Editing Stirling engine (section)

== Components ==
[[File:BetaStirlingTG4web.svg|thumb|Cut-away diagram of a [[rhombic drive]] beta configuration Stirling engine design:
{{legend|hotpink|1: Hot cylinder wall}}
{{legend|darkgray|2: Cold cylinder wall}}
{{legend|khaki|3: Coolant inlet and outlet pipes}}
{{legend|green|4: Thermal insulation separating the two cylinder ends}}
{{legend|lightgreen|5: [[Displacer]] piston}}
{{legend|royalblue|6: Power piston}}
{{legend|skyblue|7: Linkage crank and flywheels}}
Not shown: Heat source and heat sinks. In this design the displacer piston is constructed without a purpose-built [[#Regenerator|regenerator]].
]]

As a consequence of closed-cycle operation, the heat driving a Stirling engine must be transmitted from a heat source to the working fluid by [[heat exchanger]]s and finally to a [[heat sink]]. A Stirling engine system has at least one heat source, one heat sink and up to five heat exchangers. Some types may combine or dispense with some of these.{{citation needed|date=July 2020}}

=== Heat source ===
[[File:EuroDishSBP front.jpg|thumb|left|Point focus parabolic mirror with Stirling engine at its centre and its [[solar tracker]] at [[Plataforma Solar de Almería]] (PSA) in Spain.]]

The heat source may be provided by the [[combustion]] of a fuel and, since the combustion products do not mix with the working fluid and hence do not come into contact with the internal parts of the engine, a Stirling engine can run on fuels that would damage other engine types' internals, such as [[landfill gas]], which may contain [[siloxane]] that could deposit abrasive [[silicon dioxide]] in conventional engines.<ref name="LGET" />

Other suitable heat sources include [[Concentrated solar power|concentrated solar energy]], [[geothermal energy]], [[nuclear power|nuclear energy]], [[waste heat]] and [[bioenergy]]. If solar power is used as a heat source, regular [[solar mirror]]s and solar dishes may be utilised. The use of [[Fresnel lens]]es and mirrors has also been advocated, for example in planetary surface exploration.<ref name="Brandhorst-2005" /> Solar powered Stirling engines are increasingly popular as they offer an environmentally sound option for producing power while some designs are economically attractive in development projects.<ref name="Kongtragool-2003" />

=== Heat exchangers ===
Designing Stirling engine heat exchangers is a balance between high heat transfer with low [[viscosity|viscous]] [[Darcy–Weisbach equation|pumping losses]], and low dead space (unswept internal volume). Engines that operate at high powers and pressures require that heat exchangers on the hot side be made of alloys that retain considerable strength at high temperatures and that don't corrode or [[creep (deformation)|creep]].{{citation needed|date=July 2020}}

In small, low power engines the heat exchangers may simply consist of the walls of the respective hot and cold chambers, but where larger powers are required a greater surface area is needed to transfer sufficient heat. Typical implementations are internal and external fins or multiple small bore tubes for the hot side, and a cooler using a liquid (like water) for the cool side.{{citation needed|date=July 2020}}

=== Regenerator ===
{{Main|Regenerative heat exchanger}}

In a Stirling engine, the regenerator is an internal heat exchanger and temporary heat store placed between the hot and cold spaces such that the working fluid passes through it first in one direction then the other, taking heat from the fluid in one direction, and returning it in the other. It can be as simple as metal mesh or foam, and benefits from high surface area, high heat capacity, low conductivity and low flow friction.<ref name="e-futures" /> Its function is to retain within the [[thermodynamic system|system]] that heat which would otherwise be exchanged with the environment at temperatures intermediate to the maximum and minimum cycle temperatures,<ref name="Organ-1992-58" /> thus enabling the thermal efficiency of the cycle (though not of any practical engine<ref name="Organ-2014-4" />) to approach the limiting [[Carnot cycle|Carnot]] efficiency.{{citation needed|date=July 2020}}

The primary effect of regeneration in a Stirling engine is to increase the thermal efficiency by 'recycling' internal heat which would otherwise pass through the engine [[Reversible process (thermodynamics)|irreversibly]]. As a secondary effect, increased thermal efficiency yields a higher power output from a given set of hot and cold end heat exchangers. These usually limit the engine's heat throughput. In practice this additional power may not be fully realized as the additional "dead space" (unswept volume) and pumping loss inherent in practical regenerators reduces the potential efficiency gains from regeneration.{{citation needed|date=July 2020}}

The design challenge for a Stirling engine regenerator is to provide sufficient heat transfer capacity without introducing too much additional internal volume ('dead space') or flow resistance. These inherent design conflicts are one of many factors that limit the efficiency of practical Stirling engines. A typical design is a stack of fine metal [[wire]] [[mesh]]es, with low [[porosity]] to reduce dead space, and with the wire axes [[perpendicular]] to the gas flow to reduce conduction in that direction and to maximize convective heat transfer.<ref name="Hirata-1998" />

The regenerator is the key component invented by [[Robert Stirling]], and its presence distinguishes a true Stirling engine from any other closed-cycle [[hot air engine]]. Many small 'toy' Stirling engines, particularly low-temperature difference (LTD) types, do not have a distinct regenerator component and might be considered hot air engines; however, a small amount of regeneration is provided by the surface of the displacer itself and the nearby cylinder wall, or similarly the passage connecting the hot and cold cylinders of an alpha configuration engine.{{citation needed|date=July 2020}}

=== Heat sink ===
The larger the temperature difference between the hot and cold sections of a Stirling engine, the greater the engine's efficiency. The heat sink is typically the environment the engine operates in, at ambient temperature. In the case of medium- to high-power engines, a [[radiator]] is required to transfer the heat from the engine to the ambient air. Marine engines have the advantage of using cool ambient sea, lake, or river water, which is typically cooler than ambient air. In the case of combined heat and power systems, the engine's cooling water is used directly or indirectly for heating purposes, raising efficiency.{{citation needed|date=July 2020}}

Alternatively, heat may be supplied at ambient temperature and the heat sink maintained at a lower temperature by such means as [[cryogen|cryogenic fluid]] (see [[Liquid nitrogen economy]]) or iced water.{{citation needed|date=July 2020}}

=== Displacer ===
{{redirect|Displacer}}
The displacer is a special-purpose [[piston]], used in Beta and Gamma type Stirling engines, to move the working gas back and forth between the hot and cold heat exchangers. Depending on the type of engine design, the displacer may or may not be sealed to the cylinder; i.e., it may be a loose fit within the cylinder, allowing the working gas to pass around it as it moves to occupy the part of the cylinder beyond. The Alpha type engine has a high stress on the hot side, that's why so few inventors started to use a hybrid piston for that side. The hybrid piston has a sealed part as a normal Alpha type engine, but it has a connected displacer part with smaller diameter as the cylinder around that. The compression ratio is a bit smaller than in the original Alpha type engines, but the stress factor is pretty low on the sealed parts.{{citation needed|date=November 2020}}