System Analysis And Experimental Verification Of Compressed CO₂ Energy Storage

In recent years,many international conferences have emphasized energy conservation and emission reduction.The use of renewable energy as an important means to reduce the use of fossil fuels has been paid more and more attention by the countries,and the intermittent of renewable energy will Affect the stable operation of the power grid network and it also cause the waste of electricity.One of the feasible solutions to these problems is to use energy storage systems.Traditional pumped hydro energy storage requires strict geographical conditions,which restricts its promotion and widely use.The compressed air energy storage（CAES）system has a larger installed capacity and compared pumped hydro energy storage,more relaxed site selection conditions make it easier to promote.In order to solve the problem of global warming caused by CO₂ emissions,developed countries in Europe and the United States have proposed carbon capture and storage（CCS）technology,and then how to store or utilize the CO₂ which has been captured becomes a problem.Using carbon dioxide instead of air become a working fluid for compressed gas energy storage（CGES）system can provide a reasonable application of the large amount of CO₂.In order to study whether it is advantageous to use CO₂ instead of air as working fluid for CGES systems,this paper uses Aspen Plus to model several existing typical CGES systems and use air and CO₂ as working fluid respectively.By comparing the system round-trip efficiency（RTE）of the CAES and compressed CO₂ energy storage（CCES）simulation system and the key operating parameters of systems,it can be found that CO₂ is not suitable as a working fluid for diabatic CGES systems.Compared with CAES systems,the RTE of diabatic CCES without and with regenerators were 7.3%and 1.3%lower respectively.However,for adiabatic CGES systems and liquid gas energy storage（LGES）systems,the use of CO₂ has certain advantages.Using CO₂ as working fluid in adiabatic CGES system,the thermal energy storage（TES）temperature in the system is lower than the system using air as working fluid,which is beneficial to the design and maintenance of the system,and using CO₂ achieves similar RTE to using air,which are 1.9%and 1.5%lower for the high and medium temperature systems and 1.6%higher for low temperature systems.For the LGES system,using CO₂ results in a higher RTE than using air,which is 2.4%higher.Moreover,compared to system using air,the system using CO₂ has a higher storage temperature,which is closer to the ambient temperature and thus has a lower thermal loss.In order to further study these two types of systems that have advantages compare to air,this paper uses the method of exergy analysis to analyze the two systems.By studying the exergy loss changes with the efficiency of parameters changes,we can find that in adiabatic CCES,the second-stage compressor and turbine exergy loss in the system are relatively large,which are 1945kW and 3280kW respectively.The most effective way to reduce exergy loss is to improve the efficiency of second-stage compressor and turbine.For the liquid CO₂energy storage（LCES）system,the largest exergy loss in the system is the second-stage compressor,which is 1678.2kW,followed is the first-stage compressor,which is 1136.5kW.The most effective way to reduce the system exergy loss is to increase the second-stage compression isentropic efficiency,followed way is to increase the isentropic efficiency of the first-stage compressor.Increasing the efficiency of these two devices are more advantageous than increasing the efficiency of other devices in the system.Through the successful built of the CCES test bench and the successful operation,the feasibility of using CO₂ as a working fluid for CGES was verified.Basic the experimental results,it can be found that the pressure change speed in the high-pressure and low-pressure storage tanks in the compression process is relatively fast at the beginning and then the change speed is slowed down.The power of the compressor starts to drop rapidly at first,and then the drop speed is slowed down;The pressure change speed in high-pressure and low-pressure storage tanks during the expansion process also has the phenomenon which appear in the compression process,but it is not obvious compared with the compression process.The output power of the expander starts to decrease rapidly at the beginning and slows down at the later stage.The system RTE is 8.8%and 19.7%under the conditions of the expander inlet temperature is 25^oC and 35^oC respectively.It can be found that the system RTE has a large increase when the inlet temperature of the expander is increased by 10^oC.In this paper,a simulation model of the test system was developed by Matlab and Aspen Plus.Basic the simulation results,it can be found that there are some errors between the simulation results and the experimental results of the compression and expansion processes.The first is the higher final pressure in the compression and expansion processes under the simulation result.At the same time,the power consumption of the compressor and the output of the expander under the simulation conditions are larger than those under the experimental conditions.And under the simulated condition,the system RTEs are 8.6%and 18.7%when the operation conditions of the expander inlet temperature are 25^oC and 35^oC respectively,which are smaller than the experimental results.