Elastic Regenerator

[Tom Booth]

he primary effect of regeneration in a Stirling engine is to increase the thermal efficiency by 'recycling' internal heat .... In practice this additional power may not be fully realized as the additional "dead space" ...reduces the potential efficiency gains from regeneration.

http://en.wikipedia.org/wiki/Stirling_e ... egenerator

For a long time I've been aware of this problem (mentioned in the article above) and have been trying to figure out some way of taking advantage of the regenerator while somehow compensating for the loss from the introduction of additional "dead air space".

What I think might work is some kind of "elastic" regenerator.

Here is the theory.

When air in the engine is passed from the hot space to the cold space through the regenerator the regenerator gets Hot right? The opposite of what happens to the gas. When the gas is cooled the regenerator gets hot and vice versa. When the gas is heated the regenerator is cooled.

Certain metals like aluminum have a high coefficient of thermal expansion (expand when heated). There are probably some metals or alloys like zinc that would be more effective than aluminum.

If a regenerator were made out of some kind of solid aluminum (or other metal with a high CTE) cylinder with channels for air passage through it, and if it were coated on the outside with some elastic material like that high temperature silicone (RTV or similar) used for head gaskets, Or possibly some kind or elastic tubing could be used. then when the metal regenerator got hot it would expand. Therefore when the regenerator got hot it would increase the volume of the air space at the same time that the gas is cooled creating a higher vacuum to draw the piston in. Likewise when the regenerator was cooled it would shrink in size and reduce the air space at the same time that the gas was heated and expanding creating some additional pressure to drive the piston out, above that caused by the expanding gas.

I thought of this possibility quite some time ago but did not think aluminum would expand and contract enough to make a whole lot of difference, but it might be worth a try.

Recently however I became aware of "superelastic" metals. The kind used for medical "stints". The stints are narrow when cool but expand when heated by body temperature after insertion, so as to open up an artery or whatever.

So an "elastic regenerator" would act very much like a medical stint, enlarging the passage when heated and relaxing or contracting when cooled. I assume that this action would work against (or opposite to) the expansion and contraction of the gas in the cylinders and thereby compensating somewhat for the "dead air space" problem delivering more power to the piston.

Some other arrangement might be used, like having the expanding and contracting regenerator push and pull some additional pistons working against some metal springs or pneumatic "AIR springs", but the above, simply coating the regenerator with silicone seems like the easiest and simplest approach, for experimenting anyway.

[Tom Booth]

Another possibility might be to use some kind of bimetal spring type material which would expand when heated, if something in this category could be found which would respond quickly enough.

[fullofhotair]

I was thinking along these lines awhile back. What I saw as a problem was the lag time of the metal to contract and expand fast enough to say stay up with a 500 rpm stirling engine. Take a look at memory metal. Nitinol is one form. It changes pretty rapidly. They make heat engines out of it. Another thing ,if you could some how design it to expand in only one direction, up and down and not sideways . You dont need the displacer grabbing the cylinder walls or shrinking so much too much working fluid runs around it.

[fullofhotair]

What if the regenerator was made of 2 pieces. A piston insise a piston with both heads facing out. many nitinol springs connecting the two. The regenerator would change shape rapidly but in only one direction up and down, not sideways.

[Ian S C]

Tom, It's stents, not stints, and they are expanded by passing them up an artery on the end of a tube, the tube has a small balloon that is inside the stent and when the right place is reachedthe balloon is expanded and the metal mesh of the stent expands against the artery wall, the tube, and ballon are then withdrawn.
Aluminium is not a good material for a regenerator, it conducts heat too fast. Stainless steel, either wire or mesh, or shim/ foil, and for example .005" shim is not going to expand much in the possible 500*C temp change. Ian S C

[Geoff V]

Tom

Let me start by saying, the main problem with the Internet is, unverified information.

Firstly, your extract from Wikipedia implies that efficiency and power output are linked, this IMHO is a false assumption, the addition of a regenerator will reduce the heat input required for a given power output, increased efficiency, but will not necessarily increase the power output. If the heat exchangers in a particular engine are the limiting factor in terms of power output, then a regenerator may well produce a power increase by recycling heat which is already inside the engine.

Secondly, it has long been assumed that the compression and expansion process in a SE are isothermal (at a constant temperature) this is clearly not true due to the very short cycle time (search John Archibald experiment) therefore the process must be largely adiabatic (bicycle pump!). The nett result is that too higher compression ratio reduces the temperature change within the engine, hence lower cycle power, to the point where some simple engines will not even run until the compression ratio is lowered. The additional dead volume created by the regenerator, will in many designs, actually improve performance by reducing the heat exchanger work load and by lowering the compression ratio (lower adiabatic heating and cooling) hence increasing the temperature change on the working gas.

GeoffV (Unverified!)

[Tom Booth]

...if you could some how design it to expand in only one direction, up and down and not sideways . You don't need the displacer grabbing the cylinder walls or shrinking so much too much working fluid runs around it.

There are all kinds of different arrangements so it is sometimes difficult to speak in more than generalized terms, but what I primarily had in mind as perhaps the easiest type of engine to test this out on would be where the regenerator was an independent unit located between the hot and cold cylinders as shown here for example.

two-cylinder (or alpha type) Stirling

Then sideways expansion would be what you would want I think.

If the regenerator were incorporated into the displacer then yes. That would present a bit more of an engineering difficulty. In an alphy type, or similar engine it would just be a matter of exchanging regenerators.

[Tom Booth]

Tom, It's stents, not stints, and they are expanded by passing them up an artery on the end of a tube, the tube has a small balloon that is inside the stent and when the right place is reachedthe balloon is expanded and the metal mesh of the stent expands against the artery wall, the tube, and ballon are then withdrawn.

There are different types of stints sorry stentys ? Anyway, I was thinking of the "self expanding" or "self=opposing" type, which are infact, as fullofhotair mentioned, made of "memory metal" or Nitinol.

Here is a video comparing the differnet types of stents:

Comparing the self-apposing STENTYS stent with a balloon-expandable stent

Aluminium is not a good material for a regenerator, it conducts heat too fast. Stainless steel, either wire or mesh, or shim/ foil, and for example .005" shim is not going to expand much in the possible 500*C temp change. Ian S C

Finding an ideal material might prove difficult, but I don't think it is an insurmountable problem. The main question I think is assuming some material with the right properties could be found, would the idea work in principle. It is rather embryonic at the moment, as ideas go.

I was thinking an aluminum/nitinol bimetal. Aluminum conducting heat too fast might not be a problem if combined with a metal that retains heat. At any rate, the Nitinol, unlike other metals actually shrinks when heated. So I think a bimetal made of these two would have quite some "muscle", the aluminum expanding on one side and the nitinol contracting on the other.

I don't think aluminum would hold up to much flexing or stress tough, so maybe NiTi and Spring steel ?

[Tom Booth]

Tom

Let me start by saying, the main problem with the Internet is, unverified information.

Firstly, your extract from Wikipedia implies that efficiency and power output are linked, this IMHO is a false assumption,...

Point taken. Though I don't see much point in debating the accuracy of wikipedia here.

Secondly, it has long been assumed that the compression and expansion process in a SE are isothermal (at a constant temperature) this is clearly not true due to the very short cycle time (search John Archibald experiment) therefore the process must be largely adiabatic (bicycle pump!). The nett result is that too higher compression ratio reduces the temperature change within the engine, hence lower cycle power, to the point where some simple engines will not even run until the compression ratio is lowered. The additional dead volume created by the regenerator, will in many designs, actually improve performance by reducing the heat exchanger work load and by lowering the compression ratio (lower adiabatic heating and cooling) hence increasing the temperature change on the working gas.

GeoffV (Unverified!)

I'm not sure I follow all you said here, I'll have to mull it over a while. Searching for John Archibald experiment on google gave no results, except the physicist. A search here turned up another post making reference to it but with no specific citation or link. I did find this on TouTube posted by a "John Archibald" and thought it might be what you were talking about as it seems related:

[youtube]http://www.youtube.com/watch?v=HMnZrPKjleE[/youtube]

If so, I see the point being made, I think.

It took 5 seconds for the tank of air to cool down under cold running water. The air in a Stirling Engine would have to cool down very much more quickly than that if running at a high RPM. At best one might expect about 12 RPM.

I think the factor not taken into consideration here though is that the canister of of hot air under running water is not doing any "Work". I agree that the expansion is primarily adiabatic rather than isothermal as I agree, if I understand you correctly, that in an engine running at say 3000 RPM there is no time for the gas to be making heat exchange with the environment. Something else is going on.

But I think the reason that some engines run into the bicycle pump (ie. working against too much pressure on the return stroke ?) may be because most model Stiling engines simply are not doing much work at all.

At any rate, that the gas cools very quickly because it is performing work while expanding adiabatically (no time for heat exchange with the environment) explains why a Stirling Engine can run at faster than a few RPM. The gas isn't cooling by dumping heat into the heat sink. Simply put there just isn't enough time for that if the engine is running at more than a few RPM. Instead the gas is "self-cooling" due to using its "internal energy" to perform work.

If the engine isn't designed for performing work it can't get rid of the heat fast enough and so can never run at more than a few RPM.

Cyclopedia of engineering

"When a gas expands and does work, as by pushing a piston in a cylinder, we see from the first law of thermodynamics that the equivalent in the form of heat must be supplied from somewhere.... But if no heat be allowed to enter the gas, as would be the case if the cylinder and piston were perfect non-conductors of heat, (Or if the expansion takes place to quickly TB) the work done in expansion will be at the expense of the heat energy in the gas itself, and its temperature will therefore fall during the expansion."

Of course, I'm not sure about your reference to "John Archibald experiment" so I may be way out in left field on this.

[Geoff V]

Tom

I'm sorry to say I've obviously not explained myself clearly, my point was simply to say, you appear to be working on a solution, where no problem exists. The regenerator is primarily a device to improve the thermal efficiency by storing heat that would otherwise have to be rejected by the cooler and giving up this heat to the working gas at the start of the expansion part of the cycle. The dead space associated with the regenerator is not necessarily detremental, too higher compression ratio will easily overload the heat exchangers there by reducing the temperature swing within the engine. If you consider for a moment, high pressure commercial SE's have enormous tubular heaters and coolers with much dead space as well as regenerator cans.

If the engine isn't designed for performing work it can't get rid of the heat fast enough and so can never run at more than a few RPM.

I am not clear as to the difference between an engine designed to do work and one designed not to do work? Surely if this statement was correct an engine would run slowly off load and accelerate when a load was applied, where as all my engines run much faster off load than on!

GeoffV

[Tom Booth]

Tom

I'm sorry to say I've obviously not explained myself clearly, my point was simply to say, you appear to be working on a solution, where no problem exists. The regenerator is primarily a device to improve the thermal efficiency by storing heat that would otherwise have to be rejected by the cooler and giving up this heat to the working gas at the start of the expansion part of the cycle. The dead space associated with the regenerator is not necessarily detremental, too higher compression ratio will easily overload the heat exchangers there by reducing the temperature swing within the engine.

I don't think the addition of an "elastic regenerator" as proposed would reduce the temperature swing.

The metal, when absorbing heat and expanding cools the air when you want it cold. When it contracts it gives off heat to the air when you want it heated. Same basic function as a regular regenerator but possibly a little more so. I'm not sure what you mean by "too higher compression ratio will easily overload the heat exchangers there..."

What heat exchangers where? Your use of "overload the heat exchangers" and "reducing the temperature" in the same breath doesn't make sense to me.

Overload the heat exchangers how ?

If you consider for a moment, high pressure commercial SE's have enormous tubular heaters and coolers with much dead space as well as regenerator cans.

If the engine isn't designed for performing work it can't get rid of the heat fast enough and so can never run at more than a few RPM.

I am not clear as to the difference between an engine designed to do work and one designed not to do work?

A model engine that sits on a coffee table as opposed to an engine that drives a generator. "Work" as the term is used in thermodynamics as per the reference from the engineering book posted above.

Surely if this statement was correct an engine would run slowly off load and accelerate when a load was applied, where as all my engines run much faster off load than on!

GeoffV

Slower or faster does not equate with hotter or colder.

It is well known that a utility size solar powered Stirling tends to overheat during off peak hours when the load is reduced.

[Geoff V]

Tom

I'm sorry to say, I don't think I can add anything more to your Thread, as I was under the impression your proposal was applicable to the Stirling Cycle engine as invisaged by Cayley and subsequently patented by The Rev R Stirling in the early 18 hundreds. Clearly you are working on something much more advanced, employing thermodynamic principles way beyond my comprehension.

I wish you good luck with your endevours and very much look forward to seeing the test results once the prototype is up and running.

GeoffV

[Tom Booth]

Tom

I'm sorry to say, I don't think I can add anything more to your Thread, as I was under the impression your proposal was applicable to the Stirling Cycle engine as invisaged by Cayley and subsequently patented by The Rev R Stirling in the early 18 hundreds. Clearly you are working on something much more advanced, employing thermodynamic principles way beyond my comprehension.

I wish you good luck with your endevours and very much look forward to seeing the test results once the prototype is up and running.

GeoffV

Well, I do very much appreciate your input.

I don't think the principle involved is difficult to understand and it is applicable to a Stirling engine.

The purpose of a heat engine is to convert heat into work. The more effectively it can do that, the less waste heat there is left over to cause the engine to overheat.

Anyway, I would love it if I had a workshop to tinker around with the idea and build a prototype, but I'll have to build the workshop first and equip it with tools etc. So it would be some time before I could build a prototype. For that reason I'm sharing the idea so that if anyone is interested and has the means they can do something with it.

[Geoff V]

Well, I do very much appreciate your input.

It's a pleasure.

The purpose of a heat engine is to convert heat into work. The more effectively it can do that, the less waste heat there is left over to cause the engine to overheat.

Is That a fact.

I was thinking of asking you to publish some pictures of your engines in the Forum Gallery, from your comments I felt sure you must have worked with some amazing equipment, just as well I did't ask.

Are you planning on becoming an engineer when you grow up?

GeoffV

[Tom Booth]

Well, I do very much appreciate your input.

It's a pleasure.

The purpose of a heat engine is to convert heat into work. The more effectively it can do that, the less waste heat there is left over to cause the engine to overheat.

Is That a fact.

I was thinking of asking you to publish some pictures of your engines in the Forum Gallery, from your comments I felt sure you must have worked with some amazing equipment, just as well I did't ask.

Are you planning on becoming an engineer when you grow up?

GeoffV

I've worked as an engine mechanic of and on most of my life. I had a small engine repair shop closer to the city at one time but have now moved to the country and am doing some farming growing garlic. This is a new enterprise or direction for me but by next year I'll have increased the stock enough to start selling. Everything takes time. After that I should have enough income from the agricultural work to build a workshop. As of now the land is mostly vacant so I'm staying in a camper for the time being. Not room enough for any equipment.

Stirling engines are a new interest for me as of about six years ago. Mainly because I'm living in a camper off the grid. I use solar panels to run some lights and a laptop but I also heat with a wood stove. I've often thought how nice it would be if I could run a small Stirling engine on the heat from the wood stove during the winter months when there is not so much sunshine.

I am retired, so I have plenty of time for reading, especially through the winter. If it were possible, I would probably just go to the hardware store in town and buy a small Stirling Engine for backup power and that would be that, but since there aren't any there to buy, it seems the only alternative is to build one.

I have torn down and rebuilt plenty of IC engines, just not Stirling Engines. But I have read a book or two on thermodynamics just so when the time comes I might have some idea what I'm doing.

There does not seem to be a whole lot of information regarding small to mid size power producing Stirling Engines. Part of the problem seemed to be the issue relating to the regenerator. So I pondered it for a while and finally came up with what seemed like it might be a solution.

If someone took the idea and started marketing engines based on it I'd be happy just for the opportunity to go down town and buy one.

Anyway, thanks for asking. I don't think I ever claimed to be an expert on Stirling Engines. It's just an idea. No point in getting your feathers all ruffled over it.