|Wills, James - UNIVERSITY OF TENNESSEE|
|Raman, D. Raj - UNIVERSITY OF TENNESSEE|
|Honea, Gary - UNIVERSITY OF TENNESSEE|
|Buschermohle, Michael - UNIVERSITY OF TENNESSEE|
|Straw, Richard - UNIVERSITY OF TENNESSEE|
Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 2, 2005
Publication Date: September 15, 2005
Citation: Ray, S.J., Wills, J.B., Raman, D., Honea, G.S., Buschermohle, M.J., Straw, R.A. 2005. A one year study of a supplemental air distribution system for greenhouses. Applied Engineering in Agriculture. Vol. 21(5): 928-936. Interpretive Summary: Greenhouse tomato production has gained considerable popularity in the past decade due to high-quality fruit and off-season premium prices. However, environmental barriers exist that prevent many producers from obtaining their full yield potential--temperature and humidity gradients within the greenhouse are among the most prevalent barriers. A common method for moving air within a greenhouse is with fans located above the plant canopy that circulate internal air around the greenhouse in a racetrack pattern. As the tomato plants approach maturity, however, airflow within the canopy is inhibited, which induces temperature and humidity gradients. In order to resolve this problem, a system was that forced warmer air from above the canopy through perforated plastic sleeves called poly-tubes, to the lower levels of the greenhouse was compared to a conventional greenhouse during a single one-year trial composed of a spring season and a fall season. During the spring season, temperature and relative humidity gradients were significantly reduced in the experimental greenhouse, primarily during periods when supplemental heat was required. The experimental greenhouse also experienced a 9% reduction in fuel consumption and a 14% increase in yield over the control greenhouse. The net economic benefit of experimental greenhouse for the spring season was slightly less than $1000. However, there were no appreciable environmental differences between the greenhouses during the fall season, and the yields were essentially the same. Because of capital and operating costs, the treatment greenhouse experienced a net economic loss of slightly over $300 in the fall. The difference between fall and spring results is explained by the different relationship between plant size and climate during those seasons; the combination of small plants and cold weather in the spring can potentially benefit the most from this type of air circulation system. Tomato producers can increase their net income by $1000 annually per greenhouse by supplementing the conventional air distribution system with the poly-tube system during the spring season. Further testing under a wider range of conditions is required.
Technical Abstract: The effect of a supplemental air distribution system was evaluated in a greenhouse producing tomatoes (Lycopersicon esculentum Mill cv. Trust). The system used centrifugal fans to force warmer air from above the canopy through a perforated duct extending longitudinally through each double-canopy row, with the goal of improving yield and decreasing energy use. The experiment employed two commercial-size greenhouses, one as the treatment greenhouse with the air distribution system and one as the control using conventional air handling; the experiment duration was two growing seasons (spring crop and fall crop). Temperature and relative humidity were monitored at multiple locations throughout both greenhouses, and fuel consumption and electricity usage were tracked. The system significantly reduced vertical temperature and relative humidity gradients while the heater was on during the spring. The heater thus ran less, resulting in a 9% reduction in fuel consumption. The treatment greenhouse also exhibited 14% greater yield than the control house, presumably the treatment house did not suffer the excessively high humidity observed in the control house, because of the air circulation system. The net benefit from the treatment greenhouse was estimated as slightly less than $1000 for the spring season, and the benefit/cost ratio of the system was four to one. However, the benefits observed during the spring did not repeat for the fall season. There were no appreciable environmental differences between the greenhouses, and the yields were essentially the same. Because of capital and operating costs, the treatment greenhouse experienced a net economic loss of slightly over $300 in the fall. The difference between fall and spring results is explained by the different relationship between plant size and climate during those seasons; the combination of small plants and cold weather in the spring can benefit the most from this type of air circulation system. The results from a single spring/fall trial suggest that the system could be valuable to producers, but only in the spring.