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Posted by Michael Harrington on September 11, 2005, 9:07 pm
 
Consider a parabolic Aluminium Collector.

1. What are the techniques for getting a mirror finish?

2. What are the techniques for preserving and maintaining such a finish?

3. What are the tolerances for solar tracking in degrees of arc.
How much is efficiency affected by off beam tracking?

Thanks in advance for any advice or Links.
Cheers Mick.



Posted by Morris Dovey on September 11, 2005, 10:37 pm
 
Michael Harrington (in K21Ve.656$_q1.23107@nnrp1.ozemail.com.au) said:

| Consider a parabolic Aluminium Collector.
|
| 1. What are the techniques for getting a mirror finish?

Rag, polishing compound, work, optional clearcoat.

| 2. What are the techniques for preserving and maintaining such a
| finish?

Repolish when neded.

| 3. What are the tolerances for solar tracking in degrees of arc.
| How much is efficiency affected by off beam tracking?

Depends on size of target and how much reflected light misses target.

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/solar.html



Posted by Duane C. Johnson on September 11, 2005, 11:17 pm
 Hi Michaell;


 > Consider a parabolic Aluminium Collector.

Yes, many of my customers use these.

 > 1. What are the techniques for getting a mirror finish?

1. Gluing glass mirrors on the surface.
    Plain glass bathroom mirror tiles cut into smaller
    squares work well. The back should be protected with
    the tarry roofing material that is used on trailer
    houses. Use the stuff that has a high percentage of
    aluminum flake.

    I've had some of these mirrors in the weather
    for about 20 years now.

    The reflective coefficient is about 85% or so.

2. Use reflective greenhouse Mylar type film. "Glue"
    small squares or strips to the dish with car axle
    grease. This works very well and produces a surface
    smoother than if normal "hardening glue" is used.

    Besides, reflective Mylar will deteriorate in a few
    years depending on the local pollution. The grease
    makes it easy to replace.

    The reflective coefficient is about 95% or so.

    Actually this stuff is very low cost compared to
    glass so, in my opinion, is the best solution.

 > 2. What are the techniques for preserving and
 > maintaining such a finish?

Regular replacement is easy and cheap.

 > 3. What are the tolerances for solar tracking in
 > degrees of arc.

The sun's angular diameter is about 1/2 degree of arc.
So the best and smallest image formed by a lens or
mirror can not be smaller than about 1/2 degree of arc
at the focal length.

As an example:
A 100" dish with a focal length of 100" will have a
solar image that is 1" in diameter.

Now actual C-Band dishes will not attain this small
image size. They were designed for 4GHz microwaves
and the surface is not nearly ideal.

That being said a 10' dish will have a solar image
diameter more like 4".

 > How much is efficiency affected by off beam tracking?

Not much until off quite a bit. However, this is not
the important question. We would like to use a solar
tracker that can aim to about +- 1/4 degree. Or about
half of the sun's diameter.

Light balance non optical, (no lenses), sensors are
accurate to about +- 5 degrees. The accuracy can be
greatly improved with a shadow blocker. This is
about +- 1/4 degree or so.

With the LED sensors I use this is a strip of metal
about 1/2" wide and about 2" away.

 > Thanks in advance for any advice or Links.
 > Cheers Mick.

Duane

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Posted by Paul on September 12, 2005, 2:30 am
 FYI: Here is some stuff from the solar cooker folks on parabolas.

http://solarcooking.org/parabolic-from-flat-sheet.htm



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