Stirling engine

More specifically, the Stirling engine is a closed-cycle regenerative heat engine, with a permanent gaseous working fluid. Closed-cycle, in this context, means a thermodynamic system in which the working fluid is permanently contained within the system. Regenerative describes the use of a specific type of internal heat exchanger and thermal store, known as the regenerator. Strictly speaking, the inclusion of the regenerator is what differentiates a Stirling engine from other closed-cycle hot air engines.

In the Stirling engine, a working fluid (e.g. air) is heated by energy supplied from outside the engine's interior space (cylinder). As the fluid expands, mechanical work is extracted by a piston, which is coupled to a displacer. The displacer moves the working fluid to a different location within the engine, where it is cooled, which creates a partial vacuum at the working cylinder, and more mechanical work is extracted. The displacer moves the cooled fluid back to the hot part of the engine, and the cycle continues.

A unique feature is the regenerator, which acts as a temporary heat store by retaining heat within the machine rather than dumping it into the heat sink, thereby increasing its efficiency.

The heat is supplied from the outside, so the hot area of the engine can be warmed with any external heat source. Similarly, the cooler part of the engine can be maintained by an external heat sink, such as running water or air flow. The gas is permanently retained in the engine, allowing a gas with the most-suitable properties to be used, such as helium or hydrogen. There are no intake and no exhaust gas flows so the machine is practically silent. The machine is reversible so that if the shaft is turned by an external power source a temperature difference will develop across the machine; in this way it acts as a heat pump.

The Stirling engine was invented by Scotsman Robert Stirling in 1816 as an industrial prime mover to rival the steam engine, and its practical use was largely confined to low-power domestic applications for over a century.

Contemporary investment in renewable energy, especially solar energy, has given rise to its application within concentrated solar power and as a heat pump.

History

Invention and 19th-century development

The Stirling engine was invented by Scottish clergyman Robert Stirling, who patented his air engine in 1816. Stirling's patent also described a heat exchanger that he called an "economiser". In a Stirling engine this component stores heat from the working gas during one part of the cycle and returns it during another; it later became known as the regenerator.

An engine built to Stirling's design was used in 1818 to pump water at a quarry in Ayrshire. Robert Stirling and his brother James Stirling subsequently developed improved versions of the engine. Their work included further patents and the use of pressurised working gas to increase power output.

By the early 1840s, James Stirling had constructed a larger engine for a foundry in Dundee. In 1845 he presented a description of the engine to the Institution of Civil Engineers. The Dundee installation demonstrated that the design could provide industrial-scale power, but repeated failures of components exposed to high temperatures contributed to its eventual replacement by a steam engine.

After the large Dundee engine was abandoned, Stirling-type and other hot air engines continued to be developed mainly for lower-power applications. From about the 1860s, small hot-air engines were produced for uses including water pumping and supplying air to church organs. Their relatively simple operation was an advantage where the supervision required by contemporary steam engines was undesirable.

By the beginning of the twentieth century, electric motors and small internal combustion engines increasingly replaced hot-air engines in many of these applications. By the late 1930s, Stirling engines survived mainly in specialised small-scale uses until renewed development work began at Philips.

20th-century revival

Philips began investigating Stirling engines in the late 1930s while looking for a small generator capable of powering radio equipment in places without reliable electricity supplies. Engineers at the company's research laboratory in Eindhoven selected the Stirling engine partly because it could operate quietly and could use several heat sources.

By 1951 Philips had developed the MP1002CA generator set, commonly called the "Bungalow set". The unit produced about 180–200 watts of electrical power. Approximately 150 units were manufactured, but falling power requirements for transistor radios and the cost of producing the generator limited its commercial prospects.

Philips continued Stirling-engine research for several decades. Although its prime-mover programmes did not achieve large-scale commercial adoption, the company's work contributed substantially to modern Stirling-engine technology, and Stirling-cycle cryocoolers became a commercially important application.

Submarine propulsion

Stirling engines have been used as air-independent propulsion (AIP) systems in conventionally powered submarines. The Swedish Navy began sea trials of a Stirling AIP system aboard the submarine Näcken in the late 1980s, and the system was subsequently incorporated into the Gotland class.

The Gotland-class submarines were delivered in 1995–1996 with Stirling AIP installed from the outset. Their engines use liquid oxygen so that the combustion process does not require atmospheric air, allowing the submarines to remain submerged for extended periods.

Stirling AIP was subsequently used in other Swedish submarines and in Singapore's Archer class. Japanese Sōryū-class submarines were also built with Kawasaki Kockums Stirling engines; Kawasaki stated that the system increased their underwater endurance.

21st-century developments

Stirling engines have been developed for dish-based concentrated solar power systems. In a dish/Stirling system, a concentrating reflector focuses solar radiation onto a thermal receiver, which supplies heat to a Stirling engine connected to an electrical generator.

Stirling engines have also been investigated for residential micro combined heat and power systems, in which the engine produces electricity while heat rejected from the system is recovered for space heating or hot water. A Canadian residential demonstration tested a natural-gas Stirling micro-CHP unit during the winter and spring of 2003.

Research has also examined the miniaturisation of free-piston Stirling engines. A 2013 study developed scaling laws based on six dimensionless groups to investigate the effects of reducing engine size on power density, heat transfer, leakage and other losses.

In 2025, Tristan J. Deppe and Jeremy N. Munday demonstrated mechanical power generation using a Stirling engine driven by a temperature difference produced through radiative cooling to the sky. Their experiments also demonstrated airflow sufficient for applications including greenhouse circulation and building ventilation.