F.D. Molina-Aiz

Sonic Anemometry to Measure Natural Ventilation in Greenhouses

The present work has developed a methodology for studying natural ventilation in Mediterranean greenhouses by means of sonic anemometry. In addition, specific calculation programmes have been designed to enable processing and analysis of the data recorded during the experiments. Sonic anemometry allows us to study the direction of the airflow at all the greenhouse vents. Knowing through which vents the air enters and leaves the greenhouse enables us to establish the airflow pattern of the greenhouse under natural ventilation conditions. In the greenhouse analysed in this work for Poniente wind (from the southwest), a roof vent designed to open towards the North (leeward) could allow a positive interaction between the wind and stack effects, improving the ventilation capacity of the greenhouse. The cooling effect produced by the mass of turbulent air oscillating between inside and outside the greenhouse at the side vents was limited to 2% (for high wind speed, uo ≥ 4 m s−1) reaching 36.3% when wind speed was lower (uo = 2 m s−1).

An Auto-Tuning PI Control System for an Open-Circuit Low-Speed Wind Tunnel Designed for Greenhouse Technology

Wind tunnels are a key experimental tool for the analysis of airflow parameters in many fields of application. Despite their great potential impact on agricultural research, few contributions have dealt with the development of automatic control systems for wind tunnels in the field of greenhouse technology. The objective of this paper is to present an automatic control system that provides precision and speed of measurement, as well as efficient data processing in low-speed wind tunnel experiments for greenhouse engineering applications. The system is based on an algorithm that identifies the system model and calculates the optimum PI controller. The validation of the system was performed on a cellulose evaporative cooling pad and on insect-proof screens to assess its response to perturbations. The control system provided an accuracy of <0.06 m·s−1 for airflow speed and <0.50 Pa for pressure drop, thus permitting the reproducibility and standardization of the tests. The proposed control system also incorporates a fully-integrated software unit that manages the tests in terms of airflow speed and pressure drop set points.

Experimental Evaluation by Sonic Anemometry of Airflow in a Mediterranean Greenhouse Equipped with a Pad-Fan Cooling System

The aim of the present work is to study the airflow and distribution of temperature and humidity in a multi-span greenhouse equipped with a pad-fan cooling system operating both with a well-developed tomato crop and without a crop (simulating recently transplanted plants in the greenhouse). Maximum values of air velocity were recorded at the entrance of the pads. In the first few meters of air inside the greenhouse, high levels of turbulence intensity were recorded as the air dampened by the pads mixed with the hot, dry air inside and the airflow cross-section increased. The crop has a clear stabilizing effect on the airflow, producing lower energy levels and turbulence than when the greenhouse was empty. The maximum temperature gradient was recorded in the greenhouse that was empty, with an increase of 5.2°C between the entrance of air through the pad and the exit through the extractor fans. This climate heterogeneity when young plants are transplanted in the greenhouse can produce over-consumption of irrigation water, which must be considered by growers to avoid plant damage by water stress. With a crop in the greenhouse, the maximum difference in temperature was reduced to 2.3°C.

Effectiveness of horizontal air flow fans supporting natural ventilation in a Mediterranean multi-span greenhouse

Natural ventilation is the most important method of climate control in Mediterranean greenhouses. In this study, the microclimate and air flow inside a Mediterranean greenhouse were evaluated by means of sonic anemometry. Experiments were carried out in conditions of moderate wind (≈ 4.0 m s-1), and at low wind speed (≈ 1.8 m s-1) the natural ventilation of the greenhouse was supplemented by two horizontal air flow fans. The greenhouse is equipped with a single roof vent opening to the windward side and two side vents, the windward one being blocked by another greenhouse close to it, while the leeward one is free of obstacles. When no fans are used, air enters through the roof vent and exits through both side vents, thus flowing contrary to the thermal effect which causes hot air to rise and impairing the natural ventilation of the greenhouse. Using fans inside the greenhouse helps the air to circulate and mix, giving rise to a more homogeneous inside temperature and increasing the average value of normalized air velocity by 365 %. These fans also increase the average values of kinetic turbulence energy inside the greenhouse by 550 % compared to conditions of natural ventilation. As the fans are placed 4 m away from the side vents, their effect on the entrance of outside air is insufficient and they do not help to reduce the inside temperature on hot days with little wind. It is therefore recommended to place the fans closer to the side vents to allow an additional increase of the air exchange rate of greenhouses.

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

The present work studies the effect of three insect-proof screens with different geometrical and aerodynamic characteristics on the air velocity and temperature inside a Mediterranean multi-span greenhouse with three roof vents and without crops, divided into two independent sectors. First, the insect-proof screens were characterised geometrically by analysing digital images and testing in a low velocity wind tunnel. The wind tunnel tests gave screen discharge coefficient values of Cd,φ of 0.207 for screen 1 (10 × 20 threads·cm−2; porosity φ = 35.0%), 0.151 for screen 2 (13 × 30 threads·cm−2; φ = 26.3%) and 0.325 for screen 3 (10 × 20 threads·cm−2; porosity φ = 36.0%), at an air velocity of 0.25 m·s−1. Secondly, when screens were installed in the greenhouse, we observed a statistical proportionality between the discharge coefficient at the openings and the air velocity ui measured in the centre of the greenhouse, ui = 0.856 Cd + 0.062 (R2 = 0.68 and p-value = 0.012). The inside-outside temperature difference ΔTio diminishes when the inside velocity increases following the statistically significant relationship ΔTio = (−135.85 + 57.88/ui)0.5 (R2 = 0.85 and p-value = 0.0011). Different thread diameters and tension affects the screen thickness, and means that similar porosities may well be associated with very different aerodynamic characteristics. Screens must be characterised by a theoretical function Cd,φ = [(2eμ/Kpρ)·(1/us) + (2eY/Kp0.5)]−0.5 that relates the discharge coefficient of the screen Cd,φ with the air velocity us. This relationship depends on the three parameters that define the aerodynamic behaviour of porous medium: permeability Kp, inertial factor Y and screen thickness e (and on air temperature that determine its density ρ and viscosity μ). However, for a determined temperature of air, the pressure drop-velocity relationship can be characterised only with two parameters: ΔP = aus2 + bus.

Combination of image processing and artificial neural networks as a novel approach for the identification of Bemisia tabaci and Frankliniella occidentalis on sticky traps in greenhouse agriculture

Display Omitted Image processing coupled with artificial neural network were used in a IPM system.Fed-forward neural networks were used to identify two pest species in greenhouses.Proposed whitefly and thrip identification method had a high precision, 0.96 and 0.92.Common sticky traps were used in a semi-automatic early pest detection system. Integrated Pest Management (IPM) lies at the core of the current efforts to reduce the use of deleterious chemicals in greenhouse agriculture. IPM strategies rely on the early detection and continuous monitoring of pest populations, a critical task that is not only time-consuming but also highly dependent on human judgement and therefore prone to error. In this study, we propose a novel approach for the detection and monitoring of adult-stage whitefly (Bemisia tabaci) and thrip (Frankliniella occidentalis) in greenhouses based on the combination of an image-processing algorithm and artificial neural networks. Digital images of sticky traps were obtained via an image-acquisition system. Detection of the objects in the images, segmentation, and morphological and color property estimation was performed by an image-processing algorithm for each of the detected objects. Finally, classification was achieved by means of a feed-forward multi-layer artificial neural network. The proposed whitefly identification algorithm achieved high precision (0.96), recall (0.95) and F-measure (0.95) values, whereas the thrip identification algorithm obtained similar precision (0.92), recall (0.96) and F-measure (0.94) values.

Thermography and sonic anemometry to analyze air heaters in Mediterranean greenhouses.

The present work has developed a methodology based on thermography and sonic anemometry for studying the microclimate in Mediterranean greenhouses equipped with air heaters and polyethylene distribution ducts to distribute the warm air. Sonic anemometry allows us to identify the airflow pattern generated by the heaters and to analyze the temperature distribution inside the greenhouse, while thermography provides accurate crop temperature data. Air distribution by means of perforated polyethylene ducts at ground level, widely used in Mediterranean-type greenhouses, can generate heterogeneous temperature distributions inside the greenhouse when the system is not correctly designed. The system analyzed in this work used a polyethylene duct with a row of hot air outlet holes (all of equal diameter) that expel warm air toward the ground to avoid plant damage. We have observed that this design (the most widely used in Almerías greenhouses) produces stagnation of hot air in the highest part of the structure, reducing the heating of the crop zone. Using 88 kW heating power (146.7 W·m−2) the temperature inside the greenhouse is maintained 7.2 to 11.2 °C above the outside temperature. The crop temperature (17.6 to 19.9 °C) was maintained above the minimum recommended value of 10 °C.