The concept of cooling a greenhouse with thermal bouyancy and wind goes back to the beginning of controlled environment. All greenhouses built prior to the 1950's had some form of vents or louvers that were opened to allow the excess heat to escape and cooler outside air to enter.
When polyethylene was developed with large sheets covering the whole roof, placing vents on the roof proved difficult. Engineers then came up with the concept of using fans that draw outside air through louvers in one endwall and exhaust it out the opposite end. With thermostatic control, this was, and still is the accepted method for cooling many structures where positive air movement is needed.
Proper design of a fan cooled greenhouse is to provide a summer ventilation capacity of 8 – 10 cubic feet of air per square foot of floor space. This will give about one volume air change/minute which will keep the temperature difference between the louver end and the fan end to 7 - 10ºF. Besides the initial investment in a large number of fans, the increasing cost of electricity to run them has growers moving to adopt the natural ventilation system in new greenhouses that they build.
Growers with hoophouses have found that roll-up sides work well for warm season ventilation. Both manual and motorized systems are available. A location with good summer breezes and plenty of space between houses is needed. It helps to have greenhouses designed with a vertical sidewall up to the height of the attachment rail to reduce the amount of rain that can drip in.
Greenhouses with roof and sidewall vents operate on the principle that heat is removed by a pressure difference created by wind and temperature gradients. Wind plays the major role. In a well-designed greenhouse, a wind speed of 2-3 miles/hour provides 80% or more of the ventilation. Wind passing over the roof creates a vacuum and sucks the heated air out the vent. If sidewall vents are open, cool replacement air enters and drops to the floor level. If there are no sidewall vents or if the sidewall vents are closed, cool air enters the bottom of the roof vent and the heated are escapes out the top of the vent. The transition zone between the two moving air streams slows the air movement and reduces cooling somewhat.
Buoyancy, the effect of warm, moist air rising, also aides ventilation. Heavy cool air near the floor becomes lighter as it is heated and rises towards the roof. On cool days the large temperature difference creates excellent air exchange. On hot days the temperature difference can be only 5 or 10 degrees and the buoyancy effect is almost non-existent. The trend toward taller greenhouses has helped in that it gets the hot air higher above the plants. Horizontal air flow fans should be shut off to avoid destratifying the warm air.
Roof and side vents on conventional greenhouses need to be large enough to get good air movement. The American Society of Agricultural & Biological Engineers recommends that the combined sidewall vent area should equal the combined ridge vent area and each should be 15 to 20% of the floor area. The best orientation for the greenhouse is to have the normal summer wind direction blow over the ridge so that it creates a vacuum on the leeward ridge vent. For summer ventilation, the windward sidewall vent opening should equal the leeward ridge vent opening.
Until the development of the open-roof greenhouse concept, cooling large gutter-connected structures was difficult especially in southern climates. Area for sidewall vents is usually limited, and passing cool outside air and warm inside air through the roof vents usually results in uneven cooling.
Open-roof greenhouses are available from most major manufacturers. Most designs use standard vent hardware and controls to operate the roof system. Some have roof panels that are hinged at the gutter and open upward. Others have panels that are hinged at the ridge and one gutter and slide sideways on Teflon bearings. The size of the opening can be controlled from 0% to about 75%. Most designs use rubber gasketing to seal the joints.
Open- roof greenhouses have several advantages.
• During warm weather, the temperature inside the greenhouse can be maintained within a degree or two of outside temperature with little or no energy needed. Many growers have found that this shortens production time and produces a better quality plant.
• In the spring, plants can be hardened off by opening the roof on nice days. This saves considerable labor of moving plants outside.
• Energy costs are reduced. Fan ventilation can use from 0.5 to 1 kilowatt hour/sq ft/year.
• Depending on design and orientation, the crops may receive more light during the middle of the day than in a conventional greenhouse or less light in early morning or late afternoon due to more layers of glazing that it has to pass through. Further research is needed in this area.
• Reduced irrigation due to more uniform temperature and the potential for natural rainfall.
• Adding side vents allows cooling and air movement when high winds or rain prevent the roof from being opened. The guillotine vent, available from a couple of manufacturers eliminates the conventional vent with arms that interfere with inside or outside work area.
To get adequate cooling on hot, sunny days, a shade system may be needed. It should be porous so that the heat generated below can escape up through the shade material. Evaporative cooling, either a fog system or portable evaporative coolers can give added cooling. A large number of hanging baskets tends to reduce natural cooling. Further research is needed to determine air exchange rates and ventilation patterns within open-roof structures.
Continued developments in the design of natural ventilation systems are giving growers better control of temperature and humidity at lower cost. Proper sizing, orientation and operation can provide better control than with fan systems.
John W. Bartok, Jr., Extension Professor Emeritus & Agricultural Engineer, Department of Natural Resources and the Environment, University of Connecticut, Storrs CT - 2013