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.
본 연구는 액화공기 에너지 저장 (Liquid Air Energy Storage, LAES) 시스템의 열역학적 분석을 바탕으로, 공정 개선 및 효율 최적화를 통해 경제적으로 실현 가능한 대형 에너지 저장 시스템을 제안한다. 에너지 저장 (충전) 시, 공기를 압축시키고 압축열을 열 저장 매체에 저장한다. 이 후, 냉열 저장 매체에 저장되어 있던 냉열을 제공하여 압축된 공기를 액화한다. 에너지 방출 (방전) 시, 액화된 공기를 가압한 후, 냉열을 저장하며 기화시킨다. 기화된 압축 공기는 열 저장 매체에 저장 되어 있던 압축열을 전달 받는다. 이 후, 다단 팽창을 통해 액화 할 때 저장했던 에너지를 방출한다. 세 가지 연구 주제를 통해 기존의 LAES의 공정, 효율 및 경제성을 개선한다: 첫째, 기화된 압축공기를 팽창 하기 전, 연소과정을 통해 팽창 입구 온도를 높이고 에너지 회수 효율을 높인다. 하나의 시스템이 에너지 저장 시스템과 발전 시스템의 역할을 모두 수행하여, 초기 투자비용을 줄이고 운용 비용을 감소시킬 수 있다. 둘째, 공기 액화공정에 관여하는 다양한 변수를 고려하여 효율을 최적화한다. 이러한 변수들의 상관 관계 및 열역학적 효율과 작동 조건에 주는 영향을 분석한다. 이 연구를 통해 최적화된 효율과 장비의 경제성 및 접근성을 고려한 가이드라인을 제시한다. 셋째, LAES와 열/냉열 병합 발전을 결합한 Tri-generative LAES를 제안한다. 비연소 공정의 LAES의 경우, 압축열을 이용하여 지역난방 등으로 활용하고, 흡수식 냉동기를 이용해 냉열을 제공할 수 있다. 연소공정을 결합한 LAES의 경우, 배기의 폐열이 비연소식 LAES보다 매우 많으며, 열/냉열 병합 발전에 보다 적합할 수 있다.
