Towards working fluid properties and selection of Rankine cycle based waste heat recovery (WHR) systems for internal combustion engines – A fundamental analysis

• Published in: Applied thermal engineering Vol. 142; pp. 502 - 510
• Main Authors: Su, Xingyuan, Shedd, Timothy A.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.09.2018; Elsevier BV
• Subjects: Energy efficiency; Engines; Environmental impact; Heat recovery systems; Internal combustion engines; Properties (attributes); Rankine cycle; System thermal efficiency; Thermal energy; Thermodynamic efficiency; Waste heat recovery; Working fluid properties; Working fluid selection; Working fluids; Working fluid selection; System thermal efficiency; Working fluid properties; Waste heat recovery; Internal combustion engines; Rankine cycle
• ISSN: 1359-4311; 1873-5606
• DOI: 10.1016/j.applthermaleng.2018.07.036

•A new theoretical thermal efficiency formula was derived and validated.•Fluid properties effects were analyzed from system thermal performance perspective.•A fluid with smaller Jakob number is preferable for system thermal efficiency.•A fluid with smaller Jakob number achieves higher effective evaporation temperature.•A fluid with a smaller Jakob number runs closer to the Carnot cycle. Working fluid selection is one of the most important procedures in the design of Rankine cycle based waste heat recovery systems for internal combustion engines. The system’s performance, cost, and environmental impacts can be greatly influenced by the properties of its working fluid. In this work, we present an original formulation of a theoretical thermal efficiency model to fundamentally understand how the working fluid properties affect the thermal performance of the Rankine cycle based waste heat recovery systems for internal combustion engines. The derived theoretical thermal efficiency formula is validated under several different operating conditions. Overall very good agreements are achieved between the actual and theoretical approximated solutions. The results show that when the evaporation and condensation temperatures of the waste heat recovery system are fixed, the system’s thermal efficiency is simply governed by a non-dimensionalized parameter Jakob number. A working fluid with a smaller Jakob number is preferable in terms of system thermal efficiency. The observation is also further understood from a more fundamental physical perspective. Finally, a preliminary working fluid selection approach with a solid physics is proposed to screen the fluid candidates from the perspective of system performance.