The application of Next Generation Growing (NGG, in Dutch “Het Nieuwe Telen”) in Dutch greenhouses during the last years has resulted in energy savings. Instead of screen gaps, growers close their screen(s) completely and use a Ventilation Jet system to lower the greenhouse air relative humidity (and temperature) in a controllable way.
A Ventilation Jet system blows dry and cold air from the top compartment into the greenhouse to lower the greenhouse relative humidity (and temperature). By using a screen gap above the main path, the excess heat and humidity could escape. The efficacy of this system was investigated at a commercial greenhouse; the air flow through the VJ and through the screen gap as well as the horizontal temperature profile were measured in spring 2017.
The outcome was that less than 3% of the incoming air leaves the greenhouse through the screen gap above the central path; the vast majority of the air (> 97%) passes through the screens to the top compartment. It can be therefore concluded that the small gaps above the central path should not be applied in the specific greenhouse. Horizontal temperature difference (2.0 °C - 2.7 °C per 100 m) was recorded when both screens and supplementary lights were used. Closing the gap above the main path and replacing it with more and evenly spaced gaps with controllable size can probably homogenize the temperature distribution. This horizontal temperature difference did not occur when the supplementary lights were switched off.
The described study was part of “Kas als Energiebron” program, it took place at Gardener’s Pride greenhouse in Beetgum, NL and it was funded and performed in cooperation with LTO Glaskracht Nederland and the Dutch Ministry of Economic Affairs.