Analysis of the time evolution of the intercepted radiation inside a crop using CFD

Abstract : Assessing the distribution of short wavelength radiation inside a greenhouse crop is of prime interest as absorbed radiation is one of the main factors governing the energy and water vapour transfers at plant level. The aim of this paper is to analyse the distribution of solar radiation inside an ornamental crop and to improve the crop submodel implemented in CFD tools. In this prospect, a glasshouse compartment was equipped with a set of five silicon solar cells equally distributed vertically inside a New Guinea Impatiens potted crop. Fitting the data with the exponential Beer's law made it possible the determination of the corresponding extinction coefficient. A CFD model was then used to predict the solar radiation distribution inside the canopy all day long. Contrary to the commonly adopted technique which consists in describing the vertical exponential decay of the radiation from the knowledge of the incident solar radiation, the model solves the radiative transfer equation with the Discrete Ordinates model. The model requires the definition of the absorption coefficient which is inferred from the measured extinction coefficient. The model was first validated comparing the vertical distribution of solar radiation to the measured one at midday. It showed its ability to predict the exponential decay of the shortwave radiation and evidenced the variation of the diurnal vertical extinction coefficient all day long. Dividing the crop into five horizontal layers, the model was then applied to assess the evolution and proportion of the absorbed radiation in each layer. It was shown that the relative proportion of radiation absorbed in the upper layer varied all day long, reaching its minimum at midday.