building a parabolic with mirrors.

[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]

Originally Posted by SolarFreak

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.

[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 )

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]

Originally Posted by SolarFreak

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.

[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]

Originally Posted by SolarFreak

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