Exhaust fans are an integral component in evaporative cooling.
Photo courtesy of Dr. Nadia Sabeh

Ventilation, evaporative cooling and shade curtains: These are some of the most common cooling methods in the greenhouse, and ranked in importance by Dr. Nadia Sabeh, founder and president of Dr. Greenhouse (doctorgreenhouse.com), and a mechanical and agricultural engineer.

Here, we will focus on evaporative cooling, which Sabeh says is her “specific area of expertise.”

What is evaporative cooling?

Evaporative cooling helps control temperature and humidity, Sabeh says. By themselves, fans draw heat out of the greenhouse that collects as a byproduct of solar radiation, but draw moisture out as well. Evaporative cooling works to add moisture back into the greenhouse.

Comparable to how humans sweat — water from a person’s skin evaporates and carries heat away from the skin — evaporative cooling uses water to capture heat from the air, causing the air temperature to reduce and the moisture content to increase, Sabeh says. The process involves the transfer of sensible (observable) heat to latent (hidden) heat. “We’ve moved sensible heat into hidden [latent] heat, which is trapped in that water vapor,” she says. “Now we have cool air that has also been humidified — because we’ve evaporated water — moving through the greenhouse.”

In a high-pressure fog system, nozzles throughout the greenhouse inject small water droplets.
Photo courtesy of Dr. Nadia Sabeh

Evaporative cooling works best in climates where the air is hot and dry — places like Arizona, Colorado and California — rather than humid ones, and where growers need a combination of cooling and humidification, Sabeh says.

Ventilation generally works better than evaporative cooling in more humid locales, such as Florida, Hawaii and the Midwest. However, evaporative cooling in such areas can help growers achieve a slight temperature reduction, which can be beneficial if they are willing to invest in a system that cannot provide all the cooling required all of the time.

Pad and fan systems

Pad and fan systems are the most common evaporative cooling method, Sabeh says. In this type of system, a wet wall is placed at one end of the greenhouse and fans are installed at the other end. The fans pull air from the outside and across the wet pad, evaporating the water and cooling the air before it enters the greenhouse.

A major advantage of a pad, or wet wall, is that all of the water that is used for evaporative cooling is evaporated, Sabeh says. “You never have any water moving into the greenhouse that could get on your plants,” she says. “That’s a major advantage, especially over something like a high-pressure fog system, where we’re literally injecting water into the greenhouse.”

Wet walls evaporate all of the water used in a pad and fan system.
Photo courtesy of Dr. Nadia Sabeh

Pad and fan systems are simple to operate and control, Sabeh says. A pump runs water up through a pipe to a gutter above the pad, trickles water down the pad and drains it back into a tank, which the pump picks back up and recirculates. Meanwhile, fans are managed to a specific airflow volume based on temperature needs.

Water pumps and fans are going to require the most energy consumption, but the operating cost is still low, Sabeh says. “Relatively speaking, the cost of the material and equipment, and installation, is relatively low, and also the operating costs are really low, because again, you just have that little pump that’s circulating water onto the pad, and you have fans that operate as needed,” she says.

People also often have a sense of security when working with pad and fan systems because they have installed and used them before, Sabeh says. “It’s just tried-and-true technology,” she says.

One downside to pad and fan systems is that the pad, which is usually cellulose, often gets “neglected,” Sabeh says. “Two things can happen: one, algae can build up on the pad or in the water tank, and the other thing is calcium salts can build up on the pad,” she says. “Calcium salt builds up because there’s calcium in the water and when the water evaporates through evaporative cooling, it deposits calcium salts on the wet pad.” This scenario is especially true in areas with hard water.

To prevent algae colonization, growers and greenhouse managers can place a detergent tablet in their water tank, Sabeh says. And once calcium salts gather, some people use high-pressure sprayers to remove them, while others use solvents to dissolve them, Sabeh says. However, she says these methods can be difficult considering the “stubborn” nature of the salts — which are essentially barnacles — so she recommends greenhouse managers buy new pads if they can afford them. Putting water softener on the inlet to the water tank can also help limit the amount of calcium salts entering the system.

Calcium salts can build up on evaporative cooling pads.
Photo courtesy of Dr. Nadia Sabeh

High-pressure fog systems

Another drawback of using pads and fans is that growers often experience a temperature and humidity gradient between the two ends of the greenhouse, Sabeh says. Commonly, temperatures rise from the wet pad to the fans, and by a differential of anywhere from 3 to 5 degrees Fahrenheit, depending on the run of the greenhouse.

Growers can largely avoid this issue by using a high-pressure fog system, where nozzles are distributed throughout the greenhouse and inject small water droplets (5-20 micrometers); the air inside the greenhouse then evaporates the water, Sabeh says. “The benefit of that is that you get a little bit more uniform environment — at least have the potential for a little bit more uniform conditions,” she says.

“I really recommend a high-pressure fog system as opposed to low-pressure mist,” Sabeh says. “Basically, what you’re doing is you’re injecting fine droplets of water into the air, usually over the plants. And so now you’re using ventilation fans to bring air into the greenhouse, or maybe you just are using natural ventilation with sidewalls and roof vents, and that moisture — those water droplets — gets injected into the greenhouse [everywhere] and then evaporates, creating a drop in temperature over the tops of the plants.”

Sabeh has worked on multiple research projects with high-pressure fog and low-pressure mist systems. One difficulty with these systems, she says, is to find the best way to evaporate water quickly enough so that it doesn’t land on the plants, but not so quickly that it doesn’t cool the air around the plants. “The problem with high-pressure fog and low-pressure mist systems is just trying to figure out how to balance water injection and ventilation rates,” she says. As a rule of thumb, Sabeh recommends placing nozzles no less than three feet above the top of plants to prevent water from getting on them.

Whether growers decide to use a pad and fan system or high-pressure fog system, evaporative cooling can help control temperature and humidity in the greenhouse.