Developing a Cost Effective, Energy Efficient Greenhouse Using Solar Heating to Extend the Growing Season
The inspiration for this project came from the eight team members of the Southern Middle School FIRST Lego League (FLL) in Garrett County. FIRST Lego League is a competition for elementary and middle school students that combines interactive robotics programming and real-world problem solving. This year the team’s directive was to address a problem with climate. The students researched how Garrett County’s short growing season negatively impacts local farmers. Given its altitude, mountainous climate (horticultural zone 5) and terrain, Garrett County’s growing season generally lasts four to five months. High tunnels and greenhouses help famers lengthen that season. At a maximum, locally grown foods are only available from June into October.
Year-round availability of locally grown food and food processing infrastructure are the greatest barriers to increasing the production of local food in Garrett County. With local foods being available for only four to five months per year, restaurants and grocery stores are less inclined to purchase locally grown foods. If an economical, year-round or extended food production system could be developed, restaurants and grocery stores would be more likely to purchase local foods. If local foods were produced on a more consistent basis, food processors would also be more likely to develop processing facilities.
Producing food in the winter months would require the use of a heated greenhouse. Heating a greenhouse is prohibitively expensive if using fossil fuels. With the help of local educators, scientists, and other technical advisors, the FLL team proposed a solution for local growers. They proposed extending the growing season by utilizing solar and geothermal energy to heat our greenhouse. We are pleased to announce that the judges at the FIRST Lego League Pennsylvania Championship Tournament were impressed with the team’s ideas. The team’s research won first place out of 72 competing teams. By using these renewable resources, we would be heating our greenhouse in a sustainable and environmentally friendly way and maximizing local food production for this climate. Heating the greenhouse could potentially allow us to grow and harvest fresh produce from March through December.
Increasing local food production will translate into increased earning potential and increased sustainability of small farms. In addition, we would be providing fresh, flavorful, and healthful foods for the local people in our region. The average supermarket fruit or vegetable travels great distances from where it was produced. This transportation requires a large quantity of fossil fuel, increases air pollution, and often delivers a lower quality product compared to locally grown food. If this renewable-energy greenhouse model was adopted by farmers throughout the region and beyond, it would have three main benefits: 1) increased income and quality of life for farmers, 2) increased access to locally grown, healthier food for local people, and 3) the reduction of fossil fuel usage and dependence.
We are proposing to lengthen the growing season by using active solar energy collection and geothermal moderation to heat our greenhouse. Don Bustos, a SARE grant recipient in New Mexico, used a similar approach to heat his greenhouse. He installed a root-zone thermal heating system using recycled solar collectors, which he placed facing due south at a 45-degree angle to maximize exposure to the winter sun. His panels generated enough heat to raise a glycol/water mix in the system to about 200 degrees. The heating fluid runs through a closed-loop copper pipe system to an underground storage tank. The heat exchanger raises the tank’s water temperature to 180 degrees. The water is then circulated through the greenhouse beds in plastic tubes, raising the soil temperature to 48 to 52 degrees.
Our project would differ from Mr. Bustos’ in several ways. To collect solar radiation, we will use low-cost plastic solar panels normally used to heat swimming pools (the solar panels will be donated to the project from Fafco Solar, Cape Coral) These solar panels distribute water through small corrugated channels made of black polyethylene material. The panels are light weight (a 4’X 8’ panel filled with water weighs 50 lbs) and have an excellent efficiency of transferring heat. The panels are designed to heat large volumes of water compared to traditional solar panels, which heat small volumes of quickly re-circulated water. These panels are readily available and could be purchased by anyone desiring to do a solar heating system. Our solar panels will face due south at 36 degrees to maximize winter solar energy collection for our latitude. These solar panels will be connected to an underground storage tank located within the footprint of the greenhouse.
To deliver the collected heat, we will also place solar panels on the workbenches inside the greenhouse to act as radiant surfaces. Warm water will be circulated from the storage tank to these radiators. A spring crop of strawberry plants will be grown in containers placed on top of the radiators (solar panels) so their roots will be warmed directly by the circulating warm water. A fall crop of strawberries will be grown in vertical hanging bags. In addition, Plastic tubing with warm water from the storage tank will be circulated through the soil in planting beds within the greenhouse.
We will not use a glycol/water solution with this pool solar panel system to ensure that no glycol solution ever makes contact with our plants. We will use water alone to circulate through the system. To protect our panels from freezing temperatures, we will install a “drain-back” system that will allow us to remove the water from the panels as necessary.
In addition, we will pre-heat the air entering the greenhouse using an “earth-tube” (geothermal) system. Four-inch corrugated plastic pipes will be used to bring fresh air into the greenhouse. These pipes will be buried underground. One end of the pipe will be vented to the outside and the other end will terminate at the greenhouse. A fan will be mounted at the greenhouse end of each tube to pull air through the pipes. As the outside air travels through the buried pipes, its temperature will be moderated (heated in winter and cooled in the summer) to approximately 50 degrees, which will then be circulated within the greenhouse. Several different lengths of pipe will be used to determine the most effective length.
Don Bustos’ research demonstrated that it is possible to capture solar energy to heat plant beds to 48 to 52 degrees during cool weather. We will build on that information by collecting data on how well this pool solar panel system works to help raise the plant bed temperature in our greenhouse. Data gathered from this project will help us to create a model that will help other farmers in the Northeast region of the country to determine how pool solar panels will perform during various temperatures and weather conditions as well as how many panels would be needed to generate enough solar energy to heat beds within different sized greenhouses.
A 20’ by 22’ energy efficient greenhouse was constructed during the summer of 2009. The greenhouse was built into a south facing slope. The bottom 4’ of the front and sides as well as the bottom 6’ of the back wall were constructed using insulated concrete forms. The forms were coated with epoxy and back filled with soil. The remaining portion of the back wall was constructed of plywood and insulation. The rest of the greenhouse was covered with 8mm twin wall polycarbonate.
A solar water heating system consisting of four FAFCO swimming pool solar panels each 4’ X 8’ were located south facing and at a 65 degree angle near the greenhouse. A 4’ X 8’ water tank was constructing using insulated concrete forms adjacent to the greenhouse. Water from the tank will be circulated through the solar panels using a small water pump. Three earth tubes were also installed. The tubes are 6” diameter corrugated drain pipe and range in length from 100 – 150’ long. The earth tubes are connected to a small fan that draws air from the tube into the greenhouse. The ends of the tube are located in a wooded area near the greenhouse. The tubes will be used for bring in warmer fresh air during the winter and cooler air during the summer.
All of the electrical components including a variety of temperature sensors are connected to a computer module that is connected to the internet. The system can be controlled remotely through the internet. Data is also logged and can be viewed on the internet at http://welserver.com/WEL0004/. A picture of the greenhouse and solar panels as well as locations and reading of sensors are on the website.
The only remaining part of the system to be installed is the heat distribution system. The original plan was to pump the water through tubes under the benches in the greenhouse. Due to the arrangement of the benches, this has become rather impractical. We are now planning to install a heat exchanger and pump water from the tank through the heat exchanger and have a fan blowing the air into the greenhouse.
Impacts and Contributions/Outcomes
At this point the solar panel system has been tested and we are planning to operate the system this fall and early winter to collect data.
2965 Gorman Rd
Oakland , MD 21550
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