HORTICERN - AN IMPROVED STATIC MODEL FOR PREDICTING THE ENERGY-CONSUMPTION OF A GREENHOUSE

A satisfactory model which permits a description of the thermal behaviour of a greenhouse, is a useful tool for the optimisation of greenhouse components and installations. To date, two types of model have been developed: some very simple models of low precision, and some precise dynamic models unsuitable for use over long periods and too complex to be practical in use. This paper aims to describe a new intermediate method of precise and easy application, and consists of a presentation and experimental proof of the method. The model named HORTICERN is based on a static study of the thermal energy balance. Calculation of the net energy consumption is as follows: gross thermal requirement is determined from the parameters concerning climate, greenhouse and conditions of use; then the contribution of useful solar energy to the total thermal need is calculated. Net needs are obtained by deducting the useful solar contribution from gross thermal needs. Total consumption is finally given by dividing net needs by the heating system efficiency. Each stage of the work was the subject of a theoretical study and was then checked with measurements made at the trial greenhouse of the European Organisation for Nuclear Research (CERN). The main results are summarised as follows. For a precise evaluation of the thermal needs, it is necessary to take into account the combined effect of wind and radiative losses to the sky. The new theory developed brings a new understanding of sky temperature influence and measurements at CERN confirm its validity. The solar contribution covers an important fraction of the total thermal needs (between 30 and 40% over the year in Geneva). The fraction of solar energy collected which is used for heating is determined experimentally as a function of the ratio of collected energy to daytime thermal needs. This new method takes plant evapotranspiration directly into consideration. It may be applied on a daily, monthly or seasonal basis and is versatile in being applicable to any type of greenhouse or climate. HORTICERN has been checked systematically by two complementary approaches. (1) The values predicted by the model have been compared with the measurements in CERN's greenhouse. The error in annual energy consumption is less than 10%, if wind and apparent sky temperatures are taken into account. (2) HORTICERN is compared with several dynamic models; its precision is comparable with that of more complex dynamic models. The difference is on average less than 5% and is not affected by the type of glazing used (single glazing, Hortiplus, double glazing). In view of its precision and ease of use over long periods, HORTICERN is a very powerful tool for energy and optimisation studies. The software available for computer use allows numerous applications for research workers, greenhouse designers or horticultural advisers: it allows a better understanding of the physical phenomena whilst permitting economic studies, heating system power calculation, climate control strategy or the generalisation of a specific experiment.