Performance Optimization And Experimental Investigation Of Organic Rankine Cycle System Using Zeotropic Mixtures

With the intensification of energy shortages and environmental pollution,low-temperature waste heat utilization technology has attracted the increasing attention of scholars.Organic Rankine Cycle(ORC)power generation technology is one of the effective ways to utilize low-temperature waste heat.In the research of this technology,the working fluid extraordinarily affects the system performance.The study of pure working fluids is becoming mature and there are some limitations.The emergence of zeotropic mixtures provides a new way for the optimization of system performance,so it has become a new research direction by using mixtures instead of pure working fluids.In this paper,a method of optimizing system performance by using suitable mixtures to replace pure working fluid is explored by simulation.Then the experimental study is carried out for mixtures and pure working fluids.The main research contents are as follows:(1)The ORC system model was established by Aspen Plus software.Under the condition of 120°Cheat source,five pure working fluids including butane,R245fa,R601a,R600a,and R1233zd-E were selected.Their binary mixtures in different proportions were applied to the ORC system.The thermodynamic performance and economic performance of the system were investigated at different evaporation temperatures.The results show that the system performance of mixtures is affected by the pure components,and the larger temperature glide can optimize the thermal performance and economy.When the system uses R600a/R601a or butane/R601a,the net output power,thermal efficiency,and exergy efficiency enlarge with the increase of temperature glide and evaporation temperature but the LEC decreases.(2)According to the simulation results,the system performance was comprehensively analyzed by the grey correlation method and then the optimal working fluid was found out.The method to optimize the system performance of the pure working fluid by mixtures was also found.The results show that when the evaporation temperature is 100℃,the grey correlation grade of 3R600a/7R601a is the largest and it has the optimal system performance.The method to optimize the system performance of pure working fluid is to select another pure working fluid whose net output power is similar to that of the target pure working fluid.And then their mixtures which have a large temperature glide can replace pure working fluid to improve system performance.Taking R245fa as the target working fluid,R134a and R245ca are selected by the optimization method,and R134a/R245fa verifies the reliability of the optimization method.(3)Experiments using 3R134a/7R245fa as working fluid were carried out on the existing ORC power generation system experimental platform.The results show that the net output power,thermal efficiency and,exergy efficiency first increase and then decrease with the increase of the heat source flow rate and the working fluid flow rate.When the heat source flow rate is 4 m~3/h and the working fluid flow rate is 700 kg/h they reach the maximum,but the turbine speed continues to increase.As the cooling water flow rate increases,all aspects of the system performance are improved.(4)The performance of the ORC system using 3R134a/7R245fa and pure R245fa at different cooling water temperatures was experimentally compared by adjusting the working fluid flow rate at 500-900 kg/h.It is found that the changing trend of the system performance of pure working fluid and mixtures with the working fluid flow rate is consistent.When the cooling water temperature is 10℃and the working fluid flow rate is 500,600 and 700 kg/h,the performance of 3R134a/7R245fa is better than that of R245fa.With the increase of cooling water temperature,the evaporation temperature and evaporation pressure increase,and the system performance deteriorates.It means that the rotational speed of the turbine,the net output power,the thermal efficiency and the exergy efficiency all decrease.