Investigation of organic Rankine cycles with zeotropic mixtures as a working fluid: Advantages and issues

Global warming has been a major concern, and the amount of CO2 released to the atmosphere keeps increasing due to overuse of the earth's fossil fuel reserves. Overpopulation and lack of comprehensive energy management add to the problem. New and renewable energy sources are still expensive or in the conceptual phase. The organic Rankine cycle (ORC) seems to be a viable option to address these issues by reusing waste and low-grade heat that would otherwise be released into the environment. Many ORC power plants are already in commercial use. Since the introduction of the basic ORC (BORC), there have been many revolutionary ideas to optimize the performance of power generators, but they have not been experimentally tested extensively. This study focuses on zeotropic refrigerant mixtures made of two or three refrigerants instead of a single working fluid. The main advantages of this system are increased exergy efficiency and decreased irreversibility in the evaporator and condenser, where the isothermal phase change of pure refrigerant would not match the temperature profile of the heat source and heat sink. Published experimental studies on this topic are extremely rare. This study discusses almost all the articles on this subject, the proposed mixtures, and the conclusions.