Savings through the roof

Natural ventilation saves electricity, by eliminating the use of fans. Along with roll up sidewalls or large endwall doors, roof vents can provide adequate cooling. They can be motorized so temperature within the greenhouse is automatically controlled. They can save as much as 0.5 to 1 kilowatt hour per square foot per year.

Greenhouses with roof and sidewall vents operate on the principal that heat is removed by pressure difference created by wind and temperature gradients. In a well-designed greenhouse, wind speed of 2 to 3 mph provides 80 percent, or more, of the ventilation. Wind passing over the roof creates a vacuum, and draws the heated air out through the vent. If the sidewalls are rolled up or endwall doors open, cool replacement air enters and drops to floor level. If the sidewall vents are closed, cool air enters the bottom of the roof vent and heated air escapes out of the top of the vent.

Bouyancy, the effect of heated air rising, aids ventilation. Heavy cool air becomes hter as it is heated and rises toward the roof. On cool days, a large temperature difference creates excellent air exchange. On hot days, the buoyancy effect is almost nonexistent. When the vents are open, horizontal air flow fans (HAF) should be shut off to avoid destratifying warm air.
 

Vent design

Roof vents and side or endwall vents should each total 15 to 20 percent of the floor area to get adequate air exchange. The best orientation for the greenhouse is to have normal summer wind direction blow over the ridge creating a vacuum on the leeward ridge vent. In warm climates, vents on both sides of the ridge are an advantage.

Most vents are designed using standard rack and pinion hardware, with nylon bearings in the pinion yoke to support the drive shaft. Manual operation can be used but adding a 1/20 horsepower gearmotor and vent controller provides operation when no one is around. With motorized systems, limit switches need to be accurately located and adjusted. Roof vents are expensive costing from $30 - $50/linear foot.

A less expensive vent designed for hoophouses with vertical vent openings is the poly-vent made by Poly-Tex, Castle Rock, Minn. (www.poly-tex.com). This vent uses poly tubes inflated by a blower to close the opening. They are available in both single and two-stage operation controlled by a thermostat. Cost is $10 to $15 per linear foot.

Advantages to roof vents include more uniform cooling as cool air is distributed over the length of the greenhouse. With fan ventilation the air is drawn in through shutters at one end, heating up 5°F to 10°F as it travels the length of the greenhouse before being exhausted at the fan end. Energy costs are reduced as fan ventilation uses from 0.5 to 1 kilowatt hour per square foot per year.

Disadvantages include greater difficulty in covering the greenhouse with poly, as the vent divides cover into two sections and extra extrusions are needed to attach the poly. Some growers cover the vent with polycarbonate to make recovering the greenhouse easier. It is more difficult to install shade cloth as the vent area has to remain unobstructed. And there is more maintenance than with fans. Vents tend to get out of line and may not close tightly. The rack and pinion may slip, or wind may bend the vent frame resulting in a gap when closed.

Weighing the pros and cons is well worth the time and effort before going “au naturel.”

Bartok is a regular contributor to Greenhouse Management and an agricultural engineer and emeritus extension professor at the University of Connecticut. He is an author, consultant and a certified technical service provider doing greenhouse energy audits for USDA grant programs in New England.

Have a question? You can write John at jbartok@rcn.com.