Solar energy conversions in the greenhouses

For the greenhouse thermal analysis, it is essential to know the amount of solar radiation that was utilized by the greenhouse and consequently converted into sensible and latent heat via convection and evapo-transpiration. Parameters are frequently used for this purpose are: utilization efficiency factor (π), and sensible and latent heat factors (η and δ). Previous studies considered these factors as constants, although they depend on the time of the day, the environmental conditions inside and outside the greenhouse, plants and soil characteristics and the greenhouse structure, orientation and its location. These factors in addition to a proper correlation to estimate the evapo-transpiration rate in a greenhouse have not yet been investigated under the arid climatic conditions of the Arabian Peninsula. This study was to investigate π, η and δ factors; conversion of solar radiation into sensible and latent heat; and the evapo-transpiration rate (ET) in the greenhouse under arid conditions. A polyethylene-covered, evaporatively cooled, planted greenhouse with a floor area of 34 m2 was used. The parameters required for the analysis were measured in a hot summer and sunny day. The results showed that value of π was almost constant (≅0.75); whereas values of η and δ strongly depended on the net solar radiation over the plants (Sn1). ET depended on the vapor pressure deficit (VPD) much more than Sn1. Correlations to estimate ET, as a function of VPD and Sn1 and to estimate η and δ factors as functions of Sn1, were investigated. At a plant density corresponding to a leaf area index (LAI) of 3, about 54% of the integrated solar radiation that was utilized by the greenhouse was converted to sensible heat and about 46% converted to latent heat via evapo-transpiration.