Experimental Research On Cascade High Temperature Heat Pump And System Optimization Based On Exergy Analysis

With the rapid development of industry,the input of electricity has also increased greatly,leading to the emergence of primary energy consumption and greenhouse gas emissions.At the same time,in the process of industrial production,a large amount of energy was lost to the environment in various forms of waste heat,resulting in serious energy waste.The recycling and utilization of waste heat resources can not only reduce the energy consumption and industrial cost,but also effectively reduce environmental pollution caused by industrial emissions.Heat pump technology is widely used in industrial,commercial and civil fields because of its great potential in temperature lifting and heat supply by recovering low-temperature waste heat.However,at present,the heating temperature of high temperature heat pumps is mostly below 90℃.When the temperature rises from heat sink to heat resource exceeds 65℃,the COP of this system goes down a lot and its economy is low,which greatly restricts its promotion in the industrial process.Thus,cascade high temperature heat pump has shown remarkable application potential in industrial waste heat recovery because of its ability of high temperature lifting.Theoretical calculation,experimental method and exergy analysis has been employed to study cascade high temperature heat pump in this paper.By using MATLAB and REFPROP software,the thermodynamic model of the cascade high temperature heat pump system was constructed.Under the working conditions of evaporation temperature 20-40℃ and condensation temperature 90-130℃,the law of main performance parameters of the system changing with evaporation temperature and condensation temperature was studied.The results show that the COP of the system increases with evaporation temperature,and decreases with condensation temperature.The maximum COP of the system is 4.99,and the minimum COP is 1.91.Under the evaporation temperature of 30℃,condensation temperature of 90-130℃,high and low temperature cycle using different working fluid,the influence of different working fluid on the system performance parameters was studied.The results show that R1234 yf and R124 are more suitable for the low temperature cycle,R1233 zd is more suitable for the high temperature cycle of cascade high temperature heat pump system.The experimental bench of the cascade high temperature heat pump system was designed and set up.After setting experimental conditions and selecting the working fluids,the experimental scheme was determined.R245 fa working fluid for high temperature cycle and R134 a working fluid for low temperature cycle were employed in this experiment.The highest outlet temperature of condenser is up to 130℃ in experiment.When the heat source temperature is 40℃ and the heating temperature is 100℃,the maximum COP of the system is 2.35.Then,changing the mixed working fluids of R134 a and R245 fa in low-temperature cycle,the system performance using different mixing ratios was studied.When the mixing ratio of R134a: R245 fa is 8:2,the COP of the system is higher than that of other mixing ratios.Compared with R134 a in low temperature cycle,the compressor power of using mixed working fluids is lower.That is to say,the higher system performance,the higher economy.Exergy analysis of cascade high temperature heat pump system was carried out based on experimental data.Conventional exergy analysis has optimized the performance of cascade high temperature heat pump system,but it did not reveal the interaction between components of the system.So advanced exergy analysis is used to evaluate the interaction between components in the system.The conventional exergy analysis results show that the total exergy efficiency of the system is 43.23%,and high temperature compressor and low temperature compressor have the largest exergy destruction in the system which have high priority for improvement.The advanced exergy analysis results show that low temperature compressor is the component with the highest priority for improvement,followed by cascade heat exchanger and high temperature compressor.The improvement potential of low temperature and high temperature expansion valves is lower,which is different from the results of conventional exergy analysis.Overall,the advanced exergy analysis method can provide more detailed guidance on the improvement potential of the system.