MechChem Africa February 2017

MechChem Africa

•

February 2017

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

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

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.

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).

Thermoacoustic refrigerator

with no

Associate professor Shinya Hasegawa.

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

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.”

Double loop travelling wave

thermoacoustic refrigerator

(TWTR)

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