To most, the threat of climate change to the global food supply may seem like a problem for the future, but its impact can be observed today. The latest report from the Intergovernmental Panel on Climate Change (IPCC) projects reduced crop yields as global temperatures continue to rise, while the Food and Agriculture Organization of the United Nations has highlighted the immediate need to increase the resilience of global agricultural sectors. As food production becomes more stressed by changing weather patterns, the potential to destabilize global food systems and threaten food security rises.
In addition to the long-term need to build resiliency in Earth’s food supply chain, the farm-to-table movement and increased legalization of cannabis have led to a rapid escalation in the quantity and size of grow room facilities throughout the U.S. Largely unregulated as recently as a few years ago, these facilities could operate with few code-mandated energy efficiency requirements. As municipalities have started taking notice of grow room power consumption, new regulations are slowly being put in place mandating system efficiency requirements.
By controlling the grow room environment, engineers can remove the uncertainty and risks of extreme weather while maximizing product development rates, yields, quality, and consistency. The industry is rapidly evolving to develop innovative solutions to lower energy costs to compete with traditional farming methods.
As the industry continues to expand, the need for larger, more productive facilities has become urgent. The desire for locally sourced, efficiently grown food sources and the heightened demand for cannabis products are driving the demand for energy-efficient, indoor grow spaces. Growers and developers need HVAC solutions that provide effective operation at a competitive capital cost.
HVAC System Sizing
Sizing of the HVAC system is critical to the performance and control of the grow room. Proper system performance impacts plant growth rate, yield, and product quality. Multiple factors, including temperature, humidity, air quality, air circulation, ventilation, and CO₂ levels, must be adjusted throughout the propagation, vegetation, and flowering growth stages to optimize product yields. In addition to the continuously changing requirements of the developing plants, lighting, and seasonal building loads place additional demands on the HVAC system. A failure to control any single variable can impact the final product.
The complex set of load variables introduced in grow facilities requires a slightly different approach to system sizing than typical commercial buildings. Grow room HVAC systems must account for moisture loads generated by evaporation from the plants and soil, heat output from the grow lights, and roof and wall loads from ambient conditions. Evapotranspiration — the process of moisture evaporation from the plants, soil, and irrigation system — creates a cooling effect for the plant similar to humans sweating in hot weather. Depending on the quantity and type of plants in the space, this cooling effect may have a significant impact on the sizing and operation of the HVAC equipment. The amount of water evaporated into the air varies as plants develop. Designers have not standardized a method to calculate moisture loads and the associated cooling effects; however, loads are commonly approximated using the Hargreaves formula, the Penman-Monteith formula, or plant irrigation rates with a waste factor applied. The cooling effect of evapotranspiration can reduce the required tonnage of the HVAC system by up to 30%.
The plant moisture and cooling loads are highest when the grow room lights are on to drive photosynthesis. When the grow room lights are on, plants release more water vapor to the surrounding air and provide a cooling effect to the space. When the lights are off, the heat from the lights is removed and the amount of moisture and cooling effect is significantly reduced. Figures 1A&B illustrate typical day and night grow room HVAC loads. Growers will typically adjust the HVAC set points and lighting cycles at different growth stages to help drive plant development.
Vapor Pressure Deficit — The process of evapotranspiration is the largest factor in plant development. The speed at which water moves through the plant is a function of the difference between the leaf surface and the surrounding space vapor pressure. The difference between these two conditions is known as the vapor pressure deficit (VPD). Figure 2 shows optimal VPD design conditions for different stages of plant development. Since HVAC systems cannot control to a VPD set point directly, HVAC designers must review the optimal VPD range and determine temperature and humidity set points for HVAC control. The selected temperature and humidity can be used for grow space control; however, space dew point typically provides a more accurate measure of the space moisture. Dew point is a measure of the absolute humidity in the air and is a common HVAC control method for dehumidification systems. The space dew point temperature and the relationship to the design VPD is illustrated on the psychrometric chart in Figure 3.
In addition to the optimal VPD values shown in Figure 2, each grower has specific preferences that may require adjustments to the HVAC design. Understanding how a grower intends to operate each space is critical to designing an HVAC system that will allow the production of consistent, healthy crops with high yields as expeditiously as possible.
Lighting — Lighting selection can contribute significantly to the load on the HVAC system. While review of current lighting technologies is beyond the scope of this article, it is important for designers to understand the impact lighting selection has on the HVAC system operation. For example, an LED fixture may emit 40% less heat to the space than its high-pressure sodium (HPS) counterpart. It’s especially important in renovation projects implementing LED fixtures to account for the reduction in lighting load and the impact to the HVAC system sizing or control strategy to avoid short cycling of equipment. This has the potential to be overlooked and can have a material impact on the HVAC performance.
System Selection — Proper HVAC equipment selection requires coordination between the HVAC engineer, botanist, and developer. The expanding cannabis market is quickly driving the entire indoor agriculture industry toward larger-scale operations, necessitating new HVAC solutions. HVAC organizations, like ASHRAE, have commissioned technical committees to study agricultural building energy use, but the results of those studies are pending. While the industry awaits formal study results, growers and engineers are tasked with providing cost-effective solutions that allow for precise environmental controls and efficient operation.
The first generation of grow rooms included commercial, off-the-shelf HVAC solutions. Most commercial HVAC equipment is designed for human comfort cooling applications with minimal moisture load removal capability. This design is inherently flawed for grow rooms, where moisture loads are a crucial factor in the sizing of the grow room HVAC system. To remove the large moisture loads generated by the plants, many growers use supplemental, in-room dehumidifiers. The dehumidifiers remove moisture by producing very cold air, which is then reheated with waste condenser heat. The heat from the dehumidifier is rejected into the space and then removed by the packaged HVAC system. This configuration provides imprecise temperature control, inefficient system operation, and requires maintenance of multiple pieces of equipment.
More recently, designers have recognized the performance benefits of custom or semi-custom air-handling equipment with the capacity to modulate sensible cooling, dehumidification, and heating in a single unit as required to more precisely maintain space conditions. Incorporating the required processes into a single unit eliminates the potential for multiple pieces of equipment to counteract each other and provides more stable space control. Equipment manufacturers that were previously focused on precision cooling for process loads have begun to develop optimized products for grow room use. Customized air handlers will have a higher capital cost when compared to standard commercial equipment but will provide a more stable growing environment and higher product consistency to optimize plant yields at reduced energy consumption. Current technologies incorporate components, such as digital scroll compressors and modulating hot gas reheat, to provide the ability to closely match the unit-leaving air condition to the grow room requirements.
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.