Refrigeration technology is among the modest inventions that have significantly transformed lives. Depending on the desired end application, refrigerators come in different types with different modes of operation. For instance, most domestic refrigerators and air conditioners operate at room temperature. Among the available room temperature refrigerators, vapor-compression refrigerators are highly efficient and commonly used globally. Unfortunately, the working substances used in these refrigerators have environmental problems that limit their application. Other working substances such as adsorption and absorption chillers, though suitable for low-temperature waste applications, exhibit low power efficiency and corrosion. Therefore, a good refrigerator design needs to ensure environmentally-friendly working substances, high energy saving capacity, large operating temperature range, among other vital properties.
Novel thermoacoustic refrigerators have recently emerged as efficient alternatives for room temperature applications. Due to their thermoacoustic effects, they offer great flexibility with no environmental problems since they utilize inert gases as their working substances. Additionally, they are reliable and can be powered by either electric compressors or heat-driven thermoacoustic heat engines. Depending on the acoustic field conditions used, two main types of thermoacoustic refrigerators are traveling-wave and standing-wave refrigerators. Unlike the traveling-wave type that exhibits higher efficiency due to the reversible thermodynamic cycle, the standing wave experiences low thermal efficiency due to the irreversible thermodynamic process.
Two main configurations for traveling-wave thermoacoustic refrigerators (TWTARs) for room temperature operation have been proposed. Single-unit loop configuration involves one refrigerator core with a phase shifter connecting the outlet and inlet. Additionally, the multi-unit loop configurations comprise several refrigerator and engine cores connected using resonant tubes. Nevertheless, studies on TWTARs for room temperature applications remain sparse. This can be attributed to different problems, including high power recovery losses and low power utilization. To address these issues, researchers at the Chinese Academy of Sciences: Miss. Xin Wang, Professor Zhanghua Wu, and Professor Ercang Luo, etal developed a multi-stage TWTAR and numerically studied its working mechanism using SAGE software. The work is currently published in the International Journal of Refrigeration.
In their approach, the authors commenced by first exploring the traditional single-stage TWTAR. The proposed multi-stage TWTAR is composed of multiple thermoacoustic refrigerator cores. Each core consists of a room temperature heat exchanger, a regenerator and a cold heat exchanger, which were all connected serially between the inlet and outlets of the core. Through SAGE software, the system was simulated, and the parameters of the heat engines were calculated. Additionally, various variables considered vital for enhancing the performance of the system such as performance coefficient, acoustic work utilization, and acoustic work were discussed in detail.
Results showed that the proposed refrigerator could effectively improve the system’s cooling power and acoustic work, thereby leading to high cooling efficiency. Compared to the single-stage refrigerator, the cooling power increased from 2.17kW to 6.42kW, the acoustic work utilization rate improved from 0.26 to 0.82, while the performance coefficient changed from 2.60 to 3.19. However, the authors observed that an increase in the number of stages resulted in decreased cooling power and cooling efficiency. Based on the parameter optimization of the second and third stages, a three-stage refrigerator was the most suitable multi-stage TWTAR.
In summary, the research team successfully investigated the characteristics of traveling-wave thermoacoustic refrigerators operating at room temperature. The multi-stage refrigerator generally exhibited improved acoustic work utilization rate, cooling power, and overall system performance. For room temperature operations, multi-stage TWTARs with three to five stages were identified as more appropriate since they are good for balancing the cooling power and cooling efficiency, thereby leading to high performance. In a statement to Advances in Engineering, Professor Ercang Luo said the study will stimulate future studies on multi-stage refrigerators.
Wang, X., Wu, Z., Zhang, L., Hu, J., & Luo, E. (2020). Traveling-wave thermoacoustic refrigerator for room temperature application. International Journal of Refrigeration, 120, 90-96.