Microclimate evaluation of a new design of insect-proof screens in a Mediterranean greenhouse

Abstract

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.

Wind Tunnel Analysis of the Airflow through Insect-Proof Screens and Comparison of Their Effect When Installed in a Mediterranean Greenhouse

“…The coefficient C d,φ can be calculated from the pressure drop coefficient F φ previously determined in the wind tunnel experiments (Figure 3b) [4,15]: $C_{d,\varphi }=\frac{1}{F_{\varphi }^{0.5}}$ $F_{\varphi }=\frac{2e}{K_p^{0.5}}\left(\frac{1}{{\mathrm{Re}}_p}+Y\right)$…”

“…It was determined for a reference air velocity us of 0.25, 0.5 and 1.0 m·s −1 . The value of 0.25 m·s −1 for air velocity is the maximum mean value of the longitudinal component u x , perpendicular to the vents, observed in a similar experimental greenhouse [4]. …”

“…The aerodynamic characteristics of the insect-proof screens were obtained using a low velocity wind tunnel, designed and developed at the University of Almería [4,39,40], with an auto-tuning PI automatic control system, using an open hardware and software platform [41]. In the wind tunnel, we have measured with two Pitot tubes and with a differential pressure transducer the pressure drop Δ P produced for different air velocities u s measured with a hot-film anemometer [41].…”

“…Natural ventilation is very important for optimal plant growth during the summer in Mediterranean countries [2]. For most of the year, a good natural ventilation system will allow growers to maintain suitable microclimate conditions inside the greenhouse for the crops [2,3,4]. However, Valera et al [1] point out that the 14.4% average ventilation surface (ratio of total vent surface divided by total greenhouse surface, S V / S A ) in Almería’s greenhouses is well below the minimum recommended value of 15%–30% [2,3,5,6,7,8].…”

“…Nevertheless, the screens clearly have a negative effect on the greenhouse microclimate by: (i) reducing the natural ventilation capacity [4,13,14,15]; and (ii) causing higher inside temperatures [3,4,16,17,18]. Although the negative effects of installing insect-proof screens have been studied previously, the results obtained by different authors vary greatly.…”