How plant dynamics affect grow rooms
Indoor cannabis facilities have diverse, intense requirements for humidity and temperature control that change as plants develop.
In addition, the plant dynamics caused by photosynthesis and evapotranspiration have a significant impact on system design and equipment sizing. To design and specify solutions that respond to and control conditions efficiently, HVAC designers must employ a comprehensive approach to determine the loads indoor climate control systems must monitor and manage.
The designs of cannabis growing facilities have changed considerably as the industry evolves from small, often illegal grows that once dominated cultivation to much larger and more sophisticated facilities as legalization spreads across the U.S. and Canada.
Today’s growers are moving to a more sophisticated business model with an emphasis on quality, maximized yield and lower operational costs.
There are three key HVAC design considerations necessary to achieve tight environmental control at the lowest operating costs: the evaporative cooling effect of the plants; vapor pressure deficit (VPD) and equipment sizing; and designing airflow to mitigate mold and mildew.
The evaporative cooling effect of plants
In cannabis operations, temperature and humidity control are critical for optimal plant growth and for avoiding conditions that lead to bacteria, fungus, mold and pest growth. But what most people overlook is how plant dynamics affect the ability of HVAC systems to maintain optimum temperatures and humidity levels.
From an HVAC perspective, cannabis cultivation is a process where exposing plants to light increases the temperature of the space, while plant evapotranspiration is actually cooling the space. Plant evapotranspiration also adds moisture to the grow room. The HVAC industry expresses these changes in temperature and humidity as sensible and latent loads.
Sensible loads are analogous to the heat inside a building that people feel on their bodies and are temperatures that can be measured by a thermometer. The sensible heating load is the amount of heat energy that must be added to the grow room space to maintain a target air temperature. The sensible cooling load is the amount of heat energy that needs to be removed to attain the target temperature.
Latent or moisture loads are the energy and heat stored in humidity, a product of its change in state from liquid to gaseous.
The largest sensible load in grow rooms comes from the lights, although lighting loads are often different from the cloning room to the veg room to the flower room, and change based on lighting hours. Yet few HVAC designers consider the effect the plants have on the sensible load.
The latent load is also one that most designers can easily calculate. Water put into the room that does not go down the drain must be evapotranspirated into the room as latent energy.
Thus, most HVAC designers look at cannabis grow room designs as very simple: put in enough air conditioning to handle the lighting loads and put in enough dehumidifiers to handle the latent loads. At this point, though, we should remember the writer H.L. Mencken’s advice: “For every complex problem there is an answer that is clear, simple and wrong.”
Here’s the problem: The typical grow room design includes HVAC equipment large enough to handle the total lighting load and the total latent load at the same time, but counting both energy loads seems to violate a basic law of physics. The law of conservation of energy states that the total energy of an isolated system remains constant, meaning that energy can neither be created nor destroyed; rather, it can only be transformed from one form to another. If the only energy entering the grow room is lighting energy, we can’t end up with both a sensible load and a latent load greater than the lighting energy.
However, most designs don’t consider the energy used by the plants. During photosynthesis, plants convert water and carbon dioxide into food. In a process called transpiration, plants use water to carry nutrients throughout their tissues and then release the water as water vapor via tiny openings (stomata) on their surfaces to the surrounding boundary layer air. During transpiration, water changes from a liquid to a vapor. This process takes energy, which the plants get from the lights and use to evaporatively cool themselves. The amount of lighting energy plants use depends on their growth phase.
– Cloning and vegetative phases: When the plants and canopy are small, less water is being transpirated, thus less lighting energy is being used by the plants. The sensible load in the room is generally higher than the latent load, although the sensible heat ratio may still be below what a typical air conditioner is designed to handle.
– Flowering phase: The room load changes dramatically when plants start flowering. During this stage, when plants are either at or reaching their full growth, the canopy becomes a sea of green and the transpiration rate is maximized. The room has a greater latent load than it does a sensible load. The sensible heat ratio is now very low, far below what a typical air conditioner can operate at.
In both cases, the latent energy is a one-to-one subtraction from the lighting energy because that is where the energy came from. A proper HVAC design for a grow room must take these variable sensible and latent loads into account. In some cases, particularly with water- or air-cooled lights, it is possible that the lighting energy in the room can be reduced to the point where the plants do not have enough sensible energy to evaporatively cool themselves and the stomata will shut down.
VPD and equipment sizing
HVAC designers also need to consider the relationship between VPD and equipment sizing. When growers tell the HVAC designer the ideal temperature and humidity conditions for the room, they are really talking about the ideal VPD — the vapor pressure difference between plant leaves at saturation and the air surrounding the leaves. That difference is what drives plant transpiration rates.
VPD — not a specific temperature and relative humidity — drives plant growth. This is a critical concept as cannabis businesses try to minimize operating cost, because the capacity of the HVAC equipment is reduced as the relative humidity is reduced at the same sensible temperature. However, once the focus becomes the design VPD, then the equipment size can be reduced.
Size matters because the larger the HVAC equipment, the greater the initial cost and the ongoing operating costs. It’s important to note that there is a limit to both the high temperature and the low temperature. The stomata will shut down at both high and low VPDs. However, as long as the relative humidity is increased as the temperature in the room is increased, then temperatures around 80 degrees Fahrenheit should not be a problem.
Designing airflow to mitigate mold and mildew
Many growers design their rooms with a lower dewpoint to mitigate mold and mildew. However, this also increases HVAC equipment size. This is not a good approach for growers seeking to minimize operational costs.
Growers often have systems designed where the airflow from the HVAC system blows directly onto plant leaves. When the air is below the dewpoint, water will condense on the leaves. However, a better solution than lowering the dewpoint in the room is to design the airflow so the air from the HVAC system doesn’t hit the plant leaves directly.
The air should be distributed down the walls and below the canopy. The return air intake should be placed at least six feet above the floor, so the air is pulled up through the canopy. This air will have mixed properly by then and the condensation problem will have been mitigated. A key design feature is to make sure there is enough air movement through the canopy so that all plants have air movement across their leaves. Another benefit to proper airflow is that it increases the transpiration rate of the plants.
Designing an HVAC system based on these three design considerations ensures that the recommended solution is properly sized. This reconciles changing biological processes while at the same time providing an HVAC system that minimizes costs.
If a grower is to survive as the cannabis market matures and competition tightens, then minimizing operating costs is imperative, and a properly designed HVAC system can be a key element to do so.
Jim McKillip is the western regional manager for Desert Aire. He is a LEED accredited professional who has been a member of the American Society of Heating, Refrigeration and Air Conditioning Engineers since 1988.