MechChem Africa February 2017

Thermoacoustic refrigerator with no

Shinya Hasegawa and colleagues at Tokai University in Japan have developed a refrigerator capable of generating temperatures down to -107 °C, powered by waste heat at temperatures lower than 300 °C. The heat is used to generate sound waves in an innovative multistage travelling-wave, thermoacoustic engine.

on linear analysis conducted by Hasegawa and his group.

Background and aims “TA engines do not have moving parts, are easy to maintain and, potentially operate at high efficiency at low cost,” says Shinya Hasegawa, an associate professor at the Department of Prime Mover Engineering, Tokai University, Hiratsuka, Japan. “My goals in this research are to develop TA engines that operate at less than 300 °C with more that 30% efficiency and to also demonstrate a refrigerator operating at -200 °C driven by these lower waste heat temperatures.” The TWTR consists of three etched stainless steel mesh regenerators installed at optimal positions, “close to the sweet spots”within the prime mover loop and the refrigerator loop. This configuration was designed to trigger thermoacousticoscillationsatlowertempera- tures and yield a refrigerator temperature of less than -100 °C. The diameters of the regenerators ranged between 0.2 to 0.3 mm and their lengths were 30 to 120 mm, depending on location. Furthermore, theTWTRhadheat exchangers in the formof parallel platesof copper (1.0mm thick and 27 mm long) with a 2.0 mm gap. The thermoacoustic energy conversion of this design is determined by two factors: the ratio of the diameter of the flow channel and the thermal penetration depth; and the phase difference between the pressure and cross-sectional mean velocity. The overall performance of the TWTR system is expressed in terms of the coefficient of performance (COP) and given by the ratio Double loop travelling wave thermoacoustic refrigerator (TWTR)

Associate professor Shinya Hasegawa.

T okai University’s innovative ther- moacoustic refrigerator, devel- oped by associate professor Shinya Hasegawa and his colleagues, can produce gas oscillations and refrigeration using heat at a temperature lower than the boiling point of water and, at an input heat temperature level of 270 °C, it can achieve a refrigeration temperatureof -107.4 °C. These findings arepublished in the journal of Applied Thermal Engineering , November 2016. The principle of thermoacoustic (TA) engines is based on the heating, cooling and oscillation of acoustic (sound) waves created

technology are: high efficiency systems need to be able to operate at less than 300 °C as compared to the currently possible 400 to 600 °C range; and the robustness of the designs to enable the systems to be used in a wide range of environments such as fishing boats and heavy industries. Hasegawa and his colleagues have de- signedahigh-efficiencymultistage-type ther- moacoustic (MS-TA) engine, without moving parts that operates at less than 300 °C, which is the temperature ofmore than 80%of avail- able industrial waste heat. The designof theMS-TAenginewas based

by the thermal expansion and con- traction of gases such as helium enclosed in purpose-designed tubes and cavities. The potential of TA engines for generating clean and renewable energy started being demonstrat- ed in seminal reports published in the late 1990s and early 2000s by researchers in the USA. These re- ports into the modern implemen- tations of TA engines have led to increased worldwide research on thedevelopment of highefficiency TA engines to convert heat into useful power. Two of the main hurdles pre- venting the proliferation of this

The double loop TWTR consists of three etched stainless steel mesh regenerators installed within the prime mover loop (left). This creates acoustic waves that drives the refrigerator loop (right).

42 ¦ MechChem Africa • February 2017