Study of CO ₂ /Hydrocarbons Mixture as the Working Fluids for Engine Waste Heat Recovery 2018-01-0049

Transcritical Rankine cycle (TRC) is a promising technology for the engine waste heat recovery due to its good temperature matching ability for the waste heat sources. As for the high-temperature engine exhaust, working fluids selection has been an essential issue without a good solution. It was found in this research that mixtures of CO₂ and small molecule hydrocarbons are the potential working fluids for the engine waste heat recovery, since they have good chemical stability and thermal performance. Besides, CO₂ can be used as the retardant to suppress the flammability of hydrocarbons to ensure safety. In this research, CO₂ mixed with five small molecule hydrocarbons are proposed as the working fluids. A thermodynamic model of TRC system is established to evaluate the thermal performance of those mixtures. The effects of mass fraction of CO₂, turbine inlet temperature and pressure are investigated. The influence of composition shift is also discussed. The results show that, 65% CO₂ in the mixtures is a tremendous improvement for the system safety, comparing to pure hydrocarbons. Compared with the pure CO₂ TRC system, the thermal performance of the system can be effectively improved by mixing appropriate hydrocarbons with CO₂, and the operation pressure can be decreased, which is beneficial for the future application. Take CO₂/n-Pentane for example, the thermal efficiency and net power output can reach up to 16.4% and 16.06 kW, increasing by 34.4% and 13.3% compared with pure CO₂. And the composition shift of those mixtures has little effect on the performance of TRC, but can improve the TRC system safety.