The Stirling engine, which has been around for years, has often been overshadowed by internal combustion engines due to manufacturing costs and challenges. Despite Philips' attempts to produce Stirling engine electricity generators commercially, they struggled to compete due to the high-pressure gas containment requirements.

In essence, a Stirling engine generates mechanical energy from an external heat source, contrasting with internal combustion engines that rely on combustion within their cylinders.

Though Philips's endeavors did not lead to commercial success, Stirling engines found applications in various fields. For example, in 1996, the Swedish navy implemented Stirling-driven generators in their Gotland-class submarines for underwater battery recharging and propulsion, utilizing a combination of diesel and liquid oxygen to power the system.

Recently, engineers in China have integrated the principles of Stirling engines with thermoacoustic technology, resulting in an advanced electricity generator.

A thermoacoustic generator converts thermal energy into sound waves, which are then transformed into electrical power. This technology operates by utilizing a resonator, a porous material stack, and a heat source to facilitate the conversion process.

Heat Source: The system begins with a heat source that creates a temperature differential in a gas or fluid. This can be achieved through various means, including flames or electric heaters, and potentially even nuclear energy.

Acoustic Waves: The temperature gradient generates standing acoustic waves as the gas oscillates between hotter and cooler regions.

Resonator: These waves travel through a resonator, amplifying and resonating the sound.

Porous Stack: The resonating waves then move through a stack made of highly thermally conductive materials, essential for converting acoustic energy back to thermal energy.

Electrical Generation: As these waves pass through the porous stack, they induce physical changes that can be converted back into electrical power using piezoelectric materials or similar methods.

Thermoacoustic generators can be utilized in various applications, such as recovering waste heat, converting solar energy, and generating power in remote locations. Their simplicity, absence of moving parts, and resilience in harsh conditions make them particularly attractive.

Military applications are notable due to their quiet operation, beneficial for submarines and aerospace applications.

NASA has patented a design for a thermoacoustic generator, named the Stirling Thermoacoustic Power Converter and Magnetostrictive Alternator (LEW-TOPS-80). This design features a thermoacoustic engine paired with an alternator for generating electricity in space, though no prototypes or performance data have been disclosed.

Recent Developments

Chinese scientists have reportedly created a working prototype with impressive performance metrics.

> "Chinese researchers have made significant progress in alternative energy with their latest thermoacoustic Stirling generator," reports indicate.

> "This efficient generator runs quietly, making it suitable for silent applications, such as submarines and in the aerospace sector."

In a recent demonstration, this prototype produced an unprecedented 102 kilowatts of power from a heat source of 530 degrees Celsius (986 Fahrenheit), achieving a significant milestone in generator technology.

According to one of the developers interviewed by the South China Morning Post:

> "The current thermoelectric conversion efficiency is approximately 28 percent; with a 600-degree thermal fluid, we could reach 34 percent," he noted.

This efficiency is comparable to that of steam turbines.

> "It operates quietly and efficiently, utilizing various heat sources such as solar energy, waste heat, and biomass," the developer added.

He further explained that high-pressure helium, maintained at about 15 megapascals (approximately 150 atmospheres), serves as the working medium. The generator's design minimizes mechanical parts requiring lubrication, potentially extending its lifespan beyond a decade.

> "The linear motor, driven by sound waves, features a symmetrical design that reduces harmful vibrations," he stated.

> "The motor maintains a minuscule gap, comparable to a human hair's thickness, between the piston and cylinder, ensuring no contact while preserving an airtight environment."

> "This technology represents a promising advancement for solar thermal, biomass energy generation, and distributed energy systems," he concluded.

NASA's Perspective

To better understand the acoustic aspects, I explored NASA's information on their design, known as "the Glenn design."

> "The Glenn design allows transducers to operate at high frequencies, presenting a mass rather than stiffness impedance. The magnetostrictive alternator employs stacked magnetostrictive materials under magnetic and stress-induced compression, enabling acoustic energy to traverse an impedance-matching layer connected to the magnetostrictive mass," their documentation explains.

This mechanism utilizes compression bolts to maintain structural strain, allowing for microscopic compression of the magnetostrictive material and eliminating the need for bearings. The alternating compression creates a magnetic field, inducing electric current in a surrounding coil, effectively generating electricity from the acoustic pressure wave.

External Heat Source Requirement

Solar energy can serve as the heat input for Stirling engines, negating the need for combustion, which often produces harmful gases. In Concentrating Solar Thermal Systems, working fluids can be heated to temperatures approaching 1,000 degrees Celsius.

Stirling engines powered by solar energy are already operational and achieving efficiencies exceeding 30%. The next step would be integrating thermoacoustic generators in place of traditional alternators, although the cost-effectiveness remains to be evaluated.

A Thermoacoustic Generator for My Boat?

I envision needing a generator that produces around 6 kW, fitting my boat and running on diesel. While that may be a distant goal, I currently rely on conventional internal combustion generators manufactured in China. A lifespan of over ten years would certainly be an improvement.

However, it seems these devices will likely remain niche applications, particularly suitable for submarines and spacecraft.