Daniel Stewart for Zondits, December 1, 2015. Image credit: Miguel Peixe
The legalization of medical and/or recreational marijuana in twenty-seven states has paved the way for a boom in the growing of cannabis, and growers in large numbers are the latest addition to the indoor agriculture industry. Currently, they account for nearly 2% of all electric usage in the Denver metro area and nearly 45% of that area’s load growth since 2012. The load growth is welcome in some areas and is posing challenges in others, but one thing is for sure: with the high power densities at play in this rapidly growing industry, energy savings are ripe for the picking.
Air quality control is one of the most important aspects of a successful indoor agriculture operation. Most plants stop photosynthesis when the ambient temperature exceeds a certain limit (for example, for cannabis that limit is around 90°F). Humidity is generally an enemy because humid air is more capable of carrying bacterial and fungal spores as well as promoting their growth on live plants. The internal heat and humidity gains of the space will vary depending on the lighting, watering, and growing cycles, and the ideal temperature and humidity values will vary depending on the phase of the growing cycle. All of the variability in the factors contributing to the air quality within a growing space highlights the need to intelligently design and control the HVAC systems to efficiently maintain ideal conditions, which simultaneously presents the opportunity for energy savings.
Lighting accounts for roughly 40% of total electric use in a typical indoor agriculture facility. A facility that features metal halide or high pressure sodium lighting (both more common than LED due to cost and light spectrum preferences) could have lighting power densities as high as 200 watts per square foot (approximately 250 times the connected lighting wattage to serve a typical office space of the same size) and have internal heat loads that are as high as those of a corporate data center. In order to remove the heat, manufacturers of canopy-style growing lamps have integrated ventilation hoods into the reflector housings of each fixture. These hoods are connected to a common duct with an inline fan that removes heat from the light fixtures and exhausts it outside the facility.
Charcoal filters are typically used to reduce exhaust air odors coming from duct work that originates from within the growing space. Filters should be carefully sized to balance their efficacy with the added fan power caused by the pressure drop across them. A simple ventilation system layout will draw in air from within the space through a charcoal filter, pass it through the light fixtures, and exhaust it outside; make-up air enters the space via a passive intake.
A more advanced layout could have dedicated lighting heat removal duct work that uses outside air to cool the fixtures before blowing it back outside and a separate set of exhaust duct work with a charcoal filter to exchange the air in the space.
Light fixtures can be connected in many different configurations and share common exhaust duct work.
There are advantages and disadvantages to each type of setup. The setup shown in Figure 1 brings in outside air whenever the lighting system is on because the light fixtures need ventilation air for cooling. This may save fan power during periods where the space requires ventilation air; however, it may lead to more energy being spent during periods where excess outside air is introduced to the space that needs to be heated or cooled. The setups in Figures 2 and 3 serve each of the two system functions (space ventilation and lighting ventilation) with separate fans. This solves any potential excess space ventilation air conditioning issue but results in excess fan energy being used whenever both lighting and space ventilation are required. An optimal system would enable the lighting ventilation system to draw air from outside or within the space and also have a separate dedicated space ventilation system. This approach requires more sophisticated automation to coordinate the two branches of the system and maintain space conditions most efficiency. For better efficiency and tighter temperature control, the exhaust ducting within the grow space located downstream of light fixtures should always be insulated to reduce fugitive heat loss back to the space.
When ventilation alone is not enough to maintain a suitable temperature in a growing space, a supplemental cooling system can be added. When a cooling system is added, it is important to match the system to the load it will serve and maximize its efficiency by integrating it with the facility’s ventilation and humidity control systems. In addition to installing high efficiency cooling and/or dehumidification equipment, these systems can further improve their performance by being configured to return water that has been removed from the air back to the crop.
While heat is typically a negative, in very cold climates growing facilities can take advantage of the lighting system’s waste heat by diverting the lighting ventilation exhaust air back into the space. In this configuration, cold outside air that is required for air exchange purposes is heated while passing through the light fixtures, reducing the load on the facility’s heating system if it has one. If the growing space does not require the entire heat output from the lighting system, additional heat may be pushed to other areas of the facility where it is needed or, in large enough systems, the air stream can be split and a portion will be recirculated while the remainder is exhausted.
In addition to ventilation and humidity control systems, cross-flow fans are often used to circulate air within a grow space. This air circulation reduces hot and cold spots and moisture pockets and keeps leaf pores clear, which improves the plants’ respiration. The key role of the HVAC system in an indoor agricultural facility is to maintain optimal growing conditions for the crop. In order to do this while operating as efficiently as possible, outside air should be used for free-cooling whenever possible, waste heat should be reclaimed whenever it is needed, and outdoor air exchange should generally be limited to the demand of the crop.
This emerging industry faces a number of growing pains (no pun intended). Fortunately, the equipment and control technologies that the indoor cannabis growing industry will employ to reduce their energy impact are already largely in use in other sectors today; the current challenge is to develop and promote best practices that are tailored to the specific requirements of the industry.