A MODEL OF HUMIDITY AND ITS APPLICATIONS IN A GREENHOUSE

A model of humidity within a greenhouse is developed and some new appl ications in greenhouse climate management are discussed. In the model, ambient vapour concentration results from the balance of three fluxes : crop transpiration, ventilation and condensation at the cover. Trans piration and Ventilation rates are calculated by means of models devel oped earlier, described briefly here. Condensation is given by standar d theory of mass transfer at plane surfaces, neglecting the small slop e of a greenhouse cover. Through experimental verification in a greenh ouse it was shown that the model, which takes into account the many fe ed-backs present in such a system, provides reliable estimates of both ambient humidity and crop transpiration. As an application of the mod el, the use of parameters such as relative humidity or saturation defi cit in the climate control of modern greenhouses is discussed. The mer it of each one is shown to be conditional upon the intended result. Ro ughly speaking, putting a ceiling on relative humidity is likely to re duce chances of dew formation, whereas a threshold on saturation defic it in general forces the transpiration rate above some level. However, there is no general rule linking a value of any humidity parameter to a level of the intended process, since the relationship in question i s affected by prevailing weather, as well as present leaf area. With a humidity model such as the one described here, values of the regulate d climate variables, like the temperature and ventilation set points, can be deduced from the desired level of a crop process (in this way e liminating the use of an intermediate humidity parameter in climate co ntrol). Accordingly, it is concluded that modern greenhouse climate ma nagement, which aims at steering crop processes should incorporate a s imilar humidity model.