Matching Method And Experimental Research On Organic Rankine Cycle For Waste Heat Recovery From Heavy-Duty Diesel Engine

As one of the major oil consumption sources,it is significant to improve the overall energy efficiency of heavy-duty truck diesel engines and reduce CO₂ emission.Among various energy-saving technologies,the Organic Rankine cycle(ORC)-based waste heat recovery(WHR)technology could balance efficiency and practicality.The main waste heat sources of heavy-duty truck diesel engines,including exhaust gas and engine coolant,differ significantly in terms of"quality"but are equivalent in terms of"quantity".The basic Organic Rankine cycle does not match this characteristic,which makes it incapable to efficient-use the waste heat from heavy-duty truck diesel engines.Current project focuses on the matching method of the Organic Rankine cycle,which is used to guide the design of cycle configuration and working fluid to realize the efficient and coordinated use of the waste heat in terms of"quality"and"quantity".Using pinch point analysis,waste heat sources from heavy-duty diesel engines are combined into a composite waste heat source in the t-q diagram.An ideal thermodynamic cycle that matches the composite waste heat source and the cold source is presented.Based on that,the concept of cycle perfection is defined to reflect the gap between the actual Organic Rankine cycle and the ideal one,which gives guidance on cycle configuration design.Meanwhile,physical properties characteristics of the ideal working fluid are presented from aspects of waste heat utilization and cycle thermal efficiency,which provide guidance on working fluid design.Based on the concept of cycle perfection,this paper proposed a configuration design method for Organic Rankine cycles,in which the configuration modification is performed by improving the thermal matching in the t-q diagram.Using this method,a preheating Organic Rankine cycle is presented as the basic Organic Rankine cycle(cycle perfection:47.4%),and a series of cycle configuration modification concepts are discussed.A dual-pressure Organic Rankine cycle is customized with maximum cycle perfection of 72.7%,and a regenerative Organic Rankine cycle is customized for automobile applications(cycle perfection:57.0%).Based on the physical properties characteristics of the ideal working fluid:C₁(waste heat utilization rate)and C₂(cycle thermal efficiency),three sets of working fluids are chosen,namely CO₂ with high waste heat utilization rate,hydrocarbons with high cycle thermal efficiency,and refrigerants.Evaluation model based on the first thermodynamics law,the second thermodynamics law,and component size&economic evaluation is built to comprehensively evaluate various working fluids.Comparison results based on this model show that cyclohexane has the highest net output work,cycle thermal efficiency and exergy efficiency,followed by CO₂ and R245fa.CO₂ shows miniaturization potential in turbine sizes and heat exchangers.The main challenge for the application of hydrocarbons is safety risk due to their flammability,while CO₂also has the condensation issue due to its low critical temperature.Based on the evaluation results,two technical solutions are proposed with complementary ideas:CO₂+refrigerant and CO₂+hydrocarbons.After optimization analysis,CO₂/R134a(0.4/0.6)and CO₂/propane(0.4/0.6)are selected as the representatives of the two technical solutions.Results indicate that CO₂/R134a(0.4/0.6)mixture could improve cycle thermal efficiency under the condition of limited sacrificing waste heat utilization of CO₂,resulting in better system performance.CO₂/R134a(0.4/0.6)mixture has higher output power and exergy efficiency in comparison with CO₂,and it could expand the range of condensation temperature.However,CO₂/R134a(0.4/0.6)required an additional 56%?63%total heat transfer area.Based on the theoretical analysis results,CO₂/R134a mixture is selected as the experimental research object for waste heat recovery from heavy-duty truck diesel engines.The effect of the mixture composition ratio on measured operating parameters,heat exchanger performance,and system performance is investigated.Four CO₂/R134a mixtures with R134a mass fraction of 15%,30%,40%,and 60%are selected.Along with the increase of R134a mass fraction,the net power output,cycle thermal efficiency and exergy efficiency increase at first and then decrease.CO₂/R134a(0.6/0.4)and CO₂/R134a(0.7/0.3)has better system performance.Finally,considering the comprehensive system performance and the expansion of condensation temperature range,the mixture of CO₂/R134a(0.6/0.4)mixture was selected for comparison with pure CO₂and feasibility study under ambient temperature cold source conditions.The experimental results show that in comparison with pure CO₂,the CO₂/R134a(0.6/0.4)mixture could achieve significant increase in net power output,cycle thermal efficiency,and exergy efficiency.The feasibility experiments show that CO₂/R134a(0.6/0.4)based-Organic Rankine cycle is capable of operating stably under ambient cooling condition,demonstrating that CO₂/R134a mixture can expand the range of condensation temperature and alleviate low-temperature condensation issue encountered with CO₂.Using the proposed WHR system,the thermal efficiency of the diesel engine could be increased from 41.1%to 44.3%,while the fuel consumption rate can be reduced from 205.8g/k Wh to 190.7g/k Wh.