Open access publications in the SLU publication database

Abstract

In the North European greenhouses, energy use for climatization accounts for a significant share of the operational costs. Air temperature and humidity are important factors whose control is crucial for assuring high productivity and limited use of energy.

The objective of this thesis was to examine the use of thermal energy in greenhouses and to investigate the impact of a number of climate parameters on that use. A specific goal was to compare the measured use of energy for heating with the values simulated employing a model developed in Powersim software and, by means of the model, to determine the amount of energy necessary for dehumidification of a tomato greenhouse. Another specific goal was to employ the experimentally obtained performance values for a rotary heat exchanger to estimate the potential energy savings when heat exchanger is used for dehumidification of a greenhouse.

Indoor and outdoor climate data were collected in three greenhouses located in southern Sweden, two with tomatoes and one with ornamental plants. The use of thermal energy in a tomato greenhouse was first measured and then modelled in Powersim for different levels of transpiration, i.e. for a leaf index area (LAI) of 3.5 and 4.0 m²/m². The impact of wind under no-sunlight conditions and at different outdoor temperatures was investigated. The performance of the heat exchanger operating at high humidity levels was tested in a series of measurements.

The study showed that the use of thermal energy in greenhouses with tomatoes was significantly higher than in the greenhouse with ornamental plants. Further, the amount of energy used increased together with the wind speed. The reduction of the wind speed by 50% could result in energy savings of 4-10%. The use of thermal energy as obtained in the Powersim simulations was fairly similar to the measured values, especially when the modelling was for higher transpiration levels. However, it was concluded that further work on the model is needed. The simulations indicated that 23-29% of thermal energy in a greenhouse was used for dehumidification purposes. It was experimentally shown that thermal and moisture efficiencies of the heat exchanger employed in the study were about 70% and 45%, respectively. Further, it was found by modelling that the usage of such a heat exchanger in the investigated greenhouse resulted in the energy savings of 15-17%.