Solar Engine Questions

[trewhoffman]

Hello everyone

I am attempting to build a beta type engine that uses a 3" diameter piston cylinder. I would like to heat this via fresnel lenses, but I am unsure of how much heat the lenses produce and what my piston/displacer dimensions should be relative to that. From what I've read, a lens may be able to make 200C on the hot end. I also plan to water cool the cold end, but I'm not sure how effective this will be. The piston would have an aluminum casing. Does anyone have an idea of the temperature differential I could get with a setup similar to this? And would something like this classify as a low, medium, or high-temperature differential engine?

Similarly, my plans at the moment have a 1.3:1 ratio of displacer swept volume to piston swept volume. I believe this ratio is more suited to high-temperature engines, so should I change to more like a 2:1 ratio? Or rather, should I keep my 1.3:1 ratio and just get a larger fresnel lens or parabolic mirror?

I am new to this so any information regarding solar engines would be helpful, thank you!

[Tom Booth]

In my general experience, anything hotter than boiling water is considered "high temperature". Engines that can run on a cup of hot water are considered "low temperature", though I don't know of any set standard, that seems to be the general consensus.

What size is the fresnel lense? A fresnel lense in full sun could easily reach thousands of degrees at the focal point

A fresnel lense could melt aluminum almost instantly.

[trewhoffman]

Ok good to know, thanks.

At the moment I have a 11.75x8.5 inch (kind of odd dimensions) fresnel lens. I’m located in Texas so I should have a good amount of sun intensity. I tested the lens out yesterday and it easily burnt a patch of dead grass but couldn’t do much else. I didn’t have it focused on a single point for more than a dozen seconds, though, so I’d bet it could build up a lot more heat when focused on the bottom of my engine over time.

If a lens this size can melt aluminum (or even if it can get decently close) I should have as much heat as I need!

Thanks

[Tom Booth]

That's not a very large lense, and a 3" piston is rather big.

Probably you would need a larger lense for an engine of that size.

Comparatively, the engine in the video looks like it may have close to a 3" bore but the fresnel lense looks many times larger than yours.

https://youtu.be/fUrB7KRvxUk

[trewhoffman]

You make a good point.

I will run the engine off of a flame of some sort at first for testing purposes to ensure it works somewhat efficiently and then switch to solar power later I suppose. I expect that you’re right and I will need a larger lens, or large mirror.

Do you know if a mirror is better than a fresnel lens in my case? I’ve read the mirrors are more widely used but am unsure why. As I’ve seen from my experience with astronomy, though, large parabolic mirrors aren’t cheap.

Thank you for all the help.

[Tom Booth]

What is the intended use? Any? Or just having fun

I haven't tried it, but Someone told me many years ago that they made a useable parabolic dish using cheap aluminized mylar.

[trewhoffman]

Mostly for fun (certainly not to power my home or anything), although I’d like to get at least a 10 watt output. So, I wouldn’t want to buy a $100+ mirror or lens but polishing or making one may be an option. I believe greenpowerscience has some videos about that I will look into.

[Sockmonkey]

Heck, you could line a parabolic dish with tinfoil and it would still work.
Now, since it's focusing on the end of the cylinder rather than a tiny point, you can get away with the mirror being a section of a sphere rather than a proper parabola.
Now if you still want that parabola shape, there's an easy way to do it.

See how the shape is bent more at the center? If you have a rope or heavy string hanging between two points, it naturally assumes a parabola shape.
Hang it on a sheet of cardboard or something and trace the line it forms. That will give you a template for the ribs of a parabolic reflector.

[trewhoffman]

Great info! I’ll look more into the tin foil satellite dish option and post anything helpful i find in this thread, or if i complete the project.

Thanks for all the help everyone.

[Tom Booth]

I wasn't actually looking for this, but it popped up in my YouTube feed just now, today. Probably because I had searched on the subject a few days ago.

Probably the best idea for a cheap DIY parabolic dish I've stumbled across. I may give it a try myself.

https://youtu.be/8CLRTa_ocmo

I had an idea for a solar grill, years ago, using only a smaller section of a parabolic dish, but having constructed the dish from plywood strips and aluminum foil, it didn't stay together long. It seemed to work well enough to test out the concept, but using the method in this video looks a bit more substantial.

I think there are better grades of aluminized mylar available though. In my experience, those space blankets don't last that long. The aluminum flakes off after a while.

I also saw some mirror finish spray paint at the hardware store that might be worth a try.

[trewhoffman]

Yes I just saw that video a few days ago a matter of fact! I also agree that the aluminum could be flaky...

I’ve seen that there is “mirror tape” that supposedly has 90%+ reflectivity. I haven’t run into that at Lowe’s yet but it might be worth a shot for the sake of simplicity. Sounds like a similar option to the mirror finish spray paint.