# Horticulture Principles and Practices

OUTCOMES ASSESSMENT

1. Explain the underlying principles of a greenhouse design.

2. Discuss the advantages of gutter-connected greenhouses.

3. Compare contrast glass, polyethylene, and fiberglass as glazing material for

greenhouse.

4. Discuss the management of heat in greenhouse production.

5. Describe how greenhouse producers manage light for optimal crop growth.

6. In what ways does fertilization of greenhouse plants differ from fertilization of

field crops?

7. Describe how greenhouse design can optimize the use of greenhouse space.

8. The following exercises are designed to help the student better understand greenhouse

energy issues. They were developed by Dr. AJ Both and Dr. David Mears

of the Bioresource Engineering Department of Plant Biology and Pathology

Rutgers University, New Brunswick, NJ:

Heat loss through any portion of a greenhouse structure can be calculated by multiplying

a heat transfer coefficient times the area of the portion of the structure being considered

times the temperature difference between inside and outside. The heat transfer coefficient

depends on the material covering that portion of the structure and to some extent the external

weather conditions. Approximate heat transfer coefficient for various glazing systems

can be found in the reference and generally include a factor to account for some air infiltration

for a reasonably well-sealed greenhouse. The total heat loss for any greenhouse

can be calculated by adding up the heat loss the each portion. In the English systems of

units the heat transfer coefficient is in British thermal units per hour per square foot per

degree Fahrenheit, Btu/(hr*ft 2 *°F) so when coefficient is multiplied by the area and temperature

difference it is the Btu/hr lost through that portion of the structure.

1. Consider a 96-foot long hoop house made by bending 12-foot water pipes into a

semicircle with the roof and ends covered with double-laver polyethylene. If the

heat transfer coefficient for plain double-layer polyethylene is Btu/(hr*ft 2 *°F)

and it is 0°F outside when the crop requires 60°F inside what is the hourly heat

requirement under these conditions?

2. Consider a larger, gutter-connected greenhouse with 7 bays, each 30 feet wide by

210 feet long with 12 feet height to the gutter and 19.5 feet to the ridge. The roof

section, side walls, and gable end walls are all to be glazed with a single layer of

glass with a heat transfer coefficient of Btu/(hr*ft 2 *°F). For the same inside and

outside temperature conditions as in problem 1, what is the total hourly heat

requirement for this structure?

3. To change this greenhouse to double-layer polyethylene on roof and all walls, the

straight rafters are changed to curved bows each one 34.5 feet long. Assuming the

average end wall heights is 14 feet and using the same heat transfer coefficient

and the same temperature conditions as in problem 1, what is the total hourly heat

requirement for this structure?

4. For each of the first three problems, divide the total heat requirement by the floor

area of the greenhouse to compare the heat requirement per unit floor area.

Discuss the differences of the results in problems 1 and 2 and the differences of

the results of problem 2 and 3.

5. For an advanced exercise look up the heat transfer coefficient for double-layer

polyethylene with an infrared heat-absorbing additive in the references or from

an Internet source, and discuss the energy saving possible. Do the same for the

gutter-connected greenhouse with the addition of a movable shade/heat retention

curtain system.

Outcomes Assessment 437

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