A Stirling engine is a device that takes energy from a heat source as an input and outputs mechanical energy. The pistons of a Stirling engine are generally connected to a flywheel to provide smooth motion and energy delivery.
The mechanical energy can be used to turn a drive shaft and drive machinery directly, or to drive a generator and create electricity. Stirling engines can achieve up to 40% efficiency, making them more attractive in some applications than other power generation methods.
History of the Stirling Engine
The Stirling engine concept is named for inventor Robert Stirling, who patented the concept for the engine in 1816, although some work in the development of so-called "air engines" had been performed previously.
Stirling's original engine was initially used to pump water in a quarry. Stirling and his brother James continued to refine the design and patent their improvements through 1843. At that time, their engine was used to power all of the machinery at an iron foundry.
Stirling Engine Functionality
As stated in the book "Stirling Engines" by G. Walker, "A Stirling engine is a mechanical device which operates on a *closed* regenerative thermodynamic cycle, with cyclic compression and expansion of the working fluid at different temperature levels." Historically, the working gas in a Stirling engine was air, however hydrogen and helium have also been used. Like most engine types, a Stirling engine moves the working fluid through four phases: cooling, compression, heating, and expansion.
The working fluid is passed back and forth through a hot heat exchanger and a cold heat exchanger. The hot heat exchanger is situated near the input heat source, and the cold heat exchanger is situated away from the heat source. The change in temperature of the working fluid by the input heat sources causes pressure changes, resulting in movements of the pistons. One of the figures below outlines the thermodynamic Stirling cycle.
Stirling Engine Designs
Over time, the design of the Stirling engine concept has been refined to reflect changes in technology and efficiencies.
- The original Stirling engine design, dubbed "alpha", used two separate pistons in two separate cylinders. The two pistons are connected to a single flywheel.
- The next concept, dubbed "beta", used two separate pistons in a single cylinder, with the lower part of the cylinder, containing the first piston, in contact with the heat source and the upper part of the cylinder, containing the second piston, kept at a lower temperature through coolant. The two pistons are connected to a single flywheel.
- A "gamma" Stirling engine design is similar to the beta design, except each piston is in an adjacent cylinder while keeping one consistent unit of working fluid between them. This design is less complex than the beta, but results in a lower compression ratio.
Other prototype design concepts have been explored, including a rotary Stirling engine, but none have entered into widespread use.
Categorizing the Stirling Engine
The Stirling engine is categorized as an "external combustion" engine because the input source is considered to be external to and separated from the working fluid of the engine itself. This is in opposition to "internal combustion" engines, in which the input combustion occurs within the engine mechanism. The original Stirling engines were coal fired, but theoretically any heat source can be used as an input.
The Stirling engine design can also be categorized as a reciprocating piston engine, which does give it some commonality with internal combustion engines.
The Stirling Engine and Solar Thermal Power
Stirling engine usage has seen a resurgence beginning in the late 20th century due to the application of solar thermal power. Solar thermal power plants use concentrated solar rays to heat a working fluid, generally molten sodium in large utility solar plants. The heated working fluid in turn drives a Stirling engine to generate electricity.
According to the CRC Handbook of Mechanical Engineering, "the Stirling engine is considered to be the least expensive alternative for solar energy electric generation applications in the range from 1 kWe to 100 MWe."
Susan Kristoff - Susan Kristoff is mechanical engineer by trade, but has a diverse set of professional and personal interests. The glue that binds all of ...
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