| In order to solve the problem of increasing energy shortage and environmental pollution,renewable energy such as solar and wind power is increasingly used in electricity generation.But the intermittency and instability of renewables make it difficult to connect them to the grid.Large-scale renewable energy connected to the grid increases the complexity and insecurity of the power system.Energy storage technologies can well compensate for the intermittency and instability of renewable power generation.Energy storage system is used to supplement power generation during peak hours and to store electric energy during off-peak hours to realize "peak cutting and valley filling",so as to improve the efficiency,safety and economy of conventional power system.In addition,energy storage system also has many application scenarios such as frequency modulation and distributed energy supply.In the context of the global energy crisis,the "carbon peaking and carbon neutrality" target and the rapid development of renewable energy construction,energy storage technologies and industry have huge market prospects and development opportunities.Pumped thermal energy storage(PTES)is a new large-scale energy storage technology that combines compressed gas energy storage and heat storage.PTES systems use gas as a circulating working medium and set two tanks with high and low temperature to store two heat transfer media with different temperatures.PTES systems do not require large storage devices and are free from geographical constraints.Moreover,compared with the multistage compression and multistage expansion compressed gas energy storage system,PTES system has simpler structure,lower cost and higher energy storage density.Therefore,PTES technology is one of the most promising large-scale energy storage technologies.In this paper,modeling and simulation,thermodynamic analysis and dynamic characteristics of large-scale PTES system are carried out,and the influence law of key parameters on the performance of PTES system is revealed.The main work and research results of this paper are as follows:(1)The thermodynamic model of PTES system is established,the thermodynamic simulation is carried out for the systems with output power of 10MW and 100MW respectively,and the thermodynamic performance of the systems with different capacities is compared.The results show that the cycle efficiency of 100MW PTES system is 49.26%and the energy storage density is 25.37kWh/m3.Compared with the 10MW PTES system,the cycle efficiency of 100MW PTES system is improved by 14.66%,and the energy storage density is increased by 14.54kWh/m3.Large-scale PTES system has advantages in cycle efficiency and energy storage density.(2)The exergy analysis model of PTES system is established to analyze the exergy loss of major components.The results show that the equipments with a relatively large amount of exergy loss in the PTES system are expanders,heat exchangers,compressors and radiators.Their overall exergy loss accounts for 88.09%of the total exergy loss of the system.Exergy loss can be reduced and thermodynamic performance of PTES system can be improved by improving isentropic efficiency of compressors/expanders,reducing pressure loss of heat exchangers/regenerators,and improving heat exchange efficiency of heat exchangers.(3)The influence of design parameters and component performance on thermodynamic characteristics of PTES system are analyzed.The results show that the cycle efficiency of the system can be effectively improved by increasing the inlet temperature of the high temperature compressor,increasing the pressure ratio of the compressors,increasing the inlet temperature of the compressors,improving the isentropic efficiency of the compressors/expanders,reducing the pressure loss coefficient of the heat exchangers/regenerators,improving the heat exchange efficiency of the heat exchangers and reducing the heat storage/cold storage loss.According to the analysis results,the thermodynamic characteristics of PTES system are optimized from the aspects of system parameters and components.Compared with the initial parameters,the performance of PTES system after parameter optimization is significantly improved.With the same output power of 100MW,the flow rate of working medium in the optimized energy storage process is 540kg/s,which is relatively reduced by 33.33%compared with that before optimization.The net power consumption is 162.99MW,which is 19.99%less than before optimization.The cycle efficiency is 61.48%,which is 24.81%higher than before optimization.Energy storage density is 79.99kWh/m3,which is 215.39%higher than before optimization.(4)The dynamic simulation model of 100MW PTES system and its components are established,and the influence of speed rise rate and initial pressure on the start-up characteristics of PTES system is studied.The results show that,for the start-up process,the higher the speed rise,the faster the system speed rise in the initial stage,and the shorter the time required for the start-up process.However,if the speed increase rate is too high,a large overshoot will occur.If the speed increase rate is too small,a large fluctuation will occur,and the unit cannot start normally.For the system in this paper,considering safety and rapidity comprehensively,the speed of rotation is 9r/min per second.At this time,the starting time of the energy storage process is about 1500s,and the starting time of the energy release process is about 500s.The initial pressure of the system does not affect the pressure ratio at steady state,but it does affect the pressure stability value.For the system in this paper,in order to make the system pressure finally reach the rated value,the initial pressure in the process of energy storage should be set at 4.5MPa and the initial pressure in the process of energy release should be set at 6.0MPa.(5)The regulation characteristics of PTES system are analyzed,and five load regulation modes are proposed according to the position of charging and discharging points and discharging sequence.For the energy storage process,when the rotor stability is given priority,mode 4 is the best,and its rotor speed fluctuation is the least,and the maximum speed fluctuation is 0.83%higher than the rated speed.When the load regulation range is given priority,mode 3 is the best,with the largest load regulation range.The power of high temperature compressor and low temperature expander is reduced by 3 3MW and 17MW respectively.For the energy release process,when the rotor stability is given priority,mode l is the best,and the rotor speed fluctuation is the least,and the minimum speed fluctuation is 4.73%lower than the rated speed.When the load regulation range is given priority,mode 3 is the best choice,which has the largest load regulation range,reducing the power of low temperature compressor and high temperature expander by 15MW and 48MW respectively.(6)The multi-domain physical model of PTES system is established,and the effects of component parameters and actual gas physical parameters on the system characteristics are studied.The results show that with the increase of heat exchange area,the temperature end differences of the heat exchangers and the regenerators decrease gradually.With the increase of friction coefficient,the pressure loss of heat exchange equipment also increases gradually.The pressure loss of high temperature heat exchanger increases the most,followed by the regenerator,and the pressure loss of low temperature heat exchanger changes the least.When the working medium is an actual gas,the variation trend of temperature and pressure of the PTES system is the same as that of the ideal gas,but stable values of the temperature and pressure are slightly different.The properties of working medium have little influence on the pressure of PTES system,but great influence on the temperature. |
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