A multi-airspace model to investigate airflow patterns and carbon dioxide emission sources in deep-pit cattle finishing barns

An increasing number of cattle finishing barns in the Midwest are naturally ventilated (NV) with slatted floors over deep-pit manure storages. However, there is limited gas emission data currently available for this type of cattle housing and manure storage. Emission rate measurements are particularly challenging in NV barns where there are low gas concentrations and large variations in air velocity and direction. This research explores gas emission estimation using carbon dioxide (CO₂) concentrations measured at the north and south wall openings, floor, nose level, and above the manure surface in the storage pit. With these measurements, three control volume mass balances for the barn airspace, animaloccupied zone (AOZ), and manure pit headspace were used to estimate airflows between control volumes and CO₂ emissions rates. In this study, the estimated airflow and CO₂ emission values for two NV barns were calculated for the summer, fall, and spring seasons. Nominal airflow rates calculated between the three control volumes for each barn were variable using three different solution approaches applied in this study. There was a general pattern where airflow through the barn airspace was larger than airflow between the barn airspace and animal-occupied zone, followed by airflow through the slatted floors. The range of estimated CO₂ emissions considering both barns and three solution approaches was-1.16 g/s to 29.3 g/s. Overall, while calculated airflow and CO₂ emission results were variable across three solution approaches, this study provides insight on a range of CO₂ emissions and airflow rates through deep-pit NV cattle finishing facilities. Further model validation would be needed prior to the use of this model for gas mitigation purposes.