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Key features of a proper cultivation system module in a vertical farm

"A vertical farm has to be resource efficient, scalable, adaptable, and energy autonomous using electricity generated from renewables. Using renewable energy, in combination with improved resource use efficiency (RUE) whilst achieving the highest resource, a vertical farm can achieve energy autonomy," says Eri Hayashi, President of the JPFA.

Continuing on the matter, Eri elaborates on energy autonomy, pointing out that it will be enhanced by applying recent technologies and concepts, including plant phenotyping and 3D uniform distribution of environmental factors inside the plant community.

Highlighting the requirements and features of cultivation system modules in vertical farms, Toyoki Kozai and Eri Hayashi with the Japanese Plant Factory Association will provide a revised and updated methodology toward the goal mentioned above, based on the energy and material balance equations of the CSM that were initially developed by Kozai (2013) and Kozai et al. (2021).

According to Kozai and Hayashi (2023), a cultivation room consists of one or more airtight and thermally well-insulated cultivation system modules (CSMs). Aiming to achieve the highest resource and monetary productivity (Pr and Pm) by realizing the highest annual yield at the highest quality or economic value, using minimum resources and emission of waste, including greenhouse gases (GHG) during their construction and operation.

What are the basic requirements?
As Kozai and Hayashi (2023) point out every cultivation system is independent and has no interference with other cultivation systems, while they do interact with other cultivation systems via a local information network. The cultivation room can be scaled up or down by changing the number of cultivation systems in a vertical farm.

Furthermore, the system can be scaled up or down, simply by adjusting the number of cultivation systems in the farm. On top of that, the system should be standardizable, connectable, substitutable, compatible, open, low noise, and maintenance-free. If the electricity generated and stored in batteries is insufficient to operate the whole farm, it could be run on renewable energy when reducing the number of cultivation systems.

High yield and quality with minimal resources
However, to obtain the highest yield and quality with minimum resource inputs Kozai splits them into six key requirements.

Firstly, the negligible influence of the weather on environmental factors inside the cultivation system. Secondly, the negligible loss of CO2 and water vapor to the outside through air gaps. Thirdly, the negligible loss of water and nutrient components used for hydroponics and washing or cleaning the floor and cultivation beds to the outside. Fourthly, the negligible condensation of water vapor on the inner surfaces of walls, floors, and ceilings. Fifthly, a relatively easy hygiene control of each cultivation system with no pesticide application. Sixtly, as well as an optimal environmental control that allows for maximizing the highest resources and monetary productivity with minimal electricity consumption.

To maintain a stable climate for its plants, the cultivation system should consist of lights, air conditioning, hydroponic cultivation, CO2 enrichment, and tools to measure and control the environment. "However, when multiple cultivation systems are growing the same cultivar using the same crop calendar, a part of relatively expensive sensors in some systems (except the mother cultivation system) can be left out to reduce the costs, assuming that all plants are growing similarly in the other cultivation systems," Kozai adds.

Nonetheless, when it comes to a commercial large-scale vertical farm, mechanical ventilation is required with an automatic shutter for proper air exchange through fine-mesh filters. This forced ventilation unit is only used in an emergency, such as fire and excessive accumulation of volatile organic gasses, which might cause physiological disorders in plants and foul odors to humans or harm human health. According to Kozai, these gases can be emitted by certain plants and certain additives in plastic products, including paints on the surface of structural elements (such as trichloroethylene and methyl tertbutyl ether).

Figure 1 shows the resource inputs, product outputs from the cultivation system and the environmental control factors (Kozai et al., 2021). The data measured online shown above are used for estimating hourly, daily and weekly rates of net photosynthesis and transpiration, RUE, and Pr, as described in the next section. Vertical farms gain high traceability, reproducibility, and predictability with time-series datasets. This feature can be a powerful tool to improve the productivity of each resource element of the CSM successively.

Measurements of resource inputs, product outputs and environments
"A cultivation system is characterized by high observability regarding resource inputs, product outputs and environments," Kozai states. All environmental sensors, flowmeters of CO2, nutrient solutions, wastewater, watt meters for the light source, air conditioners, and other electric equipment are relatively small, reliable and inexpensive.

The waste heat energy released from outer units of air conditioners for cooling can be estimated accurately as the sum of the electricity consumption of lighting systems, air conditioners, or other equipment such as water pumps and air circulation fans.

How to measure your elements
Therefore, Kozai explains, "the estimated waste heat under stable inside and outside air temperature conditions is accurate because of the highly airtight and thermally well-insulated cultivation system modules." To create a better overview, Kozai and Hayashi note that airtightness can be expressed using the number of air exchanges per hour or hourly air infiltration rate divided by the room air volume of the cultivation system, Nh (h-1).

When it comes to thermal insulation characteristics of walls, floors and ceilings are expressed using a heat transmission coefficient, K (W/mC). The chemical composition and volume of a stock solution are recorded when prepared. Amounts of wastewater drained from hydroponic cultivation units and other drainages can be measured separately with filtered flowmeters. The number of seeds planted and locations of the seed trays are recorded with time. Then, the area ratios of seeds, seedlings and plants to the total cultivation area are calculated separately in a vertical farm.

As Kozai and Hayashi (2023) see it, the fresh weight of plant residue, daily mounts of the used substrate and other consumables are easily measured or estimated using a weighing balance. The time of any person entering or getting out of the cultivation system from the air shower room can be automatically recorded. All the above variables are measured at time intervals of 0.1-24h.

The Advances in Plant Factories book by Burleigh Dodds Science Publishing is available here.

For more information:
Japan Plant Factory Association
Toyoki Kozai, Honorary President
Eri Hayashi, President