Brief Introduction to Thermoacoustic Energy Conversion Technology

Authors: KIT team

The thermoacoustic process involves the conversion between mechanical and thermal energy using acoustic oscillations of gas in a controlled environment. In the direct cycle, the energy harvesting effect is produced by simply placing a solid porous component with a constant temperature gradient in a closed resonator tube. The temperature gradient of the porous solid induces gas movement due to thermal expansion and contraction, thereby generating sustained acoustic oscillations. With a suitable transducer, the mechanical power of the oscillations can be harnessed to produce electric power. Figure 1 illustrates the thermodynamic processes and schematic of a thermoacoustic energy harvester. In the reverse cycle, a cooling effect is produced by placing a solid porous component of initially uniform temperature in a resonator where acoustic oscillations in the gas are set up by an external power supply. Oscillatory gas movement in a closed resonator leads to adiabatic pressure and temperature oscillations whose amplitude varies in the direction of gas oscillation. As a result of this spatial gas temperature variation, a temperature gradient is developed along the length of the porous solid. With one end of the porous solid thermally connected to the ambient using a heat sink, cooling can be achieved at the other end. Figure 2 illustrates the thermodynamic processes and schematic of a thermoacoustic cooler.

Similar to the conventional, widely established systems for power generation and cooling, such as those based on Brayton and vapour compression refrigeration cycles, thermoacoustic systems also involve heat addition and rejection to the working gas at different pressure levels. But unlike the conventional systems, thermoacoustic setups do not require complex moving parts like compressors or turbines to switch between the two pressure levels. Additionally, conventional systems utilise a dedicated component for each of the thermodynamic processes, through which the working gas (fuel or refrigerant) is circulated. Whereas in thermoacoustic setups, all processes occur within a single resonator tube without an active fluid loop. The remarkable simplicity of thermoacoustic systems can be particularly leveraged for small-scale applications, including electronics or battery thermal management, where the complexity of conventional technologies cannot be handled efficiently and economically. Another major advantage is that thermoacoustic systems function on eco-friendly gases such as air, nitrogen or helium, instead of carbon-emitting fuels or ozone-depleting refrigerants, ensuring sustainable realisation of energy needs.

In one configuration of a thermoacoustic power generation system developed by Bi et al. [1] in 2017, 4.69 kW of electricity was generated with a thermal-to-electric efficiency of 18.4% (corresponding to a relative Carnot efficiency of about 57%) for a system size of 2.5 m x 2.5 m. For the case of cooling, Wang et al. [2] demonstrated a heat-driven (electricity-independent) thermoacoustic refrigerator in 2019 capable of producing 4.5 kW cooling power at 10C. While considerable progress has been made in the development of thermoacoustic systems for mid-scale and large-scale applications (W to kW range, respectively), there is a clear gap in exploring the feasibility and challenges of small-scale applications. Some valuable efforts in this area include the thermoacoustic coolers investigated by Chen et al. [3] in 2002 and Xu et al. [4] in 2022. These systems were smaller than 10 cm and exhibited cooling powers in the range of 10^-2 W.

Through THERMINATOR, small-scale thermoacoustic systems will be developed for the thermal management of solar panels to improve their efficiency and reliability in renewable energy generation. Studies on the integration of thermoacoustic and electrocaloric energy conversion technologies for this application will be carried out to assess the combined improvement in performance and commercialisation capabilities.

Harini Nivetha Raja, Jingyuan Xu, Karlsruhe Institute of Technology.

References

• Bi, T., Wu, Z., Zhang, L., Yu, G., Luo, E., & Dai, W. (2017). Development of a 5 kW traveling-wave thermoacoustic electric generator. Applied Energy, 185, 1355–1361. https://doi.org/10.1016/J.APENERGY.2015.12.034
• Wang, H., Zhang, L., Yu, G., Hu, J., Luo, E., Ma, Y., Jiang, C., & Liu, X. (2019). A looped heat-driven thermoacoustic refrigeration system with direct-coupling configuration for room temperature cooling. Science Bulletin, 64(1), 8–10.
• Tsai, C., Chen, R.L., Chen, C.L., & Denatale, J. (2002). Micromachined stack component for miniature thermoacoustic refrigerator. Technical Digest, MEMS 2002 IEEE International Conference, Fifteenth IEEE International Conference on Micro Electro Mechanical Systems (Cat. No.02CH37266).
• Chen, G., & Xu, J. (2022). Development of a small-scale piezoelectric-driven thermoacoustic cooler. Applied Thermal Engineering, 213. https://doi.org/10.1016/j.applthermaleng.2022.118667