Performance improvement of a preheating supercritical CO2 (S-CO2) cycle based system for engine waste heat recovery

Abstract

Due to the compact structure in addition to the system safety level and environmental friendly characteristics, supercritical CO2 (S-CO2) cycle has emerged as a promising method to be used in engine waste heat recovery. This paper explores the potential of using a preheating S-CO2 cycle based system to recover the waste heat of a diesel engine. An original preheating system is presented, in which the high temperature engine exhaust gas is firstly utilized in the evaporator and then it is further cooled through preheating process together with the low temperature jacket cooling water. Though the entire heat load from these two heat sources could be entirely recovered, the high preheating temperature suppresses the heat transfer in the regenerator. An improved preheating S-CO2 cycle based system with a regeneration branch is then presented. S-CO2 flow from the compressor is divided into two parts, one of which is still preheated by the jacket cooling water and the cooled engine exhaust gas in series while the other is heated in a low temperature regenerator. The two flows converge and then continues to be heated in the high temperature regenerator and in the evaporator. The simulation results reveal that the improved system could achieve a deeper utilization of the regeneration heat load hence improve the system performance. The maximum net power output of the improved system reaches 68.4 kW, which is 7.4% higher than that of the original system, 63.7 kW. Adopting the improved preheating S-CO2 cycle based system for waste heat recovery, the engine power output (996 kW) can be increased by 6.9%.