Low Cost Self-Sustaining Year-Round High Tunnel Temperature Control

Below we have listed a number of project outcomes:

Geothermal

1. Successfully installed and operated a geothermal climate battery high tunnel system powered by wind, solar, battery and electrical backup to help mitigate temperature extremes in the high tunnel particularly during cold winter months.
2. Created an optimum design for a geothermal high tunnel system in a large high tunnel. Specifically, three 30 ft manifold pipes on each side of the high tunnel connect to 90 four inch perforated pipes at two levels at three and five foot depth and two foot width apart (45 pipes at each level).  On one side of each manifold pipe there is a vertical manifold pipe connected by a 90 degree pipe connector and on the other side a cap.  On the top of three vertical manifolds on one side are the fans and on the other side the manifolds are left open.
3. Discovered the optimum fan speeds are closer to a total of 7000 to 8000 CFM which is much faster than previous research indicated.
4. Used internal pipe connectors to connect pipes to manifolds making it easier and more secure to attach.
5. After experimenting with various 12 volt fans, used four 120 watt 12 volt radiator fans which operate at optimum air flow of a total of 7500 CFM (300 watts).
6. Learned that in a climate battery, a deeper pipe depth had no significant advantage. Pipes 3 ft or deeper provide similar temperatures as those 7 to 8 ft.
7. Discovered underground air temperature is not constant but vary throughout climate changes.
8. Learned that it is important during the winter to run fans at least four hours during the heat of the day to raise the underground temperature for night heating.
9. Found that winter underground temperature is normally in the low 40s degrees and summer temperature in the low 70s.
10. Discovered during the winter, the lower the outside temperature the greater the degree differential between the outside/control high tunnel and the geothermal high tunnel.
11. Learned that drainage pipe which was installed kept almost all the water out of the underground pipes and manifolds  
12. Found that the effect of geothermal cooling during the summer is nominal because the sides of the high tunnel must remain open to prevent overheating within the high tunnel and the cool air dissipates. The geothermal fans do provide ventilation.
13. The estimated equipment and materials costs for a geothermal system are as follows:
    1. Manifolds: 14 18 inch culvert pipes - $3602
    2. Six 90 degree 18 inch pipe connectors - $1338
    3. Six end caps - $442
    4. 30 100ft four inch perforated sleaved pipes - $1911
    5. 32 two inch insulation foam boards - $1166
    6. Three 120 watt 12 volt radiator fans - $150

        Total materials and equipment for geothermal $8609    Note this not include labor or excavation costs.  These are      current prices.  There should be no additional annual costs after setup costs.   

Solar

1. 1. Learned that solar panels should be as close as possible to controller to decrease voltage/wattage loss.
   2. Designed and built an adjustable solar frame to adjust to optimum angle based on time of year (15 degrees plus latitude in winter, 15 degrees minus latitude in summer. Panels point south direction.
   3. Discovered solar panels generate reliable high amount of wattage on sunny and partially sunny days and lower but still reliable wattage on overcast days. Much more efficient than wind energy.
   4. Ten to twelve solar panels with batteries are sufficient to power 300 watts draw from fans in most cases with electrical backup when needed.
   5. Found that in winter when there are overcast conditions and no wind energy generated for consecutive days, electrical backup or battery charging is necessary to avoid depletion of batteries.
   6. Discovered steeper sloped solar panels during winter not only generates more energy but also helps shed snow from panels.
   7. Learned heavy gage wire is necessary to connect solar panels to controller and that solar panels should always be located as close as possible to the controller.
   8. Estimated costs of materials and equipment for Solar –
      1. Ten Solar Panels - $800
      2. Solar connectors and wire - $100
      3. Charge controller if not using wind turbine - $51
      4. Pipe frame total - $492.50
         1. 15 metal 10 ft pipes ($261)
         2. 6 bags of concrete ($31.50)
         3. Pipe hardware and bolts ($100)
         4. Optional winch and wire ($100)

             Total initial solar costs without wind turbine add on - $1443.50

           There should be no annual costs after setup for the life of the solar panels (estimate 15 years).

Wind

1. To state the obvious, the key to wind energy is steady high winds. The winds in our location of West Virginia do not justify wind turbine addition. Wattage from a 1600 watt wind turbine based on wind speed are specified as follows:
   1. 1. 12 mph – 100 watts
      2. 20 mph – 200 watts
      3. 40 mph – 600 watts
      4. 55 mph – 1600 watts

     2. There were times particularly in the Spring and Fall where winds generated enough energy to produce 200 to 300 watts for a sustained period of time. The combination of solar and wind will generate enough energy to power the fans during night and day with battery backup, except when there is sustained cloudiness and little or no wind. In such case the batteries charge will fall below 50% and could damage the batteries.  It is best to have an electrical backup or remote charger in such case.  See below for electrical backup discussion.

    3. In order to maximize wind turbine efficiency there should be no turbulence. This means there should be an unobstructed path from the blowing wind to the wind turbine. Trees, buildings, etc. should not be in the winds path. This may require placing the wind turbine on a very high pole. Our high tunnel was located on the top of a mountain and was placed on a 20 ft pole, but for others to avoid wind turbulence (obstructions in wind path) the pole may need to be much higher.

  4. Wind turbines require a separate type of controller. Missouri Wind and Solar provided a combination solar and wind controller with a feature to protect the overcharging of the batteries when the wind and/or solar exceeds about 14.2 volts.

   5. Wind turbines generate alternating current whereas solar panels generates direct current. The combination wind/solar charger converts the energy to direct current and regulates the amount of voltage so as not to overcharge the batteries. 

  6. Estimated costs for wind energy are as follows:

1. 1. Missouri Wind and Solar all in one controller, 1600 watt wind turbine and wire $960.
   2. 20 ft mounting pole and concrete $125

       Total setup estimate for wind energy is $1085.  There should be little or no annual costs after setup for life (unknown) of wind turbine

Batteries

1. Batteries are necessary if relying on solar or wind to provide energy for the fans.
2. We originally used six and then eight Trojan deep cell 150 AH batteries and found them insufficient for our needs, discharging quickly when power was drawn overnight. Eight 150 AH batteries should last at least 24 hours before falling below 50% charge. They did not. The formula is Watts = Volts X Amps.  Batteries should never be run below 50% charge.  Testing of the Trojan batteries we used showed significant drain below 50%.  We then purchased six 200 AH Renogy hybrid gel deep cell batteries.  These batteries with the same rated total amp hours of 1200 Ah lasted longer with less draw below 50%. This difference could be due to the quality of the battery, gel vs standard flooded lead acid or that the Trojan batteries were older and may have partially sulfated.  Our customer service experience with Renogy was much more positive than that of Trojan.
3. Renogy hybrid gel batteries are rated to last 12 years or 750 discharges at 50%. To avoid allowing a 50% discharge in overcast days with little or no wind we added an electrical back up (see next section)
4. To retain 12 volt level each battery positive will need to be connected to the positive pole of the next battery and each negative pole to a negative of the next battery. This is called linking the batteries in parallel.  This will provide a total of 1200AH for six batteries at 12 volts (actually 12.8 volts at full charge.)
5. We learned that hybrid gel batteries are at full charge at 12.8 volts and at 50% charge at 12.2 volts.
6. We learned that it is important to charge the batteries to full charge before connecting in parallel and to use no. 4 battery cables to connect the batteries to each other in parallel and to the controller.
7. We also learned that to keep the batteries protected from the cold they should be insulated. We used two inch foam board around the top, bottom and sides of the batteries to provide adequate insulation.
8. We learned that batteries should all be of the same brand and kind and should never be mixed with older batteries or batteries of a different kind and brand.
9. We found and strongly recommend getting seal batteries so that constant maintenance is eliminated. Our initial Trojan batteries were not sealed and required periodic checking of water levels to avoid the battery becoming sulfated.
10. Estimated costs: Battery costs depends on the quality and type of the battery. There are standard flooded lead acid batteries, AGM, gel and lithium batteries currently on the market. Lithium are clearly the best but can run as much as four times the cost of gel batteries when comparing amp hours. We used hybrid gel batteries which we believe are the second best and run about $450 per battery from Renogy in current prices.   Six batteries will provide 1200 Ahs which should sustain the batteries at 50% for much of the winter without an external recharge from electricity or a battery charger. Add $73 for insulating boards and the total for battery setup is: $2773 at current prices.  There are no continuing costs after the initial setup costs until the batteries die. Gel batteries are advertised at 12 years or 750 discharges at 50% or below.

Electricity

1. 1. On grid electricity can be used as a backup during winter days without full or partial sun and no wind or to directly energize the fans with no alternative energy utilized. Of course there must be grid power connections available to utilize the grid. Assuming the farmer wants supplement the alternative energy, electricity can easily be converter from 120 volt AC to 12 volt DC with a converter for about $40 plus any wiring costs.  The annual costs of using the electrical backup depends on the number of days where there is not enough sun or wind to generate sufficient battery charging to run the fans at night. We estimate this will occur only 10 to 20 days during the winter and the electrical cost will be de minimis.
   2. Looking at current electrical rates, should the farmer wish to run the fans (total of 300 watts) solely with electricity (no alternative energy) the annual electrical cost is estimated to be about $400.  There would be wiring costs to connect the fans to the electric source and this would depend on the location of the closest electric connection device.

Using natural gas or propane to heat a 30 x96 ft high tunnel

1. 1. Farmers who wish to heat their high tunnels with gas or propane should expect the following costs: Estimated retail cost of two 175,000 BTU Renzor gas furnaces with stainless steel heat exchangers is about $5200. Installation estimated at least $1000.
   2. Estimated annual natural gas cost for 90 days, 12 hours a day is $5,560 and for propane $12,867. *Note these costs do not include the costs to ventilate a high tunnel during other months of the year

Summary observations:

1. 1. 1. 1. After installation costs, geothermal heating and ventilating (with or without alternative energy to run the fans) has virtually no annual costs and is highly preferable to the traditional heating of high tunnels with propane or natural gas.
         2. While there may be times in extreme cold weather days where the farmer will want to further protect the crops with Agribon or other row cover, the geothermal heating in our experiment kept temperatures above freezing in the geothermal high tunnel when control high tunnel temperatures were 22 and above. The temperature differential between the control temperatures and the geothermal high tunnel temperatures increased the colder the control high tunnel temperature got. In order to obtain this differential the fans needed to run at a total of about 300 watts which equates to about 7000 to 8000 cubic foot per minute (CFM).
         3. While our geothermal design was slightly different than what we recommend above, we believe our recommended design that has the same number of underground pipes will be as effective, easier and less expensive to install.
         4. Given the wind speed necessary to generate energy, it is not recommended the farmer use wind turbines unless the farmer lives in an area with significant and steady wind.
         5. If there is not significant and steady wind, we recommend the use of solar energy with batteries and electrical backup as the sources of energy to run the fans of the geothermal system.