Thermodynamic Analysis And Multi-Objective Optimization Of The Organic Flash Cycle Using Zeotropic Mixture Working Fluids

In today’s increasingly serious energy and environmental problems,in addition to new energy sources such as solar,wind and geothermal energy,the study of low and medium-grade energy sources has also attracted much attention.The comprehensive utilization of waste heat resources can effectively alleviate the energy crisis,improve energy efficiency and reduce the pollution to the environment.Organic flash cycle(OFC)is a thermal system that converts low-grade waste heat into high-grade electricity.Compared with organic Rankine cycle(ORC),OFC system improves the temperature matching between the work material and the heat source in the heat exchange process and reduces the heat exchange loss,which has become a hot research topic.In recent years,the organic flash cycle has received attention because of the small heat loss in the heat absorption process,which improves the heat transfer process of the mass in the heat exchanger and increases the system efficiency compared with the traditional organic Rankine cycle.However,due to the large heat loss in the flash process of the organic flash cycle,it cannot take sufficient advantage of it.In this paper,firstly,the OFC system was optimized by using mixed masses to optimize the condenser side heat transfer,and it was found that the mixed masses could improve the OFC system heat and power efficiency,and the OFC system heat and power efficiency changed in different patterns with the change of concentration of mixed masses at different slip temperatures.The effect of circulation parameters such as heat source outlet temperature and flash inlet temperature on the system efficiency was studied again.It was found that lowering the heat source outlet temperature could improve the system radiation efficiency,which would affect the variation pattern of the OFC system radiation efficiency with concentration for the mixed masses.As the flash inlet temperature decreases,the system efficiency shows a rising and then decreasing pattern,which is caused by the bending of the heat source temperature drop curve.The existing optimization methods of organic flash cycle include the use of organic mixed masses,optimization of cycle configuration" and increase of heat return heaters.The effect of the mixture on the efficiency of the OFC and MOFC systems was investigated by applying the non-azeotropic mixture to the organic flash cycle and the two-stage expanded organic flash cycle(modified organic flash cycle,MOFC).The organic Rankine cycle is now the main way to recycle low-grade energy such as waste heat recovery.solar energy and geothermal energy,so whether the OFC system and MOFC system can replace the ORC system needs to be further studied.The thermodynamic performance of basic organic Rankine cycle,basic organic flash cycle and two-stage expanded organic flash cycle were studied comparatively at the heat source temperature of 200 ℃.At the optimum heat source outlet temperature,the OFC breakdown efficiency is slightly higher than that of the ORC,while the MOFC system achieves a significantly higher breakdown efficiency than that of the ORC system,which improves the system breakdown efficiency by reducing the breakdown losses of irreversible processes such as flash throttling.This phenomenon indicates that the OFC system and its optimized cycle configuration can further improve the utilization capacity of low and medium temperature heat sources.To address the diversity of heat source types,this paper selects 20 work gases in the order of critical temperature from high to low and divides them into two mixing groups,and uses the seven parameters of flash inlet temperature,flash outlet temperature,mixed work gas component,mixed work gas concentration,heat exchanger pinch point temperature difference and condenser pinch point temperature difference as variables.Genetic algorithm was used to optimize the OFC cycle and MOFC cycle at three heat source temperatures with multiple objectives.With the objective of using latent heat-based heat sources,the effects of mixed work gases,cycle configurations,and the addition of expanders on the organic flash cycle were investigated.The results show that it is feasible to parameterize the pure mass name as a decision variable to participate in the multi-objective parameter optimization of the OFC system,and it has the advantages of small computational effort and simultaneous optimization of mass and cycle parameters compared with the conventional multiobjective optimization.Moreover,for a given three heat source temperatures,MOFC has an advantage over OFC system at the heat source temperature of 250℃ because it has two expansion work processes.