature of the exiting air from the pipes. In fact, the pipe data indicated random data, e.g. some deeper pipes were warmer during the winter and some were colder as compared to pipes that were closer to the ground surface.  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.  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 know whether the current soil based geothermal high tunnels that did not use sand have over the years experience 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 the farmer would have to keep tract 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. 



Participation Summary
2 Farmers participating in research

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 desgin interesting and asked basic questions but one participant brought their family from Idaho to discuss how to implement a similar design in their farm infastructure. 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 without any new growth making it difficult to maintain repeat harvesting throughout 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. 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 along the long manifold that had similar air flow 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 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, and ultimately we used a small digital thermometer attached to a wire and placed in the pipe hole.  We found this method to be by far the most accurate and consistent. These challenges and changes in methodology required more tests that originally anticipated.  In all, we did over 3000 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.