Solar energy utilization by a greenhouse: General relations

Abstract

Most of solar radiation incident on a greenhouse is absorbed by greenhouse components (i.e., the cover, humid air, plants and soil) and the remaining portion is lost to outside the greenhouse. It is essential to know the absorbed and lost energy terms for any thermal analysis of greenhouses. Existing greenhouse thermal models use the radiative properties of the greenhouse components to directly determine the absorbed energy terms. However, these models neglect the lost energy term and neglect the effects of the multiple reflections of solar radiation between the greenhouse components. The present study describes the general relations for estimating the amounts of solar energy absorbed by the greenhouse components and lost to outside the greenhouse. The relations take into consideration the interrelations as well as the multiple reflections of solar radiation between these components. Thus, the greenhouse system was treated as a solar collector having an absorber plate (i.e., the greenhouse soil) and a cover system consisting of three semi-transparent parallel layers (i.e., the greenhouse cover, the humid air, and the plants). Superposition theory and ray tracing technique were used for the analysis. The presented relations were applied to an experimental plastic-covered greenhouse with a floor area of 34 m 2. The greenhouse, located in Riyadh, Saudi Arabia, was planted with tomatoes with a leaf area index (LAI) of 3.0 and was cooled by a wet pad and fan system. Results of the presented relations were accurate and more realistic comparing to results of other relations reported in the literatures. Absorption of solar radiation by water vapor in the greenhouse was negligible. The presented relations can estimate the absorbed and lost energy terms for a greenhouse precisely with a max possible error of +1.8% on each term if the LAI was less than 1.5. The error is significantly decreased to less than +0.7% if the LAI in the greenhouse is increased to 5.