Greenhouse Cooling: Natural Ventilation (2024)

Chapter 5

Natural Ventilation in Greenhouses

Natural or passive ventilation requires very little external energy. With a passive ventilation system, hot air flows out of vents positioned in the greenhouse roof or end walls. Cooler air is brought in through side vents (or other uncontrolled openings) to replace the hot air due to the density difference between air masses of different temperature; the result is a lowering of the greenhouse temperature. Naturally ventilated greenhouses rely primarily on wind pressure to force air in one side of the greenhouse, past the crop, and out through the opposite sidewall or through the roof vents. Wind passing over the ridge vents on a greenhouse can create a “vacuum” effect and draw fresh air into the sidewall vents. A secondary, much smaller effect is that of buoyancy (the effect of hot and/or humid air rising), which becomes most important on hot, still days. Buoyancy operates on the principle that air expands when heated, increasing the pressure in the greenhouse. To release pressure, air is pushed through vents or other openings in the structure and replaced with air pulled or pushed by wind through other openings in the greenhouse. Natural ventilation will work as long as the outside air has a lower temperature or is drier than that in the greenhouse and there is sufficient outside wind velocity. In the summer, temperature differentials are not as large due to the higher outside air temperatures. It is essential for efficient natural ventilation that there is a sufficient number of inlets and multiple outlets for removing warm air above and beyond the plants. For gutter-connected greenhouses, a combination of windward side vents and continuous leeward roof vents will result in the most effective ventilation design. Even for retractable or open-roof designs, an open windward sidewall is very important to achieve good midsummer cooling. Vents can be manually opened, although most systems are automatically raised or lowered with electric motors and integrated with computerized climate control systems.

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.

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.

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. The standard gutter height is now about 14 feet (4.3 m) and taller greenhouses are used for some crops. This also allows space for energy/shade screens, tall crops such as tomatoes and cucumbers and multiple layers of hanging baskets.

Types of Greenhouse Venting Systems

There are several options to choose from when designing your greenhouse with natural ventilation. Natural ventilation uses wall and roof openings for air circulation and exchange rather than motorized fans. This set-up relies on pressure differences created by wind and temperature gradients. Sidewall ventilation may be sufficient, or a combination of side wall and roof ventilation can be used to maximize air flow. It is not recommended to use roof ventilation alone.

Sidewall Vents

Sidewall ventilation may be installed as roll-up sidewall curtains or as hinged vents (See Figure 5.1). 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. Although some curtain systems open up from the bottom (i.e., roll-up), systems that open down from the top (i.e., drop-down) are preferred. When ventilation is needed, the curtains are lowered or raised, which allows cool outside air to flow through the greenhouse.

Ridge Vents

Side wall vents paired with roof or ridge ventilation may increase air flow through a greenhouse (See Figure 5.2). Ridge ventilation can be installed as a retractable, hinged rigid-glazed vent or as a framed-poly film hinged vent. Ridge ventilation can also be installed as a roll-up roof.

Vent Operation

Vents can be opened with a manual crank or with a motorized control. In large greenhouses, motorized control is more efficient and saves labor costs. A thermostat can be connected to the vent system so that it is completely automatic; however, most systems integrate automatic vent operation with a computerized climate control system. Thermostat controls or climate control systems are set at a desired temperature. As the greenhouse warms up, the vent opens to allow the heated air to exit. Many controls have a staged or modulating feature whereby the vent opens only a few inches (5cm) at a time when cooling is first required.

Greenhouse Sun Screens

A potential problem of passively cooled greenhouses arises when sun screens are used. These are necessary for crops other than bedding plants grown in the warmer months of the year when flower scorch is a threat. Loose-weave (open) sun screens should be used to permit passage of air for passive cooling. The open screen can serve a dual role as a thermal screen in warm climates.

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