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 and winter cooling systems.

Summer Cooling Systems

Passive venting

High summer temperatures result in the need for constant heat removal from the greenhouse. This may be accomplished by replacing existing air in the greenhouse with cooler air from outside of the structure. If outside temperatures are low enough, and if temperatures in the greenhouses are not too high, warm air may be passively exhausted through roof vents. The upward and outward movement of warm air pulls in cool air from side or end vents. This system is most effective in the winter, spring and fall. It is limited in its effectiveness for summer cooling to high elevations (i.e. Colorado) and coastal areas (i.e. Half Moon Bay, California) since the incoming solar load and the outside air temperature may be too high for the capabilities of this system during summer months in most other climates.

Shading Systems

Shading systems are another example of a passive cooling system used in summer months (or the year around in tropical climates). These systems may include white latex shading, saran 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.

Fan-and-Pad System

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.

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.

Pads need to be properly maintained. Salt buildup and algae growth are the greatest threat to the 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, 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.

Positive Pressure Coolers (Swamp fans)

Positive pressure 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 into the unit. On the inside of the vents of the cooling unit are water-saturated pads. The air is pulled through these pads as it enters the cooling unit and the air is cooled. 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. Because air is being forced into the greenhouse (the vacuum is inside the cooing unit), this type of cooling is often called positive pressure cooling because positive pressure is created inside the greenhouse (fans forcing air into greenhouse).

Fog Cooling Systems

These systems (Mee Fog® systems are common examples) use evaporative cooling just as the fan-and-pad system. However, with these systems, very small droplets (approximately 0.04 inches in diameter) of water are forced into the air. 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 slowly 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. Outside of these situations, fog-cooling systems are not commonly used in the U.S., but they are common in Canada and Europe.

Winter Cooling Systems

In some areas of the country, high light levels or fluctuating temperatures might necessitate cooling even during winter days. Additionally, during early spring and late fall months, heating may be required at night and early mornings while some 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 top 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 fans may be multi-speed fans so that just enough air is exhaust from the greenhouse (an replaced with outside air) to maintain the desired temperature. If temperatures continue to increase, the fan 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 form outside into the polyethylene tube.

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.

As example, cooling specifications are outlined below for a quonset greenhouse that is 30 ft wide and 110 ft in length.

The volume of the structure is determined 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.

Fans should be space not greater than 25 ft apart. Therefore, this structure requires two fans space along the 30 ft wall. The structure will therefore require two fans with a capacity of (38,857cfm/2) 19,429 cfm. From the list of fan specifications, two 48-inch, 1 horsepower fans are selected. These fans provide a total of 39,200 cfm.

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.

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

To compensate for evaporation, 0.05 gallons are required per 1000 cfm of airflow. Therefore, (39,200 cfm/1000 gallons/cfm)0.05 = 2 gallons required to compensate for evaporation.

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

The sump capacity is determine as 157 ft² of pad x 0.75 gallons/ft² of pad = 118 gallons.

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 helps to reduce energy costs.

Read the article "Fan and Pad Greenhouse Evaporative Cooling Systems"

Read the article "Fans for Greenhouses"

Read the article "Auxiliary Power Units for Greenhouse Operations"

© 2005, M.R. Evans