Big Beta Stirling 1kw, 100degC to 400degC

[Geoff V]

Zhivko

I think you are heading in the right direction but unless the fins (lamels) are brazed to the casing there will be a thermal barrier to the heat flow, see an example in the book by Andy Ross, http://stirlingbuilder.com/ also you would have to add fins to the outside of the heater casing to take the heat from the burner flame. Even if they are a tight fit in the casing when cold, the casing will expand when hot and no longer touch the fins which would dramatically reduce the heat transfer, as I have said before, slots are for me the best solution by far for heat transfer, but are very difficult/expensive to manufacture.

GeoffV

[zhivko]

Hi everybody - just had some idea:

what if would be possible to round a trapezoidal sheet into tube of 250mm? Lke this one trapezodal 3mm height sheet.
That would give plenty of small tubes (and heat or cool area) in longitudinal pipe direction.

Of course that would require seal in cold side of displacer.

Just an idea - everybody please comment - do you see this viable, doable?

best regards,
zhivko

[dconner]

I saw you used my Stirling engine simulator (I'm the admin for http://www.solarheatengines.com). That number at the bottom of the output: isentropic compression heating, is a very important number and it just about guarantees that your engine will not run. It should not be higher than 25% and a more conservative value of say 15% would be even better for a first cut on a new engine design.

The significance of the number is that real Stirling engines do not have isothermal compression. They have a combination of isothermal and adiabatic compression and expansion. The result is that the operating gas is heated by the compression stroke. If you heat it too much during compression, then you can't transfer much heat to the gas after it is compressed. The same thing happens in reverse on expansion. The gas expands and cools and you can't extract much heat from the operating gas because it is already cooled.

I've never been able to get a Stirling engine of any kind to run with 50% isentropic compression heating. I think about 35% has been the limit for me, and that is barely running with no load.

I have a post on power piston sizing that might give you some practical values for sizing the power piston and displacer. http://www.solarheatengines.com/2012/02 ... g-engines/

I see from other comments you are already working on the heat transfer problem which is a big issue on a large engine design.

Doug

[Ian S C]

Hi Doug, good to see such a well thought out web site. Ian S C

[Ante]

my opinion is that dimensions of Peter Lynn engine is something like this in attachment:

D 250mm, stroke cylinder 50mm, stroke displacer 90mm.

If that helps you..

Salute

[Geoff V]

Zhivko

Just so you are not misled by guessing, the dimensions of Peter Lynn's model 14 engine are nothing like those suggested by Ante!

I'm not prepared to divulge the dimensions as they may have commercial implicatons but I can tell you the bore is much smaller.

All you need to do to get your engine running is, change the displacement ratio as advised and fit a displacer with a smaller gap say 3.5mm.

GeoffV

[zhivko]

I saw you used my Stirling engine simulator (I'm the admin for http://www.solarheatengines.com). That number at the bottom of the output: isentropic compression heating, is a very important number and it just about guarantees that your engine will not run. It should not be higher than 25% and a more conservative value of say 15% would be even better for a first cut on a new engine design.

The significance of the number is that real Stirling engines do not have isothermal compression. They have a combination of isothermal and adiabatic compression and expansion. The result is that the operating gas is heated by the compression stroke. If you heat it too much during compression, then you can't transfer much heat to the gas after it is compressed. The same thing happens in reverse on expansion. The gas expands and cools and you can't extract much heat from the operating gas because it is already cooled.

I've never been able to get a Stirling engine of any kind to run with 50% isentropic compression heating. I think about 35% has been the limit for me, and that is barely running with no load.

I have a post on power piston sizing that might give you some practical values for sizing the power piston and displacer. http://www.solarheatengines.com/2012/02 ... g-engines/

I see from other comments you are already working on the heat transfer problem which is a big issue on a large engine design.

Doug

Hi dconer,

Yes you are right - I used your simulator. My revised values for second try are as follows:

Second crank configuration

Solar Heat Engines » Stirling Engine Simulator.jpeg (50.77 KiB) Viewed 7737 times

Now simulator shows around 19.73% isentrophic heating.
I have also doubt with displacer length - but I will give a try with current length 290mm - as soon as I get hot gas guide (already ordered). Also when I'll get hot gas guide I will remove crank mechanism and manually try to move displacer to see real effect on power piston (as GeoffV proposed).

Will report how it goes - thanks for support!

[zhivko]

Hi everybody,

It seems I had luck today - I found somebody in my country, who works sheet metal spinning and I will send him sketches, to see what will be the price for displacer and cylinder.
I will go for longer displacer with spherical end caps, length would be 2 times displacer diameter, and tighter gap 3.5 mm between displacer and cylinder.
Of course also cylinder is spherical end cap.
Outer hot cylinder is from usual stainless steel, 2 mm thick, displacer is from stainless steel 1 mm thick.
Heater would be heating in 218 mm length, cooling (for now) is air with alu fins, also cold cylinder is in aluminium.

I hope longer cylinder and tighter gap would benefit heat transfer for heating and cooling.

Please see attached Autocad dwg (Stirling2_1.zip). If somebody has some correction that I could still change to current design - please feel free to comment criticize whatever.

Longer and tighter displacer

Stirling_2_1.jpeg (37.1 KiB) Viewed 7724 times

[Ian S C]

zhivko, I would prefer to see the displace 3X the diameter, rather than your proposed 2X dia. If you can't get it made by getting it spun, it could be fabricated by TIG weldinga bit of sheel stainless into a cylinder, then TIG welding the domed end on. I'v had ends .5 mm thick welded on tube of the same thickness. Ian S C

[Ante]

Zhivko

Just so you are not misled by guessing, the dimensions of Peter Lynn's model 14 engine are nothing like those suggested by Ante!
I'm not prepared to divulge the dimensions as they may have commercial implicatons but I can tell you the bore is much smaller.

GeoffV

Zhivko asked me why 14 engine has small ratio diameter / overall height , I was use pictures of this engine and compare dimensions on engine, only information that i have is that working volume is 2.5 l , all another informations are assumptions. So i get this type of drawing, sorry for wrong assumptions.

Zhivko from drawing it seems to me that you adding skirt from back side of displacer, that will make lots of dead space,correct me if i am wrong.

Salute

[zhivko]

Zhivko
Zhivko from drawing it seems to me that you adding skirt from back side of displacer, that will make lots of dead space,correct me if i am wrong.

Hi Ante,

The conical part is part of the heater guidance for hot air produced by gas burner. I hope that from this video it would be seen that it's not part of "working gas" volume.
Since displacer is much heavier now it would be great if displacer could be somehow guided at the hot end - that would also relieve bushings inside piston. Although if working in vertical direction that would not be a problem...
Video of longer displacer (3/4 view)

Hre is another picture of it:

image with gas heater

Stirling2_1.jpg (237.87 KiB) Viewed 7713 times

zhivko, I would prefer to see the displace 3X the diameter, rather than your proposed 2X dia. If you can't get it made by getting it spun, it could be fabricated by TIG weldinga bit of sheel stainless into a cylinder, then TIG welding the domed end on. I'v had ends .5 mm thick welded on tube of the same thickness. Ian S C

Hi Ian,

If I dared to do that - stirling would become cumbersly big for such "small" power. I would try with "short" version first - although already currently designed version is almost 1 meter long. :)

[zhivko]

Hi guys,

has anybody checked this paper Design of a 2.5kW Low Temperature Stirling Engine for Distributed Solar Thermal Generation ? This seems like interesting Stirling design for residential applications.

Also heating low temperature difference stirling with solar energy is very attractive this days...

[Ian S C]

zhivko, a motor with the bore that your design has is going to be BIG, If I was going to build a motor of this power range, I would build a much smaller motor, get that going, then build 2 or 3 more and unite them as one, the smaller cylinders are easier to heat, and cool. Ian S C

[zhivko]

Yes Ian,

I am aware of fact that smaller cylinders could be much more efectively heated or cooled. BUT the fact is that I already have cold aluminium cylinder done, also mechanism and aluminium fins...
This week I should get heater skirt, and also I found a sheet metal spinning guy Sheet metal spinning - quite near my town (few kilometers away actually)...

I must try to finish this big cylinder now...

Ian and others - have you checked paper that I referenced above?

[zhivko]

Hi guys just to inform you I made new crank that should give compression ratio of 2.

video of new crank

Still waiting guy to metal sheet spin cylinder and displacer :( Hopefully it will happen in wednesdays :).

Will report how it goes!