Theoretical analysis of exergy transfer and conversionin grain drying process

Abstract: The exergy is defined as the maximum useful work possibly during a thermal dynamic process that brings the system into equilibrium. Analysis of exergy utilization provides a fair and effective method for evaluating the energy efficiency. Since drying is a comprehensive process involving complicated interaction among different materials, exergy analysis is especially helpful in rating the efficiency of different drying strategies. However, the mechanism behind the exergy transfer and conversion during drying has not yet been fully investigated and understood. At present, the lack of theoretical analysis is hindering the implementation and progress of the sophisticated applications. The theoretical difficulties include the quantitative understanding and expression of the coupling effects in different exergies. In this article, we analyzed the exergy transfer and conversion between grain and drying medium. In a drying process, the ultimate goal is to reduce the moisture content in the grain until it is in the dryness that is the same as the environment where the grain is stored. Therefore, we define exergy as zero when the system is in equilibrium with the ambient environment. Based on the comprehensive coupling of the potential energy difference, temperature gradient and pressure gradient, the theoretical models of thermal exergy, flow exergy, diffusion exergy and exergy efficiency are given. We also studied the relationships and restrictions of different exergies based on the enthalpy-moisture diagram. Our results revealed that drying is the result of the simultaneous action of thermal exergy, diffusion exergy and flow exergy. The conversion and transfer of thermal exergy can be directly characterized by water vaporization, which is driven by the temperature gradient in the system. Most of the thermal exergy is directly converted to the latent heat of evaporation. Regardless of the number and type of the heat sources, thermal exergy transfer is always directly related to the temperature gradient of the drying system. Diffusion exergy originates from the excess water in grain. The vapor pressure difference between the wet grain and the drying medium will naturally drive the water transfer, making the wet grain to dry medium. The temperature gradient and pressure gradient both have important effects on the diffusion process. When the temperature gradient is opposite to the vapor pressure gradient, the exergy efficiency is enhanced; otherwise, the exergy efficiency is weakened. The flow exergy maintains the potential difference necessary for the transfer of heat exergy and diffusion exergy. Without flow exergy, the transfer of heat and wet exergy cannot be effectively carried out. In a ventilation and drying system, the exergy difference between the wet grain and drying medium is the driving force behind the drying process. Drying can be summarized as the process of exergy transfer and conversion, converting physical conditions of wet grain into to conditions of the drying medium. Different from isolated systems, the thermal equilibrium of a drying process is determined by the ambient environment which is always static regardless of the size of energy and mass transfer. The drying process still follows the second law of thermodynamics. However, the entropy increase of the ambient environment is negligible. The thermal exergy, diffusion exergy and flow exergy can all be expressed in state functions. We provided time-dependent state functions of exergy and exergy efficiency to reveal the change of exergy flow density and efficiency according to the environmental conditions and the conditions of the grain. These theoretical models can be applied to make fair and reasonable evaluation on the energy utilization in practical applications. Based on the theoretical analyses of exergy and its efficiency, the largest exergy loss process can be predicted and prevented. These models provide a scientific basis for evaluating the energy utilization in a drying system, and can be used to optimize the drying process.