Chapter 3

Greenhouse Ventilation and Cooling

Natural Ventilation Systems

A natural greenhouse ventilation system is a system that has no powered fans but instead relies on the principle that heat is removed by a pressure difference created by wind and thermal buoyancy for air movement. The concept of thermal buoyancy is based on the physical properties of air; as air is heated it has the natural tendency to rise. Most greenhouses utilize natural ventilation by implementing a series of ridge (roof) and sidewall vents. As the temperature rises within the greenhouse the hot air rises and escapes through the ridge vents. This process creates a vacuum that draws cooler air into the greenhouse through the sidewall vents located closer to the ground. Wind plays an important role in passive ventilation systems as well. Wind passing over the ridge vents on a greenhouse can create the same vacuum effect and draw fresh air into the sidewall vents. A greenhouse positioned so the prevailing wind blows over the ridge vents will generally be more efficient in terms of natural ventilation. Natural ventilation methods are becoming more popular because of increased energy costs. Although fans and fan and pad cooling give more positive control over greenhouse ventilation, the cost of electricity is forcing growers to consider natural ventilation.

Buoyancy Driven Ventilation

Temperature differences between the greenhouse interior and exterior or within the greenhouse cause buoyancy forces, thereby driving the airflow. Natural ventilation takes advantage of this phenomenon when the air temperature within the greenhouse increases to a value which is greater than the outside ambient temperature. The natural tendency for hot and humid air is to rise and accumulate towards the gable leads to temperature gradients in the greenhouse, which has a significant influence on the air flow patterns within the greenhouse. Vertical openings at the top of a single span greenhouse will allow exchange of warm and humid air outwards and cool dryer air inwards.

Wind Driven Ventilation

The “wind effect” is wind blowing outside the greenhouse creates small pressure differences between the windward and leeward side of the greenhouse causing air to move towards the leeward side. All that is needed are (strategically located) inlet and outlet openings, vent window motors, and electricity to operate the motors. Wind-driven ventilation is most effective when greenhouse vents are oriented to take advantage of prevailing winds. The wind effect is very small if vents on the roof are not complemented with sidewall openings.

Greenhouse Vents

Some greenhouses can be ventilated using side and ridge vents, which run the full length of the house and can be opened as needed to provide the desired temperature. Greenhouses with roof (ridge) 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 percent or more of the ventilation.  Wind passing over the roof creates a vacuum and sucks the heated air out the vent.

Roof (Ridge) Vents

Many different arrangements of vents have been used in greenhouses (See Figure 3.1). Roof vents can be classified under the type headings of continuous or non-continuous and either fixed (ridge) or fully automatic (open-roof greenhouse systems). Open-roof vents include open-panel, retractable-film, flat-roof, and low-profile systems. Open-panel systems use the entire greenhouse roof like a ridge-vent. Retractable-film systems “fold” the glazing as the glazing is retracted.

Sidewall Vents

Sidewall ventilation may be installed as roll-up sidewall curtains or as hinged vents (See Figure 3.2). Sidewall vents are typically two to three feet tall, installed at the ground level. Because natural ventilation relies on wind or pressure differences, sidewall vents should be installed on the length walls of a greenhouse and on walls free from outside obstruction such as vegetation or other buildings. It is optimal to design new greenhouses to allow sidewall vents to face normal summer wind so that wind naturally flows into the greenhouse.

Size of Vents

American Society of Agricultural and Biological Engineers standards recommend that that the combined roof vent area should equal the combined sidewall vent area and each should be at least 15 to 20 percent of the floor area.

Vent Location

The typical location of roof vents is hinged on both sides of the ridge. This allows operation so that the leeward vents can be opened to create a vacuum at the top of the ridge. Sidewall vents are usually located at bench height.

Vent Operation

With little or no wind both roof and sidewall vents can be opened on the leeward side of the greenhouse. The flow of air over this vent creates a partial suction that helps to pull air from the greenhouse. The opening of the vent facing the wind (windward) is delayed until the ventilation requirements are higher. If the wind is very strong the vents facing the wind should not be opened all the way opens, or not all if the wind is very strong. If the weather forecast predicts strong winds due to thunderstorm or gales, the ridge vents should be closed completely. It is necessary to monitor conditions with an anemometer, vane and rain detector to close the vents in case of excessive winds (depending on their direction) or in case of rain.

Insect-proof Screens for Vents

Most greenhouses equipped with ventilation openings unfortunately, these vents serve also as a major port of entry for pests and, as a consequence, growers are forced to cover the vents completely and permanently with fine mesh screens to prevent pest invasion. Since the pests can be very small (e.g., whiteflies and thrips), very fine mesh screens are required to prevent their entry; these screens impede ventilation and, in some cases, reduce light transmission. These screens reduce the air flowing through the vent, notably decreasing ventilation, so the opening area ratio must be increased or a device with a higher screen surface than that of the vent itself must be adopted.

Greenhouse Height

The trend toward taller greenhouses has helped ventilation because it increases the buoyancy effect and gets the hot air higher above the plants. It also provides buffering of the air and reduces quick changes in temperature.

Orientation of the Greenhouse

Some greenhouses can be ventilated using side and ridge vents, which run the full length of the house and can be opened as needed to provide the desired temperature. Greenhouses with roof (ridge) 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 percent or more of the ventilation.  Wind passing over the roof creates a vacuum and sucks the heated air out the vent.

Greenhouse Sun Screens

A potential problem of passively cooled greenhouses arises when sun screens are desired. These are necessary for crops other than bedding plants grown in the warmer months of the year when flower scorch is a threat.

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