Greenhouses play an important role in the field of agriculture, allowing the grower to have more control on the environmental factors that govern the behaviour of crops. The interactions of the environmental factors on the inside climate of a greenhouse are complex and involve a set of physical mechanisms that constitute challenges to modellers. To better assess the greenhouse climatic behaviour, a CFD modelling approach was developed and combined with field surveys considering a Venlo, closed greenhouse, under semi arid conditions. Measurements were undertaken at different heights along a vertical cross-section at the centre of the greenhouse. The components of the greenhouse were then integrated inside a CFD distributed climate model. The boundary conditions were inferred from the outside climatic conditions, taking account both of radiative and convective transfers through the walls. Simulations were conducted under pseudo-steady state conditions (i.e., updating the boundary conditions at each time step) all day long, and several scenarios (clear or cloudy day) were simulated to analyse to what extent the inside climate of the green¬house responds to the outside climate conditions.

The thermal behavior of the inside air of a closed Venlo glasshouse without plants was analysed under semi-arid climate conditions. The aim of the study was to investigate to what extent the characteristics of the greenhouse design and outside climatic conditions influence airflow and temperature patterns inside the greenhouse. For the purpose of the present work, a CFD modeling approach was combined with field surveys. The study focuses on the effects of (i) the thermal inertia of the soil, (ii) the movement of the interior air, and (iii) the distribution of the temperature inside the greenhouse. Two contrasted days were considered: a windy overcast day and clear day. From the results, it is concluded that when the greenhouse is fully closed with bare soil, the heat absorbed and stored by the ground during daytime represents a significant heat source which enhances buoyancy forces, the main driving forces of the movement of the air, especially during the night. The temperature of the roof was relatively low and the air temperature distribution inside the greenhouse disclosed a vertical gradient from the roof towards the ground surface due to the movement of the air above the surface of the ground absorbing thermal energy (solar energy). Maximum air velocities inside the greenhouse were observed near the ground surface, while they reached their minimum values in the middle of the greenhouse. Similar results were obtained for the windy overcast day and for the clear day.

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.

The aim of this work is to analyze the thermal performance of a greenhouse exposed to semi-arid conditions by investigating experimentally the heat transfers occurring through the walls and ground. A closed unheated Venlo greenhouse without crop in the area of Batna (southern Mediterranean basin) was considered. The study focuses on the effects of (i) the thermal inertia of the soil, (ii) the radiative losses through the cover, and (iii) the convection mode and flow regime on the heat transfer coefficients. Experiments were conducted from January to March 2008 under clear or cloudy skies, and low or high wind velocities. From the results, it is concluded that the heat stored in the ground of the greenhouse represents a significant heat source which can compensate the energy losses through the walls, especially during a night preceded by a high diurnal insulation. This process can maintain an average inside - outside temperature difference during the night within the range [1.59-2.81] K. Results also show that the radiation losses are the main component of the energy losses of the greenhouse, mainly through the outside wall surface of the cover. Conversely, the radiative heat exchange along the inner wall represents the main heat supply to this wall. The convection mode inside the greenhouse induced by the air movement seems to play a significant role on the convective exchange coefficient inside the greenhouse. These coefficients both inside and outside the greenhouse were estimated and analyzed, and a good agreement with the models reported in the literature was found. This study could help growers define and adapt their heating strategy to avoid undesired low temperatures which may damage their crops at night.

Semi-arid regions are frequently subject to major temperature changes during a 24 h period, which may drastically affect greenhouse indoor climates. In order to improve energy management of these buildings, numerical tools have been developed to predict the evolution of the inside climatic conditions. However, most of the available models neither take account of the transmittivity variation through the day nor of differences between wall temperatures. In the present paper, a model for predicting the thermal and water behaviour inside an unheated agricultural green-house is presented. The energy balance method is applied to each element: cover, indoor air and soil surface. Specific modules have been developed to calculate heat transfer coefficients for the cover of the greenhouse as well as heat transfer through the subsoil. These modules have been integrated in the TRNSYS environment. Radiative transfers and view factors were also calculated. The simulations predict two main parameters under transient conditions: the indoor air temperature and the indoor humidity in response to the outside conditions. These parameters were validated with fair agreement from experiments conducted in a monospan greenhouse located in Batna (6.11° E, 35.33° N). Based upon the results of the simulations and the measurements it was also concluded that firstly, the transmittivity was not constant in time and varied with surface orientation; and secondly, vertical surface temperatures were different during the daytime while the temperature difference between roof surfaces remained insignificant. The evolution of humidity was not correctly reproduced by the model, probably because the effects of condensation and variation of soil water content were not properly included in the equations.

    Up to now, few studies were devoted to the description of the energy balance components of a greenhouse located in the semi arid region of the southern Mediterranean basin, and no attention was paid to the prediction of the inside air temperature. In this study, experiments were undertaken to investigate the response of a greenhouse to the outside climate conditions considering a naturally ventilated Venlo glasshouse with a tomato crop. The measurements show that the difference between inside and outside air temperature is strongly linked to the incoming solar radiation as well as to the wind speed. From these results a simplified model was established to predict the greenhouse air temperature, knowing the greenhouse characteristics and the outside climate variables. The model is based on the energy balance of the greenhouse. Using a parameter identification technique, the model was calibrated against the experimental results. A sensivity analysis was conducted to assess the impact of several physical parameters such as solar radiation, wind speed and cover transmission on the evolution of the inside air temperature. This model appears to be suitable for predicting the greenhouse air temperature satisfactorily, at least during night

    In 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.