Optimization Analysis Of Engine Waste Heat Recovery System Based On Temperature Matching

At present,the problem of global energy shortage is increasingly serious.The energy saving,emission reduction and energy efficiency increase of internal combustion engines(ICEs)have far-reaching significance to alleviate the energy crisis and environmental pollution.According to research,the effective power output of ICEs is less than half of the total heat energy generated by fuel combustion.Most of the energy is consumed by exhaust and jacket water cooling.It is significant to improve the energy efficiency of ICEs by using waste heat from exhaust and jacket water.Organic Rankine cycle(ORC)is considered to be one of the most promising waste heat recovery technologies for ICEs due to its good efficiency,stability and wide application range.The waste heat of ICEs has the characteristic of multi-grade.The temperature difference between jacket water and exhaust is large,which brings great challenges to the construction of ORC and selection of working fluids.In order to adapt to the characteristics of multi-grade heat sources of ICEs,this paper matches the cold and heat sources by constructing cycle structure and screening working fluids actively,so as to improve the efficiency of cycle.In order to comprehensively evaluate the performance of structures,an evaluation model based on the first and the second law of thermodynamics,and the economic efficiency is constructed in this paper.By establishing the temperature matching T-Q diagram of the waste heat sources,it is found that the traditional preheating cycle will cause poor temperature matching in the heat exchange process.At the same time,the temperature of working fluids after expanding is very high and the temperature matching between the cycle and the cold source also needs to be improved.In order to solve the problem of poor temperature matching between the traditional preheating cycle and the heat sources,this paper actively constructed cycles based on the T-Q diagram.Four optimized regenerative cycle structures were constructed.They are low temperature regenerative cycle,high temperature regenerative cycle,dual regenerative cycle and split regenerative cycle.Through the comparison and analysis between the preheating cycle and the four optimized cycles,it is found that the optimized cycles can improve the temperature matching with heat sources,reduce the irreversible loss in the heat exchange process and achieve a significant increase in the net power output.Split regenerative cycle has the best temperature matching with heat source and obtains the highest net power output.The influence of the physical properties of working fluids on the temperature matching of the cycle is studied.From the view of critical temperature and the specific heat of working fluid,the physical characteristic of working fluid is defined for better matching with ICEs waste heat recovery.Based on this physical characteristic,this paper actively selects the suitable working fluids.The results show that when the critical temperature of the working fluid is located in the exhaust heat exchange temperature zone,the temperature matching between the cycle and the cold source can be effectively improved and the net power of the cycle can be improved.The cycle performance is optimal when it adopts ethanol as the working fluid.The maximum net power output is25.52 k W and Electricity Production Cost is 0.5766 $/(k W·h).Based on the temperature matching relationship between the waste heat recovery cycle and the heat source,cycle structures are constructed actively and the working fluid are selected actively in this paper.The results show that the cycle constructed by the temperature matching method can improve the heat transfer quality in the heat exchange process of the system and improve the performance of the system,which can provide effective guidance ICEs waste heat recovery cycle design.