The application of Next Generation Growing (NGG, in Dutch "Het Nieuwe Telen") in Dutch greenhouses has resulted in energy savings, accompanied by the extensive use of screens. The latter has affected the control of greenhouse climate which strongly depends on the air and humidity exchange between the top and bottom greenhouse compartment, as they are separated by the screen itself.
When the screens are fully deployed an air exchange/mix system (e.g. VentilationJets) that blows dry and cold air from the top compartment into the greenhouse can be used to lower the greenhouse air temperature and relative humidity in a controllable way. Significant part of our applied research focuses on analysing the effect of the aforementioned air mixing systems on climate uniformity as well as on energy use, aiming to contribute towards their most efficient and appropriate use.
Measurements in spring and winter
In spring 2017, the air flow through a VentilationJet system and through the screen gaps as well as the horizontal temperature profile were measured in a commercial greenhouse. In winter 2018, the effect of the aforementioned greenhouse climate control equipment use on vertical air temperature profile as well as on energy use was studied in the same greenhouse. Effects of artificial lighting, heating with pipes below the crop, activation of vertical fans and air exchange rate were also analyzed.
It was observed that when significant heat input (e.g. sun radiation or artificial lights) at the top of the crop occurred a vertical temperature gradient up to 2oC exists, with the bottom of the crop being colder than the top. This temperature gradient cannot be reduced by using only vertical air circulation fans but it can be minimized by additional heat input at the bottom; the latter, may result in excess heat that has to be removed through either natural ventilation or with the use of VentilationJets when the screens are deployed.
Homogeneous temperature profile
The two systems (VentilationJets and heating system) act on opposing directions but by balancing the heat input at the bottom of the crop with the heat removal from the top, a homogeneous vertical temperature profile can be achieved. During the studied period about a quarter of the daily gas use for heating took place at the same time as the VentilationJets were removing warm and humid air from the top of the greenhouse.
In winter 2018-2019, the horizontal temperature and humidity profile under different scenarios of climate control settings will be studied in a commercial greenhouse equipped with "AirMix" system.
"Kas als Energiebron" program
The described studies were part of "Kas als Energiebron" program, they took place at Gardener's Pride greenhouse in Beetgum, NL and they were funded and performed in cooperation with LTO Glaskracht Nederland and the Dutch Ministry of Economic Affairs. The upcoming study will take place at a commercial rose grower in the Westland area and it is also part of the same program.