Theoretical analysis and comparison on supercritical CO2 based combined cycles for waste heat recovery of engine

Supercritical carbon dioxide cascade waste heat recovery has proven to be a promising alternative for energy conversion applications. In this paper, four different CO2 Brayton-based dual-loop cycles are integrated to the dual-fuel engine respectively to recover the engine waste heat, including regenerative supercritical CO2 Brayton cycle/organic Rankine cycle (RSCBC/ORC), regenerative supercritical CO2 Brayton cycle/supercritical CO2 Brayton cycle (RSCBC/SCBC), supercritical CO2 recompression Brayton cycle/organic Rankine cycle (SCRBC/ORC), and supercritical CO2 recompression Brayton cycle/supercritical CO2 Brayton cycle (SCRBC/SCBC). Comprehensive parametric analysis and comparison were carried out. When the engine was operated at 1100 rpm and cycle fuel injection quantity of 14.1 mg, the SCRBC/ORC combined system presents the best performance when cyclohexane is used as the working fluid for the ORC, followed by the RSCBC/ORC, RSCBC/SCBC, and SCRBC/SCBC. The energy efficiency of the whole system is increased by 7.03% with the SCRBC/ORC, compared with the condition that engine without waste heat recovery.