Theoretical Analysis Of CO₂ Power Cycle Using For Engine Waste Heat Recovery

Considering the increasingly serious energy and environmental issues,the energy saving and emission reduction for internal combustion engine are of great significance,and the engine waste heat recovery is an important way to achieve it.CO₂ power cycle has been regarded as one of the promising technologies in the field of engine waste heat recovery with better adaptability.Since the relevant research on CO₂ power cycle was lack of comparative investigation and practical evaluation,a detailed theoretical analysis was conducted in view of cycle configurations,performance evaluation and optimization,in order to provide reference and guidance for recovering engine waste heat in an efficient and economical way.The CO₂ power cycle waste heat recovery system with various configurations was proposed,considering CO₂ transcritical Rankine cycle and CO₂ Brayton cycle as the basic framework.Meanwhile,a thermo-economic performance evaluation system involving of net power output,exergy efficiency and electricity production cost was established.The influence of different operating parameters on the system performance was also studied.The results revealed that the CO₂ transcritical Rankine cycle with better performance operated at a highest pressure of more than 12.5 MPa.Additionally,CO₂ Brayton cycle achieved the best comprehensive performance with the lowest pressure less than 8.28 MPa,the intermediate pressure less than 11.16 MPa,the highest pressure more than 19.37 MPa and the maximum temperature more than 610.72 K.Based on the established performance evaluation system,the multi-objective optimization was carried out using genetic algorithm,thus the selection of the cycle configuration and the decision of the operation parameters were realized.The results showed that compared with the B-CTRC of worst heat recovery capability,P-CTRC owned the most limited enhancement on thermodynamic performance but at the expense of the highest electricity production cost.R-CTRC had better improvements on thermodynamic performance,accompanied with the best economical performance corresponding to the lowest electricity production cost of 0.560 $/k W·h.PR-CTRC was most potential in terms of thermodynamic performance with the highest net power output of 25.89 k W and highest exergy efficiency of 40.95%.The thermo-economic performance of CO₂ Brayton cycle was slightly inferior compared with CO₂ transcritical Rankine cycle.Based on the B-CBC of poor worst heat recovery capability,the thermodynamic performance boost was unsatisfactory but the economic performance was deteriorated with simply adding a preheater,while the introduction of a regenerator was the significant means to improve the system performance.The maximum net power output was obtained in PR-CBC as 20.25 k W.The maximum exergy efficiency and minimum electricity production cost were both obtained in R-CBC of 35.67% and 0.817 $/k W·h,respectively.A more detailed discussion on the system performance under the optimal operating parameters was conducted with the consideration of component weight for R-CTRC,PR-CTRC,R-CBC and PR-CBC of higher recycling potential based on multi-objective optimization.Even considering the power loss caused by the additional weight of the waste heat recovery system,PR-CTRC was still most promising on net power output.Under the optimal operating condition,a considerable amount of net power of about 22.03 k W could be recovered from the engine waste heat by PR-CTRC.As a result,the efficiency of the internal combustion engine was improved by 8.97%.