• Energy Research

Crop cultivation in greenhouses under semi-arid climatic conditions is subject to various stresses, in particular during the winter season at night, when the interior air is poorly controlled, leading to prolonged periods of low temperature. The aim is then to evaluate and control the heat exchanges of the enclosure in order to prevent low indoor air temperatures and reduce the thermal load of the greenhouse. The objectives of this study are to investigate the convective and radiative heat exchanges at the cover in order to establish new correlations for the convective heat transfer coefficients in semi arid regions. The climatic parameters were measured inside and outside a closed empty glasshouse without crop, for three different nights during the winter season in the semi-arid land of Algeria. A physical model for analysing the convective heat transfers was implemented, and new correlations were established, parameterised, calibrated and validated thoroughly. A significant difference was observed between the correlations obtained by this study and the models obtained for other greenhouse designs under different climatic conditions. Results show that the convection mode along the inside wall of the cover is free turbulent. Conversely, the convection mode along the outside greenhouse cover remains forced turbulent. A consistent performance of the correlations was observed, both in the calibration and validation stages.

AbstractIn regions with warm and hot climates as is the case of several countries of the Mediterranean basin, it is interesting to study the energy balance inside a greenhouse and to quantify the heat transfers along the building components (roof, walls and ground) in winter and during night time. The present experimental work was conducted in an unheated glasshouse without crop in the region of Batna, Algeria. Three types of measurements were done from January to March: the first one is at a cloudy night; the second one at a windy night and the third one at a cloudless night. The results indicate that the greenhouse ground is considered as a significant heat source which can compensate the energy losses through the walls especially during a night preceded by a significant diurnal insulation. In addition, the convection heat transfer coefficients inside and outside the greenhouse were estimated and analysed. A good agreement with the models reported in the literature was found.

In greenhouses, reducing water consumption by increasing water efficiency is of high interest. Water need is linked to the plant transpiration, which itself depends on the stomatal resistance (Rs). Up until now however, prediction of Rs through models was mainly conducted for well-watered plants in open field conditions and very few models exist for greenhouse plants grown in pots. The aim of this work was to develop a dynamic model of Rs based on the full factorial design (FFD) under water restriction conditions. Then, the model was validated inside a greenhouse. The FFD is based on an optimization process to establish a polynomial relationship between Rs and the radiation, humidity and temperature. To implement the model, a set of experiments was conducted inside a 10 m2 growth chamber equipped with Impatiens New Guinea planted in 0.74-L pots under well-watered conditions. Rs was measured with a porometer and nine climatic scenarios were tested. First the FFD model was implemented for well-watered conditions. Then it was adapted to calculate Rs under water restriction, through the introduction of a new multiplicative function depending on the growing media matric potential. Once these parameters had been determined, the obtained transpiration model was validated against experimental data recorded inside a greenhouse Impatiens crop. The transpiration model derived from the FFD method showed its ability to correctly simulate Rs under water restriction conditions. Hence, it could be useful to accurately predict the evolution of transpiration.

In the context of increasing energy costs, alternative methods to the energy consuming venting-heating method must be considered for greenhouse dehumidification. In this paper the performance of a heat pump used as a dehumidifier is investigated. Contrary to the classical control aiming at maintaining the greenhouse air at a relative humidity set point, the considered device is designed as a preventive tool to avoid condensation on the crop and limit the energy consumption. The experimental set up was run during winter inside a 2350 m2 plastic greenhouse in the West of France for a set temperature of 16 C. During the experiment, no condensation occurred on the plants with a mean condensation rate of 12 W m 2 and a mean electrical power of 7.62 kW, for an overall efficiency of 4.9. Moreover, the energy retrieved by vapour condensation was given back to the greenhouse as sensible heat, contributing to the total heating of the greenhouse. While dehumidifying the greenhouse air, the device reduces, or may even rule out the gas consumption. The total energy consumption of the heat pump during the season was compared to simulated values for venting-heating dehumidification, with or without an exchanger. The heat pump dehumidifier was shown to be 6 to 8.5 times less energy consuming than the former and 3-8 than the latter, depending on the exterior climate. Using the energy cost of several significant countries, a preliminary operative cost study was conducted and showed that the heat pump can be competitive as a dehumidification alternative.