Research On Fundamental Problems Of ORC Integrated With Latent Thermal Energy Storage Under Dynamic Heat Source

Organic Rankine cycle(ORC)has shown great potential in medium-to-low grade waste heat recovery,due to its relatively high thermal efficiency,simple structure and high reliability.Most of present studies about ORC were conducted based on steady heat source conditions,however,typical waste heat sources such as engine exhaust and industrial exhaust always present fluctuating characteristics,posing great challenges to the safe and efficient operation of ORC systems.Considering the fluctuation of many waste heat sources,ORC integrated with latent thermal energy storage(LTES)is proposed in this study,aiming to use phase change material(PCM)to weaken the fluctuation amplitude of dynamic heat source and ensure the safe operation of downstream ORC system.As a novel scenario,many fundamental problems need to be investigated and solved,such as the charging performance of LTES under fluctuating heat source and the dynamic performance of LTES-ORC system.This thesis focuses on solving these fundamental issues.Main tasks and conclusions are summarized as following:(1)Research on adaptation performance of basic ORC to fluctuating heat sources and fast prediction method for safe operating limitation.The effects of characteristics of fluctuating heat sources,parameters of evaporator on the step change and cyclic operating performance for basic ORC are comprehensively investigated.The variation trend of minimum superheat degree of basic ORC during the operating process is deduced,and the fast prediction method is established to obtain the maximum limitation of fluctuating amplitude of heat sources.(2)Research on the influence mechanism of unsteady heat sources on the melting process of shell-and-tube LTES and heat transfer enhancement techniques.The influence mechanism of fluctuation characteristics of different heat sources on the melting process of LTES is explored.It is found that effects of the fluctuation heat source with smaller periods are less on the melting process,while heat sources with larger periods can promote the melting process.Heat transfer enhancement based on fins for LTES is carried out,and results show that the effect of weakening the fluctuation of heat sources becomes more significant as the height of the fins increases.(3)Research on the influence of PCM thermo-physical parameters on dynamic operating characteristics of LTES-ORC.The influence of thermal conductivity and melting point of PCM on the dynamic performance of LTES-ORC under step-change and cyclic operating conditions is analyzed.LTES can significantly reduce the fluctuation range of heat sources,and the reduction range increases with the increase of the thermal conductivity of PCM.Based on the dynamic operation law,the fast prediction correlation formula is obtained to fast predict the maximum fluctuating amplitude of heat sources for the safe operation of the LTES-ORC,and the results show that the safe operation limit of LTES-ORC is significantly higher than that of the basic ORC.(4)Construction and experimental research of LTES-ORC test bench driven by the waste heat of the internal combustion engine.The performance comparison of the basic ORC and LTESORC under steady-state operating conditions,step change operating conditions and cyclic operating conditions has been experimentally compared.When the step change ratio is small,the LTES-ORC can ensure safe operation,and when the step ratio is large,the time for the superheat to drop to 0 can be prolonged.Under the condition of cyclically changing heat source,the thermal storage ORC can ensure safe operation,and the theoretical average net work and thermal efficiency are increased by 23.5% and 23.2% respectively compared with the basic ORC.(5)Preliminary investigation on performance of different novel LTES-ORC schemes.ORC system integrated with cascaded LTES and direct-evaporation ORC system integrated dual LTES are proposed.As for the former system,the number of segments of LTES N=2 possesses the best performance to overcome the fluctuation of heat source.As for latter system,the results indicates that ORC with double LTES can provide larger total output and work for longer time than other two ORC systems with only one LTES,meanwhile,the system with two LTES can effectively overcome the problems of intermittent operation that single-LTES ORC systems have to face.