Microclimate Air Motion and Uniformity of Indoor Plant Factory System: Effects of Crop Planting Density and Air Change Rate

In a plant factory, maintaining proper and uniform air/moisture movement above the crop canopy is crucial for aiding plant growth. This research has utilized a three-dimensional computation model to investigate airflow and heat transfer in a plant factory, where airflow, heat, and humidity distributions above plant crops were calculated concerning five categories of crop planting density (Pd) and air change rate (ACH) in the crop area. Spatial uniformities of airflow velocity, temperature, and relative humidity immediately above the crops are evaluated using the objective uniformity parameter (OU), relative standard deviation of temperature (RSDT) and relative standard deviation of relative humidity (RSDRH), respectively. Furthermore, a factor of effectiveness (θ) is defined, depending on the uniformity of velocity, temperature, and relative humidity distribution, to comprehensively evaluate the impact of various ACH with Pd on overall effectiveness. Full numerical results show that air velocity, temperature, and relative humidity above the crops are notably influenced by Pd and ACH. As ACH increases, the OU of the air above the indoor crop also expands. Moreover, higher OU values are observed for smaller crop Pd. However, excessively small crop area planting densities and excessively large ACH do not result in a higher OU for the air above the crop. As ACH increases, both RSDT and RSDRH decay for the whole range of crop Pd. Moreover, smaller Pd values could achieve the uniformity of thermal fields, while having minimal effects on the relative humidity distributions. Generally, increasing ACH and decreasing Pd could enhance overall value of θ. However, excessively increasing ACH and decreasing Pd does not have a significant effect on θ, which is jointly influenced by OU, RSDT, and RSDRH. Therefore, a more suitable combination of ACH and Pd is urgently required to improve the design of agricultural system to enhance crop microclimate uniformity for optimal plant growth and productivity.