The Reality of a Small Household Thermoacoustic Refrigerator

This paper discusses the thermoacoustic refrigeration cycle and how it can be applied to real world uses, particularly its use in household refrigeration systems. The commercial viability of this technology is determined by comparing it to a vapour compression system. The paper has two goals: to determine practical applications where thermoacoustic refrigeration may prove a strong rival to current methods, and to detennine what future developments are required for this technology to be of commercial value. INTRODUCTION The thermoacoustic effect was fJISt discovered in the 19th century when heat driven acoustic osciiiations were observed in open-ended glass rubes [1]. These devices were the fust thermoacoustic engines, consisting of a bulb attached to a long narrow tube (see Figure 1). Lord Rayleigh made a qualitative explanation of the effect in 1896 [2), however a quantitatively accurate theory of the phenomena was not established until the 1970's [3). It was in the 1980's that thermoacoustic refrigeration was fJISt developed, when a research group at the Los Alamos National Laboratory [4] showed that the effect could be used to pump heat The technology has seen rapid growth since then, much of this being attributed to the Naval Postgraduate School in Monterey, California, which carried out the development of a reliable spacecraft cryocooler [ 5]. What thermoacoustic refrigerators offer is both simplicity and reliability. Unlike current commercial devices that require crank shafts and pistons, these devices use only a single moving part the diaphragm of a loudspeaker. What currently makes them very attractive as an alternative to other approaches is their use of an inert gas as the working fluid, making them environmentally clean. In order for the thermoacoustic refrigerator to become a viable commercial alternative their efficiency has to be competitive when compared to currently used systems. This paper compares the COP of a thermoacoustic refrigerator to that of a vapour compression based system for a variety of heat loads and temperature spans to investigate under what conditions a thennoacoustic refrigerator might be competitive with current refrigeration systems. · The basics of thermoacoustic refrigeration A basic thennoacoustic refrigerator consists of a stack of thin parallel plates housed withln a resonator, as shown schematically in Figure 2(a). Heat can be pumped from the cold to wann end of the stack by setting up a standing wave within the resonator. This effect, where heat is pumped up a temperature gradient by the use of sound, may be explained by considering an element of fluid as it oscillates back and forth along the stack, as shown in Figure 2(b). The element experiences a cyclic temperature oscillation about its mean temperature, due to adiabatic compression and expansion of the gas. Irreversibilities caused by a temperature difference between the oscillating working fluid and the stack result in the correct phasing between the pressure and temperature oscillations. The phasing is such that when