Study On The Performance Of Solar Organic Rankine Cycle With Stable Output Using Zeotropic Mixtures

Solar energy is green,clean and abundant,but its energy flow density is low,and it is easy to convert into low-grade heat energy.Organic Rankine Cycle(ORC)using organics as working fluids can efficiently utilize low-grade heat energy.The solar organic Rankine cycle(SORC)power generation system uses organic Rankine cycle to convert heat energy collected by solar collector into electric energy,which has huge development potential.Climate change will cause the unstability of solar radiation,which makes it difficult for conventional SORC system to maintain the rated operating condition,while the stable output is of great significance for the security of power grid and to meet the needs of users.The performance of ORC is directly affected by the working fluids.The zeotropic mixtures composed of a variety of organic pure working fluids.Because of the difference of boiling points among its components,the temperature of zeotropic mixtures in the phase change process constantly changes,which can improve the temperature matching between the working fluid phase change process and the cold / heat sources,and achieve the purpose of improving the cycle performance.In this paper,zeotropic mixtures are combined with the SORC system with stable output.The study on the SORC system with stable output using zeotropic mixtures is conducted.The main contents and conclusions are as follows:(1)The thermal performance and exergy performance models of the zeotropic mixtures simple SORC system and constant output regenerative SORC system with stable output are constructed,and the accuracy of the model is verified by matlab2019 a.Based on the established thermal performance model,the effects of heat source temperature,working fluids,circulation configuration and operation parameters on system efficiency,circulation efficiency,collector area and regenerative efficiency are studied.The results show that the efficiency of the regenerative SORC system with stable output is significantly higher than that of the simple SORC system with stable output.The increase of the superheat will lead to the increase of the collector area of the simple SORC system and the decrease of the collector area of the regenerative system.(2)The performance optimization model of the simple SORC system with stable output and the regeneration SORC system with stable output for zeotropic mixtures is established.The parameters of the system are optimized with the evaporation pressure and the superheat degree of the working fluids at the inlet of the turbine as the optimization parameters and the system efficiency as the optimization objective.The effects of heat source temperature and cyclopentane mole composition on the optimization parameters,the optimal system efficiency,the optimized cycle efficiency,the regenerative efficiency,the collector area,the mass flow rate of working fluids,etc.are studied.The results show that when the heat source temperature is low,the performance of the SORC system with stable output can be improved by using the zeotropic mixtures.In the simple SORC system with stable output,cyclopentane is better than cyclohexane when the heat source temperature is less than 275 ℃,and cyclohexane is better than cyclopentane when the heat source temperature is more than 300 ℃.When the heat source temperature is 200-325 ℃,compared with the optimal pure working fluids,the optimal system efficiency is improved by 2.18%,1.65%,1.32%,2.22%,1.84% and 0.25% respectively after using zeotropic mixtures.When the heat source temperature is higher than 350 ℃,the pure working fluids is better than the zeotropic mixtures.In the regeneration SORC system with stable output,cyclohexane is superior to cyclopentane in terms of pure working fluids at each heat source temperature.When the heat source temperature is 200-225 ℃,compared with the optimal pure working fluids,the optimal system efficiency increases by 2.15% and 0.95% respectively.With the increase of the heat source temperature,the optimal cyclopentane mole composition decreases.(3)Based on the exergy analysis model and system performance optimization model,exergy analysis of the simple SORC system and the regenerative SORC system with stable output of zeotropic mixtures is carried out.The effects of heat source temperature and cyclopentane mole composition on exergy efficiency and exergy loss distribution of the systems were investigated.The results show that the exergy efficiency of the stable output SORC system can be improved by using the zeotropic mixtures.In the simple SORC system with stable output,when the heat source temperature is low,the exergy loss of the evaporator accounts for the main part of the exergy loss of the system;when the heat source temperature is high,the exergy loss of the condenser accounts for the main part of the exergy loss of the system;the overall change of the turbine exergy loss is small.