Impact of dual channel aeration ventilation on lettuce canopy heat exchange in plant factories

A stagnant airflow area can often occur in the increasing cultivation layers in a plant factory with artificial light, leading to the low growth of the plant, and even physiologic diseases, like tipburn. Furthermore, stagnant airflow can also lead to uneven distribution of environmental factors in the plant canopy, resulting in uneven growth of plants. The optimal air velocity is in the range of 0.3-1.0 m/s in plant canopy. A ventilation system can effectively solve these challenges. Previous studies have focused on the airflow to the plant cultivation spaces. However, these ventilation systems can only provide a single direction of airflow with low penetration, which is heavily obstructed by leaves, resulting in stagnant airflow zones within the canopy. Besides, the devices can also increase the equipment complexity to decrease operational efficiency. In this study, an assembled system of dual channel aeration cultivation was designed to increase the airflow within the plant canopy using simple equipment with operation convenience. A cultivation experiment was conducted in a plant factory. There was one control group (common aeration ventilation of a plant factory generated an airflow velocity of 0.2 m/s within the plant canopy) and three experiment groups (the dual channel aeration ventilation mode generated the airflow velocity of 0.6 (T₁), 0.9 (T₂) and 1.2 (T₃) m/s, respectively). A systematic investigation was made to explore the impact of different ventilation modes on plant growth, tipburn occurrence, heat exchange with the surrounding environment, and the canopy microenvironment. The results showed that the dual channel aeration ventilation outperformed the conventional one, in terms of the lettuce canopy environment, lettuce growth, and heat exchange capacity. Specifically, the best growth for lettuce plants was observed at a canopy airflow velocity of 0.9 m/s, with a shoot fresh weight of 56.7 g. The optimal canopy environment and heat exchange capacity for lettuce were achieved at a canopy airflow velocity of 1.2 m/s. There was a decrease of 8.8% and 2.8 ℃ in average canopy relative humidity and average air temperature, compared with the control group. The airflow regime was first transitioned from the laminar to a transitional flow and then changed to a turbulent flow at an airflow velocity of 0.9 m/s. The convective heat transfer coefficient was also significantly improved with the increasing airflow velocity. The sensible heat flux in the light and dark periods increased by 48.5% and 52.3%, respectively, while the latent heat flux rose by 52.9% and 37.9%, respectively, with the airflow velocity within the plant canopy increased from 0.2 m/s to 1.2 m/s. Besides, there was no tipburn occurred in the experiment groups, while the tipburn occurrence of the control group was 20.9%. It infers that the dual channel aeration ventilation can be expected to effectively alleviate tipburn. In conclusion, compared with the conventional ventilation mode, dual channel aeration ventilation can effectively enhance the plant canopy environment, plant yield, and quality, as well as heat exchange between plants and their surroundings. And the assembled structure can improve transportation and installation efficiency. Furthermore, the integrated structure of ventilation ducts and cultivation tanks also reduced equipment complexity. This mode can provide technical support to precise microenvironment control in plant factories.