Design And Energy Analysis Of Underwater Compressed Air Energy Storage System

In the planet's sustainable energy structure,the proportion and significance of renewable energy are increasing.The marine renewable energy possesses many advantages:enormous energy potential,convenience for serving the energy-hungry coastal regions,free from valuable land resources,and mitigation of energy security issues faced by many fossil energy-poor coastal countries.Compared with the relatively mature terrestrial counterparts,the development of marine renewables remains rather embryonic.Nevertheless,both terrestrial and marine renewable energy are unsteady,intermittent,stochastic,and low energy density.The introduction of proper energy storage technologies is one of the most effective measures to overcome these drawbacks.However,currently,because of the unique marine circumstances,commercially available onshore energy storage technologies cannot meet with the demand of large-scale marine renewable energy storage.The innovative underwater compressed air energy storage(UWCAES)is suitable for flexible scale storage of marine renewable energy in coastal and offshore regions,thereby providing a new idea and feasible solution for this issue.Thus,in this study,the system design and energy analysis of UWCAES are detailed explored in order to establish a foundation for a better UWCAES development.The main studies and conclusions of this thesis can be devided into following aspects.The studies of this dissertation focus on following aspects:A schematic UWCAES system with thermal energy storage is designed.Theunique key technologies of UWCAES system are analyzed and significant equipment selections are completed.The force characteristics,flow structures,and wake vortex shedding around a balloon-shaped air accumulator model at Reynolds number of 7 × 104 are investigated using a computational-experimental dual prong method.The results show that the k-co SST can only correctly capture the time-averaged fluid dynamic characteristics while the large eddy simulation can accurately calculate both the time-averaged and instantaneous fluid dynamic characteristics,thereby confirming a reliable reference for further study.And then,taking islands energy system in China South Sea as an example,a hybrid energy storage system integrated both UWCAES and battery energy storage is designed.The techno-ecnomic study of the hybrid system shows that the penetration rate of marine renewable energy can be improved,and the diesel consumption and green house gas emission are significantly reduced,therby showing the economic and environmental advantages of UWCAES technology.Based on the pre-designed hybrid energy storage system,a comprehensive thermodynamic model of Compressed Air Energy Storage system is established by considering the dynamic properties of system components.The system performance is compared under both designed regime and off-designed regime.For bridging the gap between the underwater isobaric energy storage and tradition isochoric compressed air energy storage,a comprehensive comparison is conducted.From the perspectives of system dynamic characteristics,round-trip energy efficiency,and energy storage density,the difference between isobaric storage and isochoric storage are investigated.The results show that the round-trip energy efficiency of isobaric storage is about 6%higher than that of isochoric storage,and the energy storage volume density of isobaric storage is about 2.5 times of that of isochoric storage,thereby proving the advantages of underwater isobaric storage over traditional isochoric storage.Finally,the detailed information on energy coupling,transfer,and destruction is explored with system energy efficiency analyses.First of all,three methods of evaluating energy efficiency in compressed air systems are reviewed and compared,which shows the second-law based exergy analysis is optimum.The general enthalpy exergy expression of the real gas is deduced based on the R-K-S real gas state equations and general thermodynamic properties expressions of entropy and enthalpy.A comparison between exergy of ideal gas and real gas is conducted.It shows that the relative error is less than 5%at the pressure of 0-7MPa and temperature of 300?500K,which means the calculation based on ideal gas assumption is reliable.And then,the exergy analysis and parametric sensitivity analysis are conducted on the proposed UWCAES system.It is found that the round-trip exergy efficiency can reach 58.91%under the real condition while 80.08%under the unavoidable condition.From the results of advanced exergy analysis,it is clear that the system round-trip exergy efficiency can be improved significantly,and the interactions between system components are weak but interwoven.Through the parametric sensitivity analysis,it is found that the system round-trip exergy efficiency is the most sensitive to isentropic efficiency of compressors and expanders.While the effect of heat exchangers on system round-trip exergy efficiency shows obvious nonlinearity and non-monotonicity.