building a parabolic with mirrors.

[SolarFreak]

I’m building a parabolic reflector this coming weekend. I will be using 2in^2 mirrors.

I’m trying to calc the power output and what temperature will be at the focal point. My 1st goal is to reach 500f.

And will be using a 2in^2 mirrors, I’m also trying to calc the optimal focal point of the mirrors.

The sun is about .5 deg wide at the earth so, I can find what the max focal length would be before blurring of the reflected sun would be.
a 2 inch mirror max focal is 19 feet
(2 / tan(.5)) / 12

So far this is my estimated output power per row of mirrors.
http://forum.teamfc3s.org/parabola/output.php
Is my estimated power output correct?

A question about solar constant: 1366 ~ 1370 W/M^2 above the atmosphere and 1000W/M^2 on the ground at noon.

How do I calculate what my solar constant would be at my location?
Latitude: 26° 10' North
Longitude: 127° 40' East

I will post some pictures of the project this weekend.

Im also interested in building a 2 axis tracker for this reflector.

[Turtle]

Originally Posted by SolarFreak

I’m building a parabolic reflector this coming weekend. I will be using 2in^2 mirrors.

I’m trying to calc the power output and what temperature will be at the focal point. My 1st goal is to reach 500f.

I think you have overestimated the power from this setup. For one, in using the flat mirrors you only approximate a parabola & the reflection will scatter considerably. Second, by using second-surface mirrors you incurr loss within the glass.

http://www.edmundoptics.com/TechSupp...ics:%20Mirrors

Originally Posted by edmundoptics.com

What is the difference between a first-surface and a second-surface mirror?
A typical mirror is a flat glass substrate with a metallic reflective coating applied to one side. If the coating is applied to the top surface, it is called a first (or front) surface mirror. The other surface may be clear (during fabrication of the glass or by polishing) or ground and the mirror is oriented so that the coating faces the source. If the coating is applied to the bottom surface and overcoated with black paint, then it is called a second (or back) surface mirror. The other surface in this case must be clear and the mirror is oriented so that the glass is facing the source in order for the light to pass through the glass before reflecting off the coating. The black paint (not always used) is used to protect the coating from the other direction and prevent any minimal transmission. An example of a second surface mirror is a common bathroom mirror. A second surface mirror is usually not preferred over a first surface mirror in most applications due to many inherent characteristics. A second surface mirror suffers from lower reflectivity due to absorption by the glass (especially in the UV and IR). In addition, there are often ghost images due to two reflections (one from the glass, one from the coating) and an increased optical path length since light passes through the glass twice (once to reach the coating and once to reflect back). A second surface mirror does have the advantage of increased protection of the coating. If the coating is very delicate or the environment is harsh, a second surface mirror may be selected (typically with a thin glass substrate).

[SolarFreak]

ok thats good info.

the only mirror I could find local was 5mm and 3mm second surface mirror.
I picked the 3mm type.

I have change it from 90% to 75%. I cant see it going much less than 75%

can anyone help with finding the optimal focus point of the mirrors. is there a forumal?

I know that a 2 inch mirror max focal is 19 feet
(2 / tan(.5)) / 12

I had been thinking of setting up a test jig with a black surface and measure the distance to highest temp I can make with 1 mirror. and using white paper and see what is the clearest reflection of the sun in a 2^2 target

[SolarFreak]

does anyone know a link to a list of materials that lists their reflective index?

[Turtle]

Originally Posted by SolarFreak

does anyone know a link to a list of materials that lists their reflective index?

I ran across this in-depth study on the best reflective material when I built my experimental oven; then I lost it. Now I found it again. Here ya go:
http://www.fsec.ucf.edu/Solar/PROJEC...ialsreport.pdf

PS Web Results 1 - 10 of about 23,700 for "best material for solar reflector". (0.50 seconds)

[Jay-qu]

the focus is dependant on the shape of your setup, unless you are using non-flat mirrors.

[SolarFreak]

Originally Posted by Jay-qu

the focus is dependant on the shape of your setup, unless you are using non-flat mirrors.

Yes I understand that I select the focus point of my parabolic. I'm trying to match the focal point of the dish/ reflector to match the optimal focal length of the mirrors I’m using.

x^2 = 4py p is the focal point of the parabolic.


Im using flat mirrors and even flat mirrors have focus point.

you are reflecting an image of the sun. the sun is about .5 deg wide as seen from the earth.

so using 2in^2 mirror at 19 feet the suns image blurs
(2 / tan(.5)) / 12

http://www.amasci.com/amateur/mirror.html

MIRROR SIZE AND LONG FOCAL LENGTH
The smaller your mirror chips, the smaller and hotter the focus. After all, the hotspot is approximately the size of a single mirror. An array of 1in. mirrors a foot across will make 144 beams, but if you use 2in. mirrors for your 1ft furnace, the hotspot only receives 36 beams.

However, if you use small mirror chips and adjust your solar furnace for a very long focal length, you'll find that the hotspot grows larger, fuzzier, and cooler. This occurs because the sun is not a tiny point, instead it is a disk, and the mirror-facets act as the pinholes of a "pinhole camera." Small mirror-chips form an image of the sun, rather than an image of the mirror-chip shapes. Each little square of light will develop a blurry edge, and only the center of each square image will get "full sun."


To compensate for this blurring effect, use larger mirrors. Here's the rule: choose a focal length which is lots shorter than 120 times the width of a mirror-chip. (This 120 comes from 1/tan(.5deg), the sun being about 1/2 degree in angular size.) For example, the 1in. mirrors would give a blurry hotspot if F.L. was longer than a few feet, and at 9.5ft the blurred regions swallow the hot center of the hotspot. This "blur" is an image of the sun. If you want to burn objects from 120ft away, you'll have to build a furnace using mirrors which are wider than 1ft each. The size of the sun-disk is the cause. (If our sun was tiny, but still just as bright, then this blur would be gone, and you could form its light into an intense parallel beam like a laser!)

1 in. 9 ft.
2 in. 19 ft.
3 in. 28 ft.
4 in. 38 ft.

[SolarFreak]

I will post some drawings of my layout later today when I get home and use my camera.

basicly Im going to be putting the mirrors in rings. So I can calc the the angle at ring. then center the mirror on the ring. in using 2in^2 mirrors 1inch is before the ring line and 1inch after the ring line.

the rings work out will have
8 mirrors
16 mirrors
24 mirrors
33 mirrors
42 mirrors
50 mirrors
58 mirrors
66 mirrors

right now the plan is 297 mirrors

kinda like the attached image

[Turtle]

Originally Posted by SolarFreak

I will post some drawings of my layout later today when I get home and use my camera.

basicly Im going to be putting the mirrors in rings. So I can calc the the angle at ring. then center the mirror on the ring. in using 2in^2 mirrors 1inch is before the ring line and 1inch after the ring line.

the rings work out will have
8 mirrors
16 mirrors
24 mirrors
33 mirrors
42 mirrors
50 mirrors
58 mirrors
66 mirrors

right now the plan is 297 mirrors

kinda like the attached image

Sweet! I have thought it was a parabolic trough oven not parobolic dish oven. What is the construction method & material for the receiver at the focus? Whatcha gonna heat up with this death ray?
Ohhhhh baby.

[SolarFreak]

Im working on a parabolic through also I will post more info on it later

This one I using as a test bed to see what kinda temps I can make and also if I can make some good steam with it. and maybe test it on a small sterling engine too.