Alain Baille

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.

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.

Microclimate and transpiration of greenhouse rose crops

Abstract Measurements of microclimate and transpiration of two greenhouse crops of roses (grafted and ungrafted) grown in rockwool were carried out during late spring and summer periods of 1989 to 1992 in the South of France. Results show that the crops have relatively low rates of transpiration compared with the latent heat equivalent to the global radiation incident on the crop. Analysis of the results indicates that this can be attributed to several causes: (1) for both crops, (a) stomatal closure as a result of high levels of the saturation deficit when the misting system was not operating (a value of 1.5 kPa was found to be the critical value above which stomatal conductance of the crop falls significantly) and (b) high substrate temperature during the afternoon and poor aeration of the root system may also reduce water uptake by the roots; (2) for the ungrafted crop, the low leaf area index ( L ≈ 1) limits potential transpiration rate and also prevents cooling and humidification of the greenhouse air through a microclimatic ‘feedback effect’. The use of the Penman-Monteith formula, incorporating computed values of the surface resistance to water vapour transfer vs. global radiation and saturation deficit, gives a good prediction of the hourly transpiration rate. The results confirm the importance of adequate control of greenhouse saturation deficit and substrate temperature during summer conditions, especially when radiation load is high and canopy transpiration is not sufficient to cool and humidify the greenhouse environment.

A simplified model for predicting evapotranspiration rate of nine ornamental species vs. climate factors and leaf area

Measurements of evapotranspiration rate (E) for nine greenhouse ornamental species (Begonia, Cyclamen, Gardenia, Gloxinia, Hibiscus, Impatients, Pelargonium, Poinsettia and Schefflera) have been carried out concurrently with a survey of indoor climate (e.g. solar radiation, G, and vapour pressure deficit, D) and leaf area index (L). Correlations linking E to G, D and L are proposed, based on the formalism of the Penman-Monteith equation: E = Af1(L)G + Bf2(L)D. Calculated short-term (hourly) evapotranspiration rates agree fairly well with measured rates. The information contained in A and B coefficients was analyzed, showing that the species under study have quite different behaviour and sensitivity to G and D, relative to canopy water vapour exchanges. From A and B, estimations of the average values for the leaf stomatal resistance, r1, and leaf aerodynamic resistance, ra, were derived and gave, for most of the species, plausible orders of magnitude for these two resistances.

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.

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).

Canopy surface resistances to water vapour transfer for nine greenhouse pot plant crops

Abstract Canopy surface resistances ( r s ) for nine ornamental species (begonia, cyclamen, gardenia, gloxinia, hibiscus, impatiens, pelargonium, poinsettia and schefflera) were deduced from measurements of stand evapotranspiration ( E ) and used with the Penman-Monteith equation to predict hourly rates of E . A multiplicative relationship was used to express r s as a function of solar irradiance ( G ) and vapour pressure deficit ( D ). For most of the species, r s showed a high correlation with solar irradiance, except for begonia which had a significant correlation between r s and D , and poinsettia which had a low correlation of r s with both G and D . Minimum leaf stomatal resistance ( r l , min ) and dark resistance, r l , d were deduced from r s formulae. Significant differences were found between species: r l , min ranged from 150 s m −1 (impatiens) to 400 s m −1 (gloxinia), whereas r l , d varied from 1500 s m −1 (cyclamen) to 4500 s m −1 (schefflera). Calculated hourly E rates, gave good agreement with the experimental data. These results show that such a method based on the Penman-Monteith equation and the estimation of r s can be used to predict the short-term evapotranspiration rate of ornamental crops.

Heat and water vapour transfer in a greenhouse with an underground heat storage system part II. Model

Abstract The experimental results, obtained with a ground storage greenhouse (Part I) are used for validation of a dynamic tri-dimensional soil storage model including sensible and latent heat transfer phenomena. Outputs of the model are compared to measured values of characteristic parameters of the system. Satisfactory agreement between measured and calculated values is obtained and the model is used to investigate the influence of some design parameters of the system.

Net photosynthesis response to light and air CO2 concentration of Begonia X hiemalis: whole plant measurements and modelling

Abstract Response of net photosynthesis ( Pn ) of Begonia plants to photosynthetic photon flux density ( I o ) and carbon dioxide concentration ( C ) was measured and modelled. Measurements were performed on whole plants at different growth stages in either controlled (assimilation chambers) or in situ conditions (glasshouse). A rectangular hyperbola was chosen as the response function of net photosynthesis to I o , C and leaf area ( A ). Parameterisation and validation of the model was carried out using Pn measurements on Begonia plants, cv. “Line”. The model was then tested for its extrapolative ability with two other data sets: one obtained in assimilation chambers with different cultivars (cv. “Heidi”, and shoot cuttings of the Rosalie group) and the other from in situ measurements in glasshouse with the cv. “Line”, using a portable assimilation chamber. Prediction of Pn was satisfactory under normal conditions of air temperature ( T a ), vapour pressure deficit (VPD) and I o in glasshouse production. Discrepancies between measured and estimated rates of Pn occurred under high I o (more than 700 μmol photon m −2 s −1 ), T a (over 25 °C) and VPD (more than 1.5 kPa), and can possibly be attributed to effects of stomatal closure on the net photosynthetic rate.