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Stirling Engine

The Stirling engine is a heat engine that uses the cyclic compression and expansion of gas to convert heat to mechanical energy. Far from being a new device, the Stirling engine was actually developed in 1816 and rivaled the steam engine for a short period of time. The Stirling engine fell out of popularity in the 1860s as a result of the need to operate at high temperatures that led to frequent failure. The revival of the Stirling engine came about as concentrated solar power developed in the late 20th century. Concentrated solar power does not require high operating temperatures.

History of the Stirling Engine

Robert Stirling patented the engine that bears his name in 1816. It was developed as a safer, cheaper, less fuel hungry alternative to the steam engines of the day that used boilers.

The largest installation of a Stirling engine was in Dundee, Michigan at an iron foundry. Unfortunately, the need for the engine to run at very high temperatures demonstrated deficiencies in the materials available at the time for building such engines. The Dundee plant suffered three cylinder failures in their engine in four years, forcing them to replace the Stirling engine with a traditional boiler system.

By 1930, the Stirling engine was generally forgotten. It found application in a few places, such as children’s toys and in church organs. The expense of the system compared to boilers and the inability to run Stirling engines at high temperatures limited their application to only small energy needs.

Function of the Stirling Engine

The Stirling engine is an external combustion engine because heat is generated outside of the “working fluid.” Working fluid simply refers to the gas or liquid that expands and contracts to drive a machine. In most internal combustion engines, that gas is air. In steam engines, the gas is water vapor. In Stirling engines the gas is most commonly air, hydrogen, or helium.

The engine operates on a simple four step process that relies on the differences in temperature between a hot and cold portion of the engine to operate. To understand how the engine operates, it is useful to see it diagramed in its most basic form.

The alpha type Stirling engine is the simplest. There are also beta and gamma configurations. The operating principles are the same even though the geometry of the engines differs. In the alpha configuration there are two pistons, one hot and one cold. Both pistons are connected to a common crankshaft and share a single, seal chamber that contains the working fluid.

Stirling engine

The operation of the Stirling engine follows the principle of isothermal expansion.:

1. The expansion space is kept at constant high temperature and the gas in this space (hot side) undergoes isothermal expansion. This pushes the hot side piston out (to the left in the above diagram) and drives the crank shaft.

2. The heated working fluid, which is now at maximum volume, is moved into the cold piston, pushing it up and providing more energy to the crank shaft. The cold piston is 90 degrees BEHIND the hot piston in terms of cycle.

3. The gas is now mostly in the cold cylinder and is cooling and reaching minimal volume as both pistons comes to rest at the bottom of their respective chambers.

4. The small amount of gas in the hot cylinder will now begin to expand again and the cycle will repeat.

Resurgence in Popularity

The ability of the Stirling engine to use an external heat source makes it an ideal engine for concentrated solar power. The heat energy from a parabolic mirror can be reflected onto the hot cylinder and drive the engine. By turning a turbine, the engine can produce electricity.

Work on advancing the materials used in Stirling engines in the 1970s was carried out by Phillips and taken over by Infinia Corporation in 1986. Developments from both companies led to the production of a greatly simplified, though more sophisticated, version of the Stirling engine called free piston. These engines have lifetimes of 20+ years and a reduced number of moving parts compared to the original Stirling design.

Applications to Solar

A parabolic mirror with a Stirling engine placed at its focus is currently more efficient in producing electricity that a photovoltaic cell and is approximately the same in terms of efficiency as a concentrated photovoltaic system. 

There is currently a plant in Arizona capable of producing 1.5 MW in full sun using parabolic reflectors and Stirling engines. There is also a plant in southern California that will produce 850 MW over a 20 year timeframe.

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