Making Sense of Science
   
Jan 2011
 
 
The Reverend Robert Stirling
The Rev. Robert Stirling

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The Secret Life of the Stirling Engine 

Happy New Year! I can report that I am very excited about what this year will bring.  We begin, however, with a glance into the past with this special Video Edition of Making Sense of Science.  As the product of a bygone era overtaken by history and technology, the Stirling Engine has spent most of its existence in obscurity in the science classroom as an instructive curiosity.  In recent times however, it has seen something of a revival as people search desperately for solutions to the conflict between the expanding energy needs of the world's population and the long-term consequences of energy use.

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Though it has been around for nearly 200 years, the Stirling Engine has never made much of an economic impact on the world.  There is a reason for this, which a scientific approach quickly reveals.  To view the video, click on this link:

The Secret Life of the Stirling Engine (10 min 28 sec)

There is something about the way that a Stirling Engine converts fire into mechanical motion that fascinates both amateur tinkerers and professional engineers alike (who really ought to know better).  I too confess to having been bitten by the Stirling bug in the past, and have built a number of experimental engines while contemplating their improbable commercial application. 

As if by magic the Stirling Engine seems to violate common sense and the 2nd Law of Thermodynamics by apparently reversing the natural flow of energy.  That flow is always from concentrated forms (like mechanical motion) ever towards diffuse forms (like heat), and never the other way around.  Actually, the Stirling Engine and other devices (including refrigeration) fully comply with natural law by speeding up the diffusion of heat energy while reversing the flow of just a small proportion of it.   

While most proponents of the Stirling Engine focus their attention on thermodynamic efficiency (the amount of mechanical energy recovered compared to the amount of heat energy supplied), the engine suffers from problems of much greater significance than efficiency.

I know I will be bombarded with emails from Stirling Engine advocates pointing out "such and such" advantage or "this and that" recent development.  But none of that changes the fact that for their size, weight, displacement, cost per capacity, service life, or by any other measure, Stirling Engines simply underperform conventional engine and turbine technology.  Efficiency alone just doesn't matter as much as overall Return On Investment.

Of course the model Stirling Engine used in the video is not optimised for performance, but for simplicity.  By their nature, however, all Stirling Engines operate at relatively low internal pressures and compression ratios, and this is the crux of the matter.  Low pressure means low specific work and low power output. 

In contrast, Rudolf Diesel, a keen student of Sadi Carnot (the father of thermodynamics theory), understood this point very well and made full use of it in the design of his engine.  Diesel's engine has excellent characteristic Return On Investment as evidenced by its widespread utilization in industry.

By way of comparison, the model Stirling engine shown in the video weighs about 330g without the base and produces around 0.012W of power.  A 330g model aircraft engine can be expected to produce nearly 1000W of power.  This means one model internal combustion engine can replace 80,000 model Stirling engines of the same weight.  Surely, some improvements can be made, you say.  I quite agree.  However, an 80,000-fold improvement is a tall order, certainly costing millions in R&D. 

Undeterred, advocates of the Stirling Engine have spent many millions of dollars over the decades on improved Stirling Engine designs.  A design called a Free Piston Stirling Engine (because it has no crankshaft) is contained in a sealed, pressurized vessel and generates electricity when heated by sunlight concentrated by an array of motorized mirrors.  Will it work?  I am sure that it will.  I am equally sure that it will be ridiculously expensive for the amount of electricity ultimately produced. 

As a final note, we sometimes hear about engines that run entirely on rubbish, garbage or waste of various sorts.  Well, here's a new variation: an engine that is MADE entirely from rubbish (1 min 22 sec).  Wow!

Best Regards,

John

 
 
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