Effects of surrounding objects on the thermal performance of passively ventilated greenhouses

Abstract

The growing expansion of protected horticulture in many regions is occurring around densely populated areas where land for agriculture is scarce, expensive or is used for other purposes. Inexpensive plastic passively ventilated greenhouses are the common choice for protected cultivation in these developing regions. The objective of this work was to analyse the effect of surrounding constructions and natural obstacles on the thermal performance of two naturally ventilated greenhouses. A saw tooth type greenhouse (TCG), typical for Colombian production, and an optimised greenhouse (OG) alternative with greater ventilation areas were analysed using computational fluid dynamics (CFD) with and without the surrounding objects of a real environment. The results showed that air exchange rate of a greenhouse with restricted ventilation areas are greatly reduced when neighbouring objects are high enough. This ventilation restriction is intensified under low wind speed conditions. The temperature gradients of the OG greenhouse were lower than those of the TCG scenarios due to the increased ventilation rates. The rooftop ventilation index for the OG greenhouse was increased by 65% with respect to the TCG greenhouse index, resulting in a direct effect on the ventilation rates. An improved air exchange with the outside can be reached by increasing the greenhouse ventilation areas, especially the roof vents, to overcome the airflow restrictions imposed by the surrounding environment. This simulation exercise was validated with field temperature data collected for a real OG prototype built in the Bogota plateau, with results showing a similar pattern for the internal temperature gradient as exhibited by the CFD model.