Economization — The process of maintaining space conditions without mechanical cooling is known as economization. Economization reduces or eliminates the amount of time the HVAC unit’s compressors operate by using outdoor air when conditions permit. Energy codes dictate this approach in most commercial buildings, where space cooling is always required. Although grow rooms may currently be exempt from energy codes, the concept of economization can be similarly applied to reduce HVAC energy consumption. The type of economization and the quantity of hours it can be utilized is dependent on the required space environmental conditions, HVAC system equipment type, and desired return on investment.
The standard form of economization is directly introducing cool outdoor air to the space in lieu of using refrigeration equipment. In the context of grow rooms, however, systems directly utilizing outdoor air dilute grow room CO₂ levels, introduce pests into the grow environment, do not provide appropriate control over the space’s VPD, and require exhaust systems that can spread odors to surrounding properties. Instead, grow rooms require the use of indirect economization to provide HVAC energy savings without detrimentally impacting the space conditions. Indirect economization allows the HVAC system to utilize favorable outdoor air conditions without introducing ambient air directly to the space. This functionality can be provided by air-to-air heat exchangers, energy wheels, or heat pipe systems. In each case, outdoor air is used to remove heat from the grow space without directly impacting the grow environment.
Air Distribution — Proper air distribution throughout the grow environment is an extremely important factor in the prevention of microclimates and mold growth. The surface of the plant is assumed to be at 100% relative humidity, known as saturation, meaning it cannot absorb any more water. Since the plants cannot absorb more water, cool air from the HVAC system that is supplied too close to the plants has the potential to cause condensation on the leaves, increasing the potential for damaging mold and fungus to form. Wherever possible, air should be directed away from plants to minimize this risk. Circulation aisles, exterior grow room walls, and the area below the plant beds are each good spaces for supply air. Many grow rooms also incorporate space-mounted fans to help evenly distribute air and equalize conditions throughout the space. Sophisticated facility owners who understand the criticality of proper airflow design have begun to utilize computational fluid dynamics (CFD) analysis during the design of their grow facilities. CFD software allows a designer to model airflow from the HVAC system and graphically view the projected space airflow pattern, temperature, and velocity. Figure 4 offers a sample space temperature profile. This data helps designers determine the optimal size and location of HVAC openings to minimize microclimates and provide uniform airflow throughout the grow room.
Large-scale grow spaces are being constructed throughout the U.S. as developers attempt to eliminate the uncertainty of traditional farming methods and optimize product yield, quality, and consistency. By developing energy-efficient environmental control systems for grow spaces, the indoor agriculture industry is moving toward a more sustainable future. Proper sizing and equipment selection of the environmental system is critical to ensure the success of this endeavor. As the industry continues to develop, the design community must achieve a more complete understanding of optimal conditions for plant growth and maturation and develop new, efficient means to provide these conditions.