Reversible engines in Stirling Store?

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Reversible engines in Stirling Store?

Postby goodman_jason » Wed Aug 19, 2009 1:07 pm

I'm doing a demo on mechanical and thermal energy for a freshman
college class. Will any of the cheaper Stirling designs at the
Stirling Store on this site work in reverse? I.E., can I spin the
crankshaft with my bare hands or with an electric motor and measure
even a slight temperature drop at the heat exchanger? I've got some
good sensitive digital thermometers, a fraction of a degree would
prove my point.
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"Reversible engines in Stirling Store?"

Postby stan.hornbaker » Wed Aug 19, 2009 1:55 pm

I have both an MM-5 & an MM-7. They run in the reverse direction when the bottom plate is placed on ice cubes. The MM-5 is also available in kit version. Follow assembly directions carefully.

There should easily produce cooling when run by external power. If using an electric motor be sure that is variable speed to avoid damage to the engine. I used a piece of rubber tubing to connect my MM-5 to a variable speed motor. Running the engine backward from the normal direction of running on hot water a perceptible difference in temperature between the top and bottom plates was detected by touch. The top plate cooled and the bottom one warmed. Digital thermometers would have been preferred but not available.
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Response to Reversible engines in Stirling Store?

Postby jthorn001 » Thu Aug 20, 2009 1:34 pm

I wonder what the relative efficiency is?
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Response to Reversible engines in Stirling Store?

Postby stan.hornbaker » Thu Aug 20, 2009 4:30 pm

Quite low. Enough to keep running with a reasonable delta T. These are models and produce very little power. The MM-5 does produce a noticeable breeze from the model air plane propeller.
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Response to Reversible engines in Stirling Store?

Postby goodman_jason » Wed Aug 26, 2009 4:03 pm

To follow up on my original question:

I bought an MM-5 and assembled it today. It works quite well, spinning at about 250 rpm or so on hot water, 180 or so on ice.

I tried taping two digital thermometers to the top and bottom plates, insulating the lower plate by placing it on styrofoam, and driving the crankshaft at 250 rpm using a cordless drill.

After 3 minutes, I was able to measure a temperature difference of 0.22 degrees C. Definitely measureable, but not exactly a practical refrigerator!

Using a little thermodynamics and the laws of expansion of an ideal gas, I think the maximum possible temperature difference is roughly 2 degrees C.
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Response to Reversible engines in Stirling Store?

Postby stan.hornbaker » Thu Aug 27, 2009 7:37 am

Kudos on have achieved the objective of demonstrating the principle of applying mechanical input to a Stirling engine to reverse the flow of energy to produce a cooling effect where thermal energy is usually applied as input.

The success of your class demo is assured.
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