Study of the vertical temperature profile in a tomato greenhouse equipped with lighting, two screens and a VentilationJet system

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 (VentilationJet) 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. The effect of this greenhouse climate control equipment use on vertical air temperature profile as well as on energy use was studied in a commercial greenhouse. Effects of artificial lighting, heating with pipes below the crop, activation of vertical fans and air exchange rate were analyzed during the winter of 2018. It was observed that when significant heat input (radiation) at the top of the crop occurred a vertical temperature gradient up to 2°C 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; this may result in excess heat that has to be removed through either natural ventilation or with the use of VentilationJet when the screens are deployed. During the studied period about a quarter of the daily gas use for heating took place at the same time as the VentilationJet was removing warm and humid air from the top of the greenhouse.