Solar Powered LTD Twin Stirling Engine Generator (Gamma)

[simonchan]

Hi everyone~

I am required to design and fabricate a Stirling Engine as stated in the subject above for my Final Year Project. Since I am a newbie/beginner on this matter, I would like to get some comments/opinions from you guys to assist my project along the way.

1. From what I observed, most of the Stirling engine uses stainless steel as the hot/cold side. Can I opt for copper plate instead of stainless steel? As I would like to increase the thermal efficiency by using material with high thermal conductivity.

2. Corrugated stainless steel sheet as my regenerator, possible?

Thanks in advance.

Simon Chan

[Ian S C]

Simon, is this final year at university, or secondary school/collage?
Don't use copper. Use stainless steel for the hot cap, and the displacer, the heat through the hot cap only has at the most about a mm to travel. The important thing is that we must restrict the conduction of heat from the hot end to the cold end, copper would do this very quickly. Also the hot end would suffer from the heat causing the metal to flake, the copper would become fairly soft, and distort.
If you are going to make a regenerator, one of the best things to use is Stainless foil .001"/ .025 mm thick, dimpled with a seamstress's tracing wheel.
A good book (it's free), worth looking at is "Making Stirling Engines" by Andy Ross.
Ian S C

[simonchan]

Hello Ian S C,

It is my final year at university actually, but this is my first time building a stirling engine. =)

I wish to use PVC pipe to make the cylinder, so the conduction of heat from the hot end to the cold end will only occur thru the bolts. However, copper might not able to withstand high temperature during afternoon hot sun in Malaysia (around 90 deg Celsius). So, 1mm stainless steel plate would be sufficient? Perhaps I would paint it with black colour to absorb more heat from the sun.

Yeah thanks for the suggestion on the regenerator! And managed to find your recommended book in the net already. Thanks!

By the way, based on your experience, is ball bearings necessary for LTD stirling engine?

[Ian S C]

If you can, use thinner than 1 mm for the stainless, a SS cylinder with an end on it can be made by TIG welding a disc on the end of a tube.
If you are using PVC at the cold end, I imagine you are going to use a diaphragm instead of a piston on the power side of the motor.
Are you going to use a parabolic reflector, or a Freznel lens, a large lens will drive the motor marked as my second motor in my gallery. Ian S C

[simonchan]

Wow, thinner than 1mm stainless steel? Will it bulge easily after prolonged operation? By the way, my uni don't have TIG welding, only have SMAW. However, silicone glue is possible right? As I plan to use PVC pipe as both the power cylinder and displacer cylinder.

Yeap you are correct. I plan to use a diaphragm on the power side. However, I not sure what kind of material is best suits on this purpose. Few materials come into my mind such as balloon, duct-tape, and shock-absorber dust boot.

Nope, I am neither using parabolic reflector nor Freznel lens, as these are too complicated for me already. So, I suppose ball bearings for crankshaft and con-rod are optional in my case right?

[tomostre]

Hey guys,

I saw this earlier, its pretty hard to beat I think... Twin LTD Solar stirling

[Ian S C]

Have you looked at any low temperature engines, you will note that the displacer cylinder is a large diameter compared to its hight, and the diametric ratio between that and the power piston (on my one), is about 6:1. For your diaphragm, a balloon, or surgical glove is about the right weight.
The short, large diameter displacer cylinder is best made of PVC, or some form of plastic. Glass may also be used. Make the top and bottom plates from aluminium sheet (I used the bottoms of 2 frying pans). Don't join the 2 plates directly together with metal bolts, as this short circuits the heat path. If you heat the top, and cool the bottom, the motor will run in one direction, and if you turn the motor over, and heat the bottom,etc etc the motor will run the other way. If you have a PVC plate on the bottom, and sit the motor on a bowl of boiling water, it will go soft.
Don't worry about stainless, you won't need it for a LTD motor, it's only when you get on to high temp motors that you need to get on to this sort of stuff. Ian S C

[simonchan]

Halo tomostre, I guess there's a disadvantage on the stirling engine design as shown in the video right? As there will be "shadow" created by the parts, which reduces heat received by the top plate.

And thanks for the great advice Ian S C!! I plan to use 6''x3" PVC tube for displacer cylinder and 1.5"x1" for power cylinder. As for the crankshaft as shown in the image, is it better to cut and paste using Loctite as shown in your gallery or by bending it directly from a 3mm diameter shaft?

https://www.dropbox.com/s/xsyctso7xc4g5 ... 0image.png

[Triangle.Stirling]

Hi simonchan,

1 mm stainless is a bit thick, if you look up our engine '2-Cylinder Gamma Stirling - Project' somewhere down on the index page, our cylinders are .45 mm thick. However, you're going to use aluminium i think.

The pvc, what is the diameter of it, on the inside? I'm not so familiar with the imperial system. Is the diameter 6 and 1.5 inch?

a crankshaft made with loctite can be better. You can assemble it straight, it might be easier than bending, however, it wasn't strong enough for our engine. That had also to do with the gap between the parts, they must be very thight. In your case it's maybe better than bending, more accurate. Bending is also possible, but it's less precise, and maybe more flexy.

Do i understand right that you're going to build something similar as in the link from Tomostre? Keep us updated!

[Ian S C]

My LTD motor has a displacer cylinder about 6 1/2" /160 mm diameter x 1 1/4" / 31 mm high. The displacer is 6"/ 150 mm dia. x 1/2" / 13 mm thick, made of foam plastic. The crankshaft is a short rod mounted on ball races, with a crank at each end. Ian S C

[simonchan]

Hi Triangle.Stirling,

Yeap, after further considerations and consultation with my supervisor, I will use copper/aluminum coated with black paint.

Yeap you are correct, 6 inch diameter for displacer cylinder and 1.5 inch diameter for power cylinder.

Yeah the problem by cutting the crankshaft is that there are too many separate bodies already. Perhaps I would try bending it first, see how it goes later.

Okie. I will upload the model in solidworks drawing once I finished^^

Hi Ian S C, half inch thick displacer? Wouldn't it be flimsy?

[Ian S C]

If you choose the foam best you can, it's quite OK. Mine started as a block about 2" / 50 mm thick, and big enough to get a 6" / 150 mm disc. I made a hot wire cutter, and sliced it into 1/2" / 13 mm thick slices, then rigged up a jig to cut the slices into discs, 3 spare just in case. Mine was a little crumbly, but I had some spray paint that was for painting Christmas decorations made of foam, I used that, and it all holds together.
Another good way of making a light weight displacer is to use balsa wood. A thin disc top and bottom, some ribs across inside, and a thin strip around the edge. Ian S C

[simonchan]

Owh okay. In fact, there's a stationery shop that sell disc-shaped foam with the appropriate size that I need over here. So it is pretty easy for me on the displacer part. ;)

However, I still haven't figure out how to connect the foam displacer to the connecting rod yet. Any thoughts on this?

[Ian S C]

The way I connected mine, I put a hole through the middle, the rod I used was a bike spoke, I put a nut on first, then an aluminium washer about 1" diameter, then a short bit of tube, this went through the foam, the another aluminium washer and a nut. Ian S C

[simonchan]

Huh? But there's no thread on the bike spoke. You mean you place the nut by putting glue?? or weld??