CO₂ ENRICHMENT IN A FAN-VENTILATED GREENHOUSE UNDER DIFFERENT VENTILATION MODES

CO₂ enrichment in warm climates requires a delicate balance between the need to ventilate and the desire to enrich. Furthermore, strict regulations on CO₂ emission to the atmosphere, which were issued in recent years, emphasize the need for better control on CO₂ enrichment in ventilated greenhouses. Experiments were done in a fan-ventilated small scale greenhouse, to check the feasibility of CO₂ enrichment during summer time, when greenhouse ventilation is crucial, and to study the effect of different ventilation modes on the CO₂ level attained within the house. The greenhouse had a floor area of about 64 m2 and was ventilated by two fans capable of delivering a maximum ventilation rate of 0.07 m³ s^-1 m^-2. Enrichment was done under three modes of on-off ventilation (high: 0.07 m3 s^-1 m^-2, low: 0.042 m³ s^-1 m^-2 and interchanging high/low rates), and also under continuous variable rate ventilation. The continuous variable rate ventilation was accomplished by connecting the fans to a variable speed drive unit and changing the speed of rotation of the fans according to the heat load on the greenhouse. The CO₂ was injected to the greenhouse from a bottle at a rate of about 1.8 kg h^-1. Greenhouse microclimate (air temperature, relative humidity and solar radiation), amount of CO₂ supply, energy used for ventilation and CO₂ concentration between 6:00 and 9:30 am were monitored and average values are presented for all modes of ventilation. A criterion taking into account the CO₂ level retained in the greenhouse relative to the atmospheric level, the amount of CO₂ supply, the energy used for ventilation and the attained temperature difference between the greenhouse and the surrounding is offered and used for comparison between the different modes of ventilation. The results suggest that the best performance between 6:00 and 9:30 am was obtained with the variable ventilation mode. The results further show that under the operation conditions of the present study, enrichment beyond 10 am was not practical since the fans had to be operated continuously due to high ventilation requirements.