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)]

A naturally ventilated crop protection structure for tropical conditions.

This study presents the theoretical and experimental results of natural ventilation rates induced by stack, wind and the combination of both stack and wind effects for a typical crop protection structure suitable for the tropics. The structure consists of simple structural frame, transparent roofing and insect screen side walls. It was found the relative importance of the stack and wind effects is dependent on the ratio between wind speed and the square root of the inside-outside temperature difference (u/AT°.5). In this study, the wind effect dominates over the stack effect when the ratio u/AT" becomes greater than 0.5. Ventilation rate induced by the stack effect was found to increase with increasing temperature difference between inside and outside of the crop protection structure according to a power law, with an index of 0.5. The wind effect ventilation rate was found to increase linearly with increasing outside wind speed measured at eaves level. In addition, the combination of the stack and wind effects could be represented as the vectorial sum of two the independent effects (40sw = [43)k2 + (1)„,nd21 0.5). However, the result of the wind effect in the combined effects was insignificant when the ratio of ventilator opening to the total wall area is higher than 20 %. Different methods have been used to determine the natural ventilation rates. The dynamic tracer gas was used as the control; direct airspeed measurement, energy balance and neutral plane methods were used to quantify ventilation induced by the stack effect. Pressure field measurements were used to quantify ventilation by wind effect. In addition, the dynamic tracer gas, energy balance, and stack and wind methods were used to quantify ventilation induced by the combined effects. However, these methods have their constraints and limitations because of statistically significant differences in the comparison between the methods. The tracer gas method was found very difficult to use in the highly porous structure. In addition, the ventilation rate measured by this method was 30-40 % less than the other methods. The energy balance method has the advantage that it estimates many important climatic and crop parameters, however, the errors were found to be the highest. The neutral plane method was suitable for measuring ventilation induced by stack effect, the simplest method, requiring only the measurement of the inside and outside temperatures. The direct airspeed measurement method was much easier to handle and the result was comparable to other methods suitable for determining the ventilation induced by the wind effect. The physical properties of the covering materials, namely light transmission, coefficient of discharge and airflow characteristics were also determined in this study. It was found that the light transmissions of transparent polythene film and insect screens were close to each other. The coefficient of discharge and light transmission were dominant parameters in the ventilation rate calculation. It was found that when air flows through a screen, the pressure drop increases linearly with the square of approach airspeed. Airflow distributions inside the crop protection structure induced by the stack and wind effects are also presented in this study. Finally, this study presents information on natural ventilation for tropical greenhouses that was not previously available

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

Factors affecting greenhouse microclimate and its regulating techniques: a review

This paper reviews factors affecting greenhouse microclimate and its regulating techniques towards upgrading the greenhouse applications in the area of southeast China which have little or very basic technology integration. The microclimate of greenhouse is apparently influenced by the shape and its orientation, the wind direction, the property of covering material, and the use of insect-proof screen as they eventually affect the total solar radiation, the thermal characterises, and the flow pattern inside. The natural ventilation and sun block are the most common method to cool the greenhouse, but more efficient evaporative cooling such as pad-fan system, misting/fogging system and roof sprinkler are required with extreme temperatures. The earth to air heat exchanger and the heat storage using phase change material may be used for heating or cooling throughout the year which are more economic and energy-saving than other traditional thermal technologies. The reviewed knowledge provides insights into upgrading greenhouse applications in Ningbo area towards more sustainable and efficient greenhouse farming

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

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

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.</p

Three-dimensional numerical simulation of the thermal and aerodynamic behavior of a roof structure built on a slope and used for horticultural production

In tropical countries, part of the horticultural production is carried out in hillside soils. In recent years, due to the adverse effects of climate change and other biotic factors that limit and affect agricultural production, the use of roof structures has been promoted as a technological means to improve production in this type of production system. The microclimate study of structures built on slopes is scarce, therefore farmers continue to build the same type of structure without technical design criteria and without knowing if the microclimate conditions generated are suitable for the crops. In the present research work, an experimentally validated 3D CFD numerical model was implemented to analyze air flows and spatial temperature behavior in a roof structure built on a site with broken topography. The results obtained allowed us to find that the air flows are strongly affected by the longitudinal and transversal slopes of the land, which produces low ventilation rates that generate thermal gradients above 8 °C and highly heterogeneous thermal behavior, factors that are not suitable for horticultural production

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

A Review of Ventilation and Cooling Technologies in Agricultural Greenhouse Application

This article presents a comprehensive review of the literature that deal with ventilation and cooling technologies applied to agricultural greenhouses. The representative application of each technology as well as its advantages and limitations are discussed. Advance systems employing heat storage in phase change materials, earth-to-air heat exchangers and aquifer-coupled cavity flow heat exchangers have also been discussed. For an agricultural greenhouse equipped with cooling and artificial ventilation system, availability of uninterrupted electric supply is important. To achieve grid independence, dedicated power generation and storage systems need to be integrated with the greenhouse. The relevant literature on such power generation system for greenhouse application has been reviewed and is discussed here. This review concludes by identifying some important areas where further research needs to be undertaken

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)