Greener Gardens Greenhouse Update

Hi all, I’m Kirstee Schutte. I’m a Master Gardener and Nebraska native with years of greenhouse experience and a background in landscape management. In 2022, I started as the Greenhouse Special Project Coordinator with the North Platte Natural Resources District. Upon employment, I eagerly applied my experience and skills to the new position. Though not much has changed in the grand scheme of things, there have been some electrical updates and project expansions. The bulk of the updates within the greenhouse address the heating and ventilation targeting summer high and winter low temperatures. Further project updates will provide opportunities for the expansion of food production on the outside grounds of the greenhouse through an edible landscape and raised bed garden.

Heating and cooling a greenhouse can be complicated, but the Greener Gardens Greenhouse has a few massive advantages. Its walipini, passive solar design allows for capturing and storing daytime solar radiation to then be released diurnally via thermal mass. In this case, the thermal mass is the ground around it and below it. The geothermal system facilitates the transfer of air through tubing located below the frost line. This tubing is trenched 8 ft below ground with the intake and outtake beginning and ending in the greenhouse. As the air travels from the greenhouse through the system, it is warmed (or cooled) to a stable temperature near 45°F before reentering.

To heat a greenhouse, it must be designed with a heating capacity large enough to meet the needs of heat loss per hour to maintain a desired temperature.

To calculate heat loss per hour use H=KA(T1-T2)

• H= Heat required in BTU
• K=Heat transfer coefficient for glazing
• Greener Gardens: K= 0.58 (twin wall polycarbonate 8mm)
  • A= Ft2 exposed glazing (surface area)
• Greener Gardens A= 1820 ft2
  •  T1= Desired inside temperature in Fahrenheit
• Greener Gardens T1= 50°F
  • T2= Outside temperature in Fahrenheit
• Greener Gardens T2= -35°F

Interestingly, on January 17th, 2023, we experienced a low temperature of -35°F while the greenhouse maintained a temperature of nearly 50°F provided by a heating capacity of 60,690 BTU. Theoretically, the heating capacity requirement to maintain 50°F on a -35°F night is 89,726 BTU. This demonstrates that the unique design of the greenhouse offsets the heating capacity requirement by 29,036 BTU.  These findings prompted me to update the greenhouse such that it could support higher electricity more efficiently.

Ventilation is equally as important as heating in a greenhouse. It ensures the plants get the carbon dioxide they need to photosynthesize. Proper ventilation is also necessary for air circulation. The air movement works to cool the climbing temperatures during summer when the sun can radiate as much as 277 BTU/hr/ft2. Up to 85% of that solar radiation can enter the greenhouse.  Most of that infrared radiation becomes trapped heat. Moving as much heat out as possible is critical. The electrical updates also provided the ability to add an exhaust fan to the end of the greenhouse. This gave the greenhouse the capability to meet higher air exchanges per hour which equates to more cooling.

Measuring the cost of heating and cooling is the defining factor of whether these changes warrant the cost to implement them. During the month of August 2023, (the peak month for electric and water use), 324 kWh was used to cool the greenhouse. This came out to a total power expense of $27.95. Water usage reached 2,400 gallons (about the volume of a storage unit) or about 77 gallons a day. In contrast, the average American household uses 300 gallons (about 1135.62 L) per day. If I’m being honest, I was surprised by the efficiency. I didn’t expect the total cost to be under $1/day. The water usage covered sale plants and the greenhouse itself. The humble water use can be accounted for by consistent humidity expressed as lower water needs.  Overall, Greener Gardens Greenhouse’s exceptionally unique design supports its goal of sustainability and community outreach through food donation, and I’m eager to continue building on those key elements.