# Tough Parabola Model

Posted by Erdemal on October 11, 2007, 12:24 pm

Is there somewhere a model for tough parabola solar
reflector ?

I'd like to design an optimized one for the hotest
focus point given a known surface imperfections.

I could write it and probably reinvent the wheel.

Cant find any searching with 'tough parabola model'

Erdy

Posted by Morris Dovey on October 11, 2007, 12:42 pm

Erdemal wrote:

| Is there somewhere a model for tough parabola solar
| reflector ?
|
| I'd like to design an optimized one for the hotest
| focus point given a known surface imperfections.
|
| I could write it and probably reinvent the wheel.
|
| Cant find any searching with 'tough parabola model'

Hmm. I'm not sure what you're after with "tough"...

I rather like the one I built. If you follow the link below you may be
able to get a few ideas from mine.

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

Posted by Erdemal on October 11, 2007, 3:31 pm

Morris Dovey wrote:

I meant 'trough', 'tough' was close enough :)

Yes exactly like yours, I visited your site long ago and
appreciate it a lot.

Not being a great 'handyman/DIY', 'imperfections' are expected
on that parabola :). The purpose is too to evaluate the quality
of the sunlight focus given :

- apparent sun diameter
- quality of the 'real' parabola surface
which will not be perfect.
- parabola parameter (p   in   y^2 = 2px)

In example, divide the 'trough parabola' surface in
squares of side of one centimeter (or more or less),
suppose these surfaces plane, set a random tilt error
(1, 5, ... degree) for each of these surfaces,
apply reflection law to them then compute the
total 'ligth' received by a surface positionned at
focus or elsewhere.

To an old man like me, it would take few+ hours to
write it. A model or charts doing that must exist
somewhere. Such plots for UHF and SHF can be found
(gain versus surface average 'bump', size, frequency, ...).

Erdy

Posted by Morris Dovey on October 11, 2007, 7:20 pm
Erdemal wrote:

| Not being a great 'handyman/DIY', 'imperfections' are expected
| on that parabola :). The purpose is too to evaluate the quality
| of the sunlight focus given :
|
|     - apparent sun diameter
|     - quality of the 'real' parabola surface
|       which will not be perfect.
|     - parabola parameter (p   in   y^2 = 2px)
|
| In example, divide the 'trough parabola' surface in
| squares of side of one centimeter (or more or less),
| suppose these surfaces plane, set a random tilt error
| (1, 5, ... degree) for each of these surfaces,
| apply reflection law to them then compute the
| total 'ligth' received by a surface positionned at
| focus or elsewhere.

Ok. The "easy" way to approach this one miight be to take a
representative crosswise "slice" that's one square wide, then pick a
target pipe diameter. Now pick numbers of squares in that
representative slice that have 0-degree, 1-degree, 2-degree, etc.
errors and calculate how much of the light reflected from that square
will hit (or miss) the target.

Note that focal length and target sizes are significant. Since for a
given target size a small angular error might give a partial hit if
the focal length were 'small', but might be a complete miss if the
focal length were 'large'...

| To an old man like me, it would take few+ hours to
| write it. A model or charts doing that must exist
| somewhere. Such plots for UHF and SHF can be found
| (gain versus surface average 'bump', size, frequency, ...).

Yeah right. An experienced old duffer like you could probably knock
off a better fast approximation than an inexperienced young guy like
me could compute. :-)

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

Posted by Jeff on October 11, 2007, 10:36 pm
Morris Dovey wrote:

Living in a hazy area myself, I wonder about the indirect fraction of
sunlight. I suppose that for a normal geometry absorber (ie round) that
this would simply be thrown away. Anyone done any more looking into
those compound parabola reflectors?

What do you figure the working temp for your stirling should be
about? The fact that you are getting 700F almost seems like you should
be using a different working fluid than water. At any rate, it seems
like the parabola is a big success!

Jeff

•
• Subject
• Author
• Date
 Re: Tough Parabola Model Morris Dovey 10-11-2007
 Re: Tough Parabola Model Morris Dovey 10-11-2007
 Re: Tough Parabola Model Morris Dovey 10-12-2007
 Re: Tough Parabola Model Morris Dovey 10-12-2007