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Stirling engine
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=== 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}}
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