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Tadj N.,Bechar University | Bartzanas T.,Institute of Technology and Management of Agriculture Ecosystems | Fidaros D.,Institute of Technology and Management of Agriculture Ecosystems | Draoui B.,Bechar University | Kittas C.,University of Thessaly
Transactions of the ASABE | Year: 2010

Convective and radiative heat transfer from heating systems significantly determines greenhouse microclimate during the cold period of the year. These mechanisms are complicated because they combine free and forced convection modes, most often turbulent with different characteristics. The aim of the present study is to analyze the internal convective flows in a closed greenhouse caused by buoyancy forces from different configurations of heating systems. Numerical results obtained by the use of a commercial computational fluid dynamics code (ANSYS CFX) are compared to experimental measurements carried out in a full-scale experimental greenhouse with a tomato crop. The greenhouse was heated with a network of heating pipes and/or with an air heater. The standard k-ε turbulence model was adopted to describe the turbulent nature of the flow and transported properties. The resistance of the crop to airflow and the heat and mass exchanges of the crop with the surrounding air were simulated using the equivalent porous medium approach. In general, good agreement was found, since the mean error between measured and simulated values for air velocity, air temperature, and air absolute humidity distribution was 16%. The combined use of heating pipes and air heater enhanced plant activity and reduced the condensation rate. This heating method led to an increase in energy consumption of up to 19%, but it also created a more heterogeneous climate distribution compared to the case in which only heating pipes were used. It was shown that the greenhouse air volume is split into two regions: one occupied by the crop where natural convection dominated, and one above the crop where the hot air from the air heater resulted in a different microclimate from the lower part of the greenhouse (crop level), and the convection mode changed to mixed or forced depending on the distance from the air heater. © 2010 American Society of Agricultural and Biological Engineers.


Katsoulas N.,University of Thessaly | Kakavikakis G.,University of Thessaly | Kittas C.,University of Thessaly | Bartzanas T.,Institute of Technology and Management of Agriculture Ecosystems | Savvas D.,Agricultural University of Athens
Acta Horticulturae | Year: 2012

The recycling of greenhouse effluents in soilless cultivation systems is environment-friendly, since the waste of water and the disposal of nutrients to the environment is drastically reduced or even eliminated. However, a long-term recycling of the effluents may result in accumulation of Na+ and Cl-, in the recycled nutrient solution. Previous research has indicated that the concentration of Na+ and Cl-, in the recycled nutrient solution may be predicted by employing mass-balance models and monitoring the plant water consumption. The major aim of the present research was to test the performance of this model when operating as part of a decision support system in a tomato crop grown in a closed-cycle hydroponic system. An additional objective of this research was to compare tomato crops grown in fully closed, semi-closed and open hydroponic systems in terms of nutrition and yield performance. The crops were planted in January and scheduled to be terminated in June 2011 and irrigated with nutrient solutions, which contained 2 mM NaCl originating from the irrigation water. During the vegetative growth stage, the young tomato plants exhibited much higher uptake concentrations of nutrients than those suggested for North Europe. As a result, the electrical conductivity (EC) in the root zone of the crops grown in closed and semi-closed systems was maintained at lower levels than in the crop with free drainage for several weeks after planting. However, progressively the EC in the crops grown in closed and semi-closed systems was increased to higher levels than in the open system due to Na+ and Cl- accumulation. The efficiency of the model to minimize discharge of nutrient solution effluents in semi-closed hydroponic systems thereby maximizing water use efficiency in hydroponic tomato crops is discussed.


Katsoulas N.,University of Thessaly | Kittas C.,University of Thessaly | Bartzanas T.,Institute of Technology and Management of Agriculture Ecosystems
Acta Horticulturae | Year: 2012

The aim of this study was to characterise the microclimate distribution in a greenhouse cooled by a mist system in comparison to the microclimate distribution of a roof ventilated greenhouse. For this purpose, air temperature, vapour pressure deficit and air velocity were measured in several positions inside an even-span greenhouse with a soilless rose crop, during periods with: (i) natural ventilation only, by means of a continuous roof vent and (ii) a mist-system operating associated with natural ventilation. The results indicated that fog induced a more homogeneous field for temperature and humidity, providing a more intensive mixing of the inside atmosphere, while natural ventilation induced high vertical temperature gradients.


Katsoulas N.,University of Thessaly | Kittas C.,University of Thessaly | Bartzanas T.,Institute of Technology and Management of Agriculture Ecosystems | Savvas D.,Agricultural University of Athens
Acta Horticulturae | Year: 2014

The recycling of greenhouse effluents in soilless cultivation systems is environment-friendly, since the waste of water and the disposal of nutrients to the environment is drastically reduced or even eliminated. However, a long-term recycling of the effluents may result in accumulation of Na+ and Cl- in the recycled nutrient solution. The major aim of the present research was to test the performance of a Na+ mass balance model operating as part of a decision support system in a tomato crop grown in a closed-loop hydroponic system. Three treatments were compared: a completely open, a completely closed and a semi-closed hydroponic system during one experimental period lasting from October 2011 to January 2012. The fresh water used for the preparation of the nutrient solution contained 2 mM NaCl. During the total experimental period, the water uptake by the crop was relatively low, near to 130 L m-2, and thus, the concentration of Na+ in the closed system did not surpass the level of about 20 mM during the total experimental period. Comparison between measured and simulated Na+ values gave fair results. The system was efficient at minimizing the discharge of nutrient solution effluents in semi-closed hydroponic systems, thereby maximizing the water use efficiency.


Bartzanas T.,Institute of Technology and Management of Agriculture Ecosystems | Kitta E.,Institute of Technology and Management of Agriculture Ecosystems | Katsoulas N.,University of Thessaly | Tsouknidas A.,University of Thessaly
Acta Horticulturae | Year: 2012

The aim of this work was to study the effects of a new polyethylene (PE) film with high reflectance and absorption in near infrared radiation (NIR-PE film) and of a PE film with low transmittance in infrared radiation IR (IR-PE film), on the greenhouse microclimate, energy consumption and on growth and yield of a hydroponic tomato crop. Compared to a common PE film (C-PE), the IR-PE film, had 75% lower transmittance in IR, while the NIR-PE had 78% lower transmittance in IR and 42% higher reflectance in NIR. Films were evaluated experimentally in three similar arched roof greenhouses, two covered with the new covering materials and the third one by a C-PE film. The greenhouse and outside microclimate parameters along with crop growth and production were recorded. During the winter period, no significant differences of air temperature and relative air humidity between the tested greenhouses were found. The air vapour pressure deficit (VPD) values observed were relatively low (i.e below 0.5 kPa) during the night period, leading to condensation over the cover. The total energy consumption was 10% lower in the IR-PE covered greenhouse than in the C-PE covered greenhouse. During the summer period, the air temperature in the greenhouse covered by the NIR-PE was 2°C lower than the air temperature of the C-PE covered greenhouse.

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