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)

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)]

Characterization of air velocities near greenhouse internal mobile screens using 3D sonic anemometry

In Dutch greenhouses, different screen types are used for different purposes (shading, energy saving, black-out, light emission, etc.). In order to quantify the energy and mass transfers through screens, characterization of air permeability through the screens is required. In the case of energy-saving screens, it is an essential parameter to estimate the energy saving of each screen. Air permeability can be measured under defined conditions in a laboratory. In order to select the appropriate equipment for air velocity measurements, the air velocity vector near screens in a practical situation in a greenhouse needs to be identified by measurements. Sonic anemometry techniques have been used extensively in different types of greenhouses: a) to study natural ventilation, with and without insect screens, and in different positions; b) to study airflow patterns in greenhouses with mechanical ventilation/pad and fan systems; c) to study airflow patterns induced by different types of heating systems, and d) for the estimation of crop evapotranspiration (i.e., eddy covariance). However, to the best of our knowledge, no research has been carried out to study the airflow near different types of screens in a greenhouse. Many Dutch growers are increasingly using various types of fans with different positions in the greenhouse for dehumidification and improved climate uniformity purposes. The effect of such fans on the air velocity near screens, and therefore the effect on energy and mass transfer, is unknown. For this purpose, air velocities near different types of screens in commercial greenhouses were measured using ultrasonic 3D anemometers. The results show that, in the absence of fans, air velocity near the screens is affected by vent opening. With vents closed, air velocities are hardly ever above 0.2 m s-1. Therefore, a simple air-suction device can be used to characterize permeability of screens at a very low Reynolds range.</p

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

Effects of ventilator configuration on the flow pattern of a naturally-ventilated three-span Mediterranean greenhouse

Natural ventilation used in agricultural greenhouses is important to control greenhouse microclimate. The effect of the ventilator configuration on the flow pattern of a three-span Mediterranean greenhouse with an obstacle to airflow (a neighbouring greenhouse) was investigated. Two different ventilator configurations, two or three half-arch roof vents with two roll-up side vents, were evaluated using sonic anemometry. It was observed that the flow pattern through the greenhouse depends of the ventilation surfaces distribution and the obstruction to the ventilation system. Moreover, the magnitude and distribution of ventilation surface affected the overall ventilation rate and the ventilation rate at plant level. The ventilator configuration with two roof and two side vents improved air movement at the plant level, although the overall volumetric flow rate was lower than that with three roof and two side vents

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)

Analysis of turbulent air flow characteristics due to the presence of a 13 × 30 threads·cm^−2 insect proof screen on the side windows of a Mediterranean greenhouse

Insect-proof screens are a frequent passive method to restrict the entrance of insects into greenhouses. However, the installation of these screens also has a negative effect on natural ventilation, which is reflected in the turbulence and velocity of the airflow inside the greenhouse. The turbulent characteristics of airflow through an insect-proof screen installed in the greenhouse windows have not been studied thoroughly in the literature. The present work focuses on the use of two simultaneous 3D sonic anemometers to study the impact of the use of a 13 × 30 threads·cm−2 insect-proof screen on the turbulence properties of the micro and microscale airflow turbulence. Four tests have been carried out in windward-oriented side windows of a Mediterranean greenhouse. Results demonstrate that the approach of using two simultaneous 3D sonic anemometers for the first time allows one to observe that the effect is different for the three components of the velocity vector field, and there is a strong connection between the simultaneous conditions inside and outside of the greenhouse. Useful information and data on the effect of using a 13 × 30 threads·cm^−2 insect-proof screen are also provided. To give details on the impact of screens in the turbulent properties of ventilation is essential for any commercial distribution, as well as providing important data in the design and development of more efficient insect-proof screens

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

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