Energy quality and energy grade: concepts, applications and prospects

A more comprehensive view on the energy properties could contribute to the deepening of thermodynamic theories and efficient utilization of energy, which has both theoretical and practical significance. As derivative concepts of exergy, energy quality and energy grade, which characterize the quality of energy, have been fully applied in the field of engineering thermophysics during the past four decades. However, their concepts and calculations have not been fully clarified, and summaries of related applications are very rare in the literature. This has led to a considerable degree of ambiguity and confusion in the utilization of these concepts, which is not conducive to the application and promotion of the concepts, and would also hinder the process of the cognition for energy properties. Based on literature research, this paper explains the history of the energy quality and energy grade concepts and expands the calculations and related benchmark issues (environmental reference states) in detail. Moreover, in order to better understand the functions of energy quality and energy grade, analysis methods derived from the concepts and their applications are reviewed. Through summarizing existing research, future research needs on energy quality and energy grade is elaborated. This paper will help to clarify the physical meanings and calculations of energy quality and energy grade, as well as contribute to in-depth thinking on the energy quality and energy grade and other potential energy properties.