Thermodynamic analysis, process improvement, and efficiency optimization of cryogenic energy storage system

Abstract

This dissertation investigated Liquid Air Energy Storage (LAES) system aiming thermodynamic analysis, process improvement, and efficiency optimization. This system gives the flexibility to the power grid and is capable of storing surplus electricity from renewable energy sources. The system comprises three main regions: Energy charge, energy store, and energy discharge. At the off-peak time, the low-priced off-peak electricity generated by the RESs or grid is supplied to the liquid-air energy-storage site. Air is liquefied using the supplied and previously-stored cold energies. In the storing region, liquefied air is stored in a cryogenic tank with high insulation to mitigate thermal losses. At the on-peak time, the liquid air is pressurized, and the cold energy is transferred to the thermal media. The thermal media with cold energy are stored in cryogenic tanks with insulations. The air is re-gasified by heat from a hot medium. The heated and compressed air generates high-priced electricity using turbo-machinery. In the present work, the economic and thermodynamic performances are improved and analyzed in three research topics: LAES system integrated with natural gas combustion, thermodynamic analysis and optimization with comparative analysis for LAES, Tri-generative cryogenic energy storage system. In the first topic, An LAES and generation system combined with LNG is proposed for distributed energy storage-generation. The system shows improved thermodynamic and the economic efficiencies. This LAES system takes advantages of natural gas fuel or LNG to create synergistic effects for storage and power generation systems. The proposed system was compared thermodynamically and economically with CAES systems and an adiabatic LAES system. The round-trip and storage efficiencies of the proposed system were 64.2% and 73.4%, respectively. The LCOE of the proposed systems ranged from 142.5 to 190.0 $/MWh depending on the size and the storage time. In the second topic, Linde-Hampson cycle, Modified Claude cycle and Modified Kapitza cycle are thermodynamically optimized selecting critical process variables by partially enumeration. With this method, the contour maps for the independent variables are illustrated, that gives intuition to the behavior of the LAES systems. Beside, interaction between the variables can be found thermodynamically and be analyzed considering equipment limitations. The optimal values for the independent variables are proposed considering the limitations for the equipment and industry infrastructure. The charge pressure and the discharge pressure of the process have main effects on the selection of critical equipment such as compressors, expanders, and cryogenic heat exchangers. The relatively low pressure operations would be the most economic processes considering life-cycle cost. In the last topic, a storage/tri-generation system with cryogenic energy storage is investigated. The integrated system efficiently recovers the waste heat from flue gas and compression heat and generates energy for district heating and cooling. The overall efficiency of the proposed system is 80.8% and 74.3%, respectively for heating and cooling mode. These are 24-30% higher than the current optimized standalone LAES system.