Stirling Engine
What I'd really like to do now is put together a concise explanation of the challenges that make a Stirling engine impractical.
Impracticality
https://www.youtube.com/watch?v=kpxTdLWmZVc
Above is a 1W Stirling engine. A decent gas generator produces about 1500W and costs about $1000. So we'll set a conservative target of 1000W for $2000.
The problems with Stirling engines show up when we try to scale up the engine. Let's start by looking at the options individually before we look at the drawbacks and finally the fatal blow - how these options tend to be exclusive.
Base Improvements
Size
The energy flowing through the engine that is converted to power is limited to the surface area of the hot and cold side. So increasing the size of the engine only grows the power relative to the surface not the volume. From this perspective, the most efficient shape is very flat (a pancake).
Heat Transfer
The engine can be made more efficient by increasing the heat transfer. We can do this by changing the material to increase thermal conductivity or adding internal fins.
Pressure
By increasing the gas inside the engine, we increase it's power out put. This means increasing the pressure. Doubling the pressure would double the efficiency.
Working Fluid
The working principle behind a Stirling engine is changing the volume of the gas by changing the temperature. But this ratio is not fixed. Helium and hydrogen will produce more change with the same heat differential than atmosphere.
Temperature
Increasing the difference in temperature will also increase the power.
Drawbacks
Material
It would be appealing to use copper because of it's high thermal conductivity. But it's both very expensive and not very strong. So if the internal pressure is going to be much higher than the surrounding temperature, this won't work. Steel would provide a lot of strength, but it has very low thermal conductivity. The best bet is probably aluminum. It's a happy medium in terms of both strength and thermal conductivity and it's cheap.
There's a temptation to built body from aluminum and add copper fins, but then you deal with galvanic corrosion. Though if the working fluid is helium or hydrogen, this won't be an issue because there's no oxygen to from "rust".
Seals
If the internal pressure is higher then we need to address leaks. There are three good solutions to this.
The first is to have seals that don't leak. But this is expensive and drastically increases friction.
The next is to use good seals that only leak a little and actively pump fluid back into the system to maintain pressure. This requires a pump and drastically increase complexity and steals some of the power.
The third is to create a pressure vessel that the entire engine sits in. This increases the material cost.
Best Design
Roughly speaking, the best design I could come up with was a high pressure aluminum tank filled with helium. The entire engine, including a generator sits inside the pressure vessel.
Version 0.1
Design
Two beta style engines coupled together.
Piston
I want to use a piston instead of a diaphragm because it has a larger displacement, more displacement equals more power. I want to build my diaphragm using a steel wool core and some small pieces of metal (coat-hanger?) to reinforce it, pour concrete over that and seal the whole thing with silicon. This should allow me to make any shape I want while still getting a good smooth seal.
Notes
I saw a video using metal water bottles for the chamber - metal and already very smooth.
Saw a cool design that used a 90° linkage so the piston and displace could run off the same crank. But I might still need bearings for all the joints.
Another idea would be to give each cylinder it's own small crankshaft and transfer the power back to a central shaft via a timing belt.
Combustion Heat Transfer
I would like to build a wood burning model and attempt to produce a very efficient heat transfer.
The cold portion will be water cooled. The pump is operated by the engine. The water reservoir will have a heat sink. A fan will pump air over the heat sink. That air (now slightly warmed) passes through a heat exchange with the exhaust. It is then forced in to the combustion chamber (wood burning). The exhaust from the combustion chamber is then forced back through the heat exchange, where most of the energy is reclaimed.
I'd also be interested in finding a way to reduce the emissions by filtering the particulate out of the exhaust. A filter would be an easy way to do this, but I'd also be interested if it could be done with centrifugal separation or ionization.
An Alpha configuration may work better. The entire hot section could be in the chimney of a rocket stove, for very high heat transfer. The cool section could be water cooled, far enough away from the heat. On second thought, making the piston would be hard and the shaft would still be right in the heat. Better would be a heat exchange. And the Stirling engine could power the fan. Rocket stove could be smallish, probably use a metal trash can insulated with perlite. Use metal ducts for the inside. The Stirling engine chambers would be pretty easy, probably metal pipes. The piston end shouldn't get too hot, so a concrete piston should work just fine.
Design 2015-10-05
The power source is a wood burning rocket stove. The outside will be a metal trash can. The burn chamber will be made from metal ducts and the whole thing will be insulated with perlite.
In the chimney will be a heat exchange. It looks like the best design is a coil of copper tubing. Rather than some complex multi-coil, I'll but another perlite insulator in the center. The intake will be hooked up to centrifugal blower, run by the engine.
The engine itself will be a beta design with 3 cylinders. The pistons cylinder will just be pipe, probably PVC because that part shouldn't get too hot. The piston itself will be concrete sealed with silicone. I'm not yet sure what the main cylinders will be made of. They need to stand up to a lot of heat and a medium amount of pressure. I'd like to find a 4" metal pipe, but that seems to get pretty pricey. I'll keep looking. The displacer will be filled with aluminum shavings, which I can get very cheap. These will act as a heat reclaimer. The cylinders will sit in a structure to manage the heat. It has 3 layers, the middle only exists for insulation (again, perlite). The bottom chamber takes hot air from the heat exchanger. The top has cold water running through it. The hot air and cold water should run in opposite to directions to help keep the temperature difference in the cylinders even. The water will be circulated with a basic pump, run from the engine.
I want to make the bearings myself for the crankshaft, it looks pretty easy (pvc pipe + BBs). The bearings will be held in place by 3 screws allowing for fine alignment. The crankshaft will be a little tough, but I don't think there's actually anything tricky about it. Lastly, I'll make a flywheel out of concrete.
The outlet of the hot cavity will still be quite warm, it should be fed back into the rocket stove for efficient combustion.
I'd really like to shoot for something in the 5hp range, then it can be used in some other projects I'm interested in.
Use Aluminum Cups for the cylinders. They're a bit tapered, but I can shape the displacer to match.
Pressure
These engines perform much better when the air is at pressure. I'd like to experiment with this. I should be able to get a good seal on the shafts with silicone. I can do the tests with a bike pump. When I have an engine that operates near the power I want I'll hook up an electric pump and a pressure sensor.
Fins
It would be ideal for heat transfer if I could put some fins in this. For the heat chamber, horizontal plates to transfer heat from the air. Inside the chambers, vertical fins that could mesh with the displacer.
Cost
- Trashcan $16
- Ducts $7.60
- Aluminum Cups $22
- Perlite (ebay) $20
- Copper tubing $15
- Concrete $1
- Aluminum shaving $12
There's a lot of additional, small assorted hardware, but I don't expect that to be more than $30. Total: $100
Questions
- What is the proper volume ratio between the cylinder and the piston for a given temperature difference? this may help
- What is the expected power output based on piston volume and temperature difference?
Calc
Cups have a volume of 2cups, so 4 cups total per cylinder. I expect the hot side to be around 700F (just a guess) and the cold side to be around 80F.
One calculator suggested 60w/cylinder, but I don't know if I used it right.
I should be able to measure the power if I know the flywheel weight. By measuring the angular acceleration (and max rpm) I should get the power.
2015-10-06
Already significantly rethinking my design from yesterday.
The core problem in a stirling engine is heat transfer. So surface area and thermal conductivity are key. I also want a high pressure, but it doesn't need to be super high, probably 20psi would be great (needs more research).
Thermal Conductivity
Btu/(hr F° ft)
Need to do cost calculation - for the same conductivity how much more aluminum than copper is needed.
- Copper: 220
- Aluminum: 120
- Steel : 20
Design
Now I'm thinking pressure cookers. Or an aluminum hemisphere - pans are available, but I'm not sure they're thick enough. Or, if I'm deciding on aluminum, I can go back to looking at forges because aluminum is the easiest to work with.
Crankshaft
This will be difficult, but not tricky. Proceed carefully. Each segment is the same a centered piece, off-center, another off-center 90° out of phase then back to center. So each segment requires 3 bearings, plus one more at the end. 10 bearings. The parts will be steel. To connect two segments, I'll drill a small hole and put a pin in it. During the testing phase, I can lose fit everything. When I actually assemble it, I'll get the pin hot, then hammer the ends down. As the pin cools, it will shrink making a very tight fit.
As I build each segment, I just need to make sure it's balanced. That shouldn't be too hard with the bearings.
Cost
- Aluminum plate: $0.30 per sqin @ 1/4"
- Aluminum Angle: $0.12 per in @ 1", 1", 1/8"
- Heat resistant Silicone: $18
- Bearings: ~$0.40