IX. REDUCING GREENHOUSE TEMPERATURE

A. temperature regimes– range 50-80 deg, stress above 90 deg

Greenhouse effect

B. Temperature in greenhouse determined by:

1. Solar energy– Greenhouse effect:

• Solar energy- short wave radiation passes through

glass

• Thermal radiation- longer waves trapped

– shading in greenhouse

– time of year

– time of day

2. Exterior ambient temperature

3. Moisture transpired– Temperature in empty greenhouse greater than

in greenhouse with plants

4. Evaporation of water from soil, walls etc.

C. Techniques to reduce temperature

1. Shading - reduce energy absorbeda. Shading compound

• Permanent shade after application• Removed Aug – Sept

b. Shade curtain• Remove during cloudy weather

2. Greenhouse ventilationa. Definition• movement of outside air through

greenhouse

b. Characteristics of ventilation system1) capacity to reduce temperature2) uniform temperature3) automated4) minimum and maximum ventilation

c. Time of year

1) Winter ventilation • Late fall, winter, early spring: • outside air cool • Minimum to maximum ventilation

usually adequate

2) Summer ventilation

1) Temperature control difficult• Shading and ventilation• Maximum ventilation

d. Types of Systems

1) Natural ventilation

–Establish convection currents

• Ridge vent and side vents

• Open area

-40% of floor space

• Removable side wall

• Removable roof

Natural ventilation

Natural ventilation

Natural ventilation

Natural ventilation

2) Motorized Systems - minimum • Mix cold with warm ghs. air• Poly tube system with vent• Low speed fan with vent• One fan or several turned

on, with vent

3) Motorized systems – maximum• Outside directly into

greenhouse • Temperature dependent• Air flow rate

-3/4 to 1 1/4 air ex. /min -7-8° rise

Air exchange vs temperature

e. Control of Systems1) Natural ventilation

Manual

Automatic

Thermostat

Computer

2) Motorized ventilation– thermostats– fan(s) 1 or 2 speed– Vents

• Fully open or closed

• modulating

3) End wall ventilation1) Fans on one end wall2) Vents on other end wall3) Fan spacing 25’4) 1 air exchange/min

4) Side wall ventilation

–problem mixing cold air

–Air exchange rate vs velocity

• 1 exchange 30’ x 100’

-end wall - 100 ft/min

-side wall - 30 ft/min

–Fan spacing- max 25 ft

Side wall ventilation

Natural ventilation