High tunnel temperature control is critical to year-round crop growth; yet can be both challenging and expensive. The objective of this study is to develop a cost-effective self-sustaining temperature moderation system for large (defined as 30ft by 96ft) high tunnels utilizing a geothermal network, powered by a configuration of solar panels, wind turbine and batteries to energize fans that will circulate moderating air into the high tunnel from geothermal piping. We plan to evaluate whether this system can support year-round plant growth including during extreme hot and cold temperatures.
The geothermal high tunnel will have four high speed fans powered by twelve solar panels, a wind turbine and batteries with component adjustments made as needed. Temperature data inside the geothermal high tunnel and a second “control” high tunnel will be collected during designated summer and winter days and months in 2020 and 2021 when the temperature in a control non-geothermal second high tunnel exceeds 85 degrees or is below 40 degrees. Data will also be documented on estimated cost of materials and operation for three possible options for farmers who wish to grow year-round crops: 1. Cost of propane/natural gas heating systems with fuel (no cooling/air circulation with this option), 2. Cost of geothermal network that is not self-sustaining and 3. Cost of a self-sustaining geothermal/solar/wind energy system. Upon completion of testing, an analysis of the temperature moderation and relative cost data will be made.
Where possible within Covid 19 safety considerations, the project will be publicized through presentations, farm visits, news releases and internet postings.
This project seeks to discover if a geothermal high tunnel powered by self-sustaining solar/wind energy is a viable option for farmers wanting to farm year-round in climates susceptible to extreme hot and cold temperatures. In studying this objective, this project seeks to determine the following:
- Whether a self-sustaining geothermal solar/wind energy system can “sufficiently heat” a geothermal high tunnel when the control high tunnel temperatures are below 40 degrees without a supplemental heat source such as natural gas. “Sufficiently heat” is defined as maintaining the geothermal high tunnel temperature above freezing at night when the control high tunnel temperature is 22 degrees or above; and maintaining the geothermal high tunnel temperature at least ten degrees higher when the control high tunnel temperature is below 22 degrees. Note that night control high tunnel temperatures should equate to outside temperatures.
2. Whether a self-sustaining geothermal solar/wind energy system can “sufficiently cool” a high tunnel during temperatures at night above 75 degrees with side curtains closed and if so, can it do so for extended periods during the hot summer months in West Virginia. “Sufficiently cool” is defined as maintaining the temperature in the geothermal high tunnel at 75 degrees when the control high tunnel is 85 degrees and lowering the geothermal high tunnel temperature to at least ten degrees below the control high tunnel temperature when the temperature in the control high tunnel is above 85 degrees.
3. The optimum design configuration for such a system in terms of solar, wind turbine and battery interface.
4. The relative materials and operating costs of traditional geothermal, self-sustaining geothermal, and natural gas/propane systems both in start-up and annual operating costs.
5. Whether the sides of a high tunnel should be open or closed when cooling the high tunnel using geothermal energy.
6. The optimum fan air velocities (measured as cubic feet per minute) in the underground pipes to effectively transmit and moderate the heating and cooling in a large high tunnel.
High tunnel temperature control is critical to year-round and specialty crop growth; yet it is challenging and expensive. Currently, farmers utilize natural gas/propane, wood burning stoves, oil furnaces, electric heat, low tunnels using Agri-bon fabric, and geothermal systems to provide additional heat to high tunnels beyond the effects of passive solar heating. For cooling and air circulation, farmers use fans, evaporative cooling systems and geothermal systems and for ventilation, they use end vents, side curtains and roof vents to release heat from the high tunnels and circulate air. None of the common active heating and cooling systems are self-sustaining, i.e. all require expenditure of an external energy source and some have additional labor costs. In addition, while ventilation can be self-sustaining and will provide limited heat relief during hot summer days, a geothermal system will dispense and circulate cool air in and around the high tunnel.
During our 2019 Farm tour Dr. Lewis Jett from WVU indicated that he is unaware of any geothermal high tunnels in West Virginia and has a strong interest in our project.
Geothermal systems powered by wind/solar energy can heat and cool high tunnels with virtually no maintenance or additional cost after system installation. One of the outcomes of this project is to determine if geothermal systems provide the best option for farmers wishing to grow year-round in areas that experience extreme hot and cold temperatures. The project will examine whether a geothermal system can sufficiently heat a large (30 ft by 96 ft) high tunnel during weather below 40 degrees for several days without a supplemental heat source such as natural gas. In addition, the project will provide data and analysis on whether it is functionally and economically feasible to power a geothermal high tunnel with self-sustaining solar/wind energy. With this information farmers can determine if they wish to utilize traditional heat sources such as the burning of gas or wood, a traditional geothermal system utilizing the electric grid to power the fans, or a self-sustaining geothermal system utilizing solar and wind. For farmers who require cooling during the summer to protect heat sensitive crops, this project will also provide data and analysis to determine if geothermal cooling will contribute to cooling a high tunnel during hot weather extremes (temperature above 75 degrees). It will also determine whether the side curtains should be open or closed while the geothermal system is circulating the cool air.
If a self-sustaining system can be developed, farmers without access to the electrical grid can farm year-round. Moreover, farmers with access to the grid may decide to forgo the energy costs of on-grid electricity and go off grid. While there is much research and understanding about how much energy a solar panel can generate in various areas of the country, there is little or no research as to optimum solar/wind turbine battery design necessary to power geothermal/climate battery high tunnel fans in winter months when solar energy production is diminished and in summer months when the geothermal fans would be needed to be run for longer periods during the day. Another aspect of this research will be to provide data and analysis of a cost-effective design. We have identified a commercial product package that permits up to 12 solar panels and one wind turbine. We already own six 12-volt batteries we purchased previously for an ATV but did not use. We intend to adjust the number of solar panels, wind turbines and batteries to ensure the fans do not stop from lack of energy while seeking to minimize the number of solar panels, wind turbines and batteries required. We may find for example that we only need five solar panels, one wind turbine and three batteries which would mean greater cost savings for farmers who install the self-sustaining geothermal solar/wind energy system or we may need to add to the configuration. In designing the system, we will also need to adjust the geothermal fan velocity (CFM) to provide optimum temperature-controlled air flow. We will record and analyze this data as well. We are using both solar and wind energy because snow can cover the solar panels curtailing their usefulness and because on high overcast days and at night solar energy is not sufficient.
In summary, we hope to provide a viable design for a self-sustaining geothermal system powered by a solar/wind energy and demonstrate that this system is a cost-effective ideal solution to year-round temperature control in a high tunnel.
T. L. Fruits and Vegetables LLC has been a farm business since 2010. We grow and sell fruits and vegetables through an online farmer’s market year-round, sell at multiple local farmer’s markets, offer a yearly CSA program, sell berries to a locally winery for a variety of berry wines and meads, donate produce to local food pantries and local feeding programs, offer small pop-up markets at area schools and senior centers with support from our statewide Extension service, and we participate in a local FARMacy program using food as one way to combat diabetes. Our farm has four high tunnels which are used all year long as well as one acre in fruit trees, one acre in berry production and a half acre in field production of vegetables. Both my spouse and I are retired educators and farm full time. We hire one part time seasonal employee and have two retired ladies who help us as needed. Farm resources of significant value dedicated to this project include the following: High tunnel, land below high tunnel, eight 150 Ah 12 volt batteries and six 200 Ah 12 volt replacement batteries, excavation work, gravel, piping, 2 controllers, 12 solar panels, wind turbine, wiring, fans, nine pipe collection container boxes, tractor and a USDA Renewable Energy Efficiency Improvement Grant.
April 2020 – July 2021 – Initially four 100 watt (920 CFM) fans were installed to blow air through the geothermal pipes. After analysis of air
flow, fans were upgraded twice, first to four 90 watt/2000 CFM fans and then to four 120 watt/3000 CFM fans. Four rheostats were installed to vary fans speeds. Initially ten solar 100 watt panels, one 1600 watt Missouri Wind and Solar wind turbine and all in one controller were installed. Two additional solar panels were added due to the increased wattage needed for testing. This was the maximin wattage limit for the wind and solar controller. Initially 6 used 12 volt/150 AH Trojan deep cell lead acid batteries were installed. After analysis, two additional new batteries of the same brand were installed. After further analysis, five new deep cell Renogy 200 AH batteries were substituted and another one is on order and will be installed shortly. Also installed was an Advancing Alternatives controller that served to regulate the fans, side curtains and other functions unrelated to this project. Remote temperature gauges were installed, one for the control high tunnel (non geothermal) and one for the geothermal high tunnel. All these components were in place by January 2021. Up until that time, hundreds of tests were done to determine the optimum methodology and component configuration. Beginning in early January 2021, testing with the new components has began utilizing the following revised procedure: The geothermal fans are running for at least four hours during the daylight hours. With the side wall curtains closed at night on the control and geothermal high tunnels, fans are set at one of four speeds. These speeds are estimated at 100, 240, 400 and 480 watts with the corresponding CFM of 2500, 6000, 10000 and 12000 respectively. During winter nights where the temperature will be below 40 degrees, temperature data for the control and geothermal high tunnels will be recorded at four different times (early evening, midnight to 2:00 AM, 4:00 to 6:00 AM and at sunrise. There will be at least five measurement nights for each of the fan velocities (2500, 6000, 10000, and 12000 CFM). Thus a total of at least 20 nights (four times a night) of measurements will be made during the winter 2021 months.
During the summer 2021, the procedure will be as follows: when the temperature in the high tunnels exceeds 75 degrees at 6:00 PM, the sides of the control and geothermal high tunnels will be closed and the same process of varying the velocity of the fans and recording the temperatures in the control and geothermal high tunnels will be followed for 20 days, five days at each of the four speeds.
March 2021 – Relative materials and operating costs for three types of technologies to heat similar size high tunnels will estimated and recorded. The three are: 1. a non self-sustaining geothermal high tunnel, 2. a self-sustaining geothermal high tunnel and 3. a natural gas/propane high tunnel.
March 2021 – August 2021 – Analysis of the data and conclusions will be made and submissions for presentations developed. Presentations and farm visits will be conducted where possible within COVID 19 guidelines. It is possible this outreach may have to be totally internet based until vaccines are readily available and utilized in West Virginia.
September 2021 – Preparation of report to be submitted to SARE.
The following has been discovered thus far:
Experimental design: It became apparent that the original experimental design would not work, particularly during summer days. This is because the passive solar heating from an enclosed high tunnel (sides down) made measuring the effect of geothermal cooling impossible. With the sides down during sunny days the temperatures in high tunnels rose quickly and exceeded 100 degrees. i Thus, there could be no comparison during the day between the control high tunnel and the geothermal high tunnel. With the sides open on the geothermal high tunnel, temperature measurements were made at each of the 90 exit points of the pipes and these temperatures were lower than the temperature in the high tunnel, but there was no way to determine if this has any real effect on cooling the high tunnel, since the sides were open and the cool air quickly dissipated. To address this concern, measurements will be made at night with the sides closed as described in the procedure above (Materials and Methods). This experimentation will occur during the summer 2021. Regardless of the outcome of the experimentation, the fans do provide air circulation and we have found the temperatures are cooler exiting the pipes than the air in the high tunnel. Whether there is a measurable cooling effect or not, air circulation is helpful to plant growth and disease prevention.
Experimentation is occurring this winter as described above. Initially the experimentation was to focus on measuring the temperatures in each of the 90 geothermal pipes. Temperatures in the pipes vary and this data did not address the main intent of the project, i.e. whether the geothermal high tunnel can be heated using the geothermal piping and wind/solar as a energy source to power the fans and the optimum fan CFM, and configuration of the solar panels, wind turbine and batteries. The new experimental design will provide useful data necessary to report valuable findings.
Solar Panels and Wind Turbine: Solar panels during the day produce significantly more energy (measured in watts) than the wind turbine, except when there is heavy overcast and the wind is blowing hard (at least 15 to 20 miles per hour). Neither produce the wattage specified in their rating (a total of 1200 watts for solar and 1600 for wind), but solar is much more efficient. Of course at night when the wind is blowing in excess of 15 mph, there is wind energy and no solar energy being produced. But unless the geothermal high tunnel is located in a high and consistent wind area, readings demonstrate solar is much more efficient.
Fans: The fans have been a challenge in the experimental design. Originally, four 25 watt, 920 CFM were installed. However, the air flow from the geothermal pipes was minimal and utilizing these fans would not provide strong enough CFM to compare speeds. Therefore higher velocity fans were utilized at 90 watts each. While these fans did increase the air flow, it was decided to ultimately install use four 120 watt radiator fans (3000 CFM each). We have ordered an additional Renogy battery that we hope will provide the additional amp hours needed to run the fans at night at 480 watts (see battery discussion below).
Batteries: Originally six standard deep cell 150 Ah (amp hours) Trojan batteries were installed. These batteries removed from a newly purchased ATV vehicle. Testing during the summer days indicated acceptable voltage rarely dropping below 50% of charge (12.4 volts). Lead acid 12 volt batteries can be damaged if the voltage goes below 50%. Testing during the late fall showed frequent voltage drop to impermissible levels. To remedy this, two additional Trojan batteries were purchased. This added an additional 300 Ah. As testing continued during the winter the batteries began to lose charge and on two occasions went dead. We believe this was due to the older batteries not being in good condition when purchased or not maintained properly after purchase. After research, we decided to purchase Renogy deep cell hybrid gel 200 AH batteries. We initially purchased 5 batteries but decided that a total of six would be needed to do this experimental testing at 480 watt draw. Hybrid gel batteries provide excellent performance and long life. Specifically, the are able to better withstand repeated charge discharge cycles. We calculated that six batteries were needed because this would allow for use of 600 Ah per cycle (50 % of the 1200 total Ah). We have not determined the optimum speed (CFM) to provide the highest temperature increase during the winter, but as noted above, our testing range is 100 to 480 watts. At 480 watts, six 12 volt batteries (total of 1200 Ah) should be able to run safely for 15 hours at 50% or greater. As the sun rises or if the wind blows, the batteries will recharge, especially since the fans do not need to run more than a few hours during the day. It should be noted that ultimately fewer batteries and lesser solar panels may be an acceptable configuration, but if we are to test up to 480 watts, we need the additional batteries and solar panels. Given the controller limitation, we can only have one wind turbine.
Controller: A controller is needed to be an interface between the solar panels, wind turbine and batteries. We chose the Missouri Solar and Wind “All in One” controller because it has a dump function to prevent overcharging and damaging the batteries and because it allowed up to 1200 watts of solar panels (twelve 100 watt panels) and a 1600 watt wind turbine. Other controllers we researched did not allow for both wind and solar energy. We also purchased an Advancing Alternatives controller that can automatically turn on and off the fans as well as control side curtains, side vents and other unrelated needs. The Advancing Alternatives controller is not critical for this project, but the equipment does help regulate temperature within the high tunnel.
Preliminary testing: Preliminary testing has shown that the geothermal effect does moderate the temperature within the high tunnel and that fan velocity does affect the temperature. As expected, during the winter nights, the colder the outside temperature, the greater the temperature gap between the control and the geothermal high tunnels. Whether the temperature is moderated to a level acceptable to the grower will depend on the crops grown and the expectations of the farmer.