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]

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/TechSupport/DisplayArticle.cfm?articleid=285#Optics:%20Mirrors

 

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]

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/PROJECTS/SolarCooker/reflectivematerialsreport.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]

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]

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.:doh: What is the construction method & material for the receiver at the focus? Whatcha gonna heat up with this death ray?:eek: ;)

Ohhhhh baby.:eek: :cup: :hyper:

[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.

[SolarFreak]

well I stated testing the focal length some mirrors.

 

for test I have

 

12 X 12

6X6

4X4

2X2

1X1

 

I only got to run a few tests then the clouds rolled in

I placed a mirror at each distance at the same time and lined up the reflection side by side so I could compair each of them

 

2x2 test

12 feet full on blur

9 feet blur

7.5 a little blur around the edge

6 feet sharper

3 feet sharp and clear

2 feet very clear

 

Im sure some ninja has already come up with a forumal, I just have not one yet.

 

if the sun comes out again i will get some run. I want to try 2.5 and 1 foot

[Turtle]

well I stated testing the focal length some mirrors.

 

Im sure some ninja has already come up with a forumal, I just have not one yet.

 

Good work Solar! As to finding ready data on the heat collected for a specific oven, I think the commercial manufacturers play those cards close to the vest as they are in competition with one-another.

Because so many variables affect the temperature of any solar oven, such as surface area of reflector, type of reflecting material, cleanliness of reflector, hours & intensity of sunlight available, size of receiver, material of receiver, contents of receiver, insulation of receiver, ambient air temp, etc., I expect the type of experimentation you have started is the best source for developing a rough range of expected temperatures.

Please by all means continue posting your measurements as you have them.:cup: :cup:

[SolarFreak]

Well a little more data. I’m using 2-inch mirrors and 18” seams to be about the best.

 

Today I finished up 3 rings of mirrors total of 45. I just got in from adjusting them.

My focal point is 18” inches high

 

I had a sterling engine in my focal point. I will post pictures of it later must get them off the camera.

 

Anyway after I spent the time to get them I could smell something like melting/burning plastic parts of this sterling engine are made with plastic and looks like it started to melt a little.

 

It ran for about 30 sec and just stopped dead. the top and bottom are both metal plates and black. That’s the way it came in the kit. I could not touch the top with my hand.

 

I think that the bottom and top plate got so hot that there was not a big temp difference in them and it stopped work.

 

So I’m still waiting on my laser thermometer to show up so can get some good readings. I have a oven thermometer. Using set of long handle pliers I held it at the focal point and

 

after 20 sec it was reading 325F.

 

Its case is shiny so most of the light was reflecting off of it. After about 45sec the sun went behind some clouds. So I’m letting it cool down and going to paint it flat black and give a go again. Its almost 530pm here so much sun left today.

[SolarFreak]

I need to mod this sterling engine kit I have so it does not use plastic parts.

 

 

anyone know where you can get a block of copper say 12 X 12 X 2

I was thinking of milling it out to use as heat exchanger to make steam when I build a larger one using 12 inch X 12 inch mirros.

[Sajuuk]

Hehe, I built a similar machine last summer with a friend. We had around 500+ 3"x3" mirrors mounted on a 6'x6' wooden plate. The focal lenght was ~5ft and the focal point about 6 inches wide (we had so much mirrors that we didn't took the time to align every single one of them perfectly). It took us a few weekends (and a lot of beer) to build this monster.

 

We calculated that a black body should reach a theoretical temperature of 1400C at the focal point but I don't know if we actually reached that. Wood ignited instantly and cell phones melted in a few seconds, but we could not get aluminium to melt completly (probably because it has such a poor emissivity). The first thing we burned was an empty box of Sunlight Soap ... it actually took us a few hours to realise the irony of the situation.

 

I know this is shameless ranting but I'm kind of proud of this thing. :)

(I have a few picture but I don't have enough posts yet to upload them :circle: )

 

Anyway, if you want to calculate a theoretical temperature of an object at the focal point you can use Stefen-Boltzmann law. If you want to maximize its efficiency find some black paint with a very high emissivity.

[SolarFreak]

well busted out 550F this afternoon. after I adjusted the mirrors a little bit more.

[Turtle]

well busted out 550F this afternoon. after I adjusted the mirrors a little bit more.

Sweet! How many mirrors is that? Do you plan to make any charcoal? Is the Sterling engine still kaput? Thanks for keeping us up to date on the progress. Standing by to burn Solar Freak.:hihi: