Thermo-fluid dynamic analysis of the air flow inside an indoor vertical farming system

Abstract

In this study, computational fluid dynamics (CFD) is used to assess the thermo-fluid dynamic behaviour of an indoor vertical farming system. Experimentally driven three-dimensional k-ε steady simulations are performed using a species transport model to account for relative humidity. In-field sensor measurements are used to set the boundary conditions for the simulations and to validate the results in terms of temperature and humidity distribution within the growing cell. The entire cell, consisting of eight levels of growing tables is simulated under daylight conditions, including the heat source from LED lights in the thermo-fluid dynamic air flow field. Results help to understand the main features of the air flow distribution. The comparison of the numerical results with measurements demonstrate the ability of the numerical approach to characterize the thermo-fluid dynamic flow field of the indoor farming system and the reliability of well-calibrated CFD simulations in controlling the air flow distribution, which is necessary to reduce energy consumption and to improve plant growth quality. Critical regions for the plant growth are identified within the farm based on the study of air speed uniformity and on the analysis of vapor pressure deficit (VPD). A growing table efficiency index is introduced based on the definitions of two dimensionless objective uniformity parameters. The analysis of these indices helps identifying differences in the fluid dynamic behaviour of the upper and lower floors of the cell and some deficiencies in the ventilation system ability to provide uniform air conditions to all the growing tables. Results show a symmetric behaviour of the left and right layers for upper floors, with tables in front of the evaporators showing better efficiencies. An opposite behaviour is observed on the lower floors, exhibiting a strong asymmetry between left and right layers. On the other hand, the VPD analysis reveale that certain regions near the evaporators and walls (with good flow uniformity) experience extreme conditions that could affect plant growth. The approach presented in this paper provides a detailed understanding of the indoor vertical farming environment and its impact on growth and yield of leafy greens. This, in turn, helps in the effort to optimize the vertical farm ventilation system and thus its energy, which remains the main issue of such cultivation plants.