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Thermoacoustic Stirling Engines (TASE) might sound like a lot to digest in one go. But, in principle, they are acoustic equivalents of Stirling engines. Therefore, before delving deeper into TASEs, we must start with Stirling Engines.

What is a Stirling Engine?

The name comes from its inventor, Reverend Robert Stirling, who came up with an external heat engine vastly different from the traditional internal combustion engines used in cars. It is different in its use of the stirring cycle, which has three core features. 

• Gasses used inside a Stirling engine never leave the engine. Unlike gasoline or diesel engines, these engines do not have exhaust valves to vent high-pressure gasses. It does not have explosions taking place, either. Resultantly, Stirling engines are very quiet.
• Since it is not an internal combustion engine, it requires an external heat source. And that heat source could be sunrays, geothermal heat, gasoline, or solar energy. The heat could even come from a decaying plant. 
• Finally, all one needs is a flywheel propeller with a gentle spin to start the engine. 

In this tradition of building Stirling engines comes Thermoacoustic Stirling Engines or TASEs, the acoustic equivalents of Stirling engines. 

TASEs: The Acoustic Equivalents of Stirling Engines

Thermoacoustic Stirling Engines have gained momentum and traction in the field of scientific research because of their low manufacturing cost, high efficiency, maintenance-free features, and self-starting capabilities. 

One of the most popular categories of Thermoacoustic Stirling Engines are Thermoacoustic Stirling Heat Engines or TASHEs. These devices can convert heat into acoustic power at a very high-efficiency rate. Its potential rests in the fact that it does not involve moving parts, and the components are relatively simple. These systems are less costly to manufacture and maintain. It is preferred by many as a means to generate clean and effective energy. 

How TASHEs Work?

The energy conversion process in these engines happens in the regenerator. A regenerator is a porous metallic block between a hot heat exchanger (HHX) and a cold or ambient heat exchanger. Such placements at the two ends help maintain a mean temperature gradient in the axial direction. Acoustic waves propagating through it – with the right phase – can be amplified by a thermodynamic process resembling a Stirling cycle. 

Why are Thermoacoustic Stirling Engines Beneficial?

The scarcity of fossil fuel is a challenge that faces humanity in the near term. Fossil fuels are also not beneficial for the health of the planet and come with a host of sustainability concerns. 

Scientific communities have been contemplating the use of alternative sources of fuel, including solar energy, geothermal energy, biofuels/biomass, radioisotopes, and more. 

In this scenario, Stirling engines have shown good results owing to their high efficiency, closed thermodynamic cycle, silent operation, low vibration, long operating life, and low maintenance. 

There could be two types of Stirling engines: the conventional and the advanced.

The Evolution of Thermoacoustic Stirling Engines: The Innovations and Breakthroughs

Knowing the history of the TASEs is crucial to understanding how they serve their purpose and what the nature of the technology has been. 

We’ve already talked about acoustic waves passing through the gradient between hot and cold or ambient heat exchangers on two sides. 

Until the 1980s, the efficiency of most designs in this field typically did not exceed 5%. Marking a turning point in 1979, a significant breakthrough was achieved by Ceperley. He showed that traveling waves can extract acoustic energy more efficiently, which led to the design concept of traveling-wave TASHEs used today. 

What happens in this more efficient scheme of things is that part of the generated acoustic power goes back to the regenerator via some form of feedback loop and, in part, is directed towards a resonator for energy extraction.

The first decade of the millennium saw further improvement in the technology behind TASHEs. In 2011, Tijani & Spolestra designed a traveling-wave TASHE that achieved a remarkable overall efficiency of 49% of the Carnot limit. To add context, the Carnot Limit sets an absolute limit on the efficiency with which heat energy can be turned into useful work.

In the latest development in the field of TASEs, China recently developed a high-efficiency thermoacoustic Stirling generator, which can deliver 140 hp or 102 KWs of power from a heat source of 986 degrees Fahrenheit. The development came from the researchers working at the Technical Institute of Physics and Chemistry at the Chinese Academy of Sciences. It was the first time such a Stirling generator could go over 134 hp or 100 KWs power. 

This Chinese innovation is seen as a potential game-changer by many for its versatility. It can go with a range of different heat sources and could change how energy is generated, providing solutions for various energy needs.

According to the team that innovated it, its reliability, simple design, and compatibility with diverse heat sources can make it compete with the efficiency of steam turbines. The motor’s design rids the system of vibrations and helps maintain an airtight seal. The innovation could help make China ultra-quiet, non-nuclear submarines. 

Another 2017 study proposed thermoacoustic Stirling power generation from LNG cold energy and low-temperature waste heat. The study resulted in the design of a thermoacoustic Stirling generator operated with 4 MPa helium gas capable of generating 2.3 kW electric power with the highest exergy efficiency of 0.253 when the cold and hot ends are maintained at 110 K and 500 K. 

The evolution of the Thermoacoustic Stirling Engine as a solution to introduce higher levels of energy efficiency has been studied closely by researchers and high-end technologists around the world. And much work has then been done on the large-scale enterprise solution front. 

Organizations Leveraging Thermoacoustic Stirling Engines

#1. NASA

NASA has made significant progress in the area of Thermoacoustic Stirling Engines. The solution, known as Stirling Thermoacoustic Power Converter and Magnetostrictive Alternator, eliminates all moving parts for maximum efficiency and reliability. 

Novel Technology Developed by NASA’s Glenn Research Center

This technology makes Stirling engines more efficient and less costly. It leverages thermoacoustic power converters where sound is used to turn heat into electric power. The system uses heat-driven pressures and volume oscillations from thermo-coustic sources to power piezoelectric alternators or other power-converter technologies. This device is capable of generating electricity with unmatched efficiency. 

The impact of NASA’s innovation has been far-reaching. The thermoacoustic power converter has helped reshape the conventional Stirling engine from a toroidal shape to a straight collinear arrangement. This innovation ensures that further systems would not have to depend on failure-prone mechanical inertance and compliance tubes. The goal can be achieved by using acoustical resonance by using electronic components. 

The innovation has resulted in something efficient, reliable, low-cost, compact, and versatile. One can use it in distributed generation and residential power systems, combined heat and power systems, concentrated solar power generation, hybrid electric vehicles, refrigeration systems, heat pumps, underwater and marine power systems, and auxiliary power units. 

#2. SpaceX

There is a high probability that SpaceX, another space and aviation technology giant, may explore TASEs shortly. It could help them achieve enhanced efficiency in converting heat to mechanical work. 

Low manufacturing and maintenance costs could lead to bringing out lighter spacecraft at lower costs. It would also help SpaceX to manage heat effectively, with enhanced power generation capabilities in deep space missions. 

According to reports published during mid-August 2023, SpaceX’s surging revenue has made it profitable in Q1 2023 after two annual losses. The Elon Musk-owned company generated $55 million in profit on $1.5 billion in revenue during the January-to-March period. It was valued at nearly US$150 billion at a recent employee stock sale.

#3. Sierra Lobo, Inc.

Based out of Fremont, Ohio, Sierra Lobo, Inc., provides specialized space and aerospace test, evaluation, and engineering services worldwide. It has developed full-fledged thermoacoustic Stirling heat engines that can operate with a variety of power/heat sources, with high efficiency and reliability, containing no moving parts. It is compact and scalable and can be used in space applications for its gravity-independent operation. 

These heat engines are expected to find applications in many areas, including acoustic power and pressure wave generation and electrical generation for ground, underwater, and in-space applications. It can simultaneously produce electrical and cooling power, drive a linear alternator for electrical power generation, and refrigerator and cryocooler for cooling generation. 

Challenges and Opportunities in the Road Ahead

Thermoacoustic Stirling Heat Engines (TASEs), a specific category of Stirling engines, have shown great potential in developing power sources suitable for deep space travel. NASA, particularly through its Glenn Research Center, has been at the forefront of leveraging the technology’s efficiency and low-maintenance attributes. This technology is expected to progress further, supporting power systems that could be instrumental in numerous space exploration projects, potentially including powering bases on the Moon and Mars.

These engines are preferred for their ability to offer very high thermal-to-electric efficiency compared to other heat engines. The absence of bearing systems and moving parts in TASEs significantly reduces the risk of failure and fabrication expenses.

However, to become the most preferred choice, there is still substantial work to be done in refining this technology. TASEs currently face challenges with volume and weight, primarily because their design, involving heat exchangers at both hot and cold points, results in a bulkier and heavier formation than traditional internal combustion engines with equivalent power output.

Additionally, TASEs are often challenged by a slower startup due to inherent thermal inertia. Their application is limited in scenarios requiring rapid starts or quick changes in speed. Nevertheless, ongoing technological innovations aim to overcome these challenges, positioning TASEs as a crucial component for high-tech space operations and beyond.

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