Mitigating heat stress for agricultural workers using computational fluid dynamics (CFD) simulations

The Venlo ventilation system is one of the most widely adopted designs for greenhouses, and it is known for its distinctive roof structure that promotes natural airflow. However, despite its widespread use, it is often inadequate in maintaining the desired temperatures during the summer. This increases the risk of heat stress for greenhouse workers, particularly under the high humidity conditions typically encountered in aquaponic greenhouses. In this study, we developed computational fluid dynamics (CFD) models simulating temperature and air velocity distributions using a commercial-scale Venlo-style aquaponic greenhouse as a reference. The study aimed to evaluate the impact of applying reflective whitewash and installing positive pressure ventilation tubes (PPVT) in worker areas on the heat stress experienced by the workers. The standard k-epsilon turbulence and solar ray tracing models were employed in CFD simulations. A total of 1,776 lettuce at three different growth stages were placed inside the aquaponic pools. The CFD models were validated using an experimental-scale greenhouse, and with these validated models, air velocities at the worker's height were calculated to be 3.1 +/- 0.13 m s-1, 3.2 +/- 0.16 m s-1, and 3.8 +/- 0.04 m s-1 for the control, whitewash, and PPVT + whitewash conditions, respectively. When whitewash and positive pressure ventilation tubes were both in use, the air exchange rate increased from 27.3 to 31.8 per hour. Although there was only a 4.5 increase in air exchanges, strategically placing the ventilation tubes resulted in a 7.3 degrees C decrease in temperature at the average worker height. This reduction shifted the heat index from the "extreme danger" to the "caution" zone, allowing workers to safely work up to four consecutive hours with adequate water intake. The annual cost for running the PPVT inline fans was calculated to be as low as $245, which was 2.6 times less than the previously reported operating costs for greenhouse ventilation systems.