The greenhouse environment is designed to maximize solar energy intake, which is beneficial for plant growth during cooler months. However, this efficient trapping of heat, often called the greenhouse effect, can quickly lead to overheating when outside temperatures rise in the summer. Uncontrolled high temperatures can cause plant stress, leaf burn, reduced flowering, and even plant death. Managing the internal climate requires a multi-pronged approach addressing air movement, solar radiation, and thermodynamic principles to maintain a healthy growing environment.
Maximizing Airflow and Ventilation
Controlling the internal temperature begins with managing air exchange, involving both passive and active systems. Passive ventilation relies on the principle of thermal buoyancy, known as the stack effect. This system uses low-level intake vents and high-level exhaust vents, typically located on the roof ridge. This setup allows heated, less dense air to naturally rise and escape, pulling cooler, denser air in from below.
While the stack effect provides constant air circulation, active ventilation is necessary for rapid temperature drops during peak heat. Active systems rely on powerful exhaust fans paired with motorized intake shutters located on the opposite wall. Fan capacity is measured in Cubic Feet per Minute (CFM). For effective summer cooling, the entire volume of air should be exchanged at least once per minute. Calculating the required CFM involves determining the greenhouse’s total cubic footage, and the fan capacity must meet or exceed this volume per minute.
The strategic placement of the exhaust fan and intake shutters creates a cross-ventilation path. This forces a continuous, uniform flow of air across the crop canopy, ensuring heat and moisture are removed consistently. Internal circulation fans should also be used to move air within the canopy, preventing stagnant pockets of humid, heated air that can lead to fungal diseases and heat stress.
Reducing Heat Intake Through Shading
An effective cooling strategy includes measures that reduce the amount of solar energy entering the structure. Shading works by intercepting solar radiation, reducing the structure’s overall solar load before it converts to heat. This is a passive method of temperature control.
The most common method uses shade cloth, a material rated by the percentage of sunlight it blocks. For general-purpose greenhouses, a density between 30% and 50% is often suitable, balancing light reduction with the plant’s need for photosynthesis.
Shading materials can be woven, knitted, or made with reflective materials like aluminized fabric. Positioning the shade cloth externally, above the glazing, prevents heat from building up between the cloth and the roof material. Internal screens are less effective because they allow heat to enter the structure before interception.
An alternative approach involves applying shading compounds, such as specialized whitewash, directly onto the exterior of the glazing. These temporary coatings scatter and reflect sunlight. They are designed to gradually wear off over the summer season and can be manually removed afterward to restore maximum light transmission for winter.
Utilizing Evaporative Cooling
When ventilation and shading are insufficient, evaporative cooling introduces water into the air using a thermodynamic principle. This process converts sensible heat into latent heat—the energy consumed when water changes phase from liquid to vapor. This energy is drawn directly from the surrounding air, resulting in a temperature drop.
One common application is the fan-and-pad system, often called a swamp cooler, which is highly efficient in hot, dry climates. This system uses exhaust fans to pull outside air through thick, porous pads saturated with water. As air passes through the wet pads, water evaporates, cooling the air before it enters the growing space.
Another method involves high-pressure misting or fogging systems that inject extremely fine water droplets directly into the air. Because the droplets are small, they remain suspended and evaporate rapidly before settling on the plants, cooling the air without wetting the foliage.
The effectiveness of evaporative cooling is limited by the existing humidity. In environments already high in relative humidity, less water can evaporate, significantly reducing the system’s cooling potential. This method is most effective in arid and semi-arid regions.