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Venting naturally in the summer

Sometimes the simplest forms of venting make the most sense.

Sometimes the simplest forms of venting make the most sense.

Temperature and humidity – maintaining this balance can be a constant source of struggle for greenhouse producers. As summer time rolls around, sometimes the simplest forms of ventilation can make the most sense in time and expense.

“Natural venting is by far the most popular form of ventilation for modern greenhouse growers in the Northern Hemisphere,” says Leigh Coulter, president of GGS Structures, based in Vineland, Ont. As she explains, natural venting is equally popular for gutter-connected and ground-to-ground freestanding greenhouses, where this type of ventilation can excel. To understand why, Coulter draws a literal picture shown here.

“Roof vents operate on the principal of the chimney effect. Hot air inside the greenhouse rises naturally. By placing a venting on the leeward side, wind travelling over top the vent creates suction pressure that pulls the hot air out of the greenhouse. Vents closer to the ground on the windward side let cooler outside air come in to replace the hot air that is being pulled out.”

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With vents on the leeward side, wind travelling over top creates suction pressure that pulls hot air out of the greenhouse. Vents closer to the ground on the windward side let the cooler outside air into the greenhouse

Built from taller posts, the height of gutter-connected structures allows for a greater chimney effect. Freestanding greenhouses can make use of both roof vents and rollup sides to increase the amount of natural ventilation. In a correctly designed greenhouse, the results would be even cooling within the crop, for a more uniform product.

That being said, there are situations where natural venting may not be as suitable.

“If you need your greenhouse temperature to be below ambient temperature and you are in a dry climate zone, then fan and pad cooling is better than natural ventilation,” Coulter explains. “But fan and pad do not work as well in humid climates, so natural ventilation is better in those situations.”

Before the summer heat peaks, Coulter recommends checking airflow from vents, having racks greased and conducting a proper maintenance check. “To visualize airflow you can use coloured smoke bombs as a low tech solution to see how the air flows through the greenhouse. This technique can also be used to see where you have air leaks that you may want to eliminate or reduce before winter.”

She emphasizes that ventilation is only one part of environmental control. “Humidity and temperature and light levels also play a roll in determining how the greenhouse systems operate. A good environmental computer will take inputs from both the outside environment and the growing environment and determine what needs to be adjusted to maintain the ideal environment for the crop.”

For those looking to retrofit older greenhouses, Coulter says they’ve been able to add natural venting to many older greenhouses over the years.

“All that is required is for us to know the structural details of the existing greenhouse, the geographic location for climate data, and the crops you are growing. From there we can build a plan with the grower.” 

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In January’s issue of Greenhouse Canada, readers were introduced to the concept of ‘Growing by Plant Empowerment’ (GPE). Combining grower experience and knowledge of plant physiology, the goal of GPE is to optimize the behaviour of plants in the greenhouse environment by maintaining critical balances involving energy, water, CO2 and assimilates within the plant.

These balances can be monitored by sensors, combined with crop measurements, then interpreted in the context of plant physiology and physics to help finetune and improve the crop.



British want better insulated greenhouse without glass

Glass in a greenhouse has well-known advantages, but also disadvantages.

For example, glass offers little insulation. With thermal insulation, growers in cold climates could benefit much more from greenhouse cultivation.

That is why the British Crop Health & Protection (CHAP) recently organized a webinar about the Envirup Insulation System Project, a research project that aims to provide a solution to this problem. “Houses are expected to be well insulated to keep the heat in and the cold out, so why can’t greenhouses do the same?” stated Assim Ishaque, energy advisor and keynote speaker at the event.

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The project was led by Envirup, a UK clean technology consultancy. This is a feasibility study aimed at verifying the performance of the Envirup Insulation System (EIS) on different crops at various locations and to see how optimal greenhouse conditions can be maintained for as long as possible with the lowest possible heat loss. The project was supported by CHAP and Cambridge HOK, among others, and was based on the perspective of the grower trying to reduce the energy costs in a cold climate.

Panel system
After a short introduction, Assim Ishaque, one of the project leaders, gave some more details about the system. It consists of a panel and hood design that can be built on top of each other.

In addition to the high energy costs, Assim also cited the increasing consumer demand for sustainably and locally grown fresh products as a reason to insulate greenhouses. People want fewer food kilometers and governments often want growers to pay taxes on their energy consumption. So there is a double benefit in reducing energy costs. “Horticulture is an interesting sector for the insulation industry, because innovation quickly pays for itself. So we decided to build greenhouse walls from polycarbonate. When testing the panels, we found a total light transmission percentage of 80.28%, so there wouldn’t be much sunlight are lost.”

Another advantage of the panel structure is that it is much easier to install and involves fewer risks and fewer personnel. “Only two people are needed to assemble the two-metre-long panels. The glass structure becomes superfluous in this way and no one will be able” explains Assim.

In addition, with global raw material shortages, it is only an advantage that much less steel is needed for the construction, because the panels are self-supporting. “Setting up the greenhouse also saves the grower 10 construction days, so this time can be spent on cultivation.”

Smaller local cultivation
One of those present asked whether the fact that only the walls can be insulated in this way is not a disadvantage for larger greenhouses, since the heat loss in such is greater via the ceiling.

“The smaller the greenhouse, the smaller the heat loss because the walls retain a relatively large amount of heat. Our starting point is therefore the small-scale greenhouse, although the project does not stop there,” explains Assim. “With this system, growers can work all year round, even in winter in cold climates, without sacrificing the cost-effectiveness of their operations.”

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“This system also contributes to the diversification of greenhouse cultivation. What we are going for is local, sustainable cultivation. Several smaller greenhouses that produce a variety of products can help us better than one large that achieves a huge yield for export. This system supports the local cultivation model.”

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