Natural Ventilation And Fog Increase Cooling Efficiency

This research greenhouse at the University of Arizona is being used to develop advanced climate control strategies for a naturally vented greenhouse equipped with high-pressure fogging.

For greenhouse ventilation, two main methods are used — mechanical and natural. Mechanical ventilation includes fan-and-pad systems, which are a type of evaporative cooling system. Fan-and-pad systems cool air by passing outside air through a wet pad, lowering the temperature and humidifying the greenhouse. This type of system works best in hotter, drier climates, because the capacity for cooling with a fan-and-pad system is very limited in humid climates.

University of Arizona associate professor Murat Kacira says a drawback of using a fan-and-pad type system is that it requires a lot of energy and water.

“Water is needed for the pad system, and energy is needed to run the pump for water circulation over the pad as well as the energy to run the exhaust fans,” Kacira says. “Water is a precious resource with limited availability, especially in arid and semi-arid regions.

“Another issue is there can be a temperature gradient from the cooler wet pad side [of the greenhouse] to the warmer exhaust fan side. This gradient may affect plant quality and crop yield.”

Natural ventilation is the other ventilation system for cooling greenhouses. For the last three years, Kacira has been involved in an international project to study naturally ventilated greenhouses integrated with a fogging system.

“With funding from the United States-Israel Binational Agricultural Research and Development Fund, we focused on developing advanced climate control strategies to operate in naturally ventilated greenhouses with high-pressure fogging systems,” Kacira says. “Our partners in this project at Technion-Israel Institute of Technology focused on developing climate control strategies using mechanical ventilation with only fans and high pressure fogging.”

The ultimate goal, he says, was to develop advanced control strategies to maintain the desired greenhouse microclimate while saving water and energy.

“With the strategies we developed using a naturally ventilated greenhouse with a variable high-pressure fogging system, we were able to maintain the greenhouse climate close to the desired range, around 77ºF (25ºC) and 75 percent relative humidity for tomato production. The outside temperatures were about 95ºF to 104ºF (35ºC to 40ºC) and 10 to 25 percent relative humidity. With the system operating with variable rate fogging and ventilation rate strategy, we were able to save about 20 to 25 percent of the water and 15 to 20 percent on energy compared to a control strategy using a constant fogging rate and a fixed vent opening. We have not compared the energy and water savings to a mechanically ventilated greenhouse at our facility. That will be done with a future study.”

Using Fog For Cooling

The fogging system that Kacira used in his research was a variable high-pressure fogging system that delivered fine droplets under a 4.5 to 10.4 MPa operation pressure range.

“For greenhouse applications, a fine droplet size is best because the water droplets should evaporate as quickly as possible before they can fall onto the canopy and wet the plants,” Kacira says. “One of the major concerns with high-pressure fogging systems is free moisture on the plants, which can lead to disease development. That is one of the main concerns with fogging systems. Thus proper system design, installation and control strategy are needed.”

Kacira says another challenge with using high-pressure fogging systems is they require high-quality water (i.e., potable water) so that the nozzles don’t get clogged. If there isn’t a source of high-quality water, a water treatment system like reverse osmosis is needed.

The high-pressure fog works best in a dry climate, he says. If the outside air is humid and hot, the cooling capacity of an evaporative cooling system is very limited to just a couple of degrees Celsius.

“In a climate like what we have here in Arizona there can be a huge temperature drop in the greenhouse compared to outside,” he says. “In places where the climate is humid and hot like Alabama, this system is not nearly as effective. The effectiveness of this system is a function of the dry and wet bulb temperatures of the outside air and the desired indoor air temperature.”

Improving Climate Control

For the last decade, Kacira has studied the development of climate control strategies that take into consideration different greenhouse environment variables along with the greenhouse’s aerodynamics. Using two- and three-dimensional computational fluid dynamics (CFD) models, Kacira and other university researchers have studied the aerodynamics of greenhouse systems to improve climate uniformity and offer design recommendations to greenhouse and climate control system manufacturers and growers.

Using CFD models, the researchers have been able to take actual greenhouse dimensions and put in the geometry and determine how to improve airflow patterns, greenhouse and canopy zone air exchange rates and various vent configurations through a particular structure.

“We can take an existing greenhouse design and analyze the geometry, the configuration and the aerodynamics of the greenhouse under a worse-case scenario. We can take any greenhouse shape or size, vent configuration, climate conditions and plant canopy and do analyses with CFD models. Then we can look at various design options and operational conditions or other greenhouse details,” Kacira says.

The analysis can be done with any type of controlled environment agriculture structure.

“We have been working on complex CFD models that take into consideration the greenhouse airflow pattern, air exchange rates, vent configurations and designs, plant canopy, solar radiation and high-pressure fogging. All of these are integrated into CFD models,” he says.

Future Use Of CFD Models

Kacira says CFD is an engineering analysis tool that can be used with any controlled environment agriculture system producing ornamental and vegetable crops.

There is an interest in using CFD models for indoor and urban greenhouse systems including plant factory operations and vertical farming setups.

“We are seeing interest from companies that are working with indoor plant factory and vertical farming systems and airflow patterns,” he says. “Airflow is challenging in those kinds of designs. You have to make sure there is uniform airflow with proper air exchange rates to ensure uniform, healthy, high-quality crops. That is something we want to work on.”

Another area of interest using CFD models is comparing the effectiveness of vertical airflow vs. horizontal airflow systems in greenhouses.

“In the Netherlands, growers tend to prefer using vertical fans. Vertical airflow fans are mounted in the roof structure and push the air vertically toward the plant canopy.” Kacira says. “The question is which type of fan is better — horizontal airflow or vertical fans — under a given crop arrangement and greenhouse system? There is interest coming from the growers and greenhouse manufacturers about comparing airflow patterns, climate uniformity and aerodynamics between the two fan systems.”

The purpose of horizontal airflow fans is to mix and homogenize the air above the canopy.

“I have not seen any data as to which of these fans creates an optimum environment for a given crop in a given greenhouse setting,” Kacira says.

CFD models can also be used with some of the new glazing materials being developed.

“There are new glazing materials coming into application including some that are wavelength selective that can block some of the radiation. These types of glazing materials can also be analyzed using CFD models to evaluate their effects on greenhouse climate and crop responses. Our research interest is also focused in this direction.”

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