Greenhouse Cultivation Leaves Carbon Emission Impacts

magazine.cannabisbusinesstimes.com, 9/23/2020.

Authors’ Note: 

This article is the third in a five-part series by Resource Innovation Institute (RII), a nonprofit that works to advance resource efficiency in cannabis cultivation. In Part I of the series (available at bit.ly/CBT_Resource_Guides), we introduced the “LED Lighting for Cannabis Cultivation and Controlled Environment Agriculture Best Practices Guide” and “HVAC for Cannabis Cultivation and Controlled Environment Agriculture Best Practices Guide,” which were examined by 29 peer reviewers. Key terms introduced in the article are italicized and described in more detail in the guides at ResourceInnovation.org/Resources.

The next two series installments will feature snippets from RII’s Best Practices Guides to highlight more important considerations for growers and the supply chains serving them.

Reducing production costs by optimizing resource efficiency and conveying that sustainability story are becoming central factors in the valuation of cannabis cultivation operations.

While the controlled-environment cultivation industry does not yet have enough information to fully understand the energy consumption of grow facilities using various methods and equipment, recent research reveals new insights into the carbon impacts of cultivating cannabis in greenhouse environments.

How Do Greenhouses Operate?

Many consider greenhouses an environmentally superior and less energy-intensive way to grow plants because natural light can be used for a portion of the grower’s target daily light integral (DLI) for their cultivars. However, there are trade-offs with heating energy use, building envelope integrity and quality, and environmental control with greenhouses due to their unique construction.

When greenhouse building envelopes are designed to let in the sun, they incorporate materials and construction methods that make them more sensitive to their location’s ambient conditions than indoor facilities. Their geographic latitude impacts the length and strength of available daylight, and ambient conditions include outdoor air temperature and relative humidity (RH).

“Greenhouses” can take many forms. From ventilated polycarbonate structures with no thermal curtains to tightly sealed and well-insulated, high-performance buildings with large skylights, these facilities can range widely in how they perform in various climates.

Infiltration, when outside air enters a building, is higher in ventilated greenhouses and lower in sealed greenhouses. Higher infiltration in ventilated structures makes them more sensitive to outdoor temperatures and humidity than sealed structures. The primary driver of infiltration is how leaky the construction is. Outdoor temperature plays a role, as does greenhouse size, but those influences can be all but eliminated if the envelope is well sealed.

Infiltration can be measured using the air leakage rate in air changes per hour (ACH); a lower ACH means less infiltration of outdoor air into a building. For example, an ACH under 1 means a half air change per hour. According to RII research and industry data, sealed and ventilated greenhouses may have these infiltration rates:

Sealed greenhouses: 0.3 – 0.5 ACH

Average ventilated greenhouses: 0.5 – 3.0 ACH

Leaky ventilated greenhouses: 3.0 – 6.0+ ACH

How Greenhouses Use Energy

Like most cannabis operations, greenhouses in colder climates use energy primarily for horticultural lighting. Energy also is used for heating, ventilation, air conditioning (HVAC), dehumidification and the control systems responsible for maintaining target environmental conditions.

Cultivation processes are generally exothermic, meaning they need to reject excess heat into the outside environment.

Several sensible (dry) loads and latent (wet) heat loads, the amount of heat and moisture, respectively, need to be removed from greenhouse air to attain optimal conditions:

• Passive solar heat gain: The sun adds sensible heat regardless of whether facilities are ventilated or sealed.
• Building envelope: How leaky or well-insulated a greenhouse is substantially affects sensible heating loads in colder climates where weather conditions are more extreme. In warmer climates, poor building envelope integrity can also impact latent heating loads.
• Evapotranspiration: The moisture from both plant transpiration and evaporation from water in cultivation spaces is a large source of latent heat.
• Horticultural lighting: Lighting is both an energy end-use and a source of sensible heat.

Temperature Controls for Greenhouse Environments

The environments inside both ventilated and sealed greenhouses traditionally are controlled using both hydronic (water-based) and convective (air-based) HVAC systems. Most greenhouse heating systems use fuel, not electricity; typical heating equipment used includes unit heaters, under-bench heating, forced hot air, and radiant heating systems.

Ventilated greenhouses: Traditional ventilated greenhouses use end-wall ventilation fans operated in stages and evaporative cooling wall systems installed on the wall opposite the ventilation fans. These systems pump water onto pads, and as air passes through the media, it is cooled via evaporation. Ventilation equipment and/or roof vents are typically employed to reduce humidity and cool cultivation spaces, and dehumidification equipment is not commonly employed.

It can be a challenge to meet target environmental conditions with coarse controls because many ventilated greenhouses rely on outdoor air and relatively simple fan systems for cooling and dehumidification. Because ventilated greenhouses do not often use mechanical cooling or dehumidification equipment, cultivators are unable to precisely control the conditions to the varying environmental targets for different weeks of flowering.

Field data demonstrates operating conditions regularly vary +/- 10 degrees F from the target temperatures and 10 percentage points from the target relative humidity values. Temperature differences of 7 degrees F have been recorded between the intake and fan (exhaust) ends of the same greenhouse bay, meaning cultivars are experiencing wide temperature variation across cultivation spaces.

Sealed greenhouses: High-performance sealed greenhouses use much different equipment for HVAC and dehumidification due to their sealed nature. While these greenhouses benefit from sunlight (compared to indoor facilities) and improved environmental control (compared to ventilated greenhouses), they must manage solar heat gain using mechanical systems. These tightly built facilities use mechanical cooling systems similar to those used by indoor operations, including commercial-grade hydronic and convective cooling systems. Sealed greenhouses also dehumidify using the same equipment as indoor facilities, such as standalone portable dehumidifiers and integrated HVAC and dehumidification (HVACD) systems. HVACD systems can provide conditioned air to better match the loads of the space, providing greater environmental control. Centralized HVACD systems can leverage sophisticated automation systems, providing precise control of supply air conditions to match the dynamic loads of the space.

Sealed greenhouses are more capable of achieving target environmental conditions because they make use of mechanical cooling and dehumidification equipment, more sophisticated HVACD controls and strategies, and are less sensitive to ambient conditions due to less outdoor air infiltration and better thermal performance than ventilated greenhouses.

Sealed greenhouses can also operate with enriched CO₂ atmospheres (because they are sealed), while ventilated greenhouses can only hope to introduce supplemental CO₂ during cold weather months, when ventilation is reduced or eliminated as cooling needs are reduced. However, reducing ventilation to either preserve heating energy or operate enriched CO₂ can often result in high humidity.

The Greenhouse vs. Indoor Energy Mix

A year-long study of cultivation facilities in Boulder, Colo., assessed how greenhouses use energy. The study gathered electricity consumption and demand data of several greenhouses at 15-minute intervals; monthly energy bills and fuel delivery data; and annual production data, along with a complete inventory of facility equipment and modeling.

The study compared Boulder’s facilities to the performance of indoor and greenhouse facilities in RII’s Cannabis PowerScore Ranked Data Set across North America to understand how greenhouses compared to indoor operations when measuring energy and carbon emissions impacts. The researchers found greenhouses in Boulder typically use less electricity and more fossil fuel on average than indoor operations (which should hold true for greenhouses operating in other cold climates).

Figure 1 shows the breakdown of energy use from electricity and all fuels used in different systems in Boulder greenhouse facilities. Natural gas consumed by greenhouses in colder climates for heating loads, on average, can make up 47% of the total MMBtu, with electricity used to power the lighting, HVAC, fans and other production area systems accounting for the other 53%. When looking at greenhouse electricity use only, lighting energy load was found to account for 61% of greenhouse electric energy use, with HVAC energy loads driving 29% of greenhouse electricity consumption.

Figure 2 shows the breakdown of energy use allocated to different systems in Boulder indoor facilities. Natural gas can make up as little as 2% of the total MMBtu consumed by indoor operations in colder climates, with electricity for lighting driving 69% of total energy use, compared to 32% for greenhouses. HVAC energy loads contribute nearly the same amount in indoor and greenhouse facilities.

Because greenhouses use sunlight for plant cultivation, electricity demand can be reduced in the middle of the day as solar radiation increases. The peak electric load from greenhouses in the Boulder study was recorded between 8 a.m. and 9 a.m., when electric lighting is turned on, but the morning sunlight is still intensifying. Once the sun sets, electric load increases again until lights are shut off overnight. Facility electric load is intimately linked to solar radiation, and generally as solar radiation increases, greenhouse electric demand decreases.

Greenhouse gas (GHG) emissions from fuel consumption are higher for greenhouses in colder climates. When greenhouses use fuel-based heating systems, as the Boulder greenhouses do, when heating degree days increase (meaning the facility experiences more hours of colder weather), natural gas consumption increases. The amount of cold weather and fuel used may change depending on where the greenhouse is located, but the relationship between greenhouse fuel use and outdoor conditions will always exist.

How Do We Measure Sustainability?

How do we define and measure cannabis cultivation operations’ sustainability, and determine which operations are the most environmentally friendly? One important step is to verify or challenge past assumptions using data that is available.

To enhance sustainability claims and business valuation, greenhouse operators, like those of all cultivation facilities, should balance electricity and fuel costs with their carbon impacts. The site-specific GHG emissions from any industrial operation are dictated by the regional electric utility generation assets and grid transmission losses, in addition to the carbon content of delivered fuels used for processes. Some fuels, like propane and fuel oil, have higher GHG emissions (measured using equivalent carbon dioxide (CO_2e)) than other fuels, like natural gas. Given the large amount of fuel used by greenhouses, the carbon footprint of this consumption is important to understand.

The Boulder study concluded that, on average:

1. Greenhouse productivity in grams/MMBtu of site energy was 15% better than the indoor facilities. (Note: Highly efficient indoor cultivation is also possible—the study found the productivity of the best-performing indoor facility was 25% better than the best-performing greenhouse); and
2. Greenhouse grams/lb. CO_2e was 71% better than the indoor facilities.

How is it possible that greenhouse site energy productivity is only 15% better, but the emissions are 71% better? It comes down to the energy mix used on-site, and the fuel used to generate the electricity that serves the facility.

If you were to compare the emissions of two greenhouses with identical site energy productivity, one based in Boulder and one based in Massachusetts, the Massachusetts facility would produce nearly 50% less CO_2e. This is due to a large portion of Boulder’s electricity generation coming from coal-fired generation facilities, while Massachusetts electricity is generated largely through renewables and natural gas, according to data from the U.S. Environmental Protection Agency.

Toward Sustainable Greenhouse Cannabis Cultivation

We see from this sample of projects that the electric savings achieved through leveraging sunlight and outdoor air ventilation in the cultivation process are largely offset by heating needs in cold-climate ventilated greenhouses. However, while greenhouse facilities’ average productivity in grams/MMBtu of site energy was slightly better than that of indoor facilities, greenhouses’ CO_2e emissions were substantially lower. For this geographic region with this electric grid, and viewed through the lens of CO₂ emissions, greenhouses far outperform indoor facilities.

Regardless of the location of your greenhouse, high-performance sealed greenhouses can maximize your productivity and reduce your CO₂ emissions. It is important to understand the role of geography (infiltration, ambient conditions, sunlight) and the source of electricity serving the facility when assessing the performance and emissions of any new greenhouse facility, or when assessing existing facilities for energy, productivity, or emissions improvements.

Gretchen Schimelpfenig, PE, is the technical director of RII and manages the organization’s Technical Advisory Council.

Nick Collins, PE, is a member of RII’s Technical Advisory Committee and a contributor to the “HVAC Best Practices Guide.”