This article explores how vapor pressure differentials (VPD) affect transpiration rates in cannabis plants. Transpiration is a process in which water and essential nutrients move through the plant from cell to cell. Understanding of VPD at different temperatures and relative humidities, and its effect on this process, is important to gain maximum plant growth. We also cover the impact of set points on growth environments as well as energy costs, along with an analysis on how data surrounding watering rates and transpiration can affect equipment selection.
The key to healthy VPD and transpiration rates is to provide controlled environmental conditions, which will come into play when you are selecting equipment for your operation.
The role of HVAC equipment is to keep the room as close to the design temperature and humidity setpoints as possible because this will have the greatest impact on the product. In the beginning of a project, we send an HVAC survey document to our clients to determine their project requirements. The information provided allows the engineers to determine the “design conditions,” which tells the engineer what they need to design around. Information such as temperature and humidity setpoints as well as type and quantity of lights, watering, and run-off rates are gathered.
This information allows the designer to estimate the transpiration rate of the plants and the heat load from the equipment, which are called latent and sensible loads, respectively. The HVAC equipment will remove the heat load and humidity load from the space, so the more accurate the information provided by the grower, the more accurate the sizing of the system will be.
Determining Transpiration Rates and Humidity Control Strategies
One of the best, most straightforward methods for determining transpiration rates is to use known watering rates to indirectly quantify transpiration. Once irrigation water is introduced into a space, it can do two things: stay within the plant or pot, increasing its overall mass, or leave the plant or pot as runoff or transpiration. The general assumption is relatively little of the water mass is left behind in the pot or plant, and the rest of the water is lost to runoff and transpiration between waterings. Quantifying watering rates is typically straightforward, but quantifying runoff can be difficult depending on which grow method is used. Data can be gathered at the grow facility level, stage of growth level, room level, or plant level.
Properly designed HVAC equipment will not only control the temperature of a space, but it will also remove the moisture transpired by the plants to keep the relative humidity ratio at the appropriate set point. There are numerous humidity control strategies. On the most basic level is the dehumidifier, which utilizes a refrigeration process to subcool the air to its saturation point to extract the moisture from the air. While effective, this is an inherently inefficient process as the compressor heat is rejected into the space, adding to the heat load that the HVAC equipment needs to take care of. On the most complicated level, your designer may add the dehumidification scheme into the overall HVAC design, whereby the air may be subcooled at the coils to extract the water out of the air, then the air may be reheated so as not to overcool the space. For maximum energy efficiency, the reheating of air can be done with hot water or hot gas produced by the heat rejected from the cooling equipment.
What happens when we vary the humidity and temperature setpoints? If the temperature and humidity setpoints are decreased, the peak load on the HVAC system increases, as does the energy required to run the equipment. Similarly, the reverse is true—if the temperature and humidity setpoints are increased, the peak load on the system decreases, and the energy required to run the equipment also decreases. It is in the best interest of the grower, both from a first cost and an operating cost perspective, to run their operation as hot and humid as possible.
Let’s refer back to Table II. As you can see, the calculated VPD at 70 °F and 60% RH is similar to the calculated VPD at 75 °F and 65% RH. Therefore, the plants may perform just as well at the higher temperature and humidity setpoint than at the lower temperature and humidity setpoint, but the effects on the first cost, physical size, and energy use of the HVAC system will be significant. In our experience, a setpoint difference of just 10% can have a dramatic effect.
Real World Case Study
The issue of equipment selection came to life in a recent project with very specific goals. Clients regularly come to us with specific temperature and humidity setpoints and ranges in mind. Often times these setpoints are determined by experience in the field, and not necessarily by gathering information on the VPD required. The growers know that their plants thrive in certain conditions, but normally they’ve determined this through a trial and error process. In this case study, our client—a cultivation and extraction facility—needed precise temperature and humidity control for their grow spaces, with a major focus on energy efficiency. Like most of our clients, they had specific temperature and humidity setpoints in mind: 76 °F and 55% RH.
The proposed HVAC system was a water-cooled chilled water system, with an economizer operation to allow for compressor-free operation when outdoor conditions allow. The hot water for the facility is provided via heat pumps that utilize the heat rejected from the chillers as the source for the heating loop. Fan coils inside the cultivation rooms utilize variable frequency drives to dehumidify without overcooling the spaces.
Because sustainability was a core goal of the client, we decided to start with a different approach—a shoebox energy model to narrow in on the implications of different setpoints on the size and energy use of the HVAC equipment. We were able to show the client that by adjusting their temperature and humidity setpoints upwards, they would be able to maintain the same VPD in the space, yet the size of the HVAC equipment decreased by 33% and the energy use associated with the HVAC equipment decreased by upwards of 35% per year. By looking at the VPD specifically, and using that metric to make decisions around the required temperature and humidity setpoint, we were able to design an HVAC system that met their sustainability goals and decreased the size of the equipment, while still ensuring that their plants would thrive.
The initial criteria the client provided us, a temperature setpoint of 76 °F and a relative humidity setpoint of 55%, would have resulted in an HVAC plant size of 600 tons of cooling. By looking first at the VPD, and then utilizing that to determine the setpoints, we were able to downsize the equipment to 400 tons of cooling. This equipment is roughly ¾ the physical size and 66% of the cost of the larger 600 ton system.
Proper HVAC equipment design --> consistent temperature and relative humidity in the space --> consistent vapor pressure deficit --> plant transpiration --> plant growth.
It is understood that proper HVAC equipment design is necessary to keep consistent temperature and relative humidity within a space. It is also generally recognized that the plant growth is impacted, both positively and negatively, by space temperature and humidity and therefore vapor pressure deficit. What is often overlooked or unknown is that small changes in temperature and humidity setpoints can have an imperceptible impact on the vapor pressure differential, and a large impact on the size, first cost, and operating costs of the HVAC system.
We aren’t growers, we are engineers. As such, we do not claim to understand the complete effects of an operation running at higher temperature and humidity setpoints on plant quality and quantity. The purpose of this article is to illustrate the effects of temperature and humidity setpoint differences on HVAC sizing and energy costs, to allow growers to make a more informed decision when determining the appropriate setpoints for their space.
Laura Breit, PE, Michael Leavitt, PE, and Adam Boyd, PE, are professional engineers with Root Engineers in Bend, Oregon. Direct correspondence to: [email protected]
How to Cite This Article
L Breit, M Leavitt, A Boyd, Cannabis Science and Technology 2(2), 52-61 (2019).