Date of Award


Degree Type


Degree Name

Master of Science


Biosystems Engineering

Major Professor

Michael J. Buschermohle


The effectiveness of a supplemental internal air distribution system for closed greenhouses with dense plant canopies was evaluated. The system employed centrifugal fans and perforated ducts to move relatively warmer air from above the canopy to within the canopy. Preliminary tests demonstrated the potential to improve net profits in commercial tomato greenhouse production by increasing yield and accelerating fruit maturity. It was hypothesized that reductions in temperature and humidity variations contributed to reduced disease susceptibility. The purpose of this project was to determine the validity of the preliminary results by testing the effectiveness of the system in a controlled experiment.

Two 270-m2 greenhouses, a treatment house and an identical control house, were constructed. A fall tomato crop from August to December 2002 and a spring crop from January to July 2003 were evaluated. Electricity and fuel consumption were separately metered for each house. The same variety of beefsteak tomato (Lycopersicon esculentum Mill cv. Trust) at identical growth stages was placed in each house on the same day. Set points for thermostats controlling ventilation and heating were also identical in both houses. Relative humidity, temperature, carbon dioxide concentration, and light intensity were logged in identical locations in each house. All harvested tomatoes were graded and weighed.

During the first crop grown in the newly constructed greenhouses, several start-up problems were addressed in the equipment, structures, and cultural practices. Because of these start-up issues, the environmental and yield results for the fall crop were questionable. In contrast, the spring data was considered reliable.

During the spring season, the bulk of environmental differences between the houses occurred at night. Vertical and longitudinal thermal gradients were significantly less in the treatment house during the first half of the growing season. Reduced vertical and longitudinal relative humidity gradients were also observed in the treatment house during nighttime for most of the growing season. The north side of the control house experienced relative humidity (rh) levels from 95 to 100% from March to the end of the season, while the treatment house generally remained drier, at 90 to 95% rh. An elevated carbon dioxide concentration was found in the treatment house during nighttime hours, which is hypothesized to be due to higher respiration rates. The fuel consumption in the treatment house was reduced by 9%, resulting in a fuel savings of $177, while the electrical consumption followed the opposite trend. The treatment house used 3,550 kWh ($243) more electricity than the control house, primarily to supply power to the supplemental air distribution fans. The treatment house yielded 14 % (518 kg) more marketable fruit than the control. At a wholesale price of $2.20 per kg, the difference in yield returns was $1140 per year (assuming one crop per year). Amortized capital cost of the system was estimated to be $178 per year, based on 5 years at 10%. The net benefit of the system was therefore estimated to be $896 per year.

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