Photos: Adobe Stock

As our world faces challenges associated with population growth, climate variability and the loss of arable land and fresh water, 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 air speed 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.

In this third article of a four-part series, researchers from Michigan State University share science-based information about indoor production of leafy greens and herbs. To read part one, click here. To read part two, click here.

In recent years, researchers and growers have been mainly focused on quantifying the effects of sole-source light quality on crops grown in indoor CEA warehouses and containers. However, one commonly overlooked environmental parameter that has the potential to increase growth and yield is carbon dioxide (CO₂). Atmospheric (ambient) CO₂ concentration has been increasing over the years from below 320 µmol·mol^-1 (or parts per million, ppm) in 1960 to current values where CO₂ levels outdoors comprise 0.04% of atmospheric volume, or around 400 µmol·mol^-1. However, CO₂ concentrations in a “tightly sealed” greenhouse or indoor growing operation can quickly dip down to 200 µmol·mol^-1 as plants use CO₂ during photosynthesis. You may think that maintaining the CO₂ concentration at ambient levels is as easy as venting or introducing fresh air. It can be during certain times of the year, but this can be difficult when outdoor temperatures are very low. Increasing CO₂ to concentrations above ambient and up to 1,200 µmol·mol^-1 has been shown to increase photosynthetic rates, growth and yield. There are several commercial methods to increase CO₂ concentrations above atmospheric levels. However, these methods should be deployed during periods when ventilation is minimal to reduce the loss of the added CO₂ outside of the growing area.

For CO₂ enrichment above ambient, growers can deploy CO₂ burners that produce CO₂. This method produces some heat, moisture and CO₂ by burning natural gas or propane. Incomplete combustion or contaminated fuels can lead to the introduction of toxic gases for both plants and humans. To improve CO₂ uniformity, burners should be dispersed throughout and horizontal air flow (HAF) fans can be deployed to circulate air. Another method of CO₂ enrichment for both greenhouses and indoor farms is injecting compressed or liquid CO₂ from a tank. The compressed CO₂ is converted from a liquid to a gas and then released into the growing area. These tanks can be rented or purchased through local gas distributors. When delivered in this form, CO₂ is often dispersed through polyethylene tubes. Remember that CO₂ is heavier than the other air components, so concentrations tend to be greater closer to the floor.

Experimental protocol

The goal of our research program is to develop indoor and greenhouse environmental management protocols for different stages of culinary herb production. Given that CEA production is energy-intensive, we have focused our efforts on young plant production since inputs such as light and CO₂ can be delivered across more plants at the seedling stage when plant density is greater compared to the finished stage when plant density is lower. The objectives of our research were to determine if indoor CO₂ enrichment during the seedling stage influences: 1) sweet basil seedling growth and development; 2) morphology, growth, and yield at harvest in the greenhouse; and 3) volatile oil content and flavor (this research is in progress and we will report the results in an upcoming article).

Sweet basil ‘Nufar’ seeds were sown in Grodan rock-wool cubes and placed in walk-in growth chambers with an average daily temperature (ADT) set point of 73° F (23° C) and CO₂ set points of 500 or 1,000 µmol·mol^-1 throughout the day and night. We maintained these concentrations by injecting compressed CO₂ into the chambers, and by scrubbing CO₂ with soda lime when concentrations were too high. In each chamber we had four light intensity treatments of 100, 200, 400 and 600 µmol·m^–2·s^–1 that operated 16 h per day to create daily light integrals (DLIs) of 6, 12, 23 or 35 mol·m^–2·s^–1. This allowed us to determine whether there was an interaction between CO₂ concentration and light intensity. The seedlings were grown for two weeks, after which plants were transplanted into deep flow technique (DFT) hydroponic systems in a greenhouse with an ADT of 73° F (23° C) and a DLI of 14 mol·m^–2·s^–1. With the plants growing in a common greenhouse environment, we could evaluate if differences or higher inputs at the seedling stage would result in increased yields down the road.

In theory, by increasing CO₂ concentration, we should have seen increased growth; however, this was not the case. As can be seen in Figs. 1 and 2, light intensity had a much larger impact on seedling growth than CO₂, but we will discuss that in the next article of this series. Increasing CO₂ concentration did not influence growth and development. Why would increasing CO₂ from 500 to 1,000 µmol·mol^-1 not actually improve growth and development of basil seedlings?

There are three main limitations to photosynthesis: the supply or utilization of CO₂, of light, or of phosphate (also referred to as biochemical limitation; Fig 3.). Theoretically, if CO₂ concentration starts at 0 and increases, you will hit a CO₂ compensation point above which plants have positive net photosynthesis. As the CO₂ concentration further increases, the photosynthetic rate will increase until the CO₂ concentration reaches a species or cultivar-dependent saturation point. In our case, 500 µmol·mol^-1 CO₂ may have been near the saturation point for basil seedlings. Therefore, increasing the concentration to 1,000 µmol·mol^-1 did not significantly increase growth. In addition, if photosynthesis is light-limited, increasing CO₂ concentration will not result in large increases in photosynthesis. Conversely, if photosynthesis is CO₂-limited, increasing light intensity will not result in large increases in photosynthesis. However, increasing CO₂ can have some other positive effects, including reducing oxygenation and photorespiration. In our case, increasing the light intensity further or growing the plants past the transplant stage may have resulted in differences between CO₂ treatments.

Do our results mean we should write off CO₂ enrichment as a means of increasing photosynthesis and ultimately growth and yield? No. We are currently evaluating several other culinary herbs at lower and higher CO₂ concentrations and a range of light intensities to see if there is a species-dependent response. In addition, it is possible that elevating CO₂ concentration would have a larger impact during the finish stage. More studies are needed to parse out which horticultural crops and at what stage of production CO₂ enrichment would be the most beneficial to improving growth and yield.

Take-home message

Other environmental factors besides CO₂ concentration may have a larger impact during the seedling stage. Read the last article in this four-part series to understand the benefits of increasing indoor CEA light intensity during propagation on subsequent yield in the greenhouse.

Kellie is a PhD student and Roberto is assistant professor and controlled environment/floriculture extension specialist in the Department of Horticulture at Michigan State University. The authors gratefully acknowledge Sean Tarr and Nate DuRussel for assistance, Fluence Bioengineering for LEDs, JR Peters for fertilizer, Grodan for substrate, Hydrofarm for hydroponic production systems, Dramm for irrigation equipment and MSU AgBioResearch, MSU Graduate School and the USDA-NIFA for funding.

One of the biggest costs for any hydroponic business, regardless of the crops they produce, is labor. Mechanizing processes to reduce the amount of labor required for crop production is a sure way one way to reduce input costs. While the primary reason for using mechanization and automating processes is to reduce labor costs, there are other benefits to automation. For example, in addition to saving on labor costs, it can also make more labor available for other jobs in the greenhouse, make certain tasks easier to execute and increase employee productivity. But benefits of automating extend beyond labor. Automation also reduces the time it takes to accomplish tasks and expedites processes and may open additional space in your facility and help you turn crops.

Regardless of what scale you are producing on, there are steps you can take to improve your efficiencies by integrating automation and mechanization to improve productivity. Plus, there are some relatively easy steps to take to start automating processes in your greenhouse. Investing in a seeder is going to be one of the first things any producer should do. A vacuum seeder is going to be one of the first and easiest ways to incorporate equipment to improve productivity. Manifold seeders seed one row of plugs at a time, whereas plate seeders will seed an entire flat at once; large facilities may benefit from using the larger drum or cylinder seeders.

In the greenhouse, regardless of size, automation can be key to maintaining a productive growing environment. Nutrient solutions for recirculating systems require adjustment to keep the pH and electrical conductivity within target ranges. This can be achieved by hand, spot-checking nutrient solution EC and pH and making the adjustments as-needed. However, automated pH and EC measurement of nutrient solution properties and subsequent adjustments moderates root-zone conditions and minimizes unwanted fluctuations. Sanitizing the troughs for nutrient-film technique (NFT) systems and rafts for deep-flow technique (DFT) or raceway systems can also be a laborious process. The opportunity to sanitize NFT channels and DFT rafts keep your system clean and reduces disease incidences and increases food safety, but it can be a laborious task. Equipment simplifying routine sanitation reduces labor costs and helps maintain best management practices (Fig. 1).

The larger the scale an operation and production, the possibilities to automate become even greater; the return on investment increases with the scale of production. In order to centralize labor, from planting to harvest, in the same part of the facility, some production systems are designed to move crops through a growing environment throughout production. For instance, the lettuce grown in the NFT system in Fig. 2 is planted in the same area of the greenhouse where it is harvested. The NFT troughs move throughout the greenhouse and returns to be harvested. There are also opportunities to automate post-harvest processing and packaging. For common Dutch cucumbers produced hydroponically, conveyor belts facilitate transporting fruits from harvesting carts to shrink-wrapping machines, then onto cardboard containers for bulk packaging.

And there are some very exciting robots in the CEA industry. Grafted tomato plants are increasingly popular, but grafting is a very labor-intensive process. While these grafting robots still require labor to operate, they can complete up to 800 grafts per hour, greatly improving productivity. On the other end of production, we are not far from having automated harvesting as well! There are robots designed to selectively harvest hydroponically grown fruits such as cucumbers and strawberries.

Christopher (ccurrey@iastate.edu) is an assistant professor in the Department of Horticulture at Iowa State University.

Transportation has long been one of the most unregulated aspects of the food supply’s chain of custody, particularly in relation to food safety. That all changed with the passage of the Sanitary Transportation of Human and Animal Food rule of the Food Safety Modernization Act (FSMA). Among the key aspects of that rule are requirements for ensuring the vehicle and equipment are capable of maintaining temperatures needed for food safety, and having monitoring mechanism and written procedures and records to ensure that food is transported under adequate temperature control.

For produce, temperature is often more a factor of quality than food safety. However, there are several instances in which food safety can be impacted by out-of-spec temperatures and environmental conditions, including:

• If the produce has been subject to pathogenic contaminated in the field or during harvest (such as the 2018 E. coli contamination of romaine lettuce), higher temperatures can enable greater pathogenic growth.
• Fresh-cut, bagged or packaged produce — as well as some more perishable fresh fruits and vegetables including strawberries, lettuce, herbs and mushrooms can require temperature controls. In fact, FDA advises that consumers store these at a temperature of 40° F or below and only purchase fresh-cut, bagged or packaged produce that is refrigerated or surrounded by ice. More information is available here.

But whether the produce is susceptible to food safety risks, its quality and shelf life is dependent on environmental controls. Thus, the continuing advancement of technologies designed for temperature control, maintenance and validation throughout the food’s transport can be invaluable, from temperature sensors to remote monitoring and GPS tracking.

“Keeping fresh produce ‘fresh’ requires maintaining a careful combination of temperature and humidity,” says Dr. Jennifer McEntire, the United Fresh Produce Association vice president of food safety and technology. “Data loggers have historically been used, and the evolution of technology now allows the capture and real-time analysis of conditions to maximize the quality of the product, which means reducing shrink.” While temperature sensors, remote monitoring and GPS are not new, the continuing improvement of these technologies enables growers to gain greater control over downstream traceability for the quality and food safety of their produce. As I discuss in Chapter 10 of the upcoming book “A Chain of Linked Nuances of Food Traceability,” not only can these monitor the temperature of the product, but with GPS tracking, the trucks can be monitored for location, speed, unscheduled breaks, etc. When drivers know they are being tracked, they may tend to be more diligent, resulting in added protection for food safety and food defense as well.”

The technologies have begun to be used recently, according to Produce Marketing Association vice president of supply chain efficiencies Ed Treacy.

“Over the last three to four years, the technology has really improved with the environmental and GPS sensing,” he says. Treacy is seeing the food industry using such technologies to provide full-chain visibility of the supply chain: to tie transparency all the way through the chain.

For growers in particular, he says, it can provide a record of what occurred between loading and unloading of the produce. Not only does this help ensure the maintenance of temperature, the remote sensors can provide an assessment of the environmental conditions, monitor for issues such as shock and ethylene levels, and record GPS coordinates in near real-time.

“I’ve seen companies use that to predict freshness,” Treacy says.

When drivers know they are being tracked, they may tend to be more diligent, resulting in added protection for food safety and food defense as well.

Some of the sensing equipment also will provide alerts. The grower or receiver can select the device by entering a serial number, then set standards, such as minimum or maximum temperatures, and corresponding alerts. The alerts can be sent to one’s phone, with notifications as soon as a condition goes out of range. In some cases, the alert may even precede the driver’s knowledge of an issue and enable the grower to contact the truck and potentially save the load.

“Sensor technology is evolving very, very quickly and getting relatively cheap,” Treacy says. In fact, he says, use has become fairly standard for long-distance loads moving across the country.

The GPS-enable devices also can increase operational efficiencies through the setting of geofencing, Treacy says. That is, the setting of a virtual geographic boundary, enabling an alert when the truck enters or leaves a specific area. For example, the grower or recipient could set the device to provide an alert when the truck is 50 or 100 miles from its destination. The notification enables the customer to prepare for the delivery and the grower to know if the truck has met its appointed time — or alert the customer that the truck is off schedule. This technology, Treacy says, “helps you manage your driver and see any exceptions.”

While the use of technology in transportation can help maintain the quality and value of fresh produce, it also is critical that growers ensure that drivers are following the requirements of FSMA’s Sanitary Transportation rule as well, Treacy says. “The Sanitary Transportation Rule is the best thing for food safety.” This is particularly true for produce because it is generally unpackaged — with even the containers used for items such as mushrooms and strawberries having vent holes through which the product can be contacted. To help prevent the possibility of contamination in transportation, he says, you need to know:

• The history of the truck — what it has carried prior to picking up your load
• The truck’s cleanliness — and what was used to clean it. Some cleaners and sanitizers can contaminate food it contacts.
• If it has and does follow good transportation practices
• The way it is loaded (e.g., fresh meats or allergens should never be placed above produce)

No matter how conscientious the grower is about food safety and quality from seed to harvest, a lack of controls during transportation can reduce all that to naught. And the perishable, unpackaged nature of produce simply adds to the risk.

“For many perishable items, maintaining quality is a race against the clock, and things like GPS tracking give greater insight into how to manage resources and inventory,” McEntire says. “Embracing these technologies can help improve efficiencies of perishable products.”

With all the advances of technology, Treacy sees a continued evolution of their deployment and use.

“Now that that information is made available, there are a lot of creative ways that people are using it,” he says.

Lisa is editor of Quality Assurance & Food Safety (QA) magazine and a Food Traceability chapter author. llupo@gie.net