Study on a heat-driven thermoacoustic refrigerator for low-grade heat recovery

Significance 

Utilization of waste heat from industries and low-grade heat from renewable energy sources, such as solar and geothermal, is particularly important for global energy efficiency. Recovering low-grade heat helps cut down on energy demand, energy costs, and carbon emissions, and also helps improve overall system efficiency. The ability to recover low-grade heat for cooling purposes would be attractive for cooling in buildings, preservation of food and medical supplies, among other applications.

Cooling using thermoacoustic wave-driven thermodynamic cycle presents an emerging heat-driven cooling technology involving no mechanical moving parts or electrical power, therefore no harmful carbon and other ozone-depleting emissions. A thermoacoustic refrigerator is made up of a thermoacoustic cooler and a thermoacoustic engine. It works by converting heat energy into acoustic work required to move heat from lower to higher temperatures.

Onset temperature difference at two ends of a well-designed thermoacoustic engine of a thermoacoustic refrigerator should exceed a critical value so that acoustic gains exceed loses to initiate a spontaneous gas oscillation. Therefore, to better utilize low-grade heat, the onset temperature should be kept to the minimum in a thermoacoustic engine.

Despite the promising results posted in previous studies, existing thermoacoustic systems using gaseous phase-matching elements still show high onset temperatures and unacceptable cooling efficiencies, which limit their low-grade heat utilization capacity. Fortunately, a gas-liquid resonator may provide an effective solution, with previous studies showing that it can reduce the onset temperature significantly, and improve cooling efficiency.

Scientists at University of Cambridge, Dr. Jingyuan Xu and Professor Simone Hochgreb together with Professor Ercang Luo at Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences explored the use of a gas-liquid resonator in a thermoacoustically-driven refrigerator with an objective of resolving high onset temperature and low cooling efficiency affecting the implementation of thermoacoustic cooling systems in low-grade heat recovery. They were looking to improve cooling efficiency and cooling power density, and extend the required heating temperature to the lowest. The original research article is published in the journal, Applied Energy.

First, theoretical analyses on multi-stage systems were performed to investigate steady performance and onset features. Then the research team studied carefully the effects of liquid volume ratio, mean pressure, and expected liquid mechanical damping coefficient on working frequency and onset temperatures in systems with different stages. A comparison of the system’s onset performance was also made with conventional gas-only resonator systems.

The investigators observed that a thermoacoustic refrigerator using gas-liquid resonators outperformed a conventional gas resonator system to a greater extent on both onset and steady performance. Using a gas-liquid resonator to replace a gas-only resonator it was observed that the onset heat temperature difference was reduced from 144.1 K to 35.5 K while cooling power and efficiency were improved by a factor of 5.6 and 1.5, respectively. It was also observed that higher pressure amplitudes and low working frequencies created by the gas-liquid resonator contributed to an improvement in performance.

The new system’s cooling steady performance was mainly affected by liquid volume ratio and the number of stages. Increasing the number of stages lead to a higher cooling power but didn’t significantly affect cooling efficiency. The liquid volume ratio optimum value fell within a range of 0.3 and 0.5 for both cooling efficiency and cooling power. Onset temperature difference is closely related to liquid damping coefficient, liquid volume ratio, and mean pressure. Most systems considered for the study were observed to begin oscillation at onset temperature differences below 50 K.

When implemented, the novel thermo-acoustically driven refrigerator reported in the study will enable for efficient low-grade recovery from renewable energy sources and waste heat from industrial processes.

Study on a heat-driven thermoacoustic refrigerator for low-grade heat recovery - Advances in Engineering
Fig. Schematics of a looped multi-stage thermoacoustically-driven refrigerator using gas-liquid resonators for recovering low-grade heat (taking 3-stage as an example): (a) 2D sketch; (b) 3D drawing. AHX is the ambient-temperature heat exchanger, REG is the regenerator, HHX is the high-temperature heat exchanger, TBT is the thermal buffer tube, CHX is the low-temperature heat exchanger, PT is the pulse tube, and GLR is the gas-liquid resonator.

About the author

Dr. Jingyuan Xu is a Research Associate in the Clean Energy Processes (CEP) Laboratory, Imperial College London. She received her B.Eng. degree from Huazhong University of Science and Technology in 2013 and received her Ph.D. degree from Technical Institute of Physical and Chemistry, Chinese Academy of Science in 2018. In 2017, she gained research experience at the University of Giessen supported by Germany DAAD short-term research grant. Prior to joining Imperial College London, she worked as a Postdoctoral Research Fellow in the Department of Engineering at University of Cambridge from 2018.

Dr. Xu’s previous research interest focuses on clean energy conversion based on thermoacoustic and Stirling technology. Her research including but not limited to the topics on power generation, solar power and/or heating technology, heat pump, refrigeration, cryogenics, low-grade heat recovery, and cold energy recovery. Dr. Xu has been awarded Carl von Linde Award Young Researcher Award by International Institute of Refrigeration, and Gustav and Ingrid Klipping Award by International Cryogenic Engineering Committee.

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 Technology Invention Prize in 2006 and the Hugangfu Prize of Chinese Association of Physics in 2007.

Email: [email protected]

About the author

Prof. Simone Hochgreb is a professor of engineering at the University of Cambridge since 2002. Her research interests include reacting flows, laser diagnostics, combustion, thermoacoustics, and aerosol formation and transport. Prior to Cambridge she held positions at Sandia National Labs, Exponent Inc. and at MIT.

Email: [email protected]

Reference

Jingyuan Xu, Ercang Luo, Simone Hochgreb. Study on a heat-driven thermoacoustic refrigerator for low-grade heat recovery. Applied Energy, issue 271 (2020), pages 115-167.

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