Effects of different ventilation strategies on the microclimate and transpiration of a rose crop in a greenhouse
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In this study, the water vapour balance method was used to evaluate the ventilation rate to calibrate and validate the ventilation sub-model of the Gembloux Dynamic Greenhouse Climate Model (GDGCM) in a naturally ventilated three span Azrom type greenhouse in Zimbabwe. Two ventilation strategies were considered to investigate their effects on the microclimate and transpiration of the rose crop: the configuration with roof vents only (while the side vents were closed) and the configuration with both roof vents and side vents. Crop transpiration was evaluated using the Penman-Monteith method. This allowed continuous and automatic determination of the ventilation rate and leakage rate using the water vapour balance method. The model was fitted to experimental data for ventilation rates, and the parameters for the model, the discharge and wind effect coefficient were determined using statistical analysis. The results showed that there was a good fit between measured and predicted values (R2 = 0.702 and 0.729 for the model of ventilation for the greenhouse operating respectively with both roof and side vents and with roof vents only for the summer period considered), although there was a general overestimation of the air renewal rates, particularly during the night. The air renewal rate was found to be influenced by the ventilation regime in practice. The greenhouse was found to have higher air renewal rates for the configuration with both roof and side vents. On a typical hot day, the maximum simulated air renewal rate was 15.6 hr-1 for the configuration with roof and side vents at 1600hrs, while it was only 6.5 hr-1 for the configuration with roof vents at the same time. The difference between the air renewal rates for the different regimes resulted in different microclimates, since the ventilation affects both the energy balance and mass balance of a greenhouse. The greenhouse inside air temperature was reduced significantly for the configuration with both roof and side vents as it had lower simulated air temperatures than the configuration with roof vents only. On a typical hot day the maximum simulated air temperature for the configuration with roof and side vents was 29.7 ºC, while it was 31.5 ºC for roof vents configuration at 1400 hrs. On typical selected days the maximum differences between the inside air temperature and the external air temperature were 3.7 ºC and 4.6 ºC for the configuration with both roof and side vents and the configuration with roof vents only, respectively. Thus during summer periods it is necessary to have a greenhouse with both roof and side vents so that plants will have a better physiological and morphological development, as the air renewal rates influence crop behaviour largely through their effects on gas exchanges, particularly transpiration and photosynthesis. The transpiration of the rose crop was found to be influenced by the ventilation strategy. The simulated maximum canopy transpiration flux density was 166.5 W m-2 for the configuration with both roof and side vents, while it was 152.9 Wm-2 for the roof vents only on a selected hot day. The simulated night-time relative humidities were higher for the configuration with roof vents only. The simulated relative humidity was above 90 % for the configuration with roof vents only, while it was 84 % for the configuration with both roof and side vents. To prevent excessive humidity build up, the ventilation strategy with both roof and side vents needs to be employed in order to prevent condensation in the greenhouse.