March 25, 2019

 Dr. Nadia Sabeh

As our world faces challenges associated with population growth, climate variability and the loss of arable land and freshwater, we humans need to find new and efficient ways to grow food. Vertical farming has emerged in the past decade as one of the tools we can implement to face some of those challenges. In most cases, the vertical farm (VF) is characterized by an array of shelves stacked vertically and side-by-side, filling the volume of a warehouse-type space with lettuce greens and culinary herbs that are usually spaced tightly together. Under this configuration, a farm can grow upwards of 5 to 10 times the number of plants for a given footprint as compared to field-grown crops. Theoretically, the number of plants grown is only limited by the height of the building.

This unlimited growth potential is a lucrative proposition and has created a “sky is the limit” mentality for many prospective VF growers and developers who envision a space filled to the brim with plants. Unfortunately, this perspective has led many down an unrealistic path of ”packing them in,” considering only how much space is required between each vertical rack to fit the growing media, the lights, the full-height plant and sometimes even the arms of the automated harvesting machine. The space requirements for HVAC equipment and air circulation around the plants are often overlooked, resulting in the two most commonly identified difficulties when controlling the environment: humidity control and air movement.

Challenge No. 1:

Temperature and humidity

The first climate management challenge that vertical farmers must overcome is figuring out how much cooling, dehumidification, and heating is required to manage the temperature and humidity of the grow space. In a VF, lighting contributes the greatest source of heat, followed by motors used to operate fans, pumps, and automation. Because VFs are often well insulated and designed to operate day and night throughout the year, cooling is usually required 24/7 and year-round to remove the heat generated inside the space.

Dehumidification is also constantly required to remove the moisture added to the air via evapotranspiration (Et) from the plants and irrigation system. The rate and quantity of Et depends on several variables, including light intensity, air temperature and humidity (or vapor pressure deficit), air movement and the irrigation method. Although Et is greatest when plants are mature and the lights are on, Et does not stop when the lights go out. Plants continue to respire and give off moisture when the lights are off, and for continuously recirculating irrigation systems (e.g. NFT and aquaponics), evaporation from these systems can remain constant all day. Therefore, the size and operation of the dehumidification system should take into consideration both the maximum and minimum Et rates expected inside a VF.

Heating systems in the VF are rarely required, due to all the heat generated inside the space by lights. However, if the air conditioning (AC) system is used to both cool and dehumidify the space, then the AC system will create very cold air to remove (or condense) water out of the air. Typically, we don’t want to deliver that cold air (eg. 45° F) back to the plants, so we reheat it before sending it to the room. This is the most common use of heating in a VF.

Challenge No. 2:

Air circulation

The second biggest challenge is figuring out how to deliver the conditioned air everywhere within the vertical farm to create a (hopefully) uniform growing environment. When racks are spaced tightly together — both vertically and horizontally — it is difficult to create uniform conditions everywhere. In the horizontal direction, the plants and lights obstruct the flow of air from Point A to Point B, often resulting in temperature, humidity and airspeed differences from one end of the rack to the other. When the vertical height is very short (say 4 inches or less), this obstruction of air movement is magnified. On top of that, the air moving across the shelf is picking up heat (from lights) and moisture (from plants), causing it to become hotter and more humid along its path. Shorter vertical heights exacerbate this trend, as they limit the volume of air that can be squeezed between the rack levels, thereby restricting the amount of heat and moisture that can be absorbed and causing the air to heat up and humidify even more quickly as it travels from end to end. The result: large differences in temperature and humidity levels from Point A to Point B. And the longer the shelf, the longer the path of air and the greater the difference.

Several strategies can be applied to facilitate air movement in the VF. Many farmers employ the use of small circulating fans, installing them at incremental positions within the racking system and above the plants to help boost airflow from one end to the other. Air movement can also be enhanced by considering where conditioned air is introduced into the space and where it is then removed after loading up with heat and moisture. The type of air diffusers can also help distribute and push air into desired locations, as well as help mix cold air near the floor with warmer air near the ceiling to prevent the stack effect.

Challenge No. 3:

HVAC equipment location

Space considerations aren’t limited to moving conditioned air through the racking system; they also include where to put HVAC equipment. Depending on the design, HVAC equipment can include air conditioners, dehumidifiers, circulation fans, ductwork, chillers, boilers, pumps, and pipes. Cooling and dehumidification equipment are best located outside the building, where heat and moisture can be rejected to the outdoor air. Some equipment (air conditioners, dehumidifiers, etc.) are ideally located on the roof of the building or on the ground outside and next to the room it is serving, helping to limit ductwork. Other equipment, such as chillers, need a designated area away from the building to accommodate the larger equipment. Inside the building and the VF itself, better air movement can be realized if adequate space is provided for ductwork, fans, and air delivery and mixing in general.

Conclusion

No matter what crop is grown, managing humidity control and air movement in a vertical farm is essential to plant productivity, harvesting schedules, quality control and, ultimately, profitability. Every developer, designer and dreamer would do well to include climate management as a foremost systems consideration — in line with lighting, racking, irrigation and automation — during the conceptual and facility planning stage. Only then can vertical farming rise to its full potential.

Nadia is the president and founder of Dr. Greenhouse, Inc., an agricultural and mechanical engineering firm that specializes in the design of HVAC systems for indoor plant environments.

Lead photo: Photos: Adobe Stock

 Dr. Nadia Sabeh

Vertical farms  Aquaponics Lettuce NFT Herbs Temperature Climate control