Methods for determining heat exergy for the study of low-temperature and cryogenic processes
At present, the method of exergic analysis for the study of various technical systems has been developed quite well. The exception is low-temperature and cryogenic processes occurring at temperatures below ambient temperature (installations for providing microclimate in buildings for various purposes, air separation systems and other gas mixtures, etc.). The results of studies of such technical systems often look unconvincing, or even contrary to common sense. The reasons lie both in the formal transfer to this area of the methods of exergetic analysis, which work well in the study of high-temperature processes, and in the interpretation of the term "exergy", without taking into account the features of the functioning of low-temperature processes. Based on the analysis of publications, the authors of the article proposed a new method for determining the exergy of heat for processes occurring at any temperature, including low one. For this purpose, the concept of a cumulative isolated system, including the technical system and the environment under consideration, is introduced. The process of establishing an equilibrium in such a system depends on the ratio of the source and receiver temperatures. If the temperature of the considered system is higher than the ambient temperature, the equilibrium in the cumulative system is established due to the transfer of heat from this system to the environment. Part of the transferred heat can be converted into work. With the opposite temperature relationship, equilibrium in the cumulative system can be established only at the expense of the heat of the environment. In this case, the environment acts as a source of heat, and the technical system becomes its receiver. Such a representation of the processes occurring in the cumulative isolated non-equilibrium system consisting of a technical system and the environment allows us to correctly determine the exergy value at any temperature.