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ABSTRACT
The design and functionality of thermoacoustic refrigerators has been the focus of considerable attention from the
research community since the 1980’s. This environmentally friendly technology has the potential to become another
option for refrigeration, as improvements in the design and technology are realised. Heat-exchangers are used to
increase the efficiency of thermoacoustic systems, however they are typically complex to manufacture, expensive,
and limitations of heat-exchangers exist in terms of efficiency and durability. Reducing or eliminating the use of
heat-exchangers through the use of flow-through designs dramatically reduces the cost and complexity of
thermoacoustic systems, potentially with minimal efficiency loss. In this review paper of flow-through
thermoacoustic refrigeration, the developments of flow-through design and its potential benefits will be discussed.
INTRODUCTION
Thermoacoustics is a term used to describe the effect arising
from sound waves creating a heat gradient, and vice versa.
Thermoacoustic devices are typically characterised as either
‘standing-wave’ or ‘travelling-wave’ configurations, where
the thermodynamic processes occur in a closed vessel. A full
background and introduction to the subject of
thermoacoustics is found in the textbook by Swift (2002).
An example of a standing-wave thermoacoustic refrigerator
as a schematic, is shown in Figure 1, and in Figure 2 as a
commercial application. An example of a travelling-wave
thermoacoustic refrigerator is shown in Figure 3 as a
schematic, and Figure 4 shows a commercial application.
Figure 1 shows a thermoacoustic air-conditioner that
comprises four heat-exchangers and two heat transfer devices
that connect the heat exchangers. The cost of the heat-
exchangers and heat transfer devices is a significant cost of
the overall system. Thermoacoustic refrigerators are in
general 20-30% less efficient than the equivalent vapour-
compression systems (FAQ about Thermoacoustics, Penn
State University). Flow-through design in thermoacoustics
could be considered a relatively unexplored concept amongst
the thermoacoustic community of 2005, and this will be
discussed later in the article.
This paper will outline standing-wave and travelling-wave
thermoacoustic refrigerator designs, and then review papers
of the various open-cycle, or flow-through thermoacoustic
refrigerator systems. A proposed concept from Swift (2002)
will then be discussed to completely eliminate the use of
heat-exchangers, followed by conclusions of this paper.
STANDING-WAVE REFRIGERATOR DESIGN
A design of a thermoacoustic standing-wave air-conditioner
is shown in Figure 1. The design of this air-conditioner is
similar to a conventional split-system, only the vapour-
compression system has been replaced with a thermoacoustic
system. The main components of the heat-pump are shown
in the middle tube in Figure 1 and comprises a closed
cylinder, an acoustic driver, a stack, and two heat-exchanger
systems. The length of the closed cylinder is typically a half
or quarter wavelength of the driving frequency. The vessel
becomes resonant after the application of the acoustic driver and the pressure and particle velocity distributions are shown
on the right hand side of Figure 1. The upper tube system in
Figure 1 is used to dissipate heat from the thermoacoustic
system to ambient air. The lower tube system is used to
transfer heat from the indoor air to produce a cold air stream.
The heat-exchangers are used to improve the transfer of heat
between the sub-systems. Although the plant shown in
Figure 1 is an effective thermoacoustic system, the heat-
exchanger sub-systems are typically expensive to
manufacture; hence there is an opportunity to develop a
cheaper alternative that does not require any heat-exchangers.
Reid et al. (2000) considers that a lower capital cost of a
refrigeration system is often more important to industry than
the efficiency, or its long term ongoing cost. Reid uses the
example of the refrigeration process used in aircraft
environmental control systems, where the weight saving
justifies the low efficiency (10%).
An example of a standing-wave thermoacoustic refrigerator is
shown in Figure 2, developed at Penn State University
(Triton, 2005). The device provides 10kW of cooling power,
and uses a double Helmholtz resonator design to increase the
cooling capacity. The product has cooling output that is
suitable for a small business or large home.