Review: Effect of ventilator configuration on the distributed climate of greenhouses: A review of experimental and CFD studies

At the moment, there are a large number of agricultural net types on the market characterized by different structural features such as type of material, type and dimensions of threads, texture, mesh size, porosity / solidity and weight; by radiometric properties like color, transmissivity/reflectivity/shading factor; by physical properties like air permeability and several mechanical characteristics such as tensile stress, strength, elongation at break, and durability. Protection from hail, wind, snow, or strong rainfall in fruit-farming and ornamentals, shading nets for greenhouses and nets moderately modifying the microenvironment for a crop are the most common applications. A systematic review of the current state-of-the-art of structural parameters, standard and regulations, most common agricultural net applications, and their supporting structures has been developed by means of a literature study, technical investigations, concerning characteristics and use of nets. As a result, the survey highlighted that in many cases different, not even similar, net types were adopted for the same application and the same cultivations by various growers. Results show that neither growers nor net producers have clear ideas about the relationship between the net typology optimization for a specific application and the construction parameters of the net. The choice often depends on empirical or economic criteria and not on scientific considerations. Moreover, it appears that scientifically justified technical requirements for nets used in specific agricultural applications have not been established yet.

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Plastic nets in agriculture ; a general review of types and applications

The effect of screened openings on greenhouse microclimate

Efficient use of energy in greenhouses has been subject of research and development for decades. The final energy efficiency, e.g. the amount of energy used per unit of product, is determined by improvements in energy conversion, reductions in energy use for environmental control and the efficiency of crop production. The new European targets on reduction of CO2 emission have resulted in a renewed interest in innovative technologies to improve energy efficiency in greenhouses designed for Northas well as South European regions. In this paper an overview of the recent developments is presented from both the Northwest European as well as the Mediterranean perspective. The developments range from new modified covering materials, innovative and energy conservative climate control equipment and plant response based control systems, to integrated energy efficient greenhouse designs. INTRODUCTION After the first “energy crisis” in the early 1980ies, where the limited supply of energy caused the first significant increase in energy prices, the energy use of greenhouses has again become a major research issue. The need for (energy) cost reduction becomes of higher importance when energy forms a substantial fraction of the total production costs. For Italy, it has been calculated that energy use for conditioning is already about 20-30% of the total production costs (De Pascale and Maggio, 2004), while in France it is 12-14% on average (Boulard, 2001) and 22% for vegetables, which is comparable to the northern regions (van der Knijff et al., 2004). The absolute use, however, differs between the specific locations: e.g. for Finland the total energy consumption, has been estimated at 1900 MJm per year (Olofsson et al., 2006), for The Netherlands 1500 MJ m (van der Knijff et al., 2004) and for southern France 500-1600 MJ m (Vesine et al., 2007). With the recently more pronounced interest in global warming and climate change, the use of fossil fuel is on the political agenda again (see Al Gore’s recent movie: An inconvenient truth). The international Kyoto protocol resulted in a new worldwide goal and many governments have set maximum CO2 emission levels for different industries, e.g. for The Netherlands: -30% CO2 emission in 2020 compared to 1990. The greenhouse industry is thereby again confronted with economical, political and social pressure to reduce the energy use and improve the energy efficiency. There are mainly two ways to increase the energy efficiency: • reduction of the energy input into the greenhouse system • increase the production per unit energy. The first strategy can be divided in two different parts: 1) efficient conversion technology to maximize the conversion of the energy source into a usable form (e.g. heat, 49 Proc. IS on Greensys2007 Eds.:S. De Pascale et al. Acta Hort. 801, ISHS 2008 cold, light) and 2): the reduction of the energy loss to the environment. The increase of the production per unit of energy, can be achieved in many ways, especially under sub optimal growing conditions. In fact, all cultivation measures which increase the production like improved irrigation, better nutrition, pest and disease control, a better utilization of the available greenhouse area etc, in the end also improve the energy

Innovative technologies for an efficient use of energy

Using computational fluid dynamics simulations to determine pressure drops on woven screens

The aim of the present study was to evaluate geometrical characteristics and airflow resistance of eleven different insect-proof screens by three different experimental procedures: equipment based in water-flow suction, low-speed wind tunnel, and CFD simulations. The two first arrangements had the same principle, in that air was forced through the test samples in order to create a pressure drop. Last analyses were carried out by numerical simulations of airflow through insect-proof screens using a commercial fluid dynamics code based in Finite Element method (ANSYS/ FLOTRAN v8.0). Previously, an analysis images system, called EUCLIDES v1.1, was designed with MS Visual Basic 6.0 running under MS Windows, for the analysis of the screens samples images captured with a microscope. A geometrical characterization of the eleven screens materials was carried out using this software tool. The software allows to determine all the geometric parameters that characterize the screens, as thread diameter and distances between two adjacent threads in two directions, from the four coordinates that defined each pore. The results obtained in this work show that the eleven screens can be classed in three groups, corresponding with the fibre density, with similar porosity and airflow properties (permeability and inertial factor). However, sample 8 has a small thread diameter and screen thickness that decreased the pressure drop coefficient. The results suggest that equations based on the porosity of the screen and the Reynolds number can be used to calculate the pressure drop coefficient. INTRODUCTION In an integrated pest management system, exclusion of pests should be one of the first tactics considered to reduce the need for other control measures. Whiteflies (Bemisia tabaci) and thrips (Frankliniella occidentalis) are among the most important pests of greenhouse crops in Almeria (Acebedo, 2004). As in other parts of the world (Taylor et al., 2001), most of the losses produced in Spain by Bemisia tabaci are due to its role as a virus vector (Guirao et al., 1997). Tomato yellow leaf curl virus (TYLCV) was reported for the first time in Spain in the autumn of 1992 (Moriones et al., 1993). Because of pest-acquired resistance, management practices that rely on insecticides are growing increasingly less effective, and less environmentally and economically appropriate. Reductions in pest populations (Baker and Jones, 1989) and lower incidence of insect-transmitted diseases (Baker and Jones 1989, 1990) have been documented when screening is used. Exclusion screens for the greenhouse may become a necessary alternative to pesticide use. However, airflow resistance, primarily a function of hole or mesh size, reduces the ventilation rate. Many efficacious screens have a small hole size and are more resistant to airflow than are more open-meshed screens (Bethke and Paine, 1991; Bell and Baker, 2000). The aim of the present study was to evaluate geometrical characteristics and airflow resistance of eleven different insect-proof screens by three different experimental procedures: low-speed wind tunnel, equipment based in water-flow suction, and CFD simulations. Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS 2005 442 MATERIALS AND METHODS In order to obtain the airflow characteristics of porous screens we measured the pressure drop caused by the insect-proof screen for different velocities in the range 0.1 to 12 m s. The first experiments were carried out in a wind tunnel with a cross-section of 420 mm × 360 mm and 5.2 m long. A helicoidally fan of 460 mm diameter driven by multi-speed 2.2 kW 3-phase induction electric motor HCT-45 (Sodeca S.A., Sant Qurze de Besora, Spain). Airflow was controlled by a Micromaster 420 AC inverter (Siemens Energy & Automation Inc., Alpharetta, USA) that allow decreased the fan motor speed from 0 to 2865 rpm, with digital microprocessor control and a set point resolution of 0.01 Hz. The static pressure drop through the screen was measured by a pressure transducer SETRA (Setra Systems Inc., Boxboruogh, USA), connected to two Pitot tubes, one 430 mm upstream and one 430 mm downstream from the tested screens (Terres-Nicoli et al., 2004.). Air velocity was determined connecting the static pressure and total pressure tapings of the upstream Pitot tube to another pressure transducer MKS (MKS Instrument Inc., Andover, USA). For air velocity lower to 1 m s the screen samples (diameter 115 mm) were mounted in a test duct (length 220 mm) separated by PVC rings of 10 mm thickness, 70 mm internal and 115 mm external diameter containing 20 screen samples (Fig. 1). Originally, the downstream tube was connected to the upper side of a water reservoir by a flexible pipe. The measurements are based on the pressure drop caused by natural suction of air through the samples as a result of water flow induced by gravity (Miguel et al., 1997). Subsequently, we repeated the test for the sample number 1, for air velocity between 0.1 and 1 m s, connecting the downstream tube to a fan NMB-4715KL (NMB Technologies Inc, Chatsworth, USA). The airflow supplied by the fan was regulated by controlling the rotational speed of the fan, function of the voltage that was varied from 3 V to 12 V with a DC power supply HY-3010 (DavJones Technology, Singapore). No statistical differences were observed between the tests made with the fan and the water reservoir for sample number 1. For reason of simplicity, we use the fan for air supply with the rest of samples (from 2 to 11). The pressure drop was measured using an inclined tube manometer AIRFLOW type 504 (Airflow Developments Limited, Buckinghamshire, England). The manometer, with a full-scale range of 125 Pa and an accuracy of 1 Pa, was connected to two Pitot tubes (150 mm upstream and 90 mm downstream). The measurement of air velocity was taken using a multifunction digital handheld instrument TESTO® 445 (Testo S.A., Cabrils, Spain) with a hot-bulb probe. This instrument has a measurement range from 0 to 10 m s with an accuracy of ±0.03 m s and resolution of 0.01 m s. The equipment also contains a temperature probe (thermistor NTC) with a range of –20 to 70oC and an accuracy of ±0.4oC. Last analyses were carried out by numerical simulations of airflow through insectproof screens using a commercial fluid dynamics code based in Finite Element Method (ANSYS/FLOTRAN v8). In the simulation, it was assumed that a woven screen comprises a large number of small pores, and a similar flow passed through each. The pore was modelled as the intersection of four cylinder (Teitel and Shklyar, 1998). Previously, an analysis images system, named EUCLIDES v1.1, was designed with MS Visual Basic 6.0 running under MS Windows, for the analysis of the screens samples images captured with a microscope DMWB1 (Motic Spain S.L., Barcelona, Spain). We used a plan 4X achromatic lens that provided images with a resolution of 0.0105 mm/pixel. Once the image was captured a different program (Photo Finish 4.0) was used to convert the images in true colour to black and white. A geometrical characterization of the eleven screens materials was carried out using the EUCLIDES software. This software allows to determine all the geometric parameters that characterize the screens, as thread diameter, d, and distances between two adjacent threads in two directions, Dhx and Dhy, from the four coordinates that defined each pore, recognised automatically by the software. The program also measures the porosity as the ratio of geometric pore area, calculated from the vertices coordinates, to whole area. For all screens we analysed three samples of 1 cm size.

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J.A. López

Papers

Contribution to characterisation of insect-proof screens: experimental measurements in wind tunnel and cfd simulation