> When you talk of a regenerator on the Stirling Engine, what kind of
> thing are you talking about? What materials do you use?


The "Regenerator", which is arguably the heart of any serious Stirling 
Engine, is a thermal store interposed between the hot and cold spaces. 
Gas flows through it from cold to hot and expands thereby raising 
overall system pressure in all spaces, or from hot to cold and 
contracts and thereby lowers the overall system pressure in all 
spaces.

The regenerator is ideally a matrix with high thermal capacity per 
volume, high to infinite resistance to thermal "heat" flow in the 
longitudinal direction (ie in the direction of gas flow) and high 
thermal conductivity in the transverse direction. It has ideally zero 
dead space and no pneumatic resistance. The best material is 
nonexisteum.

Practical alternatives are wire screens packed together under 
pressure. These can be sintered together to make a rigid block with 
appropriate properties. . At low temperatures (more for Stirling 
coolers) material such as lead shot has been used. With wire screens 
heat can flow across each screen through the wires but less easily 
from screen to screen as they are not in total contact at all points 
and points of contact are not necessarily optimum for heat transfer. 
Lead shot is used in Stirling cryo coolers as lead retains more 
thermal capacity at cryogenic temperatures than almost any other 
substance. Other unlikely substances such as cotton wool have been 
used successfully in some regenerators.

Imagine a cylinder filled with hundreds of circular screens each a 
good fit to the circular cross section. These are pressed in tightly 
and end caps fitted. Place a "hot space" at one end and a "cold space" 
at the other. Blow gas from hot to cold via the holes in the screens. 
The first screen will be heated to the temperature of the hot gas and 
there gas will be slightly cooled. The next screen will be slightly 
cooler and this effect will progress with the temperature of each 
dropping and the gas being cooled. If properly designed the gas will 
enter the cool end totally cooled to the cool end temperature. Now 
reverse the flow. Cold gas entering the screen pack will encounter 
increasingly warm screens and will get hotter as it flows towards the 
hot end. If an equal quantity of gas flows from cold to hot as flowed 
from hot to cold then it will enter the hot space at about the same 
temperature as the other gas left. The screen temperatures will be 
decreased. If the thermal capacity of the screens is several ties or 
more of the thermal content of one charge of gas then the screen 
temperatures will not vary too widely during each cycle.

If work is done at the cold end by expanding he gas to drive a piston 
then a net flow of "coolth" will flow up the regenerator and the hot 
end must supply a net downwards flow of heat energy to balance this. 
But overall the energy stored in the regenerator is several times at 
least the energy flow per piston cycle.

If the regenerator has "dead space" in it then system pressure 
increases which are intended to drive the piston will instead go to 
increasing the pressure in these dead spaces. So a R will ideally have 
zero dead space. However, as R "dead space" falls the air passages get 
smaller and pneumatic resistances rise and losses increase. As with 
most aspects of Stirling engines, actual dead space is a compromise. 
Dead space also occurs in the hot and cold ends when the "displacer" 
which sweeps the gas into or out of the space, is not a perfect fit. 
The displacer also needs some wall clearance and this is also dead 
space. Also, large gaps around the displacer encourage gas to flow to 
and from past it and bypass the regenerator. Some systems actually use 
the walls of he displacer tube as the regenerator, while others have 
the regenerator inside the displacer with the gas flowing through the 
inside of the displacer as it goes from hot to cold and back.

Heat which flows from hot to cold or coolth which flows the other way 
(same thing) via the regenerator structure, and not carried by gas, is 
waste energy which will increase overall system energy consumption 
without benefits.



            Russell

 

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