Section 1: Introduction

Greenhouse glazing materials allow shorter wavelength radiation (i.e. visible light) to pass through but long wavelength radiation such as infrared (heat) is trapped inside the greenhouse. The temperature inside of a greenhouse may be up to 30° F higher than the ambient temperature outside of the greenhouse (hence, the greenhouse effect). Because of this property, greenhouses may require both summer (or during high temperature seasons generally) and winter (or during cool seasons generally) cooling systems in order to maintain optimal temperatures for the crops being grown.

In the summer, both high temperatures outside the greenhouse (ambient air temperatures) and solar heating of the air inside of the greenhouse necessitates the use of various cooling systems to remove hot air from inside the greenhouse and to bring in air from outside of the greenhouse. In some cases, if the outside air temperature is appropriate, simply replacing the air inside of the greenhouse with outside air might be adequate to maintain acceptable air temperatures inside of the greenhouse. In other cases, the outside air temperature may be above the optimal temperature for the crop and the air brought from the outside of the greenhouse may be cooled as it enters the greenhouse (to increase the cooling potential). In the winter, cooling may be required because of the solar input. However, since the air temperature outside of the greenhouse is usually lower than the optimal temperature for the crops being grown, exchanging inside air with the outside air is usually adequate to cool the greenhouse. Therefore, the cooling system (or cooling strategies) used in the summer and those used in the winter are often different.

Cooling systems used in both summer and winter may be generally classified into two categories. The first is passive systems. These cooling systems do not use energy to cool the greenhouse. In contrast, active cooling systems use energy in some way to cool the greenhouse.

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 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 a 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:

T_cool=T_out - (% efficiency)(T_out - T_wb)
where: T_cool = temperature of air exiting cooling pad;
T_out = temperature of the outside air;
T_wb=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%.  The figure below 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.

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.

Section 3: Winter Cooling Systems

In some locations, high light levels (high solar input) or fluctuating temperatures might necessitate cooling even during winter days. Additionally, during spring and fall months, heating may be required at night and early mornings while cooling may be required during the day when solar loads are high. Passive venting as discussed above is one method that may be used for this type of cooling need. However, if the solar load is too high, an active cooling system may be required to increase the rate at which warm inside air is replaced with cool air from outside of the greenhouse. In this situation, the roof vent may be closed and fans in the greenhouse walls activated. Louvered vents in the opposite walls open to allow air to move into the greenhouse. The exhaust fans may be multi-speed fans so that just enough air is exhausted from the greenhouse (and replaced with cooler outside air) to maintain the desired temperature. If temperatures continue to increase, the fans' speed can be increased.

Another method used for winter cooling utilizes fans placed in the gable of the greenhouse and combined with a polyethylene tube extended the length of the greenhouse in the gable. The inlet vent is louvered and opens only when the fans turn on. There is an additional set of louvered vents at the opposite end of the greenhouse that allows warm greenhouse air to escape while cooler outside air is forced into the structure. The cool air is forced through the polyethylene tube to allow for a more even distribution of the cool air. In some cases, a unit heater may be used in conjunction with this system. The unit heater is placed in front of the louvered inlet. If heat is needed, the louver and vents are closed. The heater draws in air from within the greenhouse, heats it and forces the warm air through the polyethylene distribution tube. If cooling is needed, the louvered vent and exhaust vent open, and the unit heater fan turns (without ignition of the flame) on to force cool air from outside into the polyethylene tube and into the greenhouse.

Section 4: Calculating Greenhouse Cooling Requirements for a Standard Fan-and-Pad System

To determine specifications for cooling system, the greenhouse volume must be determined, and based upon several assumptions, a flow rate per minute (air exchanges) requirement is determined.

Determining greenhouse volume and airflow requirement
There are several ways of calculating required air flow capacities. This is one method to serve as an example. In this example, cooling specifications are outlined below for a single-span quonset greenhouse that is 30 ft wide and 110 ft in length. Some basic geometric equations allow the determination of the volume of the greenhouse.

The volume of the quonset structure is determined (assuming it is half of a cylinder) as: 0.5(πr2L)

= 0.5[(3.14)(152)(110)]

= 38,857 ft³

An air exchange of 1 to 1.5 times per minute is required. The value of 1.5 would be used if the greenhouse were to be used during summer months where very high solar loads and temperatures will be experienced. In this example, 1 exchange per minute is used so that 38,857ft³/min (cfm) is required.

Determining fan numbers, sizes and specifications
Fans should be spaced not greater than 25 ft apart (center to center as a general rule) and should never be greater than 200 ft from the pad wall. Therefore, this structure requires two fans spaced along the 30 ft wall. The structure will therefore require two fans with a capacity of (38,857cfm/2) 19,429 cfm each. From a list of fan specifications (taken from the table below but available from supply catalogs or manufacturer specifications), two 48-inch, 1 horsepower fans are selected. These fans provide a total of 39,200 cfm which meets the required air flow.

Determining pad wall specifications and dimensions
Aspen pads, 4-inch or 6-inch cellulose pads could be included to create a fan-and-pad cooling system. In this example, 4-inch cellulose pads are included. One square foot of this type of pad will accommodate 250 cfm. Therefore, 39,200 cfm/250 = 157 ft² of pad wall is required. The pad wall should extend the entire length of the way. Therefore, the pad should be 30 ft wide and (157 ft²/30ft) 5.2 ft tall. In reality, a 30ft x 6ft pad wall would be constructed (pads come in heights of 1ft increments).

Determining pump and sump tank specifications and capacities
The pump capacity must take into account the volume of water flow required by the system (pipes and pads) as well as water loss through evaporation from the pads.

To accommodate the system, 0.50 gallons/min are required per linear foot per minute. Therefore, 30 ft x 0.5 gallons/ft = 15 gallons/min to accommodate the system.

To compensate for evaporation as water moves through the pad, 0.05 gallons/min are required per 1000 cfm of airflow. Therefore, (39,200 cfm/1000 gallons/cfm)0.05 = 2 gallons/min required to compensate for evaporation.

The total pump capacity is 15 gallons + 2 gallons = 17 gallons/min.

The sump (water storage tank) capacity is determined as 157 ft² of pad x 0.75 gallons/ft² of pad = 118 gallons.

Section 5: Integration and Staging of Cooling Systems

During spring and fall, temperatures are cool at night and increase during the day due to the solar load. During these seasons, the cooling system may be integrated so that as the temperature reaches a certain set point (set by the greenhouse manager), the roof and side vents open to allow for passive venting. If the temperature continues to increase, the roof and side vents close, the louvered vents opposite the fans open, and the fans turn on to actively bring in cooler air from outside. If the temperature continues to increase, the sump pumps associated with the pads turn on and begin feeding water to the pads to lower the temperature of the air entering the greenhouse and to increase the cooling potential. This cyclic or integrated approach to cooling helps to reduce cooling costs. Two examples of staged cooling are shown below.

• Typical staging of cooling when greenhouse has vents and fan-and-pad system

Roof and side vents open → Roof and side vents close and exhaust fans on low with louvers open → Fans shift to high with louvers open → Fans on high with lovers open and water applied to pads

• Typical staging of cooling when greenhouse has vents and positive pressure cooling

Roof and side vents open → Roof and side vents open and positive pressure unit fans on → Roof vents open, side vents closed and positive pressure cooling on with water applied to pads

© M.R. Evans, 2008, 2009, 2011, 2014