| Nowadays,energy and environmental problems have become the bottleneck of the long-term development of human society.Recover the low-grade waste heat can effectively improve the energy efficiency and alleviate the current energy shortage and environmental pollution.Organic Rankine cycle(ORC)employs low boiling point organics instead of water in conventional Rankine cycle as working fluid,which has the advantages of simple system structure,high waste heat recovery efficiency,low investment cost and reliable operation,and has a good application prospect in low-grade waste heat recovery and utilization.In view of the important application value of ORC technology,this paper studies the optimization of ORC system and its key component radial inflow turbine.A small ORC experimental system that can be connected to the grid for power generation was established,and operation characteristics of the ORC experimental system was studied.A regenerative ORC system was established with the low temperature flue gas as the heat source.The influence of evaporation temperature and degree of superheat on the thermoeconomic performance of the system was discussed.Considering the system thermodynamic and economic performance and the limitation of the volume flow ratio of the turbine,the optimal working fluid and operating parameters of the regenerative ORC system were determined through a fuzzy multi-criteria evaluation method.The results show that with the increase of evaporation temperature,the net output power and total investment cost of the system first increase and then decrease,while the specific investment cost first decreases and then increases.Among the six working fluids,butane is the best one for the regeneration ORC system,and the optimal evaporation temperature and degree of superheat are 373.15 K and 5 K,respectively.A moderate degree of superheat is conducive to slowing down the variation trend of volume flow ratio with evaporation temperature,reducing the fluctuation of turbine efficiency among different operating conditions,and improving the comprehensive performance of the regenerative ORC system.A radial inflow turbine efficiency calculation model was coupled with the ORC system,and the constant turbine efficiency was replaced with dynamic turbine efficiency.The thermodynamic and economic performance of ORC system with different turbine efficiency models were compared,and the effect of dynamic turbine efficiency on working fluid selection and parameters optimization was discussed.Then,the sensitivity analysis of the heat source inlet temperature was conducted.The results show that the turbine efficiency increases with the decrease of evaporation temperature or the increase of condensation temperature,and there are significant differences in turbine efficiency among different working fluids with different evaporation or condensation temperature,with the maximum of 0.151.When the constant turbine efficiency is replaced with dynamic turbine efficiency,the optimal working fluids and the optimal evaporation temperature and condensation temperature of each working fluids have changed,which indicates that in the working fluids selection and parameters optimization process,using constant turbine efficiency will cause some errors to the analysis results.With the increase of heat source temperature,the difference in optimization results between the dynamic turbine efficicnecy ORC system and the constant turbine efficiency ORC system increases gradually,indicating that the higher the heat source temperature is,the greater the error caused by the constant turbine efficiency is.In order to expand the optimization range of radial inflow turbine design parameters,an optimization design method of radial inflow turbine coupling with radial inflow turbine one-dimensional aerodynamic design,ORC system thermodynamic model and multi-objective grey wolf optimizer was proposed.The optimization design of radial inflow turbine with six different working fluids under a given heat source is carried out based on this method,and the optimal working fluid and its structural parameters were determined.The internal flow characteristics and the off-design performance of radial inflow turbine were investigated by numerical simulation.The results show that multi-objective optimization can make a good trade-off between the turbine performance and the ORC system performance.The numerical simulation results are in good agreement with the one-dimensional aerodynamic design results of radial inflow turbine,which proves the accuracy of the multi-objective optimization design method adopted in this paper.When the radial inflow turbine operates at the design conditions,it has good off-design performance and good adaptability to the variations of rotation speed,inlet temperature and outlet pressure,and can maintain a high turbine efficiency within a certain variation range.A 20 kW radial inflow turbine and ORC experimental system were designed independently,and a small ORC experimental system that can be connected to the grid is established.The off-design operating characteristics of the radial inflow turbine and ORC experimental system were studied experimentally,and the performance of radial inflow turbine and the whole system under different working fluid heat absorption were analyzed.The results show that when the ORC experimental system is in stable operation,except for the pressure fluctuation at the outlet of the working fluid pump and the inlet of the turbine,the other thermal parameters and the grid connected voltage are almost constant.The isentropic efficiency and the power output of the radial inflow turbine increase with the increment of the working fluid heat absorption,and the difference between the power output calculated by the dynamic turbine efficiency and the experimental value is smaller,which proves the accuracy of the method that using dynamic turbine efficiency in the ORC system analysis process.With the increase of working fluid heat absorption,the power consumption and the isentropic efficiency of working fluid pump increase,the net power output and the thermal efficiency of the whole system also increase gradually.Compared with the power produced by the turbine,the power consumption of the pump is small,thus the influence of it on the net power output of the system can be ignored. |
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