Building a parabolic with mirrors.

[SolarFreak]

it 45 2" mirrors focal point 18"

 

Im setting up a blog and gallery software so I can post better pics, maybe later today it will be up. yeah that one sterling engine is done for.

[Corona]

it 45 2" mirrors focal point 18"

 

Did you buy the mirrors that size or did you cut them out of a larger mirror?

[SolarFreak]

In my area only a few places sell mirror and glass. Here its not like in the states where you can go into almost any home center and buy mirror tiles. The home centers here told me to goto a glass and mirror suppler. I ordered 6 12" X 12" mirrors, around $50 usd here, crazy. If I was in the states they would have been about $1.50 each.

 

anyway I was going to have them cut them down to 2" X 2" for me. but that was $35 more. I picked up a glass cutter and and cut my own mirror down size

[Corona]

You are right on about the price. I just bought a bunch of 12" x 12" mirrors tonight and they were $1.48 each.

 

You used a focal point of 18" on your system. If you changed the size of the mirrors to 4" x 4" would that change the focal length you would use?

[SolarFreak]

well I got the gallery started on my site

http://heliospheric-labs.com/gallery/v/solar/mini+parabolic/

I didnt have much time to work on it this weekend but here a few pictures.

[SolarFreak]

You are right on about the price. I just bought a bunch of 12" x 12" mirrors tonight and they were $1.48 each.

 

You used a focal point of 18" on your system. If you changed the size of the mirrors to 4" x 4" would that change the focal length you would use?

 

 

yeah I wish I could them that cheap here.

 

4" X 4" mirrors I would go for a focal point of 36" - 40"

but it really depends on how wide your radius of your mirror array is going to be.

 

how many rows are you planning on doing?

[Corona]

Thanks for posting all the pics of your project. I will take pics of my attempt when I start in a couple of days.

 

I was going to do a type of trough design but while in the shower this morning I had an idea I want to try that is more like what you did.

 

I believe the material needs to be headed up to around 450 F to become charcoalized. Do you know for how long a period we need to keep it at that temp?

[EStein]

yeah I wish I could them that cheap here.

 

4" X 4" mirrors I would go for a focal point of 36" - 40"

but it really depends on how wide your radius of your mirror array is going to be.

 

how many rows are you planning on doing?

SolarFreak, WONDERFUL PICTURES!!

I think a some of us are trying to understand the concept of a flat mirror's focal point. I think you mean aperature and the associated distortions. The larger the aperature, the less distortion. This is not much concern when focusing heat and light for your purposes. The greatest concerns are finding a material that will take the abuse and not be too reflective. Larger units actually use a coolant in the collector, liquid sodium, for example, which in turn is used as a working fluid. The working fluid could power your Stirling engine(s) but would need to be returned to the collector after the heat was extracted by the engines. Otherwise, you could experiment with using a fan to cool the radiative side of the Stirling engine. The fan would use a relatively small part of the total energy produced.

[Turtle]

SolarFreak, WONDERFUL PICTURES!!

I think a some of us are trying to understand the concept of a flat mirror's focal point. I think you mean aperature and the associated distortions.

 

See post #7 of this thread for an explanation of a flat mirror's focal length. :cup:

[arkain101]

I have seen someone build one out of tin or other kinds of shiny sheet metal and it created alot of heat!

 

You could construct a wooden face and then place any kind of reflective material on that face. The more smoother and equaly curved you make it the more powerful the device will become, obviously by taking a few square feet and condensing it into a square inch.

[EStein]

See post #7 of this thread for an explanation of a flat mirror's focal length. :cup:

I looked at the link in post #7. Technically speaking, that is not a focus. Smaller mirrors will create a smaller dot but under magnification any individual smaller dot is no more in "focus" than the larger one. The difference in using smaller mirrors is that the more compact arrangement of smaller mirrors used more closely approximates a true parabolic shape. Then, the combined smaller dots at the focal point produce a sharper image. Then, there is the whole thing about aperture vs wavelength affecting acuity which, for the discussion here, is irrelevant.

[EStein]

I have seen someone build one out of tin or other kinds of shiny sheet metal and it created alot of heat!

 

You could construct a wooden face and then place any kind of reflective material on that face. The more smoother and equaly curved you make it the more powerful the device will become, obviously by taking a few square feet and condensing it into a square inch.

A friend and I made a beautiful one in the 1960's using cardboard, thick construction paper, tape, and aluminum foil. It was about 5 ft in diameter, had about a 4 ft focal lenght, and roasted a mean hot dog.

[arkain101]

hehehhe,,,

 

get'er'done.. !

[EStein]

hehehhe,,,get'er'done.. !

Think about this. The surface of a rotating cylinder of fluid assumes a parabolic shape. This can be shown by calculus taking into account these forces:

1)The constant downward force due to gravity.

2)The outward force that is proportional to the rotational velocity times the square of the radius.

Picture a fruitcake tin filled halfway with water, centered on an old record player turntable that is spinning at 33 1/3 rpm. Would anyone care to compute the surface focal length?

What would the surface look like if the tin was off centered?

[SolarFreak]

Think about this. The surface of a rotating cylinder of fluid assumes a parabolic shape. This can be shown by calculus taking into account these forces:

1)The constant downward force due to gravity.

2)The outward force that is proportional to the rotational velocity times the square of the radius.

Picture a fruitcake tin filled halfway with water, centered on an old record player turntable that is spinning at 33 1/3 rpm. Would anyone care to compute the surface focal length?

What would the surface look like if the tin was off centered?

 

 

I dont think water would be a good choice, but liquid mercury on the hand would work fine.

 

I think 33 1/3 might be ok

where; L = focal length g = gravity w = angular speed ( in radians/sec )

2 L = g / 2w

 

The problem that I think of 1st is with the liquid spinning it would make tracking the sun or your traget if it was used in a telescope. the liquid would have gyroscopic effect so if you move it to track an object it would make waves or ripples.

 

 

but I have heard of people building a parabolic using using epoxies in spining dish. after it sets up you have your parabolic

 

here is a good link

 

http://sci-toys.com/scitoys/scitoys/thermo/liquid_metal/4m%20International%20Liquid%20Mirror%20Telescope%20Project.html

[EStein]

I dont think water would be a good choice, but liquid mercury on the hand would work fine.

 

I think 33 1/3 might be ok

where; L = focal length g = gravity w = angular speed ( in radians/sec )

2 L = g / 2w

 

The problem that I think of 1st is with the liquid spinning it would make tracking the sun or your traget if it was used in a telescope. the liquid would have gyroscopic effect so if you move it to track an object it would make waves or ripples.

 

 

but I have heard of people building a parabolic using using epoxies in spining dish. after it sets up you have your parabolic

 

here is a good link

 

http://sci-toys.com/scitoys/scitoys/thermo/liquid_metal/4m%20International%20Liquid%20Mirror%20Telescope%20Project.html

What a great response, SolarFreak. Thanks also for that wonderful link. Yes, water is not the best choice. When I first heard of the spinning liquid problem, it was in a calculus class and we were to prove the surface was paraboloid. I wanted to see what it would look like so I put a pan with water on a record player. I remember having a candle in my hand and turning out the light. When I put it over the spinning pan, an image of the candle was floating over the focal point. I later used some kind of polymer and hardener to make a permanent "mirror". Later when some college students from MIT did it, they had a whole article in Scientific American(February 1994). There have been many projects done using the principle and several "spinning telescopes" using mercury have been made.

As far as the off centered tin of water, it makes an off centered focus. This is perfect when building a telescope that needs no secondary mirror. I actually built a very crude telescope using this technique using a magnifing glass for an eyepiece. Without the off centered focus, one's head keep getting in the way.:hammer:

[Turtle]

I was going to do a type of trough design but while in the shower this morning I had an idea I want to try that is more like what you did.

 

I believe the material needs to be headed up to around 450 F to become charcoalized. Do you know for how long a period we need to keep it at that temp?

 

The problem with making charcoal in a dish oven is that it allows only a limited volume of material in the receiver, wheras the parabolic trough oven is extendable to accomodate a large volume receiver.

 

The period of time required to charcolaize a given volume of material is going to vary widely depending on the dryness of the material, the type of material, how compact the material is in the receiver, as well as the amount of Sun light available & the ambient air temperature surrounding the receiver.:)