Farm Operations Management
CO2 and HVAC Management in Vertical Farms: The Profitability Lever That Gets Overlooked
Articles for Farm Operations Managers
When yield or quality isn’t improving in a vertical farm, the first things that get reviewed are light and nutrient solution. Both matter — but when air design is neglected, you’re leaving gains on the table.
CO2 is the raw material for photosynthesis, and airflow is the channel that delivers that CO2 to the leaf surface. Raise concentration alone, and without airflow, gas exchange stalls around the leaves. Keep the air moving without balancing temperature, humidity, and CO2, and the effect fades.
This article looks at CO2 concentration, airflow, temperature, and humidity as an integrated system — and lays out a way of thinking about the air environment that connects directly to vertical farm profitability.
Air Environment from the Plant’s Perspective: The Mechanics of Photosynthesis and Growth
CO2 is the raw material for photosynthesis and the foundation of plant growth. At the current atmospheric CO2 concentration (approximately 400 ppm), plants cannot operate at full photosynthetic capacity. Research indicates that photosynthetic rate peaks for most crops at 1000–1200 ppm. Proper CO2 management creates the conditions for meaningful yield gains.
Plants suffering from CO2 deficiency show characteristic symptoms: thinner leaves, paler color, and delayed growth.
Without adequate airflow, plants cannot grow well. On the leaf surface sits a stationary layer of air called the leaf boundary layer. When this layer is thick, it impedes gas exchange. Air movement of around 0.3–0.7 m/s thins this boundary layer and promotes both CO2 uptake and water vapor release.
Airflow also promotes transpiration — cooling the plant at high temperatures and activating the transpiration stream that carries water and nutrients from roots to leaves. It strengthens and thickens stems, preventing lodging. In still-air conditions, plants become leggy and weak; steady airflow is indispensable for growing robust plants.
For more on the basics of photosynthesis and the light environment:
LED and PPFD in Vertical Farms — Learning the Basics to Optimize Your Light Environment
Photosynthesis consists of the light reactions and the dark reactions (Calvin cycle); the latter draws CO2 from the air and fixes it into sugars. Photosynthetic rate is limited by whichever factor is most scarce — light, CO2, or temperature. Higher light intensity demands more CO2, and CO2 fixation peaks in the optimal temperature range (20–28°C). Balancing all three factors is what matters.
The effect of CO2 management varies by crop, but expected yield increases are roughly 30–40% for leafy vegetables (lettuce, komatsuna, etc.) and 20–30% for fruiting vegetables (tomatoes, peppers, etc.). Achieving the optimal balance of light, CO2, and temperature draws out the plant’s full potential.
Building the Best Air Environment in a Vertical Farm
Creating an appropriate air environment requires more than simply “moving air.” The air design has to be grounded in plant physiology.
Airflow Design for Your Crop
The ideal airflow velocity and direction varies by plant species. Leafy vegetables (lettuce, komatsuna, etc.) generally do well at around 0.3–0.5 m/s, while fruiting vegetables like tomatoes and strawberries benefit from slightly stronger airflow at 0.5–0.7 m/s. Too little airflow thickens the boundary layer and inhibits CO2 exchange; too much causes mechanical stress and excessive transpiration.
Directional Airflow: Vertical vs. Horizontal
Vertical airflow (top-down or bottom-up) suits multi-tier grow racks, equalizing temperature differences between tiers. It is especially effective at preventing condensation in winter. Horizontal airflow is best suited for wide, single-level grow beds — it creates a uniform environment and efficiently distributes CO2 across the entire growing area. In most cases, combining both types achieves optimal air circulation.
The Golden Rule for Fan Placement: Eliminate Dead Spots
The goal is to avoid “dead spots” in airflow. Dead spots create locally high-humidity zones and raise the risk of disease. Use opposing-fan arrangements to create uniform airflow, and pay special attention to corners. For spaces between grow racks or areas where plants are dense, adding small supplemental fans is effective.
Note that airflow design in a vertical farm has different goals from standard industrial ventilation design. The optimal environment for plants is not the same as a comfortable working environment for people.
Three main types of equipment create the air environment in a vertical farm:
- Circulation fans (circulators):
- Advantages: easy to install, cost-effective
- How to use: primarily for air circulation; adjust direction with season and time of day
- Air conditioners:
- Advantages: handle temperature control and airflow simultaneously
- How to use: run in combination during periods when temperature adjustment is needed
- Dehumidifiers:
- Advantages: handle humidity control and airflow together
- How to use: during high-humidity periods and for condensation prevention at night
Combining these well creates an optimal environment that adapts to season and time of day. For large-scale vertical farms in particular, optimizing airflow patterns using airflow simulation is important.
Condensation Risk Elimination Through Air Design
Condensation risk in a vertical farm can be greatly reduced through air design. Periodically direct air toward walls and ceiling surfaces to prevent condensation from forming; secure adequate air circulation in the morning (when temperature rises). Maintain light airflow at night to prevent air stagnation, and direct supplemental airflow at thermal bridging points.
Simply optimizing airflow can dramatically reduce the risk of mold and disease from condensation.
The CO2 Supply System for Optimal Concentration
CO2 Supply System Types and Selection Criteria
| CO2 Cylinder | Liquid CO2 Tank | Combustion CO2 Generator | |
|---|---|---|---|
| Suitable Scale | Small–medium (up to 100 m²) | Medium–large (100–1,000 m²) | Large (1,000 m²+) |
| Advantages | · Easy to install · High purity · Flexible placement | · No cylinder swapping · Better long-term cost efficiency · Stable supply | · High-volume supply · Low long-term operating cost · Heat also usable |
| Disadvantages | · Cylinder swap labor · Rising cost at scale · Storage and safety management required | · Significant upfront investment · Space for tank · Periodic inspection required | · Waste-heat removal required · Incomplete combustion risk · High installation and maintenance cost |
| Initial Cost | Low | Moderate | High |
| Operating Cost | Moderate–high | Moderate | Low |
Selecting the right CO2 supply system for your scale and purpose is essential. Regardless of which system you choose, without adequate airflow, CO2 cannot reach the plants and the benefit of supplying it is undermined.
The air environment in a vertical farm is hard to visualize and therefore easy to neglect — but it is a critical factor in profitability. Combining proper airflow design, temperature equalization through airflow, and efficient CO2 supply optimizes the plant growing environment and creates the conditions for improved yield and quality.
The “Golden Balance” That Unlocks CO2’s Full Power
Supplying CO2 alone is not enough to get the most out of it. The balance with other environmental factors determines the effect.
Combining CO2 with Light
Plant growth is the product of multiple interacting factors: CO2, light, temperature, airflow, and humidity. Each factor works best in combination with the others, not in isolation.
The relationship between light and CO2 is fundamental to plant cultivation. The light reactions of photosynthesis produce ATP (chemical energy), which is then used in the dark reactions to convert CO2 into sugars. Higher light intensity produces more energy and enables more CO2 to be processed. The basic principle, then, is to schedule CO2 supply around periods of high light intensity.
Temperature and CO2: How They Interact
Temperature also has a major effect on CO2 utilization efficiency. For most crops, CO2 fixation efficiency peaks in the 20–25°C range. Outside this range, additional CO2 supply is not fully utilized. At the same 1000 ppm CO2 concentration, effectiveness decreases 30–40% at 17°C, and can drop more than 50% above 30°C.
The Environmental “Balance Sheet”
To understand how environmental factors interact, here’s a simple reference of their mutual effects:
| Factor Changed | Effect on CO2 | Effect on Temperature | Effect on Humidity | Effect on Airflow |
|---|---|---|---|---|
| CO2 concentration increase | — | Slight decrease | Slight decrease | No effect |
| Temperature increase | Tends to decrease | — | Decreases | Convection increases |
| Humidity increase | No effect | Slight increase | — | No effect |
| Airflow increase | Equalizes | Equalizes | Decreases | — |
The adjustment priority order: first, maintain temperature within the appropriate range (the baseline condition for photosynthesis); next, optimize CO2 concentration (supplying the raw material); then, adjust airflow to create a uniform environment (promoting gas exchange); and finally, maintain appropriate humidity (optimizing transpiration).
How Airflow Determines CO2 Effectiveness
Even with abundant CO2 supply, the effect is limited if it doesn’t reach the leaf surface. A thin layer of air called the boundary layer surrounds the leaves; when it is thick, CO2 movement is impeded. Adequate airflow thins the boundary layer and promotes CO2 absorption.
Optimal airflow velocity varies by crop, but for leafy vegetables at 0.3–0.5 m/s and fruiting vegetables at 0.5–0.7 m/s, CO2 utilization efficiency can improve by 20–30%. When positioning circulation fans, place them so air flows above or alongside plants rather than directly onto leaves. Sustained direct airflow onto leaves can cause leaf damage.
The Break-Even Point for CO2 Investment
CO2 supply has a cost. The relationship between CO2 concentration and yield is not linear — beyond a certain point, gains plateau. From atmospheric levels (around 400 ppm) up to 800 ppm, yield increases nearly linearly and the return is highest. From 800 to 1200 ppm, the effect gradually diminishes. Above 1200 ppm, the cost-benefit ratio deteriorates.
For most vertical farms, 800–1000 ppm is the cost-effective target concentration. Concentrations above 1000 ppm rarely justify the cost outside of specialized high-value crops.
Reading Plant Signals to Identify the Optimal Environment
Beyond the data, don’t miss the signals the plants themselves are sending. Leaves that are deep green and thick indicate a good CO2 environment; thin leaves with a slight yellow cast suggest CO2 deficiency. Rapid new leaf expansion and compact internodes are signs of a good environmental balance; elongated internodes and leggy growth indicate CO2 is insufficient relative to light.
Building the habit of reading plant signals lets you catch environmental balance problems that data alone can’t reveal.
Conclusion: HVAC and CO2 Management Determine Vertical Farm Profitability
The fundamental reason HVAC and CO2 management are the key to profitability improvement is that they are “invisible environments.” Light and nutrient solution are easy to quantify, making their improvement cycles faster. The interactions between CO2 concentration, airflow, temperature, and humidity are invisible — so when problems arise, identifying the cause tends to be slow.
The most important perspective in CO2 and airflow design is the “combination” view. Even when CO2 is maintained at 1000 ppm, uneven airflow can leave the effective concentration at the leaf surface as low as 400 ppm. Conversely, when airflow is well-designed, you can keep CO2 concentration modest and still extract yield returns that outperform the cost. To maximize return on investment, balancing the interaction between factors takes priority over pushing any single factor to its limit.
The 800–1000 ppm target concentration is the cost-effectiveness convergence point for most crops and facility scales. Before considering investment in higher concentrations, eliminating “blockers” first — dead spots in airflow, temperature unevenness, poor equipment placement — delivers a greater profitability impact.
Combining theory with on-the-floor observation in a continuous improvement cycle is what makes the air environment of a vertical farm reliably translate into profitability.