Best scales for Stirling? & Why do coal fired plants use steam?

[dank___6823]

Aren't most coal and oil fired electric plants steam powered? What
advantage does steam have over Stirling at this scale, and is it a
matter of scale?

Is there an intermediate scale at which Stirling is most competative?

[stan.hornbaker]

It is a matter of economics. Coal, gas, and oil fired steam plants are most cost effective. The largest commercially available CHP unit generates 1.2 KW @240 V. It is for special purpose application.
http://www.whispergen.com/main/products/
There is also a 35 KW research unit @ http://www.vok.lth.se/~ce/Research/stir ... _TA2_5.pdf
The Stirling Engine concept is deceivingly simple. Yet has consumed many hours of research and development to achieve meagre results.
John Erricsson's Stirling powerd ship of circa 1850 had 2 power cylinders of 14 ft. dia. X 6 ft. stroke and moved very slowly.
2xScale doubles linear dimesions, squares areaa, and cubes volume. Sn engine that runs at a small size simply cannot be made in a much larger scale to run at all. Materials for the larger size will not meet the requiremenst.
There is an ongoing Solar Powered Stirling Project described at: http://www.stirlingenergy.com/whatisastirlingengine.htm
For a good introduction to Stirling Power look for the book at the Stirling Store elsewhere on this site.

[d.jinkerson]

2xScale doubles linear dimesions, squares areaa, and cubes volume is what you have said but...

2 times Scale doubles linear, areas are * 4 and volumes are * 8.

I am quite sure

[stan.hornbaker]

You are correct. That is exactly what I said, perhaps badly stated;
2 x Scale is linear 2x, area 2^2=4, and volume 2^3=9

3.5 x Scale is linear 3.5x, area (3.5)^2=12.5, and volume (3.5)^3=42.87

You square or cube the scaleup factor to find relative areas and volumes.

[scottthomas]

Wow! Let's test that.

Assuming we start with a 4 inch stroke and 4 inch bore our area is 12.57 and volume is 50.27.

Now let's make it 8 by 8. Our area now becomes 50.27 which is 4 times original. Squaring the original would be 157.91! Clearly not correct. Volume becomes 402.13 which is 8 times original. Cubing the original would be 127,035.96! Also not correct!

[scottthomas]

OK. I re-read your post. Squaring and cubing the scale-up factor (in this case 2) would work.

Referencing the assertion that scaling an engine won't work, it would work assuming you compensate for the larger volume of gas that must be heated. Certainly not impossible.

[stan.hornbaker]

Scaling up a design by a factor of 2x increases the area available for heat transfer by 2^2 or 4 and the volume 2^3 or 8. The energy to heat the air thus increases dramatically. Heat transfer involves surface to gas film factors and turbulance of the gas for efficient heating.

A scaled up Stirling engine thus has to be completely redesigned to accomocate changing conditions. As far as I know the most efficient scale for a particular general design is a problem that has not been addressed.

You might find the Erricson side wheeler ship built by John Erricson,
Launced in Sept. 1852 of interest. It was a caloric or hot air engine. It had four cylinders equipped with cast (iron?) pistons 137" dia. and a stroke of six feet, running at 6 rpm. The pistons moved a slow pace such that guests on the trial voyage rode up and down on them. For more details refer to "Hor Air Caloric and Stirling Engines" by Bob Sier, ISBN No. 0 9526417 12, 1999.

It should be noted that most of the Stirling engines being built for market are modest in size and built, for specific applications, i.e. niche markets. There are NO general purpose machines being built/sold.

Also note that scaling down works to advantage. Some very small Low Temperature Difference Stirling engines have been built to run on the heat of your hand. Some are really tiny, such as a 10 to 20 mm dia. displacer chamber. A job for a jeweler or hi tech model maker!