Cool sound: the future of refrigeration? Thermodynamic and heat transfer issues in thermoacoustic refrigeration

During the past two decades the thermoacoustic refrigeration and prime mover cycles gained importance in a variety of refrigeration applications. Acoustic work, sound, can be used to generate temperature differences that allow the transport of heat from a low temperature reservoir to an ambient at higher temperature, thus forming a thermoacoustic refrigeration system. The thermoacoustic energy pumping cycle can also be reversed: temperature difference imposed along the stack plates can lead to sound generation. In this situation the thermoacoustic system operates as a prime mover. Sound generated by means of this thermoacoustic energy conversion process can be utilized to drive different types of refrigeration devices that require oscillatory flow for their operation, such as thermoacoustic refrigerators, pulse tubes and Stirling engines. In order for a thermoacoustic refrigeration or prime mover system as well as a thermoacoustic prime mover driving a non-thermoacoustic refrigeration system to be competitive on the current market, it has to be optimized in order to improve its overall performance. Optimization can involve improving the performance of the entire system as well as its components. The paper addresses some of the thermodynamic and heat transfer issues relevant in improving the performance of the thermoacoustic system, such as optimization for maximum COP, maximum cooling load and the role of the heat exchangers. Results obtained using the two optimization criteria are contrasted in the paper to illustrate the complexity of the optimization process.