Elastic Regenerator

Discussion on Stirling or "hot air" engines (all types)
Geoff V
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Re: Elastic Regenerator

Post by Geoff V » Wed Dec 19, 2012 3:22 am

Tom

I started life as a car mechanic before spending 4 years designing small hovercraft, then 22 years as an Airline pilot and finished with 7 years running a precision engineering company as a sub contractor to one of the Formula 1 teams. During this time I have designed, produced and marketed a fully machined miniature gas turbine in kit form and spent 32 years with Stirling cycle engines, as an amateur, not to mention my many other interests and activities.

In light of the above, you will understand, I'm sure, why I do not respond well to being quoted elementry thermodynamic theory as if I was still at Primary school.

OK, that's out of the way.

From the start, my only concern is to offer you (and other readers) the benefit of my experience when it comes to the effects of increased dead space on the Stirling cycle, with a view to minimising the time and materials being wasted. The text books would have us believe that the heat change process is isothermal and therefore minimising the dead volume can do nothing other than increase the power output, wrong! Due to the very limited time available, the process is mostly adiabatic and as the compression ratio is increased, the amount of heating and cooling by compression and expansion also increases. This adds to the work load on the heat exchangers and can exceed their capacity which means the gas in the cooler is higher than desired and the gas in the heater is cooler than desired which equals a reduction in temperature swing and lower power output. In extreme cases, where the heat exchangers are very limted, i.e. HAE's, they will not even run until the compression ratio is lowered.

The regenerator is one of the few parts of a SE which can be made by the amateur and function at efficiencies approaching that of its professional couterpart.

The heat exchangers however are a different 'kettle of fish' and these are where the real difficiencies lie, crack the problem of good heat transfer with a home made heat exchanger and that would be a real contribution the the SE fraternity.

GeoffV

Tom Booth
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Re: Elastic Regenerator

Post by Tom Booth » Wed Dec 19, 2012 12:31 pm

OK, I think the problem or misunderstanding is due to the fact that when I think of a Stirling engine I'm picturing something I can build out of old 16 ounce camp stove propane canisters. (I've got dozens of those laying around).

So when you talk about heat exchangers, I'm just scratching my head, "what heat exchangers?"

Your talking about industrial size, pressurized engines, I'm thinking in terms of something along the lines of a slightly beefed up "Tin can" engine I could set on top of my wood stove to charge a 12v deep-cycle battery. In other words I'm just trying to think of a way to get a little more power out of something I could build out of scrap metal scrounged from the local junk yard. Anything other than that, like using high pressure gas is pretty much off my radar.

I'm sure your experience and insights are applicable to most any Stirling engine of whatever size but I'm really not grasping what you mean when you say for example:

"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 asked "what heat exchangers where?" out of genuine perplexity and asked for clarification because I really don't know where these heat exchangers would be in the type of engine you are taking about or exactly what function they would serve.

At the regenerator?

The only regenerator type I'm aware of is basically just a wad of steel wool in a tube. It is a heat exchanger but doesn't have heat exchanger(s) [plural]. So then I'm wondering if your talking about the ends of the displacer chamber ? Or something to get rid of excess waste heat at the piston cylinder ?

So exactly how or why the heat exchangers would be "overloaded" is a mystery to me at this point not knowing where they might be located or which function they might serve. Or are you talking about the regenerator itself ? Do these industrial size engines use multiple heat exchangers for the regenerator ?

I haven't looked into these larger engines much as they are outside the scope of my immediate interest (an engine that could be built in a garage or small workshop) so I'm not able to follow your explanations without asking what must seem like idiotic or provoking questions.

For some reason I'm just not able to visualize in detail the scenario you are describing: ""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'm not suggesting eliminating the "dead space" or the regenerator. Just using a different material and different housing for it.

Theoretically it would increase the pressure changes that power the piston.

I'm also not sure what you mean exactly by "compression ratio".

I'm thinking in terms of a small non-pressurized engine where the piston would be open to the atmosphere, (like an LTD engine) so there would be no "compression" in the sense it would be used in an IC engine - at the cylinder head. The compression I'm thinking about or talking about is in the displacer chamber on the other side of the piston.

There are so many different types of Stirling Engine and so many different possible configurations it is difficult for me to know what you are talking about without narrowing things down to specifics in some one specific type of engine.

Geoff V
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Re: Elastic Regenerator

Post by Geoff V » Thu Dec 20, 2012 12:10 am

Tom

So many questions, where do I start?

OK.

A heat exchanger is a device which transfers heat from one fluid to another through a solid membrane. The part of the SE you heat, transfering heat (from the external heat source) to the working fluid (air) inside the engine, is a heat exchanger. Similarly the part of a SE which is externally cooled, transfers surplus heat from the fluid (air) inside the engine to the outside world.The heat exchangers normally sit either side on the regenerator which behaves like a battery (or capacitor) such that as the working gas is transfered from the hot part of the engine to the cold part of the engine, some of the heat is absorbed by the regenerator, this reduces the amount of heat to be rejected by the cool HE. On the second half of the cycle, the cooled gas is transfered back to the hot HE passing through the regenerator and picking up the heat it deposited there a few milli seconds earlier. The better the regenerators ability to collect, store and release heat (recycle), the lower the amount of heat that has to be transfered each cycle by the HE's.

Dead space (volume) is any volume not swept by either the compression piston or the displacer.

Compression ratio is the ratio of maximum internal volume to minimum internal volume.

If we can accept for now that the biggest problem with a SE is getting heat into and out off the working gas (air is a very good insulator which is why our clothes work so well) we might understand that the better the heat exchangers the more likely we are to get some useful power at the crankshaft. So any actions which increases the load on the heat exchangers will be detremental to the power output. Likewise, the lower the efficiency (ability to recycle heat) of the regenerator the greater the 'load' on the heat exchangers.

Raising the crompression ratio by reducing dead space, increases the amount of heating of the gas within the engine by adiabatic compression, this occurs when the air is largely in the cooler and just when the engine is trying to reject the surplus heat, so it increases the work load on the cooler HE. The same occurs during the expansion phase of the cycle and hence the 'heat exchangers can be overloaded' ie. their capacity to transfer heat is exceeded.

I hope the above is of help in your quest for a remote battery charger.

GeoffV

Tom Booth
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Re: Elastic Regenerator

Post by Tom Booth » Thu Dec 20, 2012 8:49 pm

OK, thanks.

The intent though here is to increase the efficiency of the regenerator. not take it out or reduce the "dead air space".

I think that an "elastic" regenerator would absorb and release heat more efficiently, especially if it were composed of that "super-elastic" memory metal. It does not just heat up and cool down like ordinary metal such as stainless steel, it actually goes through a state change, similar to a refrigerant or water into ice, or evaporation.

Dead air space and other factors are,... I won't say irrelevant, The Wiki quote is what first got me thinking about this but my intent was not to eliminate the dead air space (regardless if that is good or bad or the article is right or wrong) but to compensate for the perceived drawbacks, real or imaginary.

You may be right in what you say, but I think this would be something that would improve the performance of the regenerator not reduce it.

Also I think because the regenerator will "recycle heat" more efficiently this will also reduce the load on the heat exchangers.

There will also be some pressure and volume changes due to the regenerators "elasticity" but I don't believe that these changes will be detrimental, just the opposite. I do think it may result in the engine having more torque, but not necessarily more speed.

Of course my theory on this may be all wrong, but I don't think so. I'd have to do some experimenting with it to be sure.

vamoose
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Re: Elastic Regenerator

Post by vamoose » Thu Dec 20, 2012 10:05 pm

Hi Tom,

Could one consider 'Piezoelectricity' to utilise the stress/pressure resulting from expansion and contraction of the regenerator?
http://en.wikipedia.org/wiki/Piezoelectricity
If you were to use a bi-metal regenerator with a piezoelectric polymer (or nanostructures) between the different material interface (and different, relative expansion/contraction rates), maybe you could have a device that still acts primarily as the SE regenerator but with some additional piezoelectric generated power from the incidence of expansion and contraction per cycle.
I know its not exactly what you were indicating, but just thought I would put it out there...
vamoose

Ian S C
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Re: Elastic Regenerator

Post by Ian S C » Fri Dec 21, 2012 2:48 am

Vamoose, what are you going to use the power from the piezoelectric unit for, and how are you going to utilize it. Its good for making sparks, but as useful as static electrcity, or lightning. Ian S C

Geoff V
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Re: Elastic Regenerator

Post by Geoff V » Fri Dec 21, 2012 6:19 am

Tom

There is always room for improvement, but as the current regenerators are reported as 95% efficient in terms of heat recovery, there's not much room, so I wish you well with your endeavours.

GeoffV

Tom Booth
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Re: Elastic Regenerator

Post by Tom Booth » Fri Dec 21, 2012 9:03 am

vamoose wrote:Hi Tom,

Could one consider 'Piezoelectricity' to utilise the stress/pressure resulting from expansion and contraction of the regenerator?
http://en.wikipedia.org/wiki/Piezoelectricity
If you were to use a bi-metal regenerator with a piezoelectric polymer (or nanostructures) between the different material interface (and different, relative expansion/contraction rates), maybe you could have a device that still acts primarily as the SE regenerator but with some additional piezoelectric generated power from the incidence of expansion and contraction per cycle.
I know its not exactly what you were indicating, but just thought I would put it out there...
vamoose
Interesting. There seems to be a lot going on in the nanotechnology field. I'm hearing a lot about it lately. Can I do this in my kitchen ?

I'm kind of limited to rather low tech solutions myself, but it is certainly an interesting concept.

Tom Booth
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Re: Elastic Regenerator

Post by Tom Booth » Fri Dec 21, 2012 9:14 am

Ian S C wrote:Vamoose, what are you going to use the power from the piezoelectric unit for, and how are you going to utilize it. Its good for making sparks, but as useful as static electricity, or lightning. Ian S C
Hmmm...

Maybe run it through a transformer, rectifier circuit to step it up to very high voltage low amperage DC and use it (the ionization) for on site electrostatic cooling of the cold side of the displacer chamber. LOL...

Or if such things generate static electricity already, maybe it wouldn't need stepping up. Don't really know anything about it. Somebody mentioned electrostatic cooling to me just yesterday in another forum.

Tom Booth
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Re: Elastic Regenerator

Post by Tom Booth » Fri Dec 21, 2012 9:57 am

Geoff V wrote:Tom

There is always room for improvement, but as the current regenerators are reported as 95% efficient in terms of heat recovery, there's not much room, so I wish you well with your endeavours.

GeoffV
Thanks!

Well, hmmm...

Theoretically mind you, I'm thinking that this regenerator would still not be more efficient at heat recovery than any other, thought there is that other 5% to think about...

A regular regenerator doesn't do anything with the heat while it is in its possession. Just takes it in and gives it back.

An elastic regenerator on the other hand would not just reclaim the heat but also utilize it while it has it.

Lets say that it expands against a piston moving the piston against a spring. (possibly just an "air spring" i.e a dead ended chamber). In the process it would add some torque to the engine by momentarily changing the volume and thus the pressure/vacuum driving the piston.

When done, the air spring would push it back, so that there would be a simultaneous release of heat along with a change in pressure. The heat is still reclaimed but you get a little more torque or additional pressure to the piston as well.

The over all efficiency of a Stirling, I think is still around 35% (?) so... perhaps there is more room for improvement than might be apparent when talking about the overall efficiency of the engine, especially for something low tech/home built where the efficiency is likely quite low to begin with.

I'm thinking Tin-Can Engine with a little high tech "super elastic" material to give it an extra kick.

The stuff is going for a song on Ebay like .50 cents/foot, 50 foot rolls for $25.

It is peculiar stuff though, it needs to be "trained" to do whatever it is you want it to do, but that isn't really that difficult I don't think. Like you could make a spring (or regenerator) that either expands or contracts when heated, whichever way you want as it suits your particular application or intention/design.

Tom Booth
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Re: Elastic Regenerator

Post by Tom Booth » Fri Dec 21, 2012 10:13 am

In other words, it might act not only as a heat regenerator but a "pressure regenerator" at the same time.

When you want the heat gone, it would convert the heat into pressure storing it in the "air spring" at the same time expanding the volume of the displacer chamber to lower pressure and create a vacuum. When you want the heat back, it would "relax" allowing the air spring to push it back, reclaiming the stored heat (releasing it) AND the stored pressure simultaneously.

This might be hard to visualize, I don't know, but it seems to be working really great in my head.

Like I say, don't know how well it would actually work in practice. Seems like it should work, but don't know till you try I guess.

Ian S C
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Location: New Zealand

Re: Elastic Regenerator

Post by Ian S C » Fri Dec 21, 2012 5:57 pm

Tom, when you build the prototype, you must make the regenerator so that you can simply change is interiour, so that it can be compaired. Ian S C

vamoose
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Re: Elastic Regenerator

Post by vamoose » Sun Jan 06, 2013 2:00 am

Hey Guys,
vamoose wrote:Hi Tom,

Could one consider 'Piezoelectricity' to utilise the stress/pressure resulting from expansion and contraction of the regenerator?
http://en.wikipedia.org/wiki/Piezoelectricity
If you were to use a bi-metal regenerator with a piezoelectric polymer (or nanostructures) between the different material interface (and different, relative expansion/contraction rates), maybe you could have a device that still acts primarily as the SE regenerator but with some additional piezoelectric generated power from the incidence of expansion and contraction per cycle.
I know its not exactly what you were indicating, but just thought I would put it out there...
vamoose
Ian S C wrote:Vamoose, what are you going to use the power from the piezoelectric unit for, and how are you going to utilize it. Its good for making sparks, but as useful as static electrcity, or lightning. Ian S C
Don’t know if the idea has any real validity, and how one would apply it specifically, but Tom's thread prompted me to think of it.
Just attempting to make a positive contribution, and trying to stimulate thoughts and discussion with all our fellow readers out there.
Ian, your questions are definitely reasonable and prompted me to have a search.
Its all mostly new to me, but found some of the stuff in the links below of interest...

Pedestrians to power walkway to London 2012 Olympic park
http://www.energyharvestingjournal.com/ ... essionid=1

piezoelectric graphine
http://phys.org/news/2012-01-graphene-p ... ctric.html

http://www.sciencedaily.com/releases/20 ... 094535.htm

.....

http://www.piezo.com/prodproto4EHkit.html

http://blog.shiftboston.org/2011/03/cre ... lectricity

It seems to me that the expansion and contraction of a Stirling engine regenerator lends itself to piezoelectric power generation. Static force will give you a potential voltage with no true power, but an oscillating force will give you voltage and current ranging from a few hertz up to say possibly 50hz depending on the engine rpm.
You don’t get piezoelectric power from stress alone but from changing stresses..
If somebody in the forum is clued up on the ins and outs of piezoelectricity, feel free to share..
vamoose

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