STIRLING ENGINES

By:  David Saffer

History:

            Reverend Robert Stirling from Cloag, Scotland invented the Stirling Engine in 1816 (Origins).  He invented the engine because he feared for the safety of his parishioners who worked around steam engines (American).  Due to the lack of strength in the materials available to construct boilers (steel or Bessemer Iron was not yet invented), they would frequently explode with devastating results to the people working nearby (Origins).  Stirling engines would not explode because they did not use the high pressures that the steam engines did.  Stirling engines never really caught on.  Materials the boilers were constructed from became stronger and coal became cheap.  The best-recorded effort to use a Stirling engine was at the Dundee Foundry Company.  Robert’s brother James Sterling worked here.  James was a good engineer and driving force to getting the engine implemented.  The engine powered the foundry for a number of years, but material failures at inopportune times caused it to be replaced by a steam engine (Origins).

How Stirling Engines Work:

            The theoretical Stirling cycle is comprised of four different thermodynamic processes (Stirling).  Below is the step-by-step process of one stroke of the Stirling cycle (Diagrams below taken from www.sesusa.org/stir_aus.htm).

            The first step is isothermal compression.  In this step the cold piston moves to its intermediate position and compresses the gas in the chamber.  Any heat that is created during this is removed at the cold sink temperature, Tc (Stirling).

 

            Step two is constant volume heating. The cold piston moves to its maximum position or Cmax while the hot piston moves to its intermediate position or Hi.  The gas from the cold chamber passes through the regenerator and regains heat from the previous cycle and begins to heat up to a temperature greater than Tc.  Since the volume is constant the temperature increase causes the gas begins to expand (Stirling). 

            Step three is isothermal expansion.  The gas is heated at Th and expands.  This expansion forces the hot piston down to its minimum position of Hmin­ (Stirling).

            Step four is constant volume cooling.  The hot piston moves back to its Hmax position while the cold piston simultaneously moves down to it Cmin position.  The regenerator accumulates the heat from the gas while it passes back through to the cold chamber.  The loss of heat reduces the pressure of the gas, as the volume is kept constant.  The heat is stored in the regenerator until the next cycle. 

           When an engine is made a fixed volume of gas, usually helium or hydrogen is sealed inside the engine.  This gas is never used up and is always the same volume with only the pressure changing.  It is called a working gas since it is never consumed.  Stirling engines are external combustion engines since the heat source is applied and used on the outside of the engine.

The key to making the engine work is the regenerator.  This saves heat that would other wise be lost and puts it back to use.  It helps to preheat the air coming into the hot chamber and to cool down the air coming into the cold chamber.

Theory:

            Sterling engines derive work from the thermal expansion and contraction of a gas.  This can be seen using the ideal gas law, PV=nRT (Stirling).

            Frenchman Sadi Carnot, in the mid 1800s, figured out the maximum efficiency possible with any heat engine.  The formula is (TH – TC)/TH (American).  Using this formula you can find out the efficiency of a heat engine.  Stirling engines have been found to be between 25-50% efficient.

            An impressive fact about Stirling engines is that they were invited before an understanding of thermodynamics and the writings of Carnot (Origins).

Applications:

            There has been a revival of sorts for the Stirling engine as of late with the push of the green movement and finding environmentally friendly forms of energy.  Because they do not combust gas, they give off no pollution, only the pollution of the external heat source.  Because of this they are very flexible with the fuel that can be used.  Gas, propane, diesel, wood, alcohol, kerosene, biomass, and solar power are just a few of the options.

            Stirling engines today are used for various things.  Some research submarines use them to power themselves.  Stirling engines can be used to cool things down to low temperatures.  A Stirling engine is a reversible device.  It turns heat into mechanical work, but if you put in mechanical work you get one hot end and one very cold end.  Some have gotten as cold as below 10 degrees Kelvin (American).  These are used as coolers and refrigerator pumps.

            Because Stirling engines are very quiet, they are sometimes used as yacht generator and many places that require quiet power.

            There is work being done to use them to make electricity by using solar power.  By using solar light and heat as the heat source to heat up the engine and connecting the engine to a generator, you can make pollution free electricity.

            Scientists at the Las Alamos National Laboratory have developed a thermoacoustic Stirling engine with no moving parts.  The engine is made out of inexpensive steel pipe and is very low-tech.  This engine turns heat into acoustic sound waves that can be used to in acoustically powered refrigerators or to produce electricity.  It produces sound waves so powerful that Greg Swift said, "If you were in that wave, permanent hearing loss would be the least of your problems. It's loud enough to set your hair on fire (Lemley)."  Even though the engine is that powerful it runs quieter than an idling car. 

    They are still trying to figure out what they can do with this technology.  One idea is that they can recaptured natural gas that is now just burned off at oil rigs and liquefy it and use.  Another idea is to use it in homes to recapture unused heat from things such as hot-water heaters and turn it into electricity (Lemley).

This low-tech technology is just beginning to take off and has the potential to change the world as we know it now.     

Click here to return to Home Page


      WORKS CITED

 

American Stirling Company.  “Frequently Asked Questions.” 10 Oct. 2001

            http://www.stirlingengine.com/FAQ.asp

 

Lemley, Brad.  “Future Tech:  Thunder in a Bottle.”  Discover Vol. 21 No. 5 (May 2000)

             10 Oct. 2001 http://www.discover.com/may_00/featfuture.html

 

“Origins of the Stirling Heat Engine.”  10 Oct. 2001 

            http://www.stirlinghotairengine.com/history.htm

 

Stirling Engine Society-USA.  “Operation of the Stirling Engine.”  10 Oct. 2001

            http://www.sesusa.org/stir_aus.htm