Horticulturists with a Ventilation Jet system are worried about the horizontal temperature profile in the greenhouse. This is why readings were taken at Garderner's Pride in the spring of 2017 as part of the monitoring project of Kas Als Energiebron (Greenhouse as a Power Source).
Anemometers measure the flow rate through the opening above the central path
The idea behind the core is that the cold air that is blown into the greenhouse by the Ventilation Jets partially flows out of the greenhouse as warm air through the opening above the central path. But does a lot of warm damp greenhouse air flow up through this or is it negligible? And if it's considerable, what is the impact on the horizontal temperature profile?
Gardener's Pride has lit cultivation (200 µmol/m2/s), a shading screen, energy screen and Ventilation Jets to carry off the excess heat and moisture. The central opening is a set opening of 4 cm above the central path and on either side of the central path the paths are 100 metres long. Gardener's Pride has a version of the Ventilation Jet system in which holes have been made in cloths so that the air flows through them. The amount of air that flows through the Ventilation Jet is measured and the amount of air that flows up through the opening above the central path is measured by anemometers.
Schematic view of the air flow in the greenhouse
Based on the measurements it can be concluded that at Gardener's Pride the actual Ventilation Jet capacity with two completely open screens (so both screen settings at 0%) was much lower (8.9 m3/m2/hour) than the capacity calculated on the characteristics of the ventilator (12 m3/m2/hour). This difference can be explained by turbulence in the Ventilation Jet due to the ventilator being at the top of the case. In newer Ventilation Jets the ventilator is at the bottom which means there is a laminar air flow which causes much less losses to occur. With two completely closed screens the air debit decreased to 6.7 m3/m2/hour. This is partially due to the cloths causing resistance in the Ventilation Jet's case.
Schematic view of the air flow in the greenhouse
Based on the measurements it can be concluded that at Gardener's Pride the actual Ventilation Jet capacity with two completely open screens (so both screen settings at 0%) was much lower (8.9 m3/m2/hour) than the capacity calculated on the characteristics of the ventilator (12 m3/m2/hour). This difference can be explained by turbulence in the Ventilation Jet due to the ventilator being at the top of the case. In newer Ventilation Jets the ventilator is at the bottom which means there is a laminar air flow which causes much less losses to occur. With two completely closed screens the air debit decreased to 6.7 m3/m2/hour. This is partially due to the cloths causing resistance in the Ventilation Jet's case.
The researchers then saw that in different situations (lights on or off, one or two screens completely closed) a maximum of 2.8% of the incoming air left the greenhouse through the central opening above the path. The impact of the opening on the central path is therefore small and isn't needed at Gardener's Pride. In the case of the lights being on and both screens being closed, there a large horizontal temperature difference of 2.7 °C max. This isn't desirable and could be prevented by closing the opening on the central path. As the horticulturist has a set opening, we weren't able to close the opening in the experiment to see whether the horizontal temperature difference could be decreased.
In practice, a different grower with cultivation under light and Ventilation Jet no longer uses the adjustable opening on the central path. He suspects that the amount of air that flows out through the central opening is very small. He also doesn't want to create horizontal temperature differences due to this central opening.
Source: Kas als Energiebron
In practice, a different grower with cultivation under light and Ventilation Jet no longer uses the adjustable opening on the central path. He suspects that the amount of air that flows out through the central opening is very small. He also doesn't want to create horizontal temperature differences due to this central opening.
Source: Kas als Energiebron