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Even though this isn't a Stirling engine or even a hot air engine, I still think people might be interested and I've got nowhere else to post this haha.

I mentioned a few times that I was going to make my own post about this. Here goes.

As I mentioned in a previous post, my family and I started a homestead a few years ago.
We have a wood fired cob stove for cooking our meals.
It's a horribly inefficient, but carbon neutral way of cooking.
I'm trying to make use of a lot of the lost heat in that stove and fire to produce our own electricity supply. At least partly.

Because we would like electricity all day and we only cook at two moments of the day, lunch and dinner, we need a way to store either the heat or electricity.

Batteries are a pain, because they need maintenance and replacement.

Because I'm a mechanical engineer, and electrics are a little like voodoo to me, I like to solve things with a mechanical solution.
So storing heat in a sand battery seems like the best option to me. This will store the high heat from the fire and enable a slower release for the generator. Perfect.
And our stove has a small-ish unused area in it that would be perfect for a battery of that kind. It's a space of 60x50x30cm, but can be a bit larger if needed. Experiments will have to prove whether that will be big enough or not.

This battery will consist of copper piping looping through it, and the fire, to bring in the heat. I'm thinking of using glycerin as a medium there as it has a high boiling point and it's non flammable.
The copper piping will be surrounded by sand of course, and sandwiched between aluminium CNC machined plates. These plates are the heat exchangers for the generator and will be designed to try and turn the heat straight into kinetic energy, as opposed to pressure. I'll explain the reason for this later.
The fluid will pass from these heat exchanger plates to the turbine through a manifold which aims to increase the kinetic energy of that fluid as it moves through it.
A CD nozzle will be the last accelerating device before the fluid hits the turbine, hopefully making the fluid go supersonic.

The fluid I want to use is pure water and water vapor in a vacuum.
The reason I would like to avoid pressure and go for kinetic energy is twofold.
First because I would like it to be a self regulating closed loop system. Having a pressure difference between the hot and cold side would make this difficult/impossible without having to use an extra pump to pump the water back into the hot side.
The second reason has to do with the turbine design and the way they works best. I'll try to explain.

The turbine of choice is a Tesla turbine. It seems to me this is one of the most overlooked pieces of technology out there.
I plan to use it in a dual stage setup. Turbine and pump on a single shaft.
The fluid coming out of the CD nozzle will go through the turbine into the centre. The centre passage is directly connected to the centre passage of the pump and is basically a vacuum chamber inside a vacuum setup.
This dual stage setup is again a solution for two problems. Hopefully.
One is that the pump will help the fluid go through the turbine more efficiently. If there's no pump the turbine will create a centrifugal head pressure. While some people swear by this to turn it into a self regulating system, I see it as very inefficient and wasteful of heat. It will create an outer ring of backpressure around the turbine, preventing new fluid from coming in. This pressure ring, in my theory, massively reduces the speed of the perifery and thus reduces work output in the most important area of the turbine.
The second reason for the dual stage setup is to make the system only use the heat it can handle and turn that into electricity.
My reasoning is that the pump, turbine and nozzle will be designed to a certain output and will only take out the amount of fluid, and the heat this is carrying, it needs to put out that amount.
An adjustable nozzle will be necessary to account for changes in temperature in the battery.
If the hot side/battery would have pressurized fluid, neither of these two advantages would be possible. Hot fluid would be pushed through the nozzle and waste that heat. In stead I'm trying to only take as much heat as needed to accelerate the fluid. At the exit of the CD nozzle the fluid will hopefully be moving at supersonic speed and have cooled down considerably.

After all that, the pump exits in the cold side/condensor. From here the fluid should be able to freely flow back into the battery as needed.
If the battery is too hot, some slight pressure will build up and push down the fluid, out of the battery, only to come back when the turbine has used the energy that's in there. This way the system regulates its heat input, again to only use the heat that is needed.

I've designed the whole system already, but I'll start with testing the disc spacings of the turbine and pump. After I get that right I'm planning on testing the turbine/pump ratio.
I plan on doing this by mounting them on separate shafts and comparing the input and output power.

These tests will give an initial insight into whether this could work or not.

When I figure out how to properly add pictures to a post I'll add some screenshots of my designs.

This is the system I have in mind. This is a closer view of the heat exchanger plates and first draft of the manifold. This is my design for an initial test setup. With this I can change the pump size and disc spacing and measure the vacuum at different rpm. Later I'll add the turbine part where I can test the same variables and eventually measure work in vs work out. If I'm happy enough with all the results I'll build this vacuum setup to be able to test the whole engine as intended, but on 1/20th scale. After this produces the results I'm looking for I can build the full setup.

Jack wrote: ↑Tue Jul 09, 2024 4:13 am Experiments will have to prove whether that will be big enough or not.

Experiments are great, but we can at least make a calculation of the max possible energy it can store, if we know:
The volume;
The type of fill - is this just ordinary sand?
The temperature of the source, ie the fire itself

I did some calculations, I'll look them up later. But the conclusion was that there will be enough potential energy, but it's a matter of getting it out and converted into work efficiently. That's what the testing is for.

Jack wrote: ↑Tue Jul 09, 2024 5:31 pm ...
If I'm happy enough with all the results I'll build this vacuum setup to be able to test the whole engine as intended, but on 1/20th scale.
...
After this produces the results I'm looking for I can build the full setup.

Just curious, if you don't mind my asking, but building such a sophisticated machine even at 1/20th scale looks like a very involved rather sophisticated, complicated, challenging and potentially very expensive project, then to go on and build a full scale version 20 times the size...

If it were a DOE backed project, it would probably be allocated at least 5 or 10 million I'd imagine, but you see no obstacle to completing this project?

I'm not trying to be discouraging, I'm just amazed you have so much confidence and the necessary resources to move forward.

Just your CAD illustrations are incredibly detailed, sophisticated and professional. I took a course in 3D CAD drawing and after a year could still not produce anything of such detail and quality, and I dare say, neither could our instructor.

I'll take that as a compliment, thanks!
I'm confident I can do it, but it is to be seen if this confidence is reasonable haha.

The engineering side isn't an issue, but the financial side might turn into one.
I'm lucky that I live in Vietnam, things are comparatively cheap, just difficult to source quality parts.
I'm hoping to build the initial test model for under 2000 dollar.
Everything after that depends on the results. And I'm trying to design everything in a way that I can use the parts for the next step as well.

Bravery and stupidity are only distinguishable after the fact, so let's see which one I am haha.

Early morning rambles/thoughts.

Ever since I've been trying to find a solution for my stove heat challenge I've looked at piston engines and, being an instinctual person, they've always felt off to me.
I used to meddle in cars as a young kid, so I have experience with them, but now that I look at the theory up close they don't make sense.

Imagine my surprise when, after a long researching period, a friend pointed me back to Tesla turbines. I had dismissed them for my purpose before as being too loud and too high volume. This time things finally clicked for me.

My theory with this generator is the following. If heat turns into temperature and temperature is the kinetic energy of molecules, why are we trying to use that to make pressure? Why not try to use that kinetic energy directly.
If I can, with my nozzle, direct and straighten these movements into a condensed laminar flow, I can make use of this internal and external kinetic energy that's being pointed in the same direction. Maybe they'll even amplify each other? I'm not versed enough to be able to predict exactly what will happen there.

A Tesla turbine can capture this thrust/kinetic energy in a way no other turbine can. And in stead of just catching the energy of molecules bouncing off it, like normal turbines, it catches this energy from adhesion of the boundary layer. And the turbine creates a long path for the molecules to give off their energy as they "rub" (technically it's not rubbing, I know) along the discs until they exit the turbine in the centre. In stead of one bounce in normal turbines or a few increasingly weaker bounces in a piston engine.
The better I can straighten these molecules and give them the same direction, the more energy I can extract from them.

The pump, behind that turbine, that's doing all the work only has to put back in the energy that is needed to bring the fluid up to pressure or temperature in the cold side. Ideally no more, but that's the forever challenge I guess.
I look at the pump as a gasoline pump. The amount of fluid it pumps barely has any relation to the amount of energy that fluid is carrying.
Tesla talks about a pump:turbine ratio of 3:1 being effective, but my purpose goes a bit beyond that he was trying to achieve. So I'm making sure I can test up to 5:1 and see what the differences are.

I'd love to hear anyone's thoughts about this.

Reminds me of some old videos Jeremiah posted about 5 years ago, about his Tesla turbine "variant".

Out of order I think, and possibly not the same turbine, not sure, but having ice building up inside the turbine.

https://youtu.be/c5NzHvGxTss

Not sure that was the actual reason, but he was heating up the turbine with a heat gun and that increased the RPM.

There doesn't seem to have ever been any more about it after that though, which was disappointing.

https://youtu.be/SJ4hCCaW_Y4

I suppose the extra heat increases the rate of expansion of the gas inside the turbine.

A rather strange, or unexpected consequences, (apparently?) was a reduction in pressure. I assume that was the input pressure which was being regulated and monitored.

I can only speculate that perhaps the gas heating up inside the turbine would speed up and leave the turbine sooner? The turbine spinning faster would evacuate gas from the turbine faster? Reducing the back pressure?

Or? It crossed my mind it could simply be mechanical. The bearings/lubricant was cold, the extra heat reduced the viscosity?

Difficult to say from these videos. It's not really a Tesla turbine he's using in these videos, not on the pump side for sure.
If I had to guess I'd say the heat is giving the fluid extra energy to do work. Once a certain rpm is reached the pump becomes more efficient and helps evacuate the turbine. This all reaches a point where the regulator can't provide enough fluid anymore.


I don't know why Jeremiah steered away from testing the dual stage. I wish there was some more info about it out there.

Hey guys, long time no see.
I've been busy with some stuff around the house and the first parts that are coming in. I need to reorganize a bit so I have more work space for all this.
These and many other parts have come in and make this all very real. I'm coming down from dreaming this all up to organizing everything so I will have some very clear results to show.

This also made me realize how expensive this is getting real quick. I can carry some of it, but getting to a full functioning system might bankrupt me, even if all goes well.
So I've decided to see if I can get some crowdfunded research going.
I'll be testing the first parts soon and with the data from that I hope to prove I'm going in the right direction.

I'm not looking to get rich from this, just to be able to find out if it works as I think it will. And then spread the knowledge open source.

It's a shameless plug, but I think I'm doing it for the right reasons. And I really think I've got something here. Let's see.

No this isn't any more of that spam that's been flooding this forum. Follow the link and you'll see my face right there.
https://www.patreon.com/apengineering
Tell me what you think.

Nice parts Jack. I have my own reservations about tesla turbines but I hope it works out for you. In general I could see your system working better at large scale, kind of the opposite of most of these engines.

Thanks!

I think they're misunderstood. But I have no first hand experience with them yet.
So first I'm going to be trying a lot of things.

These turbines actually work better the larger they get. So you could be right about that.