A dynamic drying process: Mathematical modeling and energy consumption optimization of alfalfa bales using hot air drying

The objective of this experiment is to propose an innovative method for the dynamic drying of alfalfa bales in combination with a solar-assisted air source heat pump system, aiming to achieve a more efficient and energy-saving drying process while preventing the formation of mold and deterioration of the bales, thus reducing nutrient losses. However, conventional drying methods are often weather-dependent and time-consuming. To address these challenges, this study proposes a dynamic drying of alfalfa bales on chain layer movement drying bed with solar-assisted air source heat pump (SASHP) system. Kinetic modeling, optimization, and energy analysis of the alfalfa bales are performed, the improved Page model is the most effective for predicting the drying kinetics of alfalfa bales. The process parameters were optimized using response surface methodology, which included three drying factors and two response variables. The optimal process parameters for dynamic drying alfalfa bales were a drying temperature of 57–59 °C, a drying velocity of 5–6 m/s, and a bale density of 94–100 kg/m3. Under these conditions, the system achieved a specific moisture extraction ratio of the system is 27.3–27.5 g/kWh, and a dehumidification capacity per unit time is 134–135 g/h. Furthermore, In order to evaluate the performance of the SASHP drying system under the optimal combination of process parameters for the dynamic drying of alfalfa bales, the BP-DryNet model is constructed, and BP neural networks are utilized to predict the energy consumption under this process parameters. The BP-DryNet model adopted a multilayer structure and is designed to predict the moisture content, the drying time, and the energy consumption of the entire drying process during the dynamic drying of alfalfa bales. The results show that the BP-DryNet model performs well in predicting the drying behavior, with coefficients of determination (R2) of 0.893, 0.926 and 0.946. The innovative aspect of this study is the dynamic drying method combined with a solar-assisted air source heat pump, which presents one of the energy-efficient, economical, and effective processes for drying alfalfa bales. This approach provides a new solution for sustainable agriculture and promotes the energy efficiency and cost-effectiveness of the alfalfa bales drying process, while also providing a robust theoretical basis for the practical engineering application of solar heat pump drying in the alfalfa bales process.