Greenhouse Ventilation and Cooling
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 temperatures; 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.
Wind Driven Ventilation
The amount of cooling achieved by a naturally ventilated greenhouse will depend on wind velocity and direction, greenhouse orientation, the width of the greenhouse, outside air temperature, and any air blockage by surrounding trees, buildings, or adjacent greenhouses. In a well-designed greenhouse, a wind speed of 2 to 3 miles/hour provides 80 percent or more of the maximum possible ventilation. The wind blowing outside the greenhouse creates small pressure differences between the greenhouse’s windward and leeward sides, causing air to move towards the leeward side.
Buoyancy Driven Ventilation
Temperature and humidity differences between the inside and outside of a greenhouse produce forces that drive flow. The natural tendency for hot and humid air to rise and accumulate towards the upper part of a space leads to stable stratification, and this has a significant influence on the flow patterns within the greenhouse. The determining factor in the form of vertical stratification is the location of the openings.
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 (4.3m), and taller greenhouses are used for some crops.
Types of Greenhouse Venting Systems
There are several options to choose from when designing the 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 sidewall 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 (Figure 6.2) or as hinged vents (Figure 6.3). Sidewall vents are typically 2 to 3 feet (0.6–0.9m) tall and 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 obstructions 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.
Ridge Vents
Sidewall vents paired with the roof or ridge ventilation may increase airflow through a greenhouse (Figure 6.4). 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.
Open-Panel Greenhouses
Open-panel greenhouse designs, which allow for natural ventilation, rely on wind pressure and temperature differences to remove heated air and introduce fresh air, often through roof vents and sidewall openings, allowing for maximum control of humidity and ambient temperature (Figure 6.5). The design offers full plant exposure to the natural light entering the greenhouse and is ideal for hardening crops.
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