Influence of Heating System on Greenhouse Microclimate Distribution

Convective and radiative heat transfer from heating systems significantly determines greenhouse microclimate during the cold period of the year. These mechanisms are complicated because they combine free and forced convection modes, most often turbulent with different characteristics. The aim of the present study is to analyze the internal convective flows in a closed greenhouse caused by buoyancy forces from different configurations of heating systems. Numerical results obtained by the use of a commercial computational fluid dynamics code (ANSYS CFX) are compared to experimental measurements carried out in a full-scale experimental greenhouse with a tomato crop. The greenhouse was heated with a network of heating pipes and/or with an air heater. The standard k-e turbulence model was adopted to describe the turbulent nature of the flow and transported properties. The resistance of the crop to airflow and the heat and mass exchanges of the crop with the surrounding air were simulated using the equivalent porous medium approach. In general, good agreement was found, since the mean error between measured and simulated values for air velocity, air temperature, and air absolute humidity distribution was 16%. The combined use of heating pipes and air heater enhanced plant activity and reduced the condensation rate. This heating method led to an increase in energy consumption of up to 19%, but it also created a more heterogeneous climate distribution compared to the case in which only heating pipes were used. It was shown that the greenhouse air volume is split into two regions: one occupied by the crop where natural convection dominated, and one above the crop where the hot air from the air heater resulted in a different microclimate from the lower part of the greenhouse (crop level), and the convection mode changed to mixed or forced depending on the distance from the air heater.