Design Of Latent Heat Utilization Compound Organic Rankine Cycle System

China’s consumption of oil energy is increasing day by day,leading to a growing dependence on imported oil energy each year.Given the current international situation,which is marked by complexity and tension,it is essential to improve China’s oil energy utilization rate.With a high number of fuel vehicles,China consumes a significant amount of fuel.The thermal efficiency of internal combustion engines is quite low.The exhaust gas and coolant contain valuable heat energy.Therefore,making an efficient recovery of this surplus energy is crucial for enhancing energy use efficiency and meeting the goals of energy conservation and emission reduction.This paper presents a qualitative discussion and trend analysis of the efficient recovery of waste heat from a heavy-duty diesel engine.The study designed a latent heat utilization compound organic Rankine cycle system,which recovers exhaust gas residual energy at high temperatures and cooling liquid residual energy at low temperatures.The central focus of this work involves:(1)Establishing the architecture of the latent heat utilization compound system:analyzing the potential advantages of waste heat recovery with this architecture,verifying the adaptability of the latent heat utilization compound system with dual heat sources of exhaust gas and cooling liquid through simulation.(2)Selecting the working medium and system parameters:completing the selection of the circulating working medium and exploring the influence of working parameters and heat source state on system performance.(3)Expander design:designing expander parameters and variable expansion ratio demands,building a one-dimensional simulation model to explore the relationship between structural and working parameters and expander performance.The main focus and conclusions of this research are summarized below.Firstly,the study established a latent heat utilization compound organic Rankine cycle architecture.This involves the analysis,both theoretically and through simulation,of the difference between the latent heat utilization compound system and the parallel double-loop cycle in recovering tail gas and coolant energy,respectively.Results show that the latent heat utilization system has lower irreversible loss when heat is transferred,along with a net output power that is 17.4%higher than that of the parallel double-loop system.These findings demonstrate the excellent potential of the latent heat utilization compound system in recovering exhaust gas and coolant waste heat.Secondly,the study selected a circulating working medium for the latent heat utilization compound system.Six potential candidates,namely pentane,cyclopentane,n-hexane,cyclohexane,n-heptane,and acetone,were preliminary screened.Results showed that the net output power of each group was respectively 21.4kW,22.9kW,23.3kW,24.6kW,24.3kW,and 23.2kW.This analysis identified cyclohexane and n-heptane as the best-performing working fluids among the six candidates.Therefore,these two fluids were selected to be used as high-temperature cycle working fluids.Thirdly,the study designed the working parameters of the latent heat utilization compound system.This involves exploring and analyzing the effects of circulating evaporation pressure,high-temperature circulating superheat,high-temperature circulating expansion ratio,and coolant heat source temperature on the system.The following conclusions were drawn.When the system works at different high-temperature circulating evaporation pressures,the best net output power will always appear when the low-temperature circulating evaporation pressure is the maximum pressure that the coolant can heat the low-temperature working medium.The performance of the system improves with the higher evaporation pressure of high-temperature circulation.However,as the high-temperature evaporation pressure increases,losses gradually offset gains.A higher superheat of high-temperature circulation leads to a lower net output power of the system due to its significant effect on the mass flow of high-temperature circulation,resulting in reduced mass flow.A larger expansion ratio of high-temperature cycles,that is,a lower condensation pressure of high-temperature cycles,leads to a higher net output power of the system.Thus,a larger expansion ratio can be considered when designing the system.The losses of the heat exchanger,low-temperature circulating condenser,integrated evaporation/condenser,and low-temperature expander of low-temperature working fluids and cooling fluids are relatively high.The higher the temperature of the coolant heat source,the higher the net output power and efficiency of the system.This relationship depends on the ability of the coolant to heat the low-temperature working medium.Therefore,the matching relationship between the working parameters of the system and the heat source should be considered in the system’s design.Fourthly,the study designed the reciprocating piston expander with a variable expansion ratio.A variable expansion ratio device was developed according to the changes in the intake and exhaust time.The key structural parameters of the expander were calculated according to the displacement demand of the expander.The single-cylinder model of the reciprocating piston expander built in GT-Power was used to examine the impacts of both structural and working parameters.The study yielded the following conclusions.In the development of the variable expansion ratio device,an intake and exhaust mechanism was realized based on slotting on the cylinder body and matching with the movable slider group.This design enables the adjustment of the relative and absolute position of the intake and exhaust valves to achieve the goal of modifying the intake and exhaust timing and mass flow rate,thereby allowing the expansion ratio to be adjustable.Among the structural parameters,a stroke/cylinder diameter of 0.7(S/D=0.7)with the corresponding stroke S was 36.7mm and the cylinder diameter D was 56.7mm,showed the best output power(1.52kW),resulting in an efficiency of 74.5%and a mass flow rate of 0.047kg/s.Additionally,a smaller clearance ratio structure improved the expander’s working performance.The expansion ratio had a significant impact on the expansion ratio,for instance,an inlet pressure of the working medium of 2MPa yielded an optimal output power of 1.856kW at an expansion ratio of 1.54.With regard to working parameters,rotating speed significantly impacted the expander’s operation,with the best working performance of the S/D=0.7 expander at around 1500r/min.Moreover,increasing the inlet superheat of the expander improved output power but decreased its working efficiency.In summary,the design of the latent heat utilization compound organic Rankine cycle system has completed the verification of waste heat recovery potential,selection of circulating working mediums,determination of working parameter ranges,and the design of variable expansion ratio expander.These results provide theoretical guidance for the further development and application of heavy diesel engine waste heat recovery systems.