C. Kittas

NATURAL VENTILATION OF A GREENHOUSE WITH RIDGE AND SIDE OPENINGS: SENSITIVITY TO TEMPERATURE AND WIND EFFECTS

An air exchange rate model of a Mediterranean greenhouse with ridge and side openings combining temperature and wind effects as driving forces of the natural ventilation was developed and tested on experimental data. It is shown that both effects are dependent on the total area of openings, but the temperature effect depends also on the ratio of ridge to side openings. The wind effect is related to a global wind effect coefficient, relative to the whole building, which includes both static and turbulent transfers. As far as it concerns the combination of the temperature and wind induced fluxes, it is shown that the vectorial sum of these fluxes, gives a better fit of the experimental data than their addition. The relative importance of the thermal buoyancy and wind forces depends on the ratio between wind velocity and the root of the inside-outside temperature difference (u/iI ¦³ __ ). In our experimental conditions the wind effect predominates on the chimney effect when this ratio u/iI ¦³ __ becomes greater than 1. The use of the reduced ventilation flux (per unit of ventilator opening area and wind speed) shows that for typical conditions of temperature difference (= 5 K) the roof and side ventilation system is much more efficient than roof ventilation only for wind velocities smaller than 2.5 ms¨C1. Finally, a simple design procedure for dimensioning the area of the vents under extreme climatic conditions is proposed and a relative nomograph is given.

Temperature Gradients in a Partially Shaded Large Greenhouse equipped with Evaporative Cooling Pads

The main drawback of greenhouse evaporative cooling systems based on cooling pads and extracting fans is the thermal gradient developed along the direction of the airflow. High-temperature gradients of this type can markedly affect plant growth, and growers often combine cooling pads with shading. To predict the temperature gradients along a greenhouse, a simple climate model is proposed which incorporates the effect of ventilation rate, roof shading and crop transpiration. In order to calibrate the proposed model, measurements were performed in a commercial greenhouse equipped with fans and pads and shaded in the second half. Experimental data show that the cooling system was able to keep the greenhouse air temperature at rather low levels. However, due to the significant length of the greenhouse (60 m), large temperature gradients, (up to 8°C) were observed from pads to fans. The model was calibrated by fitting temperatures in the middle and at the end of the greenhouse. The model was validated on experimental data different from those used for the calibration and then it was used to study: (i) the influence of different ventilation rates combined with shading on air temperature profiles along the greenhouse length; and (ii) the influence of the outside air temperature and humidity on the performance of the cooling system. High ventilation rates and shading contribute to reduce thermal gradients. Despite its simplicity, the model is sufficiently accurate to improve the design and the management of the cooling pad systems.

Numerical simulation of the airflow and temperature distribution in a tunnel greenhouse equipped with insect-proof screen in the openings

Abstract An analysis of the ventilation process in a tunnel greenhouse equipped with an insect-proof screen in the side openings was performed with the use of a commercial computational fluid dynamics (CFD) package ( cfd 2000®). The aim of the study was to investigate how the screen influences airflow and temperature patterns inside the greenhouse. The screens on the greenhouse inlets and outlets, as well as the crop were simulated using the porous medium approach. The first simulations were carried out with a wind direction perpendicular to the side openings. Insect screens significantly reduced airflow and increased thermal gradients inside the greenhouse. Maximum air velocity values inside the greenhouse were observed near the openings, whereas air velocity was lowest in the middle of greenhouse. Airflow rates reduced by half in the greenhouse equipped with screen. These differences were also important in the region covered by crop, thus screen affected the sensible and latent exchanges between crop and air. The effect of different wind directions was also investigated. Wind direction considerably affected climatic conditions inside the greenhouse, as contrasted air flow and temperature patterns were observed for various wind regimes, especially when the greenhouse was equipped with insect screens.

Effect of misting on transpiration and conductances of a greenhouse rose canopy

Abstract The influence of greenhouse humidity control on the transpiration rate ( λE c ), sensible heat flux ( H c ) and bulk stomatal conductance ( g c ) of a soilless rose canopy ( Rosa hybrida , cv. First Red) was studied in a greenhouse located in the coastal area of eastern Greece. Measurements were carried out during several days in the summer (i) without air humidity control and (ii) with a mist system operating when the relative humidity of the greenhouse air was lower than 75%. The diurnal course of g c was determined from the relation linking λE c to canopy-to-air vapour pressure deficit ( D c ) or from inversion of the Penman–Monteith equation. The two ways of estimating g c were in good agreement, showing a significant increase of g c under mist conditions. Covariation of radiation and humidity during the day caused diurnal hysteresis in λE c and g c . The hysteresis phenomena were less marked when the mist system was operating. Normalising g c by radiation removed most of the hysteresis and indicated a curvilinear stomatal response to vapour pressure deficit. The analysis of the energy partition at the canopy showed high negative values of the Bowen ratio ( β ≈−0.7) in both conditions, indicating that canopy transpiration played a major role in cooling the greenhouse atmosphere. The contribution of the mist system to total evaporative cooling was estimated to be about 20%, with only 40–50% of the mist water being effectively used in cooling. Calculation of the crop water stress index confirmed that the crop was less stressed under misting conditions. It was concluded that the prediction of short-term variations of λE c and g c in greenhouse environments must account for the magnitude and diurnal variation of air VPD.

Influence of whitening on greenhouse microclimate and crop energy partitioning

Abstract The influence of whitening a greenhouse roof on microclimate and canopy behaviour was studied during summer in a greenhouse located in the coastal area of eastern Greece. Measurements of microclimate variables, intercepted net radiation, canopy temperature and canopy transpiration rate ( λE c ) of a well-watered soilless rose canopy ( Rosa hybrida , cv. First Red) were carried out over several days before and after roof whitening. Whitening reduced the average glasshouse transmission coefficient for solar radiation from 0.62 to 0.31. As a consequence, air temperature, vapour pressure deficit and canopy-to-air temperature difference experienced drastic changes while transpiration rate was not strongly affected, being slightly higher (about 18%) after whitening. The Bowen ratio β was highly positive before whitening ( β ≈0.6) and negative ( β ≈−0.5) after whitening. Calculations of the canopy stomatal conductance ( g c ) and crop water stress index clearly showed that the crop was significantly less water stressed after whitening. It was found that the values of g c doubled the day after whitening and tended to increase slightly afterwards. The latter may be ascribed to the recovery of the crop and the emergence of new and healthy leaves. Globally, glass whitening was inexpensive, presented positive effects on both microclimate and crop behaviour and can be considered as an efficient means for alleviating the large heat load during summer in warm countries.

Air flow and associated sensible heat exchanges in a naturally ventilated greenhouse

Abstract In order to investigate the three-dimensional nature of the air flow within the greenhouse, a three-dimensional sonic anemometer was used for the direct measurement of air and heat exchange inside the greenhouse and through the vent of a twin-span greenhouse equipped with a continuous roof vent at the gutter. Measurements were carried out both in the vent opening itself and in the greenhouse at the height of the ventilator. Wind blowing parallel to the greenhouse ridge gave rise to an inflow at the leeward part of the ventilator. The influx then follows a spiral trajectory guided by the internal surface of the walls and the shape of the roof before exiting at the upwind end. A large portion of the greenhouse situated between the centre and the upwind wall is ventilated less efficiently than the remainder of the greenhouse creating a large volume of higher temperature air that extends between the centre of the greenhouse and the windward wall. It is shown that the mean and turbulent components of the sensible heat flux through the vent amount to 58% and 42% of the total exchange between the greenhouse and its environment. Comparison with previous measurements demonstrates a large dependence of the wind-driven ventilation efficiency on wind speed. This confirms that other ventilation mechanisms, such as the stack effect, become important when the wind speed is low. These measurements are in a very good accord with flow patterns simulated by computational fluid dynamics techniques.

Quantification du taux d'aération d'une serre à ouvrant continu en toiture

Abstract Measurements of air exchange rates were performed in situ in a glasshouse equipped with a continuous roof opening. These measurements were carried out using the decay rate method with N 2 O as tracer gas. Simultaneously, inside and outside air temperatures were recorded as well as wind speed and direction. The experimental data were tested and fitted on a model which estimates the ventilation flux as a function of the temperature difference between inside and outside air (responsible for the chimney effect), of the wind velocity (wind effect) and of the surface of the opening. Results show that: (i) the effectiveness of ventilation decreases as the wind speed increases in a non linear fashion, (ii) the offset used in the linear models is partially a statistical artifact, (iii) the type of the opening and the dimension of the greenhouse affect the determination of the ventilation flux, (iv) wind perturbed by an upwind windbreak seems to be more effective for the ventilation of the greenhouse.

Maximum urban heat island intensity in a medium-sized coastal Mediterranean city

This paper studies the maximum intensity of the urban heat island (UHI) that develops in Volos urban area, a medium-sized coastal city in central Greece. The maximum temperature difference between the city center and a suburb is 3.4°C and 3.1°C during winter and summer, respectively, while during both seasons the average maximum UHI intensity is 2.0°C. The UHI usually starts developing after sunset during both seasons. It could be attributed to the different nocturnal radiative cooling rate and to the different anthropogenic heat emission rate that are observed at the city center and at the suburb, as well as to meteorological conditions. The analysis reveals that during both seasons the daily maximum hourly (DMH) UHI intensity is positively correlated with solar radiation and with previous day’s maximum hourly UHI intensity and negatively correlated with wind speed. It is also negatively correlated with relative humidity during winter but positively correlated with it during summer. This difference could be attributed to the different mechanisms that mainly drive humidity levels (i.e., evaporation in winter and sea breeze (SB) in summer). Moreover, it is found that SB development triggers a delay in UHI formation in summer. The impact of atmospheric pollution on maximum UHI intensity is also examined. An increase in PM10 concentration is associated with an increase in maximum UHI intensity during winter and with a decrease during summer. The impact of PM10 on UHI is caused by the attenuation of the incoming and the outgoing radiation. Additionally, this study shows that the weekly cycle of the city activities induces a weekly variation in maximum UHI intensity levels. The weekly range of DMH UHI intensity is not very large, being more pronounced during winter (0.4°C). Moreover, a first attempt is made to predict the DMH UHI intensity by applying regression models, whose success is rather promising.

Temperature, comfort and pollution levels during heat waves and the role of sea breeze

During the summer of 2007 several Greek regions suffered periods of extreme heat, with midday temperatures of over 40°C on several consecutive days. High temperatures were also recorded on the east coast of central Greece, where a complex sea breeze circulation system frequently develops. The more intense events occurred at the end of June and July. The highest temperatures were observed on 26 June and 25 July, while the sea breeze developed only on 25 July. Meteorological data collected at two sites—a coastal urban location and an inland suburban site that is not reached by the sea breeze flow—as well as pollution data collected at the urban site, were analysed in order to investigate the relationship between sea breeze development and the prevailing environmental conditions during these two heat wave events. The analysis revealed that sea breeze development affects temperature and pollution levels at the shoreline significantly, causing a decrease of ~4°C from the maximum temperature value and an increase of ~30% in peak PM10 levels. Additionally, several stress indices were calculated in order to assess heat comfort conditions at the two sites. It was found that nocturnal comfort levels are determined mainly by the urban heat island effect, the intensity of which reaches up to 8°C, while the applied indices do not demonstrate any significant daytime thermal stress relief due to sea breeze development.

INFLUENCE OF AN INSECT SCREEN ON GREENHOUSE VENTILATION

The influence of an insect screen on ventilation rate was experimentally investigated in a multispan glass–covered greenhouse equipped with a continuous roof vent, located at the University of Thessaly near Volos in the coastal area of eastern Greece. Microclimate variables as well as the ventilation rate were measured during summer. Two measuring techniques were used for the determination of ventilation rate: (1) the decay rate tracer gas technique, using N2O as tracer gas, and (2) the water vapor balance technique. The influence of the insect screen on ventilation rate was studied using a wind–related coefficient identified by fitting a simple linear model to the experimental values. The two measuring techniques gave similar results, but the water vapor balance technique provided a better fit to the experimental data. The wind–related coefficient significantly decreased when an insect screen covered the vent. Finally, the influence of the insect screen on the discharge coefficient was investigated. The discharge coefficient was correlated to the aerodynamic properties of the screen using porous media flow analysis.