| Global fossil energy consumption and environmental pollution and waste discharge issues have greatly restricted the rapid and healthy development of the world and our economy.Among them,the increasing depletion of fossil energy has made the international community pay more attention to energy conservation,emission reduction and development and utilization of renewable energy.Electric energy storage technology can effectively use renewable energy,which can not only solve the problem of grid connection of renewable energy,but also eliminate the hidden danger of power shortage during peak electricity consumption,which plays a role in peak cutting and valley filling.The representative technology is a type of large-scale compressed air energy storage technology,which has been commercialized much maturely.Compared with air,carbon dioxide(CO2)has good physical characteristics and is an energy storage medium with greater development potential,which provides a possibility of using compressed carbon dioxide energy storage technology on a large scale.In addition,CO2 as main source of greenhouse gases in the world is the main factor leading to global warming.In order to reduce greenhouse gas emissions,carbon capture and storage technology used on a large scale worldwide is currently stored underground,which exists a relatively large CO2 storage pressure.Therefore,from the perspective of reuse of underground carbon dioxide storage,a fossil-fuel-free trans-critical compressed carbon dioxide energy storage(TC-CCES)system based on underground gas storage reservoir is proposed.The system is designed with 10MW unit as a target,and the main work and results of the paper are:First,thermodynamic characteristics of the TC-CCES system was analyzed.A thermodynamic model was established and the study of the change law of the key parameters of the system and the optimization analysis of the TC-CCES system was proposed.Studies have shown that under the condition of 10MW output power,by increasing the adiabatic efficiency of the compressor unit and expansion turbine unit,increasing the energy storage pressure,reducing the pressure drop of the high-pressure throttle valve,outlet pressure of expansion turbine unit and the temperature difference of the heat exchange equipment,the system thermal characteristics can be improved.Through optimization analysis,the matching relationship between the energy storage pressure and the throttle pressure drop is revealed,and the optimal solution for the round-trip efficiency of the system is 67.61%.Second,key thermodynamic parameters for the TC-CCES system was optimized.Based on the TC-CCES system established in the above section,configuration research was carried out on the stages of the compressor unit and expansion turbine unit,the compression ratio of each compressor in the compressor unit,and the expansion ratio of each expansion turbine in the expansion unit.The research results show that for the TC-CCES system,when it adopts three-stage compression and three-stage expansion,and adopts a un-equal compression ratio-equal expansion ratio design scheme,the round-trip efficiency,heat storage efficiency and energy storage density are increased by 6.46%,3.2%and 0.218%,respectively.After reasonable configuration of the structure and key parameters,the system structure is not only simple and compact,but also the thermal performance is better improved.Third,traditional exergy analysis and advanced exergy analysis on the TC-CCES system with three-stage compression-three-stage expansion structure and un-equal compression ratio-equal expansion ratio was performed to calculate and analyze the energy loss mechanism of the system and its internal components,so as to determine the priority optimization components and carry out the corresponding system optimization.The calculation and analysis results show that the expansion turbine is the priority optimization component,heat pump is used to improve the work quality of the working fluid in the expansion turbine,and the TC-CCES coupled with the heat pump(TC-CCES+HP)system is obtained.The heat storage medium and the circulating working medium of the heat pump are further matched and optimized.When the TC-CCES+HP system uses water as the heat storage medium and R21 as the working fluid of the heat pump,the round-trip efficiency of the system is as high as 80.32%,and the heat storage efficiency is increased to 72.17%.Although the system is more complex,but the efficiency improvement of the system is better.Finally,financial analysis was carried out using commercial power stations on life cycle cost analysis method obtained from the analysis in different TC-CCES systems.Furthermore,a sensitivity analysis of key economic factors with the best techno-economy was conducted.In addition,a comparison analysis with the CAES system for techno-economic analysis was studied.The analysis results show that under 10MW output power,when the TC-CCES+HP system adopts the design scheme of un-equal compression ratio-equal expansion ratio,the technical and economic efficiency of the system is optimal.Moreover,the technology characteristics are affected by the sensitivity of key economic factors as follows:on-grid power price,annual operating hours,power purchase price,and bank loan rate,which can provide a theoretical reference for the project investment of energy storage system.At the same time,when the TC-CCES+HP system is undergoing techno-economic evaluation,after considering the cost of carbon dioxide reduction,the LCOE is 0.42 CNY/kWh,which is higher than that of CAES system under the same setting conditions.The energy storage system has a lower cost of 0.2 CNY/kWh and has stronger market competition potential. |
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