Study of a Liquid Metal Magnetohydrodynamic Generator for Thermoacoustic Power Generation

Significance 

Thermoacoustic engines, an emerging class of external combustion engines, possess unique attributes that set them apart from conventional counterparts. These engines can convert thermal energy into acoustic energy, ultimately converting it into electricity through an acoustic-to-electric conversion device, thereby forming a thermoacoustic power generation system. The simplicity of their structure, high reliability stemming from a lack of moving parts, and impressive thermal efficiency, comparable to internal-combustion engines, make them a promising candidate for sustainable energy production. Notably, their capacity to utilize diverse heat sources, such as fossil fuels, solar energy, and biomass, positions them as a versatile solution.

Traditional thermoacoustic power generation systems employ linear alternators, which entail intricate manufacturing processes and technical challenges for scaling to higher power capacities. In contrast, the integration of a liquid metal magnetohydrodynamic (LMMHD) generator introduces a novel paradigm. LMMHD generators, devoid of mechanical moving parts, capitalize on the interaction between electrically conducting fluids and magnetic fields to induce electric current, making them a highly reliable and efficient alternative for power conversion.

To this note, a new study, published in the peer-reviewed Journal of Applied Energy, researchers from the Key Laboratory of Cryogenics at the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, and the Imperial College London. Led by Shunmin Zhu, Tong Wang, Chao Jiang, Zhanghua Wu, Guoyao Yu, Jianying Hu, Christos Markides, and Ercang Luo, the study discusses the integration of a thermoacoustic cycle engine with an LMMHD generator, a significant advancement in the domain of thermoacoustic power generation.

The authors discussed the comprehensive understanding of the operating characteristics and loss mechanisms associated with LMMHD generators operating at higher frequencies particularly pertinent to the resonant frequencies encountered in thermoacoustic engines. The researchers recognized that while LMMHD generators had previously been explored for various applications, the dynamic and transient nature of high-frequency operation necessitated a nuanced approach.

To address this challenge, the researchers employed three-dimensional transient numerical analysis using the COMSOL multiphysics simulation environment. This approach facilitated a holistic examination of the interplay between magnetic, electric, and fluidic fields within the LMMHD generator. They investigated the impact of critical parameters, such as liquid metal inlet velocity, load resistance, and operating frequency, on the generator’s performance and loss mechanisms.

The authors’ findings offered valuable insights into the behavior of the LMMHD generator under diverse conditions. The interrelationship between inlet velocity, output power, generator efficiency, and losses was meticulously explored. Higher inlet velocities correlated with increased output power, attributed to enhanced kinetic energy transfer. However, an associated rise in Joule heating loss led to a marginal decrease in efficiency. This nuanced trade-off highlighted the significance of optimizing the inlet velocity to achieve the desired balance between power output and efficiency.

Moreover, in the study Dr. Shunmin Zhu and colleagues discussed the influence of load resistance on the generator’s performance. The complex relationship between load resistance, voltage, current, output power, and efficiency was elucidated. Notably, the optimal load resistance was identified as a pivotal parameter for maximizing efficiency or power output. This finding underscores the need for tailored design strategies based on specific energy conversion objectives.

To validate the theoretical insights garnered from numerical analysis, the researchers performed experimental characterization. A novel approach involving a linear compressor was employed to simulate high-frequency oscillating gas flow characteristics of thermoacoustic engines. The experimental prototype of the LMMHD generator demonstrated promising results, producing electric power with an efficiency of 24% at specific operating conditions. However, an intriguing observation was the discrepancy between the load current in simulations and experiments, leading to a lower-than-predicted efficiency. This divergence was attributed to factors such as leakage currents, contact resistances, and inductance effects, emphasizing the challenges in translating theoretical models to practical implementations.

By merging the principles of thermoacoustic engines and LMMHD generators, the researchers have presented a compelling alternative to conventional power generation methods. Through rigorous numerical analysis and experimental validation, the study has unveiled critical insights into the performance characteristics, trade-offs, and loss mechanisms of high-frequency LMMHD generators within the context of thermoacoustic power generation systems. The study not only contributes to our fundamental understanding of complex interactions within LMMHD generators but also provides a roadmap for optimizing their performance in the challenging domain of high-frequency oscillations. As society continues its shift towards sustainable energy solutions, the integration of thermoacoustic-LMMHD technology holds the promise of delivering efficient, reliable, and environmentally friendly power generation systems for diverse applications, including space nuclear power generation and beyond.

Study of a Liquid Metal Magnetohydrodynamic Generator for Thermoacoustic Power Generation - Advances in Engineering
Figure. Configuration of the LMMHD generator prototype driven by a linear compressor

About the author

Dr. Shunmin Zhu is a Postdoctoral Researcher working at the Clean Energy Processes (CEP) Laboratory, in the Department of Chemical Engineering of Imperial College London. Prior to joining Imperial College London, he worked as a Postdoctoral Researcher in the Key Laboratory of Cryogenics at the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, where he received his Ph.D. degree in 2020. Dr.Zhu’s research interests include novel thermoacoustic power generation technologies, hybrid renewable energy systems, and free-piston Stirling technologies. He is a holder of the Marie Skłodowska-Curie Individual Fellowship and an Associate Editor of Frontiers in Thermal Engineering.

Email: [email protected]

About the author

Assoc Prof. Zhanghua Wu was born in 1979, and is now working at the Technical Institute of Physics and Chemistry (TIPC), Chinese Academy of Sciences (CAS). He obtained his Bachelor degree from Zhejiang University in 2001 and his Ph.D. degree from the Chinese Academy of Sciences in 2006. His research interests include thermoacoustic engine, thermoacoustic electrical generator and solar thermal power generation. He has published over 80 papers and undertakes multiple national scientific research projects.

Email: [email protected]

About the author

Prof. Ercang Luo was born in 1967, and is now working at the Technical Institute of Physics and Chemistry (TIPC), Chinese Academy of Sciences (CAS). He obtained his Bachelor degree from the Department of Thermal Energy of Tsinghua University in 1990 and his Ph.D. degree from the Cryogenic Laboratory of Chinese Academy of Sciences in 1997. Then, he joined the Cryogenic Laboratory of CAS as a research scientist, and was promoted a full professor of TIPC/CAS in 2001. In 2006, he was selected the outstanding young scholar of the National Natural Sciences Foundation of China. Since 2009 he has been the head of CAS Key Laboratory of Cryogenics. In 2015, he was elected to be the deputy director of TIPC/CAS. His R&D activities are mainly involved with various refrigeration technologies including mixed-gas Joule-Thomson refrigerator, pulse tube cryocooler and thermoacoustically-driven refrigerator, etc. Also, he has been investigating and developing thermoacoustic electrical generator by using solar energy and industrial waste heat in recent years.

Prof. Luo has published over 300 peer-reviewed international journal or conference papers and has been issued over 100 patents. He has received several awards, including a Silver Medal of China National Invention Prize of Technology in 2006 and the Hugangfu Prize of Chinese Association of Physics in 2007.

Email: [email protected]

Reference

Shunmin Zhu, Tong Wang, Chao Jiang, Zhanghua Wu, Guoyao Yu, Jianying Hu, Christos N. Markides, Ercang Luo. Experimental and numerical study of a liquid metal magnetohydrodynamic generator for thermoacoustic power generation, Applied Energy, Volume 348, 2023, 121453.

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