ISHS V International Symposium on Models for Plant Growth, Environment Control and Farming Management in Protected Cultivation (HortiModel2016) Using Computational Fluid Dynamics to analyse the CO2 transfer in naturally ventilated greenhouses

This work studies natural ventilation in a Mediterranean greenhouse, comparing a new experimental screen of 13×30 threads cm-2 (porosity 39.0%) with a commercial control screen of 10×20 threads cm-2 (porosity 33.5%). In addition, both screens were tested in a wind tunnel to determine the discharge coefficients Cd of the greenhouse side and roof vents, which proved to be 0.16 for the commercial control screen and 0.18 for the experimental screen at both vents. These values represent a theoretical increase of 11% (Cd,φ-10×20 /Cd,φ-13×30 = 0.89) in the natural ventilation capacity of the greenhouse when the experimental screen is used. The greenhouse was divided into two separate sections allowing us to analyze natural ventilation in both sectors simultaneously. Air velocity was measured in the lateral and roof vents with two 3D and six 2D sonic anemometers. Using the commercial control screen there was an average reduction of 16% in ventilation rate, and an average increase of 0.5ºC in the average indoor air temperature, compared to the experimental screen. In addition, the ventilation efficiency ηT was higher with the experimental screen (mean value of 0.9) than with the control (mean value 0.6). We have designed an experimental insect-proof screen (13×30 threads cm-2) with smaller thread diameter, higher thread density, smaller pore size and higher porosity than are used in most commercial meshes. All of these factors promote natural ventilation and improve the greenhouse microclimate.

The purpose of this study was to analyse the effect of different evaporative cooling systems compared to natural ventilation on the microclimate, photosynthetic activity and yield of a tomato crop (Lycopersicum esculentum Mill.) in a spring-summer cycle. In this study, the expenditure of electricity and water caused by the different refrigeration systems and their economic cost was analysed. The study was carried out in three multi-span greenhouses: (i) a greenhouse with evaporative pads and fans and natural ventilation (PS + NV); (ii) a greenhouse with a fog system and natural ventilation (FS + NV); (iii) a greenhouse only with natural ventilation (NV). The photosynthetic activity was higher in the greenhouse with natural ventilation (14.7 µmol CO₂ m⁻² s⁻¹) than in the greenhouse with the pad-fan system (14.6 µmol CO₂ m⁻² s⁻¹; without a statistically significant difference) and in the greenhouse with fog system (13.4 µmol CO₂ m⁻² s⁻¹; with a statistically significant difference). The production was higher in the greenhouse with the pad-fan system (5.0 kg m⁻²) than in the greenhouse with natural ventilation (4.8 kg m⁻²; without a statistically significant difference) and in the greenhouse with a fog system (4.5 kg m⁻²; with a statistically significant difference). In general, photosynthetic activity and crop production increased as the maximum temperature (and the number of hours of exposure to high temperatures) decreased. It has been observed that the improvement in temperature conditions inside the greenhouses in spring-summer cycles produces increases in the photosynthetic activity of the tomato crop and, consequently, growth in production. The energy and water consumption derived from the use of active-type cooling systems have not been offset by a representative improvement in photosynthetic activity or crop production.

This study assesses the impact of irrigating with desalinated seawater (DSW) on the profitability of greenhouse tomato in south-eastern Spain, comparing different water-quality sources in both traditional sanding cultivation and soilless hydroponic production. The assessment is based on the combination of partial crop budgeting techniques with field data from the LIFE DESEACROP Project experimental activities. Our results show that the exclusive use of DSW for tomato production increases fertilization costs by 20% in soilless systems and by 34% in traditional sanding cultivation, and water costs by 30% in soilless systems and by 48% in traditional soil cultivation. As a result, production costs increase by 5% in soilless cultivation and 3% in soil cultivation, increases that are reduced when DSW is blended with brackish water. However, the lower salinity of DSW, compared with conventional water resources in the area, increases both crop yield and profitability. Soilless cultivation would also increase tomato profitability but only if good quality water is available. The materialization of the potential benefits of soilless production requires improving water quality through the increased use of DSW. Otherwise, the traditional sanding production system, better adapted to the area’s poor soils and bad quality water, would be more profitable.

The installation of insect-proof screens on greenhouse vents is one of the principal methods of protection against harmful insects for crops. Their main disadvantage lies on their negative effects on natural ventilation and greenhouse microclimate, which have been the focus of studies by several authors. However, few works have analysed the effect of accumulated dust and dirt on these screens. The present study has analysed four anti-insect screens, comparing their geometric characteristics before installation and after three to four years of use. Two negative effects have been observed and quantified: deterioration of the threads that make up the screen and reduction of porosity due to accumulated dirt in the pores. This deterioration over time gives leads to a mean increase in thread diameter of 3.1%, as well to a mean decrease in the pore size of 6.2% and 2.3% in the weft (Lpx) and the warp (Lpy), respectively. In fact, the insect-proof screen porosity (φ) decreased due to the deterioration of the threads by an average of 6.5%, in addition to an average 20.3% reduction due to the accumulation of dirt in the pores, making a total reduction in porosity of 26.8%. This decrease in porosity leads to lower greenhouse ventilation rates, and is therefore detrimental for the greenhouse microclimate. Consequently, it is recommended that insect-proof screens in arid areas such as Almería (Spain), with abundant dust suspended in the atmosphere, be washed monthly using water sprayed at high pressure.