T. Boulard

The mechanisms involved in the natural ventilation of greenhouses

Abstract Although natural ventilation is one of the major mechanisms that controls the greenhouse climate, our understanding of the underlying processes remains insufficient to allow accurate prediction of the rates of such exchanges. This paper deals with the physical mechanisms involved in natural ventilation of a greenhouse equipped with continuous lateral windows, and uses the following experimental procedures: • • air exchange rate measurements, using tracer gas or heat and water balance techniques; • • direct determination of the air and heat flows through an opening, using an eddy correlation system, comprising a sonic anemometer and a fine wire thermocouple; • • measurements of mean and turbulent pressure differences at ground level between inside and outside. The methods employed allow the prediction of greenhouse air exchange rates as well as the characterization of its components: a steady effect resulting from the combination of both mean wind-related and stack effects and a turbulent effect linked to wind speed fluctuations. Local estimations of total, mean and turbulent flows are provided: a wind parallel to the greenhouse axis produces an inflow at the leeward half and an outflow at the windward half. The mean flow of sensible heat is estimated between 55% and 80% of the total flux so that the turbulent flow does not exceed 45% of the total. Local estimations of total, mean and turbulent flows are compared with air exchange rate measurements using the decay rate method and a good agreement between both approaches is demonstrated.

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.

A simple greenhouse climate control model incorporating effects of ventilation and evaporative cooling

Abstract A greenhouse climate model, incorporating the effects of natural ventilation and evaporative cooling (fog-system), is proposed and discussed. Linearization of the greenhouse heat and water balance equations leads to a simple system of two equations with two unknowns (the temperature and humidity differences between inside and outside air) which represent quite well the complex coupling mechanisms between ventilation and fog observed in situ. The model predicts that a minimum inside temperature can be reached for a certain combination of these cooling processes. Crop temperature and transpiration were also estimated using the Penman-Monteith approach and the energy balance of the crop. Good agreement between measured and computed values of air temperature, air humidity, crop temperature and transpiration was observed. From a practical point of view this approach may permit improvement in on-line climate control.

Tomato leaf boundary layer climate: implications for microbiological whitefly control in greenhouses

The efficiency of mycoinsecticides as a means of whitefly control was examined under greenhouse conditions, focusing in particular on the high humidity levels favouring infection of the whitefly larvae infesting the under leaf surface. The theory of the moisture transfers occurring within the leaf boundary layer under laminar conditions is outlined and a model for the air temperature and humidity distribution is developed. Measurements were made of the humidity of the air in the boundary layer on the underside of leaves of a tomato crop grown in a full scale greenhouse. A considerable increase in relative humidity was detected 5 mm from the underside of the leaves, particularly during day-time when crop transpiration reached its maximum. These measurements were compared with the results of the model based on boundary layer theory and it was established that the data recorded were accurately predicted by the model during day-time from both the qualitative and quantitative points of view, but they were underestimated at night. The implications of these findings were examined and discussed in the context of the microbiological control of whiteflies under the conditions of confinement pertaining in various greenhouses and similar cultural systems. It is suggested that the concept of the microclimate pertaining in the leaf boundary layer developed here could be applied to the biological control of insects and microbial pathogens, in both protected and open crop growth systems.

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.

Air speed profiles in a naturally ventilated greenhouse with a tomato crop

Air speed distribution is a key factor influencing heat and mass transfer in greenhouses. The air speed profiles in the centre of a naturally ventilated greenhouse with a tomato crop were investigated by means of a customised multi-point twodimensional sonic anemometer system. The experimental results showed that air speed was linearly dependent both on external wind speed and greenhouse ventilation flux. Under leakage ventilation, however, the air speed remained nearly constant at a low value (<0.1 m s ˇ1 ), due to negligible wind and temperature effects on greenhouse air exchange rate. It was also shown that in the case of continuous openings parallel to the dominant wind, the air speed was smaller in the vent opening of the span situated at the greenhouse periphery than in the opening of the span situated in the middle of the greenhouse. Based on the measured air speed profiles and ventilation flux, an estimation method for calculating the average internal air speed was established. This method can be applied to crop aerodynamic resistance calculations and irrigation management. # 1999 Elsevier Science B.V. All rights reserved.

Mean and turbulent air flows and microclimatic patterns in an empty greenhouse tunnel

The turbulent air flow along with patterns of air temperature and humidity transport were studied in a classical ‘Filclair’ tunnel, situated near Avignon in the south of France. Measurements with three-dimensional sonic anemometers and rapid-response hygrometers revealed a strong heterogeneity in the windward side of the tunnel. For winds perpendicular to the axis of the tunnel, the air flow exhibited a strong current crossing the tunnel between the windward and leeward openings and moderate air velocities in the vertical section situated between two consecutive series of openings. The temperature distribution showed a north‐south gradient due to the cold air penetration through the vent opening and a vertical gradient above the soil surface due to solar energy absorption at the soil level. Air water vapour patterns were quite different from air temperature patterns, with ‘humid’ areas only concentrated along the soil surface close to the source of evaporating water. Analysis of the energy spectra showed that all the locations had similar spectral levels in the dissipation region. ©2000 Elsevier Science B.V. All rights reserved.

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.

Performance of a greenhouse heating system with a phase change material

A greenhouse with a phase change material (PCM) heat storage system containing a quasi-eutectic mixture was tested with a classical lettuce-tomato rotation. Measurements on the greenhouse microclimate and assessment of the thermal storage performances were carried out during two heating seasons, with special attention paid to evapo-condensatioe processes. It is concluded that such a heat storage system, when used in the South of France, can keep a greenhouse roughly 10°C higher than outside during typical nights of March or April. It appears however that the present phase change storage medium, with a transition point as high as 22°C, is not adequate for periods with low solar gains (January or February). On the other hand, when the greenhouse temperature exceeds the melting point of the PCM (in late winter), the present (1.6 MJ m−2) storage capacity was found to be too low to cope with actual solar gains. Humidity measurements showed the positive effect of condensation on the storage. At night, as most of the condensates are drained away, the purely convective and weaker heat transfers from the storage contribute to keep the greenhouse humidity below saturation and preserve the crops from fungal diseases.

Heat and water vapour transfer in a greenhouse with an underground heat storage system part I. Experimental results

Abstract A greenhouse with an underground heat storage system consisting of two layers of 0.125-m diameter PVC drain pipes buried 0.8 and 0.5 m deep, and a centrifugal fan circulating the greenhouse air, was tested with a classical crop rotation lettuce-tomato. Measurements of microclimatic and thermal performances of the system, including both sensible and latent heat transfers, were performed during two heating seasons. From our results, it can be concluded that this type of heat exchanger may be used in the South of France to maintain an average night inside-outside temperature difference of 7–9°C in March–April. Auxiliary heating to maintain the desired air temperature for these crops is only 20% of the whole heating season requirement. The measurements have shown the importance of water vapour exchanges which generate high humidity levels during the night-time and may enhance fungal diseases and physiological disorders on tomato plants (intumescences).