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Patent of the Month – ThermoAcoustic Engine

Patent of the Month – ThermoAcoustic Engine

| On 30, Mar 2018

Darrell Mann

Well, it’s never going to win prizes for readability, but patent of the month this month is nevertheless an elegant evolution of the thermoacoustic engine. US9,777,951 was granted to a quartet of inventors at Tokai University in Japan on 3 October. Fortunately, there’s quite a lot of coverage of the basic problem solved by the inventors in the online technical media. First up, a brief introduction to the basic technology:

Thermoacoustic engines work by heating, cooling and oscillating sound waves created by the thermal expansion and contraction of gases such as helium in enclosed dedicated cavities. TA engines were first devised in the late 1990s and early 2000’s by researchers in the US, leading to researchers worldwide beginning projects to develop high efficiency TA engines that convert heat into useful power. However major barriers to their application have been operating systems at a high enough efficiency at temperatures of less than 300°C and developing a robust enough design for everyday use.

The researchers from Japan have now successfully demonstrated a refrigerator powered by soundwaves from a thermoacoustic engine that runs from waste heat.

The coldest temperature the refrigerator is capable of is -107.4°C when the waste heat fuelling the thermoacoustic (TA) engine is 270°C.

The high efficiency multistage-type thermoacoustic engine does not have moving parts and operates at less than 300°C, the temperature of more than 80% of industrial waste heat, according to the researchers.

Shinya Hasegawa, associate professor of Prime Mover Engineering at the University said: “TA engines do not have moving parts, are easy to maintain, have potentially high efficiency, and are low cost.
“My goals are to develop TA engines that operates at less than 300°C with more than 30% efficiency, and also to demonstrate a refrigerator operating at -200°C at these low temperatures.”

The travelling wave thermoacoustic refrigerator (TWTR) consists of three etched stainless-steel mesh regenerators installed within the prime mover loop and one in the refrigerator loop. This configuration triggers thermoacoustic oscillations at lower temperatures and yield a refrigerator temperature of less than -100°C.

The diameters of the regenerators ranged between 0.2 to 0.3mm and their lengths were 30 to 120 mm, depending on location. Furthermore, the TWTR had heat exchangers in the form of parallel copper plates (1.0mm thick and 27.0mm in length) with a 2.0mm gap.

The thermoacoustic energy conversion of this design is determined by the ratio of the diameter of the flow channel and thermal penetration depth, and the phase difference between the pressure and cross-sectional mean velocity.

The coefficient of performance (COP) increased as the temperature of the heat exchangers in the primer loop was increased and the maximum value of COP was 0.029 at 260°C, and the corresponding cooling power was 35.6W.

The researchers also obtained gas oscillations at 85°C —that is lower than the boiling point of water—thereby opening up possibilities for applications of this technology for refrigeration and power generation using low temperature waste heat in factories and automobile engines. In addition refrigeration to −42.3°C was achieved at 90°C and the efficiency of the Tokai University TA engine was 18% at minus 107 °C.

The basic problem being solved is the conflict between the desire to increase efficiency of the engine being hindered by the need for higher temperatures than are easy to achieve. Here’s what that problem looks like when mapped on to the Contradiction Matrix:

The basic ‘multi-stage’ aspect of the solution offers up a good illustration of Principle 1, but the main inventive steps seem to be more closely attributable to Principle 31 (see the cross-sectional image in the opening figure) and Principle 7, Nested Doll. The secret to the success of the design, in other words, comes down to the different sizes of the adjacent segments of the engine:

It still looks like thermoacoustics are some way away from full commercial exploitation, but this invention overcomes a major technical obstacle. Being able to operate at temperatures of less than 300degC opens up the possibility to tap into a large number of waste heat sources like internal combustion engines, heating boilers and solar cells. With a following wind, these things could make a big difference to the global energy problem.