The greenhouse environment can quickly become too hot for healthy plant production, even in the winter. During the summer months, extra cooling beyond what fan ventilation can provide may be necessary.
Evaporative cooling is a tool that can be used to reduce greenhouse temperatures. A “fan and pad” system uses exhaust fans to pull air through evaporative cooling pads. This technique utilizes the cooling effect produced when water evaporates and cools the air as it is pulled through the pad.
Cooling pads cool by creating large amounts of surface area that are exposed to air being drawn into the greenhouse by exhaust fans. This large surface area allows for quick evaporation. Common pad materials include cellulosic materials or aspen fiber pads. Cellulosic pad materials are the preferred choice by most growers as they last longer than aspen pads.
Wet Bulb Depression
Evaporative cooling becomes more efficient as the relative humidity drops. In Alabama we have exceptionally high humidity, especially along the Gulf Coast. The ability to cool with evaporative cooling can be estimated by taking the difference between wet and dry bulb temperatures. This difference is the Wet Bulb Depression (WBD). WBD represents how much the air can be cooled through evaporative cooling.
As humidity increases, the gap between wet and dry bulb temperature decreases resulting in less efficient cooling. Because of the high humidity in Alabama, the ability to cool with evaporative cooling is somewhat limited, but may be significant depending on the situation. Table 1. summarizes WBD in Mobile, one of the most humid areas of the state. WBD was calculated for what was considered the warmest time of day (11 a.m. to 3 p.m.). The data suggests that during 64 percent of the hours, intake air temperature could be reduced by over 5 °F during July 2014.
Table 1. Percent of Hours Within Wet Bulb Depression Rangers (WBD)
Calculated from historic weather data from weather station located at the Mobile Regional Airport
Only includes hours from 11 a.m. to 3 p.m. during the month of July 2014.
|WBD (°F)||Percent of Hours|
The reduction in air temperature only occurs around the exhaust side of the cooling pad. Even in a well-designed greenhouse, air can easily pick up over 8 to 9 °F as it passes through the greenhouse. Cooling pads are not going to be able to achieve 100 percent efficiency. For a freestanding 30 x 96 ft. greenhouse, cooling pad equipment cost can be anywhere from $1800-$2000. Depending on your situation, the cooling capacity of a fan and pad system may or may not be significant enough to justify that level of investment.
Fan and pad systems need to be sized appropriately to achieve the maximum cooling efficiency. When a fan and pad system cannot be justified, in some situations passive ventilation with open sided greenhouses may provide adequate cooling for a given crop without using any energy. In the case where biosecurity or insect exclusion is a priority, a closed greenhouse with a fan and pad system may be necessary. Greenhouse lettuce growers in Baldwin County have had great success with cooling pads in combination with reflective shade cloths. Floriculture growers throughout Alabama have also had great success with passively ventilated greenhouses.
Cooling pads require regular maintenance to insure they remain efficient. Algae and mineral build up can reduce air movement and lower efficiency. Consult with the cooling pad manufacturer before applying any chemicals, as some can reduce the life expectancy of pad materials.