Low Cost Geothermal Greenhouse Heating System for Southern Climates

NOTE: No Final Report available for this project.

• 2010 annual report

Commodities

• Vegetables: cucurbits, eggplant, peppers, tomatoes
• Additional Plants: herbs

Practices

• Education and Training: demonstration, on-farm/ranch research
• Energy: energy conservation/efficiency
• Farm Business Management: community-supported agriculture, marketing management

The optimal minimal night time temperature for growing greenhouse tomatoes is about 60o F, but it is known that maintaining bottom heat is most important (keeping the root system at 60o F is more critical than maintaining leaf temperature). Despite the relatively warm winters in South Central Texas, greenhouse heating has been a major limitation to profitability and expansion of our greenhouse tomato operation. Our greenhouse was constructed using state of the art materials supplied by Atlas Greenhouse Systems, Inc. Insulation is provided by inflated double layer polyethylene on the top and side wall. Two 250,000 BTU high efficiency Modine heaters were installed for heating. It has been extremely rare when supplemental heating is needed during the day; in fact, ventilation is required on all except the coldest of days. Night-time heating, however, is normally required beginning in November through March. During the first year of operation, the first cold nights (temperatures dropped to near 20o F) resulted in excessive propane use to maintain temperatures near 60o F (we calculated a cost of $80 for one night based on amount used from propane tank). We added a wood burning stove (approx. 120,000 BTU) to assist with the heating, and lowered the temperature where the propane furnace kicks in, but costs are still in excess of $4,000 per season. We have found that lowering inside temperatures below the 60o optimum results in dramatic yield decreases, especially during the time when tomato prices are at their highest. The rising costs of propane and other fossil fuels are likely to make tomato production unprofitable in the future unless alternative heating sources can be found. Our goal is to discover a sustainable heating system that can be used to maintain optimal growing conditions for greenhouse tomato production in the South. We believe that this problem and our solution fit well into SARE priority category 7: Increasing sustainability of existing forming practices.

Our farm currently has a pond that holds approximately 650,000 gallons of water (approx. 87,500 ft3). We have measured the surface temperature of this water during last winter (late December, 2008 through early January, 2009) and the lowest temperature reading was 60o F. The soil temperature at 5’ depth in our area is also near 60o F during the winter months. If we can manage to increase this water temperature and transfer this heat to the root zone of the plants we should be able to more easily maintain an optimal root zone temperature and we can lower the temperature settings of the other heating system without impact yield or performance of the tomato plants. We should be able to transfer excess heat produced during the day from the greenhouse through the ground and into the pond that will be available for added heat during the night. We have calculated that the maximum flow rate for ½” pipe is 2 gallons per minute, and the maximum flow rate for 2” pipe is 40 gallons per minute. This means that we can use up to 20 runs of ½” pipe Teed between the incoming and outgoing 2” pipe to create an underwater heat exchanger to pull heat from the bottom of our pond. This same heat exchanger can work during the day to transfer the heat captured in the greenhouse to heat the pond water. The same principle will be used in the greenhouse where we will have a total of 18 lateral lines of ½” pipe tied between our 2” header line and 2” return line. The lateral lines will be in row and in direct contact with the pots containing the tomato plants in order to transfer the heat directly to the root zone. If we find that the water reservoir cannot be heated with this configuration, a third heat exchanger can be built along the southern wall of the greenhouse and across the ventilation openings and tied into the circulatory system with a valve to turn it on and off. This third system will be constructed using black plastic pipe to aid in capturing heat from the greenhouse during the day. We will then be able to heat the ground and pond water during the day by capturing excess greenhouse heat, and bring it back into the greenhouse during the night. We have calculated that we will need about 1/3 to ½ hp pump to operate this system; a ½ hp motor will use approximately 10 kwh per 24 hour period, if run continuously. Since our rate is currently ~0.11 per kwh, this system will cost less than $1 per day to operate, if run continuously. We have the potential to save up to $4,000 per season (at current propane rates) if we can replace all of our propane needs with this system, and since we normally have less than 100 heating days per season, this system will cost less than $100 to operate the entire season.

We will record daily temperatures for the pond surface and subsurface water; we will record daily greenhouse minimum and maximum temperature as well as root zone temperatures. We will continue to maintain accurate records of heating costs in terms of dollar amount and amount of propane used. We will also continue to measure yield of crop with weekly harvest data, and use all of this data to compare to historical records that are available for the previous 4 years of operation. We expect to increase the temperature of the pond water high enough to maintain a moderate greenhouse temperature during the night. We expect this system to replace all of our heating needs during the most typical cool nights in our area (outside temperatures from the upper 40’s to mid-50’s), and to dramatically reduce the propane consumption during the colder nights.

• Layout of Geothermal Heating System