SUMMARY
In regions with a colder climate the use of heat energy for climatization of greenhouses can be significant. Beside outdoor temperatures and climate also transpiration affects the use of heat energy since heating and ventilation or dehumidification may be required in order to keep the humidity in the greenhouse below levels which may lead to plant diseases. A reliable model is a powerful tool to predict energy use for greenhouse cultivation. Thus, a mechanistic model including transpiration and energy use for dehumidification and hourly output data was developed from an older simpler model. To be reliable, modelled data must fit with real data. The aim of the present study is to compare measured and modelled use of heat energy. Thus, energy use for a greenhouse with tomatoes was measured and compared to modelled data. The greenhouse in the study is of Venlo-type and the floor area is 80 000 m2. The cover material is single glass and screens are used in order to provide extra insulation during night time. The heating system is water based with heating pipes. Heat energy use was calculated from temperatures of incoming and outgoing heating water and pulses from two flow meters. Climate parameters and screening positions were registered and stored by the help of a Priva system. The model includes solar radiation according to equations provided by Duffie and Beckman (1974), and transpiration equations provided by Stranghellini (1987). Solar radiation is in the model divided into diffuse and direct insolation. Ventilation rate is calculated from moisture balance. Properties of the greenhouse in question, i.e. its material properties, areas, etc. are in combination with climate parameters and screening input data for the model. Measurement of heat energy use and climate was made on hourly basis during a period from April to September. Daily average climate values during the months April to September ranged for the separate months from 10-19 MJ day-1 for outdoor incoming solar radiation, 8.2-17.2° C for outdoor temperature, 18.3-20.3° C for indoor temperature, and 80-84% for indoor RH. Monthly values for measured use of heat energy per m2 greenhouse area for the months April, June, July, August and September were 157, 130, 74, 94, 108 and 121 MJ month-1 m-2 respectively. Modelled amounts of energy for the same months were 159, 125, 61, 69, 73 and 110 MJ month-1 m-2. A regression between measured and modelled daily use of heat energy for the whole period between April and September showed a fairly good agreement (R2 = 0.75). Data indicate that heat storage in the heating system increase the energy use. Further, the data suggest that also heat storage in mass inside the greenhouse should be included in a model predicting heat energy use in greenhouses.