Stirling engine

The Stirling engine is a closed-cycle regenerative heat engine. It uses a fixed amount of gas (air, helium, hydrogen, or nitrogen) that is alternately heated and cooled, causing pressure changes that move a piston. The key components are a hot heat exchanger, a cold heat exchanger, and a regenerator that stores heat during the cycle. The ideal Stirling cycle consists of four processes: isothermal compression, constant-volume heat addition (regenerator), isothermal expansion, and constant-volume heat removal. The engine's theoretical efficiency equals the Carnot efficiency, making it potentially the most efficient heat engine.

Robert Stirling, a Scottish minister, patented the engine in 1816 as an alternative to steam engines, which were prone to boiler explosions. His original design used a 'heat economiser' (now called a regenerator) to improve efficiency. In the 19th century, Stirling engines were used in small applications, but they were overshadowed by steam and internal combustion engines. In the 20th century, Philips (the Dutch electronics company) revived interest in the 1930s for portable power generation, and later for cryocoolers. Modern developments include the use in solar power generation and underwater vehicles.

There are three main configurations: alpha, beta, and gamma. The alpha type has two separate pistons in separate cylinders, one hot and one cold. The beta type has a single cylinder with a displacer and a power piston. The gamma type has two cylinders, one for the displacer and one for the power piston, connected in series. Each configuration has its own advantages in terms of mechanical simplicity, power density, and sealing. Additionally, there are free-piston Stirling engines, which eliminate mechanical linkages and use gas bearings or flexures.

Stirling engines are used in niche applications where their unique characteristics are advantageous. They are used in cryocoolers for infrared sensors and superconducting electronics, in submarine propulsion (e.g., the Swedish Gotland-class submarines), in solar power generation (dish-Stirling systems), and in combined heat and power (CHP) systems. They are also used in educational demonstrations and in some model engines. Recently, they have been proposed for space power generation using radioisotope heat sources.

Advantages include high efficiency, quiet operation (no exhaust valves), multi-fuel capability (any external heat source), low emissions, and long maintenance intervals. Disadvantages include high cost, complex sealing, large size and weight for a given power, slow response to power changes, and difficulty in varying power output. The need for high-temperature materials and effective heat exchangers also poses engineering challenges.

One lesser-known fact is that the Stirling engine was used in the 19th century to power small fans and water pumps, and even in some early refrigerators. Another is that the regenerator concept was later applied to gas turbines and other heat engines. The engine also has a connection to the development of the first practical heat pump. In the 1970s, NASA funded research on Stirling engines for space power, leading to the development of the 'Space Power Demonstrator Engine'. Additionally, there is a variant called the 'Fluidyne' engine that uses liquid pistons, which was developed in the 1980s for low-cost applications. The engine's quiet operation has made it attractive for stealthy military applications, such as in the Swedish submarines.

The Stirling engine is often confused with the hot air engine, but it is distinct due to the regenerator. The regenerator is a porous material that absorbs heat from the gas as it passes from the hot to cold side, and releases it on the return, improving efficiency. The engine can be reversed to act as a heat pump or refrigerator, known as a Stirling cooler. The theoretical efficiency is the Carnot efficiency, but real engines achieve about 30% due to losses. The engine is also known for its 'external combustion' nature, meaning the heat source is external, allowing it to run on solar, biomass, or nuclear heat.

Regenerator

A heat exchanger that stores heat from the working gas and returns it later in the cycle, improving efficiency.

Displacer

A component that moves the working gas between hot and cold spaces without changing the total volume.

Working fluid

The gas (e.g., air, helium) that is cyclically compressed and expanded to produce work.

Carnot efficiency

The maximum theoretical efficiency of a heat engine operating between two temperatures.