The design and operation optimization of liquid air energy storage within multi-vector energy systems

Liang, Ting (2023). The design and operation optimization of liquid air energy storage within multi-vector energy systems. University of Birmingham. Ph.D.

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Abstract

Climate changes call for the construction of a net-zero-carbon energy system across the globe. Such a massive need become more urgent due to the recent war on Ukraine, which has led to energy poverty, sharp rise in living costs and economic challenges particularly in Europe. Renewable energy represents a critical pathway towards the decarbonisation. A high share of renewable could trigger multiple problems due to the intrinsic intermittency and variability. Energy storage technologies offer the major solution to resolve such problems.

There are many energy storage technologies at different development stages; among which, Liquid Air Energy Storage (LAES) is considered as a promising large-scale energy storage technology. The key advantages of the LAES include high scalability, no geographical constraints, cost-effectiveness, and capability of providing multi-vector energy services, which is expected to play an increasingly crucial role in future energy systems with a high renewable penetration. However, there are few studies working on the optimization and discussing the functions and benefits of LAES when it is applied into net-zero carbon energy systems. This forms the main motivation of this Ph.D. work, to address the research gaps. In the first and second parts of the thesis, the thermo-economic and dynamic simulation and optimization of the LAES system were conducted, which can provide the basis for discussing its key roles in distributed and grid-scale multi-vector energy systems. The given results can provide evidence for the optimal design, operation and improvement of LAES integrated systems. Meantime, the outcome can provide the enlightening views on the business investment decisions, and on developing renewable energy policies and storage expansion plans, to help achieve carbon mitigation ambitions in the UK by 2050. The following is a brief summary of the work and major conclusions:

In the first part of this work, the multi-objective thermo-economic optimization of a stand-alone LAES system by using a Genetic algorithm was conducted, taking the round-trip efficiency (RTE) and economic indicators as the optimization objectives. The optimization has lead to a 9%~14% of increase in energy efficiency and a 14% of decrease in exergy destruction. The optimal design and operational parameters of LAES under different configurations and scenarios can be determined, including the optimal charging and discharging pressure, heat transfer areas, and mass flow rates of hot and cold storage media etc. Meantime, the design and operational guidelines of LAES can be derived. A LAES system with lower machine efficiencies requires lower charging and discharging pressure, while a system with worse heat transfer performance needs higher charging pressure but lower discharging pressure. Finally, the Pareto Front of capital costs, efficiencies and the occupied space energy density (OSDE) was obtained to provide system operators good investment advice of LAES. It indicated that a higher capital cost lead to a higher RTE, NPV and OSDE. Specifically, when the RTE increases by 1%, the optimized capital investment increases by 0.5-1%. If the investment budget is over 48 M£, a LAES system with three-stage compressors and four-stage turbines can produce better RTE than three-stage and four-stage LAES systems.

In the second part of this work, the dynamic simulation and analysis of the LAES discharging unit were conducted to investigate its dynamic characteristic and response time when integrated with wind power. The results revealed that the LAES discharging unit is more suitable for responding to the wind power component at a time scale more than its start-up time, which can help compensate the wind power deficiency and reduce the motor fatigue. Meanwhile, the combined storage scheme with LAES and battery was proposed to smooth the varying wind power. The economic comparison among different storage schemes indicated the suitable storage system for wind power integration. The annual cost of solely battery storage is more than two times higher than that of the combined LAES and battery storage system, meantime, the larger the wind farm, the more obvious the economic advantages of the combined storage system.

In the third part of this work, the multiple functions of LAES in decarbonizing a hybrid renewable micro-grid with high share of wind power were investigated. A mixed-integer linear programming (MILP)-based system design framework with the decoupled model of LAES was developed, which can determine the optimal sizes and operation of the micro-grid components and the LAES units. Specifically, the optimal charge/discharge energy to power ratio (27/14 h) and the storage tank size (608 t) of LAES in a micro-grid with 75% of wind power were obtained, leading to ~60% of carbon emission reduction on the 2016 level. The results also revealed the key roles of LAES in supporting a micro-grid with high share of wind power by providing multiple functions. The total benefits were split into six explicit revenue streams for the first time, including the time shifting (13.2%), renewable firming (11.4%), peak shaving (28%), flexibility (21%) and reserve value (20.4%), as well as the waste heat recovery (6%). It also indicated that a higher renewable percentage (over 50%) would be the major driving force to increase the attractiveness of LAES in micro-grids than the mildly reduced LAES capital cost and the enlarged electricity price differences.

In the fourth part of this work, the cost-effective pathways and the storage needs for the transition to a net-zero carbon energy system in the UK by 2050 were assessed. A MILP-based energy expansion model was developed to achieve the optimal design and operation of the system. Firstly, the results revealed that a future 100% renewable or net-zero carbon power system is feasible with levelised cost of energy (LCOE) at 65~80 £/MWh, and a net-zero carbon heat system is affordable with the levelised cost of heat (LCOH) at 45~63 £/MWh. The major expansions are onshore wind power (94.5 GW) in power sector and air-source heat pump (~80 - 90 GW) in heat sector. Secondly, storage technologies would play crucial roles in a net-zero carbon system, only ~10-12% of investments in electric storages would reduce the total annual costs by ~15.1% - 28%. The major storage expansions lie in LAES (384 GWh) in power sector and the short-term heat storage (330 GWh) in heat sector. Thirdly, the newly deployed capacities of renewables and storages in different zones are correlated with each other, the LAES and renewable capacity ratio is around 20%. It also indicated that the LAES with the charge durations at 8~10 h and discharge durations at 14~15 h is more suitable for the wind-dominated case in the UK than short-duration batteries (~4/5h).

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):