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PowerPedia:Stirling Engine

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The Stirling engine was invented in 1816 by the Rev. Robert Stirling who sought to create a safer alternative to the steam engines of the time, whose boilers often exploded due to the high pressure of the steam and the primitive materials of the time.

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Description

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"How Stirling Engines Operate Work"
Image from www.steamengine.com.au

Stirling engines convert any temperature differential directly to movement: they use a displacer piston to move enclosed air back and forth between cold and hot reservoirs. At the hot reservoir, the air expands and pushes a power piston, producing work and displacing the air to the cold reservoir. There the air contracts and pulls the power piston, closing the cycle.

In Stirling engines a "regenerator," typically a mesh of wire, is located between the reservoirs. As the air cycles between the hot and cold sides, its heat is transferred to and from the regenerator. In some designs, the displacer piston is itself the regenerator.

Stirling engines operate as a Carnot heat engine and have higher thermodynamic efficiency than steam engines (or even some modern internal combustion and Diesel engines).

Stirling engines can also work in reverse: when applying motion, a temperature differential appears between the reservoirs. Incidentally, one of their modern uses is in cryogenics.

Problems with Stirling engines

  • Stirling engines are difficult to construct and require precise machining, thus making them both more expensive and much slower to produce.
  • Stirling engines often have much less power output than other types of engines of similar size.
  • Stirling engines, especially the type that run on small temperature differentials, are quite large for the amount of power that they produce.
  • A "pure" Stirling engine cannot start instantly; it literally needs to "warm up".

Stirling engine types

Stirling Engines come in three distinct types:

  • An alpha Stirling contains two separate power pistons, one "hot" piston ring and one “cold" piston. The hot piston is situated after the higher temperature heat exchanger and the cold piston is situated after the low temperature heat exchanger. This type of engine has a very high power-to-volume ratio but has technical problems due to the usually high temperature of the "hot" piston and its seals.
  • A beta Stirling has a single power piston arranged coaxially with a displacer piston. The displacer piston does not extract any power from the expanding gas but only serves to shuttle the working gas from the hot heat exchanger to the cold heat exchanger. This engine does not require moving seals in the hot portion of the engine and can achieve high compression ratios due to pistons being able to overlap in their motions.
  • A gamma Stirling is simply a beta Stirling in which the power piston is not mounted coaxially to its displacer piston. This configuration produces a lower compression ratio but is often mechanically simpler and often used in multi-cylinder Stirling engines.

Re-emergence of Stirling Engine Technology

The Stirling Engine was originally designed to deal with a 19th Century problem; the fact that steam engines had a tendency to explode due to their need for high-pressures to operate. The Stirling Engine solved this problem by converting heat directly into movement. Stirling Engines lost favor in the 20th Century as they were overshadowed by new engine technologies such as the internal combustion engine and the electric motor.

However, the Stirling Engine is experiencing somewhat of a renaissance in the early 21st Century. New designs are more efficient and capable of safely storing more efficient internal gases to run the engine. One unexpected new use for the Stirling Engine is to use them in combination with solar technology to generate electricity from the temperature differential created by concentrating hot solar power on a Stirling Engine. The following article explains how the Solar/Stirling Engine design works and plans for future deployment of this technology to create utility scale electricity.

The Stirling Engine Used to Capture Waste Heat

The Stirling Engine can be used to capture waste heat from industrial processes and power generation, to produce electricity and make an operation more energy efficient. For more information a see the Advanced Power Generation Directory at PESWiki.

References and further reading

  • Van Wylan, Gordon J. and Sontag, Richard F. , "Fundamentals of Classical Thermodynamics SI Version 2nd Ed.", John Wiley and Sons, New York, 1976, ISBN 0471041882
  • Walker, G. , "Free Piston Stirling Cycle Engines", Springer-Verlag (1985), ISBN 0387154957
  • Hargreaves, C. M., "The Philips Stirling Engine", Elsevier Publishers, (1991), ISBN 0444884637
  • Wikipedia contributors, Stirling Engine, Wikipedia.

See Also


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