Developing a Cost Effective, Energy Efficient Greenhouse Using Solar Heating to Extend the Growing Season

Final Report for ONE09-104

Project Type: Partnership
Funds awarded in 2009: $6,960.00
Projected End Date: 12/31/2011
Region: Northeast
State: Maryland
Project Leader:
Willie Lantz
University of Maryland Extension
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Project Information

Summary:

Year-round availability of locally grown food and food processing infrastructure are the greatest barriers to increasing the consumption of local food in western Maryland. 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. The overall objective of the project was to lengthen the growing season using an economical and sustainable method of heating a greenhouse type structure that would allow farmers to produce local foods for extended periods of time. The goals of the project were:
1. Reduce greenhouse heat loss with economically available greenhouse construction materials.
2. Test commercially available hot water solar heating panels designed for swimming pools as a method of heating the greenhouse. A greenhouse was constructed using materials that reduced the amount of energy loss and solar water heating panels were added to reduce the amount of energy required from fossil fuels. The energy efficient greenhouse green house and solar heating system reduced the amount of propane by over 200 gallons. The system worked effectively throughout the fall, late winter and early spring. The system provided many days heating without the use of propane heat. The system provided a majority of the heat in October and November. The system also reduced the amount of propane required to maintain 50 degrees F during April and May.

Introduction:

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 combine interactive robotics programming and real-world problem solving. The 2009 team’s directive was to address a problem with climate in their community. 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 consumption 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 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 a 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 are able to heat 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.

Project Objectives:

The overall objective of the project was to lengthen the growing season using an economical and sustainable method of heating a greenhouse type structure that would allow farmers to produce local foods for extended periods of time. Solar hot water and geothermal earth tubes where examined as methods of providing supplemental heat for the greenhouse. While providing alternative non-fossil fuel heating is an important part of this project, constructing a greenhouse that reduces the amount of heat loss is equally as important. Greenhouse design and materials were evaluated to determine what type of greenhouse to construct for the project. The goals of the project were: 1. Reduce greenhouse heat loss with economically available greenhouse construction materials. 2. Test commercially available hot water solar heating panels designed for swimming pools as a method of heating the greenhouse.

Cooperators

Click linked name(s) to expand
  • Phil Malone
  • Harry Swartz

Research

Materials and methods:

A greenhouse design that was selected has a single south sloping roof. The greenhouse is also 3 feet below ground level and has an insulated north wall. The below ground portion of the greenhouse was construct using insulated concrete forms. (see Pic1 attached) The forms are made of 3 inch think polystyrene foam on the outside with steel connectors and are filled with 6” of concrete. The forms are 16” tall and 4’ long and are stacked and filed with the concrete. The total width of the wall is twelve inches and has an R-value of 30. Three sides of the greenhouse were formed with 4’ of concrete forms and the north wall is 5’4”. A galvanized car port frame was purchased to form the structure for the greenhouse. The greenhouse is 23’ X 25’. The car port frame was reconfigured to form a 2’ high front wall (6’ total with the concrete forms), a flat south facing roof and a 10’ high rear wall. Double wall 8 mil polycarbonate was selected for the roof and above ground side walls of the greenhouse as a glazing. A spread sheet was developed to calculate the heat loss from the greenhouse. (see Figure 6 attached)

Low-cost plastic solar panels used to heat swimming pools were donated to the project from FAFCO Solar Water Heating, Inc. 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. These panels are readily available and can be purchased by anyone desiring to do a solar heating system. Five 4’ X 8’ panels were erected facing due south at 36 degrees to maximize winter solar energy collection for our latitude. (see Picture 2 attached) The solar panels are connected to an 800 gallon underground storage tank that is adjacent to the greenhouse. A 750 gallon per hour fountain pump submerged in the tank is used to pump the water through the solar panels. The solar panels are plumbed to allow for complete drainage if the system is not operating which is an important feature for winter time operation.

To distribute heat throughout the greenhouse a second fountain pump is used to pump tank water through two pickup truck radiators. (see Picture 3 attached) These truck radiators serve as an economical heat exchanger. A simple box style fan, located behind the radiator, is used to move the heat throughout the greenhouse.

Fresh air is provided to the greenhouse through two 6” diameter earth tubes. The earth tubes are constructed of 150 feet of 6” corrugated drainage pipe. A four foot deep trench was dug for the drainage pipe and then the area was backfilled an additional 4’ to bury the earth tube 8’ deep. The external end of the earth tube was connected to a 4’ long pvc stand pipe and covered with a nursery container to keep out critters. A 250 cfm duct fan was placed in line with the earth tube in the greenhouse to pull air through the earth tube.

A microprocessor control system was developed by Phil Malone to control the operation of the system. Similar systems are used to control geothermal home heating systems. The control system is composed of sensors and electrical outlets which components such as fans and pumps plug into. (see Picture 4 attached) Temperature sensors are located in the water tank (top and bottom), in the return line from the solar panel, on the surface of the solar panel, outside the greenhouse, inside the greenhouse and inside the earth tube in the greenhouse. (see Picture 5 attached) These sensors feed information to the microprocessor. A flow meter is also connected to the return line from the solar panel. The control system turns on the pump to the solar panel whenever the temperature of the solar panel is greater than the temperature of the tank water by two degree F and turns it off when the temperature drops below the tank temperature. The controller also turns on the pump for the truck radiator heat exchanger whenever the temperature in the greenhouse drops below the desired set point and if the tank temperature is warmer than the greenhouse temperature.

A back up propane heater is connected to a simple manual thermostat which is set 2-3 degrees below the inside greenhouse temperature for the solar heating system. The earth tubes are turned on and off periodically throughout the day based on time intervals. The microprocessor controller can be connected directly to a laptop or remotely through the internet to change the set points on various controls. A long range wireless router is used to connect the greenhouse to a residential internet connection. The greenhouse is located 500 feet from the house however it must be in a direct line of sight with the router in the house. For this project the microprocessor was also developed to send data to a web energy data logging system which provides live data on the internet as well as automatically updated charts. The address for the website is http://www.welserver.com/WEL0004.

The greenhouse was constructed in the summer and fall of 2009. The solar panels were installed in the fall of 2009. The solar water heating system was not functional until the spring 2010. The system was tested during the spring of 2010. The fully functional system began operation in the October of 2010. A full year of data was collected from January through December of 2011.

Research results and discussion:

From the spreadsheets developed, the heat loss estimated from the “energy efficient” greenhouse is estimated at 31,962,447 BTU per year. The heat loss was based on the number of night hours in a given month and the average nighttime temperature assuming an internal greenhouse temperature of 55 degrees F. An estimated 348 gallon of propane would be needed to heat this greenhouse. The “energy efficient greenhouse” uses less heat when compared to a traditional hoop style double poly greenhouse of the same size which requires 44,835,451 BTU/year or 486 gallon of propane based on the construction materials.

The solar water heating panels worked very well to heat water in the storage tank. During sunny days even when the temperatures outside were cold (20-40 degrees F), the solar panel temperatures were often over 100 degrees F. The temperature of the water in the 800 gallon tank was often raised by 20 degrees F in a single day. Based on flow meter readings and the increase of the tank temperature the solar panels were able to create over 150,000 BTU on many sunny days. This is equivalent to nearly 2 gallon of liquid propane per day. With three to five sunny days the water tank temperature would often increase to around 100oF. This amount of heat reserve is able to heat the greenhouse for 3 to 4 days of cloudy days with daytime highs of 40 degrees F and nighttime lows of 20 degrees F. See the November Temperature Chart (see Figure 7 attached) for a documented example.

Research conclusions:

The energy efficient greenhouse green house and solar heating system reduced the amount of propane by over 200 gallon. The system worked effectively throughout the fall, late winter and early spring. The system provided many days heating without the use of propane heat. The system provided a majority of the heat in October and November. The system also reduced the amount of propane required to maintain 50 degrees F during April and May.

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary:

Education/outreach description:

A field day was held at the greenhouse in the fall of 2011. (see Fig8 attached) The field day was attended by 30 individuals. The project has also been visited by several individuals including a group of extension agents and producers from West Virginia University. A presentation about the project was made at the 2011 Mt. Top Fruit and Vegetable Conference and at the West Virginia Small Farm Conference. An estimated 60 individuals attended these presentations.

An abstract for a poster and professional development presentation has been submitted to the 2012 National Association of County Agriculture Agents Conference in July 2012. Information about the project is available on the internet at http://www.fiveacesbreeding.com/ambient_energy_harvesting.

Project Outcomes

Project outcomes:

The cost of the energy efficient greenhouse with the solar water heating system was compared to the cost of a traditional hoop style greenhouse. The conventional greenhouse has a construction cost of $3,000. This house will also require the plastic to be replaced once every four years at the cost of $200. The yearly cost of ownership for the conventional greenhouse given a life of 15 years equals $250 per year.

The energy efficient greenhouse has a cost of $7,000. Given a 15 year life the yearly cost of ownership equals $466. The conventional house requires 486 gallon of propane for winter heating which at the cost of $2.40 per gallon equals a yearly cost of $1166. The energy efficient greenhouse would require 348 gallon of propane, which would equal a yearly cost of $835. The total year cost of the energy efficient greenhouse is $115 less than the conventional greenhouse. The reduction in heating cost of $331 per year would pay for the additional cost ($4,000) to construct the energy efficient greenhouse in 12 years.

The solar water heating system has a cost of $2,000 for our project house. The cost includes the solar panels, panel support, pumps, water tank, control system, radiators and necessary electrical and plumbing. The cost does not include installation. The solar heating system was operated from February 1, 2011 until December 31, 2011. A total of nearly 6 million BTU of heat was utilized from the solar heating system which is equal to 66 gallon of propane or $158.40 at $2.40 per gallon. The greenhouse did require an additional 128 gallon of propane to maintain a minimum of 50 degrees F. The fuel savings would pay for the solar heating system in 12.5 years in our project greenhouse.

Farmer Adoption

The information from the project is available to farmers through the internet. Fruit and vegetable producers have shown interest in the energy efficient greenhouse and the solar heating system. Additional work needs to be done to configure the system to a larger sized high tunnel. As demand increase for local food, the project will provide valuable information for producers who want to increase the growing season. In western Maryland a group of farmers have formed a fruit and vegetable marketing cooperative, Garrett Growers. Garrett Growers marketed fruit and vegetables to restaurants and grocery stores in 2011. With a method to market local foods through outlets that are open year round, the potential exists to market local foods earlier and later in the year increasing the interest in methods of extending the growing season.

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

The energy efficient greenhouse and solar water heating system was trialed on a small greenhouse for the purpose of determining if it would be economically feasible. With positive results from the project, work needs to be done to scale up the project to a commercial sized greenhouse or high tunnel. While the use of energy efficient materials reduces the use of energy, on a large scale the initial cost of construction may deter producers from construction a greenhouse of this style. Therefore, using the solar heating system with traditional hoop style greenhouse/high tunnels would need to be examined.

The project’s solar heating system used 160 square feet of solar panels to heat 800 gallon of water. On many occasions the temperature of the water would be over 100 degrees F after two or three days of sunny weather. Since the solar panel pump was not set to run until the temperature of the solar panels was higher than tank temperature, the system could not collect heat for many hours during some sunny days. Increasing the size of the water tank would allow the system to collect more heat without warming the temperature of the water to the point where the system does not run on sunny days. More work needs to be done to determine the size of the water tank needed to the square feet of solar panels required.

The system in the project used radiators to transfer the heat from the tank to the greenhouse. The system heated the entire area of the greenhouse. With commercial greenhouses or high tunnels, the heat could be delivered to the plant rows. The plants could be covered during the night to reduce the amount of area in the greenhouse/high tunnel that needs to be heated therefore reducing the amount of heat required to maintain a minimum temperature at the plant zone.

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.