Analysis and Comparison of Typical Exhaust Gas Energy Recovery Bottoming Cycles

Internal Combustion Engine (ICE) consumes approx. 2/3 of the oil in the word and 30-40% of the fuel combustion energy in an ICE is wasted in the form of thermal energy in the exhaust gas stream. Exhaust gas thermal energy recovery demonstrates a great potential for overall system thermal efficiency improvements and fuel saving. In this paper different exhaust gas energy recovery bottoming cycles have been analyzed and discussed based on fundamental thermodynamics theory. The typical bottoming cycles are classified into two categories: i.e. direct and indirect energy recovery bottoming cycles. New terms, i.e. Energy Recovery Efficiency (ERE), Energy Conversion Efficiency (ECE) and Overall Energy Conversion Efficiency (OECE) are proposed for the purposes of easier to analyze and easier to compare among the various bottoming cycles. Simplified formulas are derived to demonstrate the key design and operating parameters which define or limit the energy recovery potential. Various typical bottoming cycles are analyzed and sorted based on their OECE from the greatest to least as: Brayton air cycle with isothermal compression, Over-heated Rankine steam cycle, standard Rankine steam cycle, Brayton air cycle with regeneration, standard Brayton air cycle, direct exhaust gas expansion in secondary expander such as turbo-compounding.