What Is Thermoacoustic Refrigeration?

Thermoacoustic technology refers to the phenomenon of acoustic self-excited oscillation caused by a medium. Thermoacoustic technology is essentially a "heat engine technology". Some even call the thermoacoustic heat engine "the fourth generation heat engine" (American Swift). Like a steam engine or an internal combustion engine, a thermoacoustic heat engine can convert heat into mechanical energy, or use mechanical energy to create a temperature difference.

In short, the thermoacoustic effect is a physical phenomenon in which acoustic self-excited oscillations are caused by heat in an elastic medium (usually a high-pressure inert gas). The simple device shown in the figure below, when heat is applied to the hot-end heat exchanger, the gas surrounded by the hot-end heat exchanger is heated. The gas expands and produces the first pressure-turbulent wavefront that travels at both ends at the speed of sound. At the same time, because the expanded gas is pushed into the gap of the regenerator stack, the temperature of the regenerator is lower than that of the hot-end heat exchanger. After the heat exchange of the gas, the volume shrinks and the contracted gas tends to move back. At the same time, the first pressure wave front propagates to the end of the cavity and is reflected back, and the reflected wave is superimposed with the contraction of the gas. Positive feedback enhancement occurs at a certain frequency (determined by the length of the resonant tube and the speed of sound). After repeated strengthening of several cycles, it reaches saturation and forms a continuous resonance wave. This process completes the conversion of heat to mechanical energy in the form of sound waves, and this process is called the "thermoacoustic positive effect." This thermoacoustic device is the simplest "thermoacoustic engine".

Thermoacoustic technology is essentially a "heat engine technology". Some even call the thermoacoustic heat engine "the fourth generation heat engine" (American Swift). Like a steam engine or an internal combustion engine, a thermoacoustic heat engine can convert heat into mechanical energy, or use mechanical energy to create a temperature difference. Therefore, it has a wide range of applications in thermal energy utilization and low-temperature refrigeration. Unlike a large number of internal combustion engines, it is not necessary to inject liquid or gaseous fuel into the cylinder for combustion work, as long as heat is applied to the thermal head, it can work. Increasing shortage of fossil energy makes it possible to use solar thermal power to generate electricity or generate mechanical work at low cost.

Among the existing engines, the closest to the thermoacoustic engine is the Stirling Engine (the earliest external combustion engine). Stirling refrigerators and Stirling engines have been used commercially for many years. The silent power source of conventional power submarines, the holder of the highest efficiency of the current high-power solar power generation, and the nuclear decay heat generator in deep space probes all have Stirling heat engines. However, due to cost and reliability issues, its widespread application has been limited. The difference between a thermoacoustic heat engine and a Stirling heat engine is that the internal gas configuration is achieved by using acoustic characteristics, rather than using mechanical pistons at high or low temperatures. This provides good characteristics for reducing manufacturing costs and improving reliability. The two have similar application fields and methods, and the intrinsic efficiency is similar.

Using the thermoacoustic phenomenon that heat generates self-excited oscillation in pressurized gas, heat can be converted into pressure fluctuations, that is, acoustic waves. Pressure waves are alternating mechanical energy, and thermo-mechanical conversion is achieved. Thermoacoustic engines are devices that generate mechanical power from heat through thermoacoustic effects.

Compared with the traditional thermo-mechanical technology, thermo-acoustic technology has the following outstanding advantages and development potential: First, high reliability: neither thermo-acoustic engine nor thermo-acoustic refrigerator has moving parts, and their compression and expansion processes are completely sonic. It can be achieved by raising and lowering; second, high efficiency: because there are no mechanical moving parts, losses caused by mechanical friction in conventional heat engines can be avoided; third, simple structure and low production cost: the main components of thermoacoustic heat engines Composed of heat exchangers, regenerators and pipes, the complexity of machining is much lower than that of traditional power machinery, so the manufacturing cost can be lower. Fourth, environmental protection and broad adaptability: Thermoacoustic technology generally uses inert gas for work The medium is also an external combustion type equipment, so it has higher environmental protection characteristics and can be driven by a variety of heat sources (solar energy, biomass energy, industrial waste heat, etc.).

Similar to thermoacoustic cooling, but fixing the temperature at the low temperature end to the ambient temperature can achieve efficient heating, that is, the heat pump function. In industrial processes and life, electricity is needed to generate heat in many places. A common heating method is to use electric heating elements to generate electricity through resistance heating to generate heat energy, but this heat generation method is uneconomical and has low energy utilization efficiency. As a new type of heating method, heat pump can improve the energy utilization efficiency several times, such as air source, water source or ground source heat pump. The existing heat pump uses the compression-evaporation process of the working medium (the reverse process of common air-conditioning systems) to achieve the heat pump function. However, due to the limitation of working medium characteristics, it is difficult for this heat pump to achieve large temperature difference pump heat (generally below 100 degrees Celsius), which limits the application at higher temperatures.

Using a thermoacoustic heat engine, a higher temperature pump heat process can be achieved to manufacture a thermoacoustic heat pump. Since the thermoacoustic heat pump uses a gaseous working medium, it is suitable to work in a wide temperature range, so the thermoacoustic heat pump is more suitable for high temperature (above 150 degrees Celsius) applications ^[1] .