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 Almería’s 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

Application of Semi-Empirical Ventilation Models in A Mediterranean Greenhouse with Opposing Thermal and Wind Effects. Use of Non-Constant Cd (Pressure Drop Coefficient Through the Vents) and Cw (Wind Effect Coefficient)

The present work analyses the natural ventilation of a multi-span greenhouse with one roof vent and two side vents by means of sonic anemometry. Opening the roof vent to windward, one side vent to leeward, and the other side vents to windward (this last vent obstructed by another greenhouse), causes opposing thermal GT (m3 s−1) and wind effects Gw (m3 s−1), as outside air entering the greenhouse through the roof vent circulates downward, contrary to natural convection due to the thermal effect. In our case, the ventilation rate RM (h−1) in a naturally ventilated greenhouse fits a second order polynomial with wind velocity uo (RM = 0.37 uo2 + 0.03 uo + 0.75; R2 = 0.99). The opposing wind and thermal effects mean that ventilation models based on Bernoulli’s equation must be modified in order to add or subtract their effects accordingly—Model 1, in which the flow is driven by the sum of two independent pressure fields GM1=√(∣∣G2T±G2w∣∣) , or Model 2, in which the flow is driven by the sum of two independent fluxes GM2=|GT±Gw| . A linear relationship has been obtained, which allows us to estimate the discharge coefficient of the side vents (CdVS) and roof vent (CdWR) as a function of uo [CdVS = 0.028 uo + 0.028 (R2 = 0.92); CdWR = 0.036 uo + 0.040 (R2 = 0.96)]. The wind effect coefficient Cw was determined by applying models M1 and M2 proved not to remain constant for the different experiments, but varied according to the ratio uo/∆Tio0.5 or δ [CwM1 = exp(−2.693 + 1.160/δ) (R2 = 0.94); CwM2 = exp(−2.128 + 1.264/δ) (R2 = 0.98)]

Effects of surrounding buildings on air patterns and turbulence in two naturally ventilated mediterranean greenhouses using tri‐sonic anemometry

The aim of the present study is to increase the available information concerning the influence of surrounding buildings on air patterns and turbulence characteristics of the ventilation airflow in greenhouses. With a view to evaluating the possible effect of different obstacles close to greenhouse vents, sonic anemometry has been used. At the side opening, the airflow was mainly horizontal, while at the roof vent it was upward or downward. The vicinity of obstacles to the greenhouse side openings reduced the incoming mean flow up to 79% and increased turbulence. Larger ventilation rates were observed for the leeward roof vent, since the wind impacts directly with the windward side opening without obstacles, with a maximum of 31.6 air exchanges per hour. However, when the roof vent is on the windward side, the wind is partially blocked by another similar greenhouse located upwind, as the outside air enters through the roof vent and exits through the two side openings. In this situation, the maximum ventilation rate observed was 14.5 air exchanges per hour. Natural ventilation was more effective in eliminating heat from the part of the greenhouse with a crop when the air entered through the side openings and exited through the roof vent. In this case, the ventilation efficiency for temperature ( T) was greater than 1. The maximum turbulence levels were associated with low air speeds and were observed mainly at the points located close to the side openings influenced by surrounding buildings. The turbulent energy levels of the airflow were higher at the windward openings without obstacles

Dispositivo de plataforma móvil para la medición automática de parámetros climáticos en diferentes puntos del interior de un invernadero

Número de publicación:ES2683920 A2 (28.09.2018) También publicado como: ES2683920 R1 (08.10.2018) ES2683920 B1 (16.07.2019) Número de Solicitud: Consulta de Expedientes OEPM (C.E.O.) P201700552 (28.03.2017)Dispositivo móvil que se desplaza bajo invernadero a lo largo de un raíl fijo y que contiene una plataforma auto-nivelable diseñada para la colocación de sensores de medida de parámetros climáticos en una plataforma autoportante que se desplaza de forma controlada por el interior de un invernadero gracias a un raíl sobre el que se desplaza mediante un sistema piñón-cremallera. Para obtener dicho movimiento el elemento móvil de la plataforma autoportante, incorpora un motor eléctrico. A este elemento móvil se acopla un husillo vertical que permite colocar una bandeja a la altura deseada y que se nivela automáticamente mediante la actuación de dos servomotores con realimentación por acelerómetros y giróscopo. El elemento fijo o rail está formado por un perfil de sección cuadrada al que se suelda una cremallera.Universidad de Almerí

Determining the emissivity of the leaves of nine horticultural crops by means of infrared thermography

he present study was carried out with the aim of analysing the variability of the emissivity values of nine of the most characteristic horticultural crops of the greenhouse productive system in the Mediterranean region. A thermographic camera was used for both qualitative and quantitative emissivity measurement by evaluating radiation emission from the leaves. The real temperature of the leaves was also measured with a contact probe in order to calculate emissivity. The differences in emissivity between crops for the upper side of leaves are below standard deviation values, the average values are all close to 0.98. For upper side of leaves we obtained the following average values of emissivity: 0.980 ± 0.010 for Lycopersicum esculentum Mill., 0.978 ± 0.008 for Capsicum annuum L., 0.983 ± 0.008 for Cucumis sativus L., 0.985 ± 0.007 for Cucurbita pepo L., 0.973 ± 0.007 for Solanum melongena L., 0.978 ± 0.006 for Cucumis melo L., 0.981 ± 0.009 for Citrullus lanatus Thunb., 0.983 ± 0.006 for Phaseolus vulgaris L. and 0.983 ± 0.005 for Phaseolus coccineus L. Considerable differences have been observed between the emissivity values on the opposite sides of the leaves in some horticultural crops, such as green bean and particularly red bean, with a difference of 0.029 in the average emissivity value. Emissivity values of 0.98 are recommended as a reference for measuring the temperature of horticultural crops other than those studied here whenever there is no other possibility to determine the emissivity

Pad-fan systems in mediterranean greenhouses: determining optimal setup by sonic anemometry

The present work studies the microclimate and airflow inside a greenhouse equipped with a pad-fan cooling system, analyzing several operational alternatives: three ventilation flow rates (18.1 m3 s-1, 20.2 to 23.5 m3 s-1, and 26.3 m3 s-1), and combining the medium flow rate with two interior fans or with a shading screen. The different airflow levels were obtained using a variable frequency drive (VFD) system to control ventilation fans (working at frequencies of 30, 40, and 50 Hz). The use of interior fans increased the velocity and intensity of the turbulent airflow, thus enhancing the mixing of air inside the greenhouse. The lowest fan frequency (30 Hz) reduced the system’s cooling capacity, increasing both the horizontal and vertical temperature gradients compared to the results obtained for the frequencies of 40 and 50 Hz. The system’s cooling capacity increased using the high-level ventilation flow rate or combining the pad-fan with a shading screen. In both situations, we obtained maximum temperature reductions of 3°C compared to the outside air. Different operational alternatives tested on sunny days show greater temperature reductions (4.4°C to 8.1°C) with respect to a similar naturally ventilated greenhouse (the most widespread type in the Mediterranean region). However, on cloudy days, when the outside relative humidity is high, the cooling capacity is more limited. Lack of system maintenance may lead to a considerable loss of efficiency (η), as this value fell from η = 0.82 when the system was installed to η = 0.65 one year later, at the time of this study. Estimated water consumption of the pad-fan system increases with the capacity to increase the water vapor content of the incoming air and with the volumetric flow rate

A study of natural ventilation in an Almería-type greenhouse with insect screens by means of tri-sonic anemometry

The wind coefficient of an Almería-type greenhouse has been calculated from direct estimation of airflow at the openings by means of three-dimensional sonic anemometry. Measurements were taken with a strong northeast wind (Levante) and with a weak westerly wind (Poniente). For the model considering only the wind effect, the coefficient of effectiveness obtained was EV = CdCw0.5 = 0.050, and the values of the mean and turbulent wind coefficients were Cw = 0.066 and Cw′ = 0.029, respectively. Two important characteristics of the Almería-type greenhouse analysed in this work make natural ventilation difficult: the presence of mature tomato plants inside the greenhouse and of an obstacle close to one of the side openings, which affected the air movement throughout it. Discharge coefficients due to the presence of screens in the greenhouse (Cd,φ = 0.156–0.245) were calculated from wind-tunnel measurements, obtaining a total discharge coefficient of Cd = 0.193. Use of the anti-aphid screen in the openings can cause an approximately 71% reduction of Cd and consequently of the wind-related coefficient EV. Winds perpendicular to the axis produce an inflow through the side opening free of obstacles and an outflow through the roof vents. In qualitative terms, this airflow pattern is in good agreement with previous simulations using Computational Fluid Dynamics

Development of a single energy balance model for prediction of temperatures inside a naturally ventilated greenhouse with polypropylene soil mulch

In this study, a semi-empirical dynamic model of energy balance was developed to predict temperatures (air, plants, greenhouse cover and soil) in a naturally ventilated greenhouse with a polypropylene mulch covering the soil in a Mediterranean climate. The model was validated using experimental data of 5 non-successive periods of 5 days throughout the crop season in the province of Almería (Spain). During the evaluation period, the transmissivity of the cover ranged between 0.44 and 0.80 depending on whitening, and the leaf area index of the tomato crops growing inside the greenhouse varied from LAI = 0.74 to 1.30 m2 m−2. The model mainly consists of a system of 6 non-linear differential equations of energy conservation at inside air, greenhouse plastic cover, polypropylene mulch and three layers of soil. We used multiple linear regressions to estimate the crop temperature in a simple way that allows a reduction in the number of parameters required as input. The main components of the energy balance in warm climate conditions are the solar radiation, the heat exchanged by natural ventilation and the heat stored in the soil. To improve the estimation of the heat exchanged by ventilation, different discharge coefficients were used for roof CdVR and side openings CdVS. Both coefficients changed throughout the time as a function of the height and opening angle of the windows and of the air velocity across the insect-proof screens. The model also used different wind effect coefficients Cw for Northeast or Southwest winds, to take into account the different obstacles (a neighbouring greenhouse at the south and a warehouse at the north). A linear regression of the wind direction angle θw was used as correction function for the volumetric ventilation flux G. The results showed that the accuracy of the model is affected mainly by errors in the cover transmissivity on cloudy days (when diffuse radiation prevails) and errors in the temperature of air exiting the greenhouse on windy days (when hot air stagnated near roof openings, that were closed by the climate controller to avoid wind damage). In general, the results of validation comparing calculated values with those measured on 25 days (with relative root mean square errors below 10%), show sufficient accuracy for the model to be used to estimate air, crop, plastic cover, polypropylene mulch and soil temperatures inside the greenhouse, and as a design tool to optimise the ventilation system characteristics and control settings

Sonic anemometry to evaluate airflow characteristics and temperature distribution in empty Mediterranean greenhouses equipped with pad–fan and fog systems

Sonic anemometry has been used to analyse two greenhouse evaporative cooling systems: a pad–fan system and a low pressure water/air fog system. These systems were used in empty greenhouses to simulate the microclimatic conditions produced inside Mediterranean greenhouses when crops are seeded in nurseries or transplanted in commercial greenhouses. Evaporative cooling systems could be necessary in the future for all Mediterranean greenhouses to reduce excess heat and to maintain certain levels of relative humidity on hot days from spring to autumn. The pad–fan system proved capable of maintaining more favourable conditions than the fog system. The best results were obtained by combining the evaporative pads with shading screens (differences of 1.4–1.8 °C between inside and outside temperature). The main drawbacks of the pad–fan system were the horizontal and vertical temperature gradients, with a maximum temperature difference between pads and fans of up to 11.4 °C, and a maximum difference of 6.7 °C between heights of 2 m and 1 m. However, inside temperature and relative humidity were more stable over time in the greenhouse using the pad–fan system. The fog system required higher energy consumption (7.2–8.9 kWh) than the pad–fan system (5.1 kWh) for continuous operations over 1 h. Nevertheless, the average water consumption of the pads (122.3 l h−1) is greater than that of the fog system (9.4 l h−1)

On the estimation of three-dimensional porosity of insect-proof screens

The two-dimensional estimation is the approach to porosity par excellence in the literature of insect-proof screens for their geometric characterisation and estimation of their aerodynamic parameters. However, this is not an accurate estimation, since the geometry of insect-proof screens consists of interlaced threads that create a three-dimensional woven structure, leading to different thicknesses and overlapping of threads. This paper suggests a mathematical approach to reconstruct computationally the 3D structure of the screens and to estimate the volumetric porosity, relying solely on easily measurable quantities such as diameter of threads, spacing of threads and thickness. The results on the application to 20 + 6 insect-proof screens in this work evidence that the suggested approach outperforms the standard two-dimensional modelling. These results also support experimental observations in the relationship between porosity and pressure drop not explainable by the two-dimensional approach. To increase the reliability on the analysis of porosity, the propagation of experimental uncertainty has been also included in the comparison between brand new and old&washed insect-proof screens. A software (Poro3D v1.0) using the methodology developed in this work can be downloaded as supplementary material to this manuscript to instantly obtain both 3D and 2D porosities, as well as the reconstruction of 3D geometries