An Engine Exhaust Utilization System by Combining CO2 Brayton Cycle and Transcritical Organic Rankine Cycle

Abstract

For engine exhaust gas heat recovery, the organic Rankine cycle (ORC) cannot be directly used due to the thermal stability and safety of organic fluids. Thus, a creative power system is given by integrating the supercritical CO2 Brayton cycle and transcritical ORC. This system can directly utilize the thermal energy of a high-temperature exhaust gas. The inefficiencies in the heat exchangers are highly reduced by using supercritical working fluid. The mathematical model of the system, covering both the thermodynamic and economic aspects, is built in detail. It is found that the highest irreversible loss takes place in the gas heater, taking 21.14% of the total exergy destruction. The ORC turbine and CO2 turbine have the priority for improvement, compared to the compressor and pump. The increase in CO2 turbine inlet pressure improves the system exergy efficiency and levelized cost of energy. Both the larger CO2 and ORC turbine inlet temperatures contribute to a decrease in levelized cost of energy and a rise in system exergy efficiency. There is a maximum value of system exergy efficiency and minimum value of levelized cost of energy by varying the ORC turbine inlet pressure. The determined exergy efficiency and levelized cost of energy in the proposed system are 54.63% and 36.95 USD/MWh after multi-objective optimization.