Section 2: Summer Cooling Systems
Passive venting
High summer temperatures and high solar input result in the need for constant heat removal from the greenhouse. This may be partially accomplished by replacing existing air in the greenhouse with air from outside of the greenhouse. In greenhouses with roof and side vents, warm air may be passively exhausted (not using energy) through roof vents. The rising warm air moving out of the roof vent creates a vacuum inside of the greenhouse that causes air from outside of the greenhouse to move into the greenhouse through the side vents. This results in the heated inside air being exchanged with cooler (although still maybe warm or hot) outside air. The cooling potential of this system is limited by the temperature of the outside air. If the outside air temperature is too high, additional cooling systems may need to be employed. This system is most effective in the winter, spring and fall or during the summer in high elevations (i.e. Colorado) and coastal areas (i.e. Half Moon Bay, California) since the outside air temperatures in these locations are cool enough to be effective at cooling the inside of the greenhouse.
Shading Systems
Shading systems are another example of a passive cooling system used in summer months (or the year around in subtropical and tropical climates). These systems may include white latex shading, saran or polypropylene cloth shading or retractable shading systems. They all function by limiting the amount of light energy entering the structure and thus reducing the solar load in the greenhouse (reduce heating of the air in the greenhouse caused by the sun). It must be remembered, however, that shading also reduces the light available for photosynthesis. The reduction in light must not be so great as to reduce light below acceptable levels for the crops being grown. However, typically in the summer, light levels are well above the optimal light levels and enough shading is used to reduce light levels to within the optimal range for the crops being grown. This in turn reduces the solar input into the greenhouse and helps with cooling.
Fan-and-Pad Systems
This is the most common type of active cooling system used in commercial greenhouses. The system takes advantage of the latent heat of evaporation. More specifically, as liquid water evaporates, it absorbs energy from the environment (i.e. surrounding air). This results in a lowering of the temperature of the surrounding air. In a fan-and-pad system, cellulose pads (or pads made of another material) are placed in one wall of the greenhouse and fans are placed in the opposite wall. The fans exhaust air out of the greenhouse. This creates a vacuum inside of the greenhouse and causes air to enter the greenhouse through the pads at the opposite end of the greenhouse. All vents, except for the pad opening, are closed when the fan-and-pad system is in operation. Water is forced through the pads, and as air moves through the wet pads, some of the water absorbs energy (heat) from the air as it evaporates. This results in a cooling of the air as it moves through the pad and into the greenhouse. Therefore, the air entering the greenhouse is actually cooled air (as opposed to outside air at the outside ambient temperature such as in the case with simply venting).
In a fan-and-pad cooling system, water is supplied to the pad from the tank (sump) that serves as a reservoir. A pump (such as a common sump pump) is used to move water from the reservoir to the top of the pads. The water is first supplied to a feed line that runs the length of the pads. Holes in the top of the feed line allow water to be forced out of the line. The water is forced upward, strikes a cover plate and trickles down to the pads. A cover material may be placed on the top of the pad to allow for more even wetting of the pad. The water trickles down through the pad, is collected in a catch basin and is recycled back to the reservoir. Because water evaporates as it passes through the pads (1 gallon per minute can be lost through 100 ft2 of pad on a hot dry day), water must be continuously resupplied to the reservoir. This is accomplished by having a water supply line to the reservoir that is controlled by a floater. The reservoir should have a capacity large enough to hold enough water to fill all pipes and saturate the pads. The water supply system should operate so that the entire pad is kept wet.
The pads need to be properly maintained. Salt buildup and algae growth are the greatest threat to the efficacy and longevity of the pads. As water evaporates, salts accumulate on the pads. These deposits physically block air movement through the pads and prevent uniform wetting. If the water supply is high in salts, a different water source or blended water should be used. Algae can also accumulate on pads. Several biocides can be added to the water to prevent algae growth. Sodium hypochlorite (bleach) may be added at a rate of 1% by volume. This provides a 3-5 parts-per-million Cl- solution. However, the bleach will tend to cause the water pH to increase, and this can damage pads by softening the glue holding together the pad layers. Calcium hypochlorite (i.e. pool bleach) and Agribrom® are preferred biocides for use with a fan-and-pad cooling system.
Because the cooling potential of a fan-and-pad system is dependent on evaporation, the cooling potential of a fan-and-pad system is limited by the relative humidity of the outside air. The higher the relative humidity, the less evaporation that occurs as the air moves through the pads. Therefore, fan-and-pad systems are most effective in locations or times of the year where outside relative humidity is low. During hot and humid summer months, the efficacy of fan-and-pad cooling systems will be greatly reduced. A psychometric chart demonstrates this principle. For example, if the temperature is 30°C (90°F) and the relative humidity is 40%, the air will reach saturation at a temperature of 16°C (66°F). This would be the maximum cooling potential of the fan-and-pad system. If the air temperature is 32°C and the relative humidity of the outside air is 65%, the air becomes saturated at 23°C (73°F) and this would be the maximum cooling potential of the fan-and-pad system. Fan-and-pad systems do not operate at maximum efficiency (provide maximum cooling possible) because the air moves rapidly though the pads. However, this example demonstrates how a high relative humidity depresses the cooling potential of a fan-and-pad system.
If the efficiency of the fan-and-pad cooling system is known, the temperature of the air entering the greenhouse from the pads can be calculated as:
Tcool=Tout - (% efficiency)(Tout - Twb)
where: Tcool = temperature of air exiting cooling pad;
Tout = temperature of the outside air;
Twb=wet bulb temperature of the outside air.
A properly designed, installed and maintained fan-and-pad cooling system may have an efficiency of up to 85%. This figure shows the temperature of the cooled air exiting an 85% efficient evaporative cooling pad as a function of the outside air for several relative humidity values. With an outdoor relative humidity of 50% and air temperature of 32°C (90 °F), the air entering the greenhouse from the pad would be 25°C (76°F). Again, as relative humidity of the outside air increases, the cooling potential decreases. It should be remembered that this is the temperature of the air as it exits the pad into the greenhouse. As the air then moves across the greenhouse, it absorbs energy and the temperature increases. This results in a temperature gradient (lower temperature at the pads and higher temperature at the fans) from the pads to the exhaust fans on the opposite side of the greenhouse.
Another term that is sometimes used when discussing greenhouse temperature, humidity and cooling is vapor pressure deficit (VPD). Vapor pressure deficit is a term used to describe how readily water will evaporate into a surrounding air mass. The higher the VPD, the more readily water will evaporate. The VPD close to a leaf surface is close too, and assumed to be, saturated or at 100% relative humidity. High VPD (low air moisture relative to the leaf surface) can lead to plant wilting and injury if plants are unable to transpire enough water to keep up with the cooling need. This is why under very high temperatures plants may wilt even if the substrate is moist. When the VPD is too low, the surrounding air is so saturated with moisture that little or no water can be evaporated from leaf surfaces. This can result in inadequate water and nutrient transport within the plant. Supplementary cooling such as with fan-and-pad systems lowers the temperature, increases relative humidity, decreases the VPD, reduces water demand and reduces evapotranspiration.
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Positive Pressure Coolers (Swamp Fans)
Positive pressure coolers (also known as swamp fans or swamp coolers) are essentially self-contained fan-and-pad cooling systems with the pads and fan contained within an enclosed unit. The fan used is a squirrel cage fan and is located within the cooling unit. The fan forces air out of the cooling unit and into the greenhouse, thus creating a vacuum inside of the unit and drawing air through the vented sides (from outside the unit) of the unit and into the unit. On the inside of the vents of the cooling unit are water-saturated pads. The air is pulled through the vented sides and through the pads. As the air passes through the pads it is cooled due to evaporation (as with a fan-and-pad system). The cooled air is then forced into the greenhouse by the squirrel cage fan. Sometimes these units are mounted outside the gable of the greenhouse and the cooled air is forced into a polyethylene tube that extends the length of the house. However, usually these units are mounted along the side walls of the greenhouse. Because air is being forced into the greenhouse (the vacuum is inside the cooling unit outside of the greenhouse and not in the greenhouse as with a fan-and-pad system), this type of cooling is often called positive pressure cooling because positive pressure is created inside the greenhouse (fans forcing air into greenhouse). Positive pressure cooling systems are affected by relative humidity in a similar way as fan-and-pad cooling systems.
Fog Cooling Systems
These systems (Mee Fog® systems are common examples) use evaporative cooling just as the fan-and-pad and positive pressure systems. However, with these systems, very small droplets (approximately 0.04 inches in diameter) of water are forced into the air (as a fog). Because of the small size of the droplets, they remain suspended in the air (and thus do not wet the plant material). The droplets evaporate while suspended in the air, thereby cooling the air through evaporation. The water-saturated air is removed from the greenhouse through roof vents or low-volume fans mounted in the greenhouse walls. These systems require some specialized equipment and are most useful for cooling structures used in propagation, seed germination and plug production or where greenhouses are too large for fan-and-pad systems. As with fan-and-pad systems, fog cooling systems are most effective where low relative humidities occur.
