Thermoacoustic Refrigeration

Garrett and Hofler (1992) pointed out that two recent events are responsible for the new em in refrigeration before the beginning of the twenty-first century. The most significant of these is the international agreement (signing of the Montreal Protocol) on the production and consumption of CFCs, which were found to be causing the depletion of the stratospheric ozone layer. The second event was the discovery of ‘high-temperature’ superconductors and the development of high-speed and high-density electronic circuits which require active cooling and hence a new approach to refrigeration, or thermoacoustic refrigeration, which was first discovered by Wheatley and others (1993) in August 1983. The simplicity of the hardware involved in thermoacoustic machines is best appreciated by examining a concrete example. In the mid-1990s, S.L. Garrett and his colleagues at the Naval Postgraduate School in Monterey, California developed two thermoacoustic refrigerators for the Space Shuttle. The first was designed to cool electronic components, and the second was intended to replace the refrigerator-freezer unit used to preserve blood and urine samples from astronauts engaged in biomedical experiments .

Thermoacoustic refrigeration is considered a new technology, attaining cooling without the need for refrigerants. The basic mechanism is very simple and efficient. A loudspeaker creates sound in a hollow tube which is filled with an ordinary gas. In fact, thermoacoustic refrigeration utilizes high-density sound waves to transfer heat due to the thermoacoustic effect (i.e. acoustic energy). Therefore, the working fluid in this system is acoustically driven gas. The process itself utilizes standing acoustic waves in an enclosed cavity to generate the mechanical compression and expansion of a working fluid (gas in this case) needed for the cooling cycle. The technique has the potential for high efficiency operation without the need for cooling liquids or mechanical moving parts. These factors make the concept amenable to miniaturization to chip-scale dimensions for thermal management of electronic components.

The interaction between acoustics and thermodynamics has been known ever since the dispute between Newton and Laplace over whether the speed of sound was determined by the adiabatic or isothermal compressibility of air. At the present time, the efficiency of thermoacoustic refrigerators is 20—30% lower than their vapor compression refrigerators. Part of that lower efficiency is due to the intrinsic irreversibilities of the thermoacoustic heat transport process. These intrinsic irreversibilities are also the favorable aspects of the cycle, since they make for mechanical simplicity, with few or no moving parts. A greater part of the inefficiency of current thermoacoustic refrigerators is simply due to technical immaturity. With time, improvements in heat exchangers and other sub-systems should narrow the gap. It is also likely that the efficiency in many applications will improve due only to the fact that thermoacoustic refrigerators are well suited to proportional control. One can easily and continuously control the cooling capacity of a thermoacoustic refrigerator so that its output can be adjusted accurately for varying load conditions. This could lead to higher efficiencies than for conventional vapor compression chillers which have constant displacement compressors and are therefore only capable of binary (on/off) control. Proportional control avoids losses due to the start-up surges in conventional compressors and reduces the inefficiencies in the heat exchangers, since such systems can operate over smaller temperature gaps between the coolant fluid and the heat load.

The research focus of the Thermoacoustics Laboratory in ARL at Pennsylvania State University in cooperation with Los Alamos Research Laboratory is the study of acoustically driven heat transport. Their goals include an improved understanding of fundamental thermoacoustic processes and the development of new thermoacoustic refrigerators and heat engines with increased power density, temperature span, and efficiency, and the commercialization of those devices. The Laboratory provides the infrastructure to support research on the basic processes required to understand this emerging, environmentally friendly refrigeration technology. This facility also supports the fabrication and testing required to produce complete, full-scale operational prototype refrigeration systems for military and commercial applications such as food refrigerators/freezers and air conditioners. Their prototypes have been flown on the Space Shuttle and have been used to cool radar electronics onboard a US Navy warship.  Thermoacoustic refrigerators with cooling powers ranging from a few watts to chillers with cooling capacities in excess of 10 kW are currently in operation or under construction. Figure 3.70a shows a thermoacoustic refrigerator developed by this Laboratory and it is operational for running a small fridge in Figure 3.70b.

Although thermoacoustic refrigerators have not been commercialized yet and are considered to be still at a developmental stage, it is known that they can be used for any kind of cooling. Conventional, single-stage, electrically operated thermoacoustic refrigerators can reach cold-side temperatures two-thirds to three-quarters of ambient, so they are not well suited to cryogenic applications below –40°. However, thermoacoustically driven pulse-tube style refrigerators can reach the cryogenic temperatures required to liquefy air or natural gas. In their early commercial stages, they will probably be limited to niche applications such as in military systems which are required to operate in closed environments and food merchandising where toxicity is an important issue. As global environmental mandates and legislations/amendments become essential, one can expect the scope of thermoacoustic applications to expand both domestically and in emerging markets.