Liquid air based energy conversion and storage: system configuration, simulation and optimization

Zhang, Tongtong (2022). Liquid air based energy conversion and storage: system configuration, simulation and optimization. University of Birmingham. Ph.D.

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The past decade has seen a significant growth in renewable energy installations driven by a global effort to combat climate change. The non-dispatchable nature of most renewable energy generation and the less predictable end-user demand imply a highly challenging supply-demand management for energy networks. Energy storage technologies provide an avenue to meet the energy supply and demand through the chain of generation, transmission, distribution, and end use. Liquid Air Energy Storage (LAES) is one of the most promising energy storage technologies, which have started large-scale commercial deployment.

There has been an increased number of studies on the LAES technology, especially since 2010. These studies include the basic characteristics and performance of the standalone LAES in the early stage of development, and the improvement of the round trip efficiency of the standalone LAES through configuration optimization in most of the subsequent publications. These studies also explored the integration of the LAES with external cold/heat sources or industrial processes for performance improvement. However, the range of applications of the LAES is still very limited, mainly focusing on the decarbonization of the power sector, especially for the large-scale power grid. This Ph.D. work aims to extend the application fields of the LAES technology, including the provision of green multi-energy-vector products, decarbonizing the industrial sector and building sector at different scales.

First, a power plant for recovering cryogenic waste energy from LNG regasification and compression heat from the LAES is proposed and the thermodynamic analyses are carried out. The results show that the power plant could achieve a thermal efficiency of 27% and an exergy efficiency of 40%. An economic analysis of the plant shows a payback period of only 2.2 years, due to the use of the excess heat from a 5MW/40MWh LAES system.

Second, apart from the use of excess compression for power generation, it can also be utilized for the provision of multi-energy-vector services. Therefore, a standalone LAES system is proposed for combined cooling, heating, hot water, and power supply. The results show that such a standalone LAES can achieve a high nominal-electrical round trip efficiency between 52% and 76%, with the maximum at a charging pressure of 5 MPa, a new finding that has not reported before.

Third, decoupled LAES provides a route to transport energy from renewable energy-rich areas to end-use sites. A new cryogenic thermoelectric generation (Cryo-TEG) method is therefore proposed to recover the cryogenic energy of the decoupled LAES during the discharging process. The results show that the Cryo-TEG has a thermal efficiency of ~9%, which is lower than that of a Rankine cycle operated over the same temperature range. However, the Cryo-TEG gives a levelized cost of electricity of 0.0218 $/kWh for small-scall applications, which is ~4 times cheaper than that of the Rankine cycle system. With the Cryo-TEG, the decoupled LAES systems could achieve an electrical round trip efficiency of ~29% and a combined cooling and power efficiency of ~50%.

Fourth, the ammonia synthesis process requires the feed gas of nitrogen, one of the ingredients of the storage mediums for the LAES system. Two new integrated methods between the LAES system and the ammonia synthesis system are therefore proposed: LAES-NH\(_3\) and LAES-AS-NH\(_3\). The LAES-NH3 is found to be more suitable for the existing ammonia production plant. The LAES is used as a nitrogen buffer to transfer the peak load of the air separation unit to off-peak time. In this way, the air separation unit used for nitrogen supply to the ammonia synthesis process can realize an operation cost decrease by ~38%. On the contrary, the LAES-AS-NH\(_3\) is more suggested for a new ammonia production plant. In such a configuration, a single distillation column is added to the LAES (denoted as LAES-AS) for air separation. Both liquid nitrogen and liquid air can be obtained during the charging process, for use as the feed gas for ammonia synthesis and the working fluid for the LAES discharging process. The nominal round trip efficiency of the LAES-AS side is shown to reach 69%, and the electricity consumption of the ammonia synthesis process can be reduced by ~31.6%. As a result, the LAES-AS-NH\(_3\) system could achieve an overall round trip efficiency of 30%, which is 13% higher than that of the standalone ammonia synthesis system.

Type of Work: Thesis (Doctorates > Ph.D.)
Award Type: Doctorates > Ph.D.
Licence: All rights reserved
College/Faculty: Colleges (2008 onwards) > College of Engineering & Physical Sciences
School or Department: School of Chemical Engineering
Funders: Other
Other Funders: China Scholarship Council
Subjects: T Technology > TP Chemical technology


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