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Positive pressure greenhouses: "Crucial for plant health in medical cannabis cultivation"

For medical cannabis cultivation, where fine-grain measures of control over the risk of contaminants are crucial, high-tech environmental control systems are an absolute must.

To achieve this, some measures need to be included in the greenhouse design to produce a localized climate around each plant as well as a positive pressure inside the greenhouse.

A positive internal pressure system creates environmental conditions that repel insects and pests that can be detrimental to plant germination and growth.

Bosman Van Zaal has developed exactly such a system: An integrated circuit that configures internal pressure optimally, ensuring maximum crop protection.

This system uses a more modern approach to the conventional pad and fan cooling systems achieving positive pressure by ensuring inlet fans have greater airflow than outlet fans while also creating an evenly distributed localized climate around each plant. It is also possible to use an energy-efficient way of creating positive pressure through roof vents (natural ventilation).

In this article, Bosman Van Zaal will compare the two ways of implementing an evaporative pad and fan cooling system: The conventional way most commonly used versus the more modern approach mentioned above.

Apart from the benefits of positive pressure and localized climate generation for medical cannabis cultivation in the modern approach, the article will also compare efficiency in water and energy consumption as well as the impact on land use, crop quality, and crop yield for each method in a hypothetical example.

How does evaporative pad and fan cooling work?
Firstly let’s look at how evaporative pad and fan cooling works. When water evaporation occurs, energy is needed. This means heat is taken out of the air cooling the environment in which evaporation takes place. This is precisely the science used to cool a greenhouse using a pad and fan system. Warm air is drawn over wetted filter pads by means of exhaust fans, and at this point, the water naturally evaporates into the air taking with it the heat required to make it evaporate, cooling the greenhouse.

Conventional evaporative pad and fan cooling
For the conventional method, we have a membrane of running water pumped over the pads on the padwall end of the greenhouse, with the hot, dry air outside being pulled through the padwall by exhaust fans on the opposite end of the greenhouse. The padwall cools the air coming into the greenhouse resulting in the internal cooling of the greenhouse.

However, because there is little control over the free air movement between the padwall and the fans, this method results in uneven temperature & humidity distribution as well as inhomogeneous crop growth with below-average to average crop quality and yields at best.

In a hypothetical situation where we have a greenhouse area of 5120m2, 256m2 of padwall area, 10 pumps, and 32 fans, below we can see the breakdown of water and energy consumption to yield 1kg of produce:

Modern evaporative pad and fan cooling
In our more modern approach, we have two climate chambers on opposite ends of the greenhouse with pads on the outside wall and fans on the inside wall of each chamber. The hot, dry air is pulled through the padwall where it is cooled and sucked into a perforated air distribution duct which then moves up vertically through the perforations.

This system creates much greater control over air flow with a more homogenous horizontal and vertical temperature distribution resulting in a-grade quality crop production with better yields.

Furthermore, the modern configuration of evaporative cooling provides continuous air circulation in the greenhouse when cooling is not needed, providing better temperature distribution and more efficient heating when required.

It is also possible to treat incoming air through the chamber, as well as recirculate and dehumidify greenhouse air.

In a hypothetical situation where we have a greenhouse area of 14720m2, 512m2 of padwall area, 20 pumps, and 128 fans, below we can see the breakdown of water and energy consumption to yield 1kg of produce:

As can be seen, the modern evaporative pad and fan cooling technique offers less water consumption, much more even growth and productivity, leading to better quality yields as well as greater efficiency in land usage (Ratio land:padwall area 28,75:1 m2 vs. 20:1 m2 for the conventional method). Furthermore, more steel is required to build compartmentalized greenhouse units to mitigate the uneven temperatures of the conventional system, whereas the modern option offers an even distribution of temperatures over a larger area requiring less steel construction.

For more information:
Bosman Van Zaal
[email protected]

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