I'm planning to build a Stirling generator to work off heat from our kitchen stove.

[Jack]

Hey guys, I'm new to this and found this forum on my quest for answers.
Since I already got some info from past topics I thought I'd do my part and share what's in my head. Maybe we can bounce around some ideas.

I have a background in professional engineering and hobbyist tinkering.
I live in Vietnam, tropics, as an immigrant. My wife, the kids and I are living a simplification lifestyle. We're trying to be as self sufficient as we can.
To (partly) achieve this we cook on a cob wood fired stove. We manage a small wooded area for the wood.

The one thing we're very conscious about is electricity. We've got the usage down quite a bit (average 1,5kw/h per day) and have ideas to further lower that.
We're looking into ways to generate our own electricity and I always looked at all that wasted heat from the stove. This led me here.

I want to build a Stirling engine into our stove. (As well as hot water reservoir to always have hot water)
After a long search for information I've distilled it down to a version of this:
https://youtu.be/jKu7K6Nu4_A?si=PBoCLBkRA92GLJuV

The main things I like are the 4 cylinders, the swash plate and the limited amount of moving parts.

I think I'd like to switch the working cylinder to diaphragms. Less lubrication needed, less wear and tear, quieter. Am I missing any disadvantages here? Maybe some risk with heat.

I'd like to build this with locally available materials. As little custom manufacture as possible.
There are machine shops around and I can get stuff laser cut if needed. I used to be a professional Tig welder and have a machine now. I'm thinking this might be necessary to fabricate heart exchangers.

I can update as I go along. For now I'm still in thinking mode. I'll start a SketchUp drawing soon enough.

I'm hoping to get around 300 to 700 watts out of this. And the engine including generator will have to fit into a space of 60x60x100.
I thought the heat "cones" can nicely fit into the area where the fire actually is. So the embers will keep heating it for a while.
I'll add a water-cooling jacket to it too. With a sizeable reservoir that we might be able to use as warm shower water.

This text is probably all over the place, my brain works like that. But what do you guys think?

[Fool]

Welcome to the forum.

The Beale number and associated equation should help with sizing.

https://en.m.wikipedia.org/wiki/Beale_number

Use it on some known engines and their parameters to get an effective range. Use those values on sizing your new engine.

Also, you may prefer the West number a sublink of the last link.

https://en.m.wikipedia.org/wiki/West_number


There is also the, Schmidt theory, and pseudo Stirling cycle, known also as the adiabatic Stirling cycle, as well as others.

All those are fairly inaccurate but they should get you in the ballpark.

The engines run better with more power per size with higher pressures and, temperature differences.

You may need to run your cooking stove quite a bit more to get the power out and water heat you desire. That is, you may consume a lot more wood/fuel.

These are just design concerns and all are workable. Good luck with your project.

[Jack]

Thanks for the information!
I have to admit that I'm better at the mechanical side of engineering than the theoretical math, I'll try to struggle through it haha.

The hot water for showering would be an added bonus if it turns out to be viable. The main priority is generating power, any power.

I read that pressurizing the cylinders adds efficiency, but I don't know how feasible it is with a diaphragm.

[VincentG]

Good luck with the build, and please keep us updated on your progress.

[MikeB]

I read that pressurizing the cylinders adds efficiency, but I don't know how feasible it is with a diaphragm.

I don't think it makes much difference whether you use a diaphragm or a traditional piston - pressurisation is a problem for both, as it is primarily the external/atmospheric pressure that returns the piston.
The solution, which is much easier if you are making a generator, is to seal a space that encapsulates the whole engine, so that both the inside and outside can have their pressure raised equally.

[Jack]

Ah yeah, I didn't even think of the atmospheric pressure returning the piston. That makes sense.

[Jack]

I've been changing my ideas in the last few days. Trying to simplify it and combine functions and delete some moving parts. Especially reciprocating parts.

I've ended up with a rotary displacer design in my head. This way my flywheel is the displacer. Nicely balanced and not reciprocating. The axle on which they turn is the rotor axle of a generator.
But this opens up a load of new possibilities which I'm still exploring.
Diaphragm piston or is a liquid piston possible in a way to eliminate more moving parts?
One or two "cylinders"?

I've done some early designing and calculations. Seems this way I could end up with 1300cc displacement with internally only about 50cc dead space.

I'll post some design pictures as soon as they're a bit more clear.

[Jack]

Also, what would be the ideal ratio between working piston and displacer displacement.
According to my thinking this is dependent on how fast the heat transfers into the air.

[Fool]

The Stirling Brothers, Robert and James, built pressurized and economiser (regenerated) engines before 1850. Two were as big as 21 hp and 45 hp.

https://en.m.wikipedia.org/wiki/Stirling_engine

James Stirling presented his engine to the Institution of Civil Engineers in 1845, [18] the first engine of this kind which, after various modifications, was efficiently constructed and heated, had a cylinder of 30 centimetres (12 inches) in diameter, with a length of stroke of 60 centimetres (2 ft), and made 40 strokes or revolutions in a minute (40 rpm). This engine moved all the machinery at the Dundee Foundry Company's works for eight or ten months, and was previously found capable of raising 320,000 kg (700,000 lbs) 60 cm (2 ft) in a minute, a power of approximately 16 kilowatts (21 horsepower).[citation needed] Finding this power insufficient for their works, the Dundee Foundry Company erected the second engine with a cylinder of 40 centimetres (16 inches) in diameter, a stroke of 1.2 metres (4 feet), and making 28 strokes in a minute. When this engine had been in continuous operation for over two years it had not only performed the work of the foundry in the most satisfactory manner but had been tested (by a friction brake on a third mover) to the extent of lifting nearly 687 tonnes (1,500,000 pounds), approximately 34 kilowatts (45 horsepower).[citation needed]

[Jack]

The last week or so I've been able to get my idea worked out a bit more. I hope the images are going to work.

https://www.dropbox.com/scl/fi/wk37wtve ... e5v3g&dl=0
This is the main rotary assembly. A "barrel" of about 25cm in diameter and close to 40cm wide. The top part would be in the fire, the bottom part is water cooled.

https://www.dropbox.com/scl/fi/vtf7wa08 ... x3lxi&dl=0
A look inside at the rotary discs.
All this is designed in a way I can get the parts made locally (we live quite remote). Mainly laser cut parts stacked on top of each other. I'm using steel on the hot side jacket, aluminium on the cold side. The discs are stainless steel.
In between the hot and cold side I've left space for an insulating space. I'll do that with layers of stainless steel and gaskets.
I've found a heat resistant glue for sealing between the plates on the hot side. And liquid gasket material between the plates on the cold side.
I have some concern about leakage, but I'm going ahead with this and see if it works.

This displacer has a swept volume of around 2000cc. The clearance around the discs is kept to 1mm everywhere. This is another thing that might or might not work, but I'm going to go ahead and try.

I'll mount a working piston on each side made out of an rolling diaphragm from a diaphragm pump. Initially I'll make an adjustable crank to play around with stroke and timing. Once the optimum position is found I'll design a permanent one that's balanced better.
The waterpump will initially be electric for testing, but I want to leave space on the axle to mechanically drive one later, if power production is adequate.

The mounting of the generator can either be directly on this axle or through gear ratio. This will depend on the torque and rpm I can get from this.

[VincentG]

That's quite a design. I've always been a fan of stacked plate assemblies for ease of manufacturing, like old school vise grips or master locks.

Leakage could definitely be an issue, but i'm sure you can work that out with good sealants. Have you thought about a small scale model to do some early testing? I think a rotary displacer will end up happiest with an opening of quite a bit less than 180 degrees to minimize hot and cold space mixing.

[Bumpkin]

Random thoughts from someone over forty years off-grid: I believe the engine linked to in the top post is the double-acting type, so a diaphragm would be difficult. I like the rotary displacer idea for simplicity. Since it eliminates regeneration it’s not really a Stirling, but that could be good or bad. The bad there as I see it is that a regenerator properly used also contributes to the heat exchange in and out of the engine; and of course without regeneration in the middle you’ll actually need more exchange at the ends. A negative double-whammy that might be difficult to overcome for serious power production. I cook on top of my wood-stove and run a thermo-syphon loop around the chimney into an upstairs insulated tank. It’s a converted electric water-heater, which has all the necessary plumbing and insulation and safety pop-off. I think a lot about capturing some watts from the side of the stove — the top and chimney are taken and I think the bottom would be too cool, but making such an engine civilized enough for in the house eludes me thus far. I’m instead working on a shop-stove project where some noise won’t bother. That said, I wish you best success Jack, so I can steal your ideas.

Bumpkin

[Jack]

That's quite a design. I've always been a fan of stacked plate assemblies for ease of manufacturing, like old school vise grips or master locks.

Leakage could definitely be an issue, but i'm sure you can work that out with good sealants. Have you thought about a small scale model to do some early testing? I think a rotary displacer will end up happiest with an opening of quite a bit less than 180 degrees to minimize hot and cold space mixing.

Yeah I need to run plenty of tests before I can commit to the final engine. I'm planning on building a single cylinder version first. But even before that I'll have to get single parts made and test the balance on things. But that's a fun process.
I've thought a lot about the 180 degree openings. The solution I've implemented here is that the ports are never both unobstructed at the same time. The first disc is designed in a way to "close" at least one of them and at the moment of switchover both will be covered for a short time. The idea behind that for me was to interrupt the flow of the air for a split second to prepare it to flow the other way.
This might be something that I can play with in the future, but for now there are too many variables already. First get it running and optimizing later.

[Jack]

Random thoughts from someone over forty years off-grid: I believe the engine linked to in the top post is the double-acting type, so a diaphragm would be difficult. I like the rotary displacer idea for simplicity. Since it eliminates regeneration it’s not really a Stirling, but that could be good or bad. The bad there as I see it is that a regenerator properly used also contributes to the heat exchange in and out of the engine; and of course without regeneration in the middle you’ll actually need more exchange at the ends. A negative double-whammy that might be difficult to overcome for serious power production. I cook on top of my wood-stove and run a thermo-syphon loop around the chimney into an upstairs insulated tank. It’s a converted electric water-heater, which has all the necessary plumbing and insulation and safety pop-off. I think a lot about capturing some watts from the side of the stove — the top and chimney are taken and I think the bottom would be too cool, but making such an engine civilized enough for in the house eludes me thus far. I’m instead working on a shop-stove project where some noise won’t bother. That said, I wish you best success Jack, so I can steal your ideas.

Bumpkin

Forty years! I'm just getting started with two years under my belt haha.
I like your chimney idea. It's something worth stealing for our hot shower water idea.

About the regenerator. I've lost track of that one in my quest for limiting dead space. The piston in a way could possibly act as a regenerator. The problem that's arising in my head is that this design makes the air flow through the piston space, in stead of back and forth. So a regenerator might be difficult to implement.
Maybe I can combine the hot and cold port on the way to the piston. Just thinking out loud here.

[Jack]

Don't be afraid to call me crazy here, but I litterally just had another idea that just sounds so simple that I'm 90 percent sure I'm missing something.

I'm always looking to combine parts and make things more efficient. Reciprocating motions are very crude in my mind. I don't like them. But often it seems we can't do without them.
I've been looking for a way to make the working piston rotary as well. It finally clicked.


This is the inside of a vacuum pump. I used to work with them a lot. And now I'm thinkin this might be the solution I'm looking for. This same concept is used for rotary vane engines that run on compressed air.

My next thought was, can I combine this with the displacer? So just make a long rotor and heat and cool one side of the cylinder/stator it runs in.

Two problems I can see:
1. Heating and cooling on opposite sides will "bend" the cylinder (don't know the correct english term here)
2. This is only a 1:1 displacer vs working piston ratio. Not sure that's enough for higher powers as I was aiming for 4:1 before. Maybe a rotating displacer can be added to support this "piston".

The vanes in vacuum pumps are made from graphite. Which has pretty good heat resistance.