Solar Concentrator Mirrors - How to attach?
 

Ok, so I've found ~80 some 12" square mirror tiles for my solar concentrator project. I have a 10 ft satellite dish in the back yard that I plan to use as the concentrator once I refit it with new actuators.

I've been thinking for a bit on how to attach the mirrors to the dish, but haven't had that 'Eureeka!' moment yet... any ideas? I'm thinking I will have to glue the mirrors to some sort of base plate for strength, and to give me something to attach anchor bolts to.. but what? plexi-glass maybe? expensive tho... I thought about 1/2" plywood, but that will likely swell as soon as it gets wet. I was thinking about ceramic tile... but I wouldn't be able to drill into it for anchoring to the dish... ARg!

ideas from anyone that has done this? Ideally I want to be able to attach adjusting bolts at 3 of the 4 corners of each mirror tile in order to allow for alignment to the focal point.

Are the mirror tiles flexible?

Maybe some waterproofed plywood? If you are going to make it 8x8 feet or larger,
1/2" is going to get a bit heavy.
It might be better to use a strong frame, with 1/4" plywood over it.?.

Or, maybe just make a 'light' wooden frame, with a strong center section (for the mount),
and use boards (1 x 1/2?) with a 8" to 12" space between them.
Weather-proof all of it, before mounting anything.


How securely you mount the mirrors, depends on your environment.

Do you live in an area where the weather will be a problem?
Like does it snow a lot there, will the mirrors each get a 20 pound ice load?
Or, do you live down south where a hurricane can rip off the mirrors??

Daox - the mirrors are 1/8" glass mirror tiles, like they used for 'mirror walls' in the 70's!

Xringer - I live in south central Manitoba, Canada. The dish is currently well sheltered from the north wind, which is my biggest concern in these parts. The mirrors could see a bit of snow load, not much problem with ice storms tho.. I want these things solid on the dish so I can brush the snow off them without messing up the alignment.

1/4" plywood may work, covered with a liberal coating of barn paint to seal it?

Xringer - btw, love your Solar Tracker Project thread...

Permanent Pre-Adjustment
 

If you mount on a flat surface, perhaps the mirrors could be glued on with epoxy.?.

I wonder if one could use a band saw to cut some dense foam (EPS?) into mounting blocks,
with the precise shapes needed to angle the mirrors?
This is starting to sound like a math job!!

Once a mirror was installed, the whole foam mount could be painted with epoxy. (Like a Surfboard).

If the back of the mirrors and the plywood is primed for the epoxy, then
foam mounting blocks (6"x6") should be pretty solid.. Even with a snow load.

This approach means you won't ever need to make any adjustments..
(You would have to saw the foam in the middle, sand it down and re-glue).

You could do a Fit-Check with light duty spay-on glue.
http://web4.hobbylinc.com/gr/mmm/mmm77-7.jpg


I assume you are going to come up with a fancy X-Y computer controlled mount??

Well, Mike Dabrowski did it by gluing thousands of 1" square mirrors to a parabolic dish. If you need the light sharply focused, your 1' square mirrors will be too coarse. I suppose you could break your mirrors into little pieces.

If you don't need the light sharply focused, then my first thought would be to use hardware, e.g. screws and clasps, to hold the glass in place.

I wonder if you could use those coarse mirrors, to melt steel etc, by sending the light to a Solar Fresnel Lens ?

YouTube - New Generation of Solar Fresnel Lens

No, because a coarse parabolic mirror has already de-parallelized and almost-concentrated the light.

If the mirrors are mounted on a flat surface, that wouldn't seem to add
up to being a parabolic reflector.

I found some pics of mirror arrays..

Solar Fire Project - Welcome

Not sure I understand what you mean... you're saying I wont generate enough heat with 75 to 80 mirrors? I'm looking at building a slightly smaller version of this, although I'll be cheating and using the satellite dish as a frame:

A Tracking Solar Concentrator for the home experimenter, Table of Contents and Notices, page_iii,iv, 10/09/99

What do you mean 'sharply focused' ?

"enough heat" for what? I don't think you've told us what the application is.

If you shine parallel rays of light on a perfectly parabolic reflector, it will focus that light to a single point. In the case of your dish, it's the point where the antenna was located. But with flat pieces of mirror, you're only approximating a parabola.

If you think about the geometry, I think you'll agree that a collector of this type with flat pieces of mirror of width x, will focus the light across an area of width x. One inch mirror fragments can concentrate light onto a 1" wide area, two foot wide mirrors will focus the light onto a 2' wide area. Is that good enough for your application?

My end goal is to replace/suppliment my electric furnace and to preheat the water going into my hot water tank.

The system will be comprised of the collector, a water/antifreeze storage tank in my basement, with copper or pex coils going to a water to air heat exchanger in my furnace ducting. There will also be a loop coming into the tank to preheat the HWT cold water feed.

The collector will focus on a single point that is the same size as the mirror tiles, 1ft square, which will be a steel plate manifold with the AF pumped through it.

The idea came from the collector detailed in the "Teton Engineering's Tracking Solar Concentrator " link I posted above. From what they've done there with flat mirrors they say they can generate 1200F...

you don't seem to think this is possible with flat mirrors?

I would make the collector (Sun-to-AF) surface area a bit larger than 1'x1',
because there is always going to be some error.. Splash over energy.

These look like flat mirrors..
http://www.evworld.com/images/sandia_solarthermal.jpg
"National Solar Thermal Test Facility at Sandia National Laboratories,
near Albuquerque NM. Sunlight is focused on the tower,
super-heating a working fluid that can be used to run an electrical generator."


When you get near this site and look up at the focal point on the tower,
it's like looking at a really small and bright sun. Sunglasses are a must! :rolleyes:
I've heard they can melt just about anything..

That's what I'm thinking! The combined heat from 75 suns should be enough to heat my house no? What's the diff between heating a ft sq or an inch sq?

The 1" target might melt.. :eek:

Some folks around here, put new Low-E replacement windows on their homes,
and I think some of the glass might be bowed inwards,
giving some of the panes a parabolic shape..
I can see that parabolic effect in some of my Harvey windows..

Anyways, I was watching the Boston TV news and they showed how the
Low-E windows from next door neighbors was melting off Vinyl Siding!!
(And the insulation underneath the siding)!

Energy Efficient Windows Melt Vinyl Siding - Boston News Story - WCVB Boston

Given a large enough number of flat mirrors, you could generate 1200F. But unless you're going to go with an exotic heat transfer fluid like the hundred million dollar projects do, you probably want to keep things near or even below the boiling point of water.

I do hope y'all know, temperature and heat are two distinct concepts.

I wonder if anyone has a calculator out there to determine just how many mirrors = what temp? I suppose that will depend largely on the surrounding conditions tho...

Not sure what you mean by 'temp and heat being different' ? I was never much of a scholar, more of a 'do it and find out' kinda guy.

I'm not planning on generating 1200F... that would be nuts! but if I could generate enough heat to store at say 200F, I think that would be sufficient.

Hmmm... those sun towers look distinctly like grain elevators... which we have alot of around here... I wonder.

You should be able to calculate the amount of energy the collectors focus fairly easily. Temperature would be a function of how quickly you dissipate or remove the energy.

:(. An understanding of the concept of energy is fundamental to any sort of home heating system. You should crack open Wikipedia or YouTube for some background information on the concept.

Heat is a form of energy, which is what you want to collect from the sun and move into your house. Temperature is less important. It's just a matter of how concentrated heat energy is, not a measure of how much energy you gather.

The rate at which you move energy is called power, and is measured in kW or BTU/hr.

Do you have the dish that has a mesh screen on the back? If you do you could glue/attach a 1/4" nut to the back of the mirror, use a short piece of threaded rod and secure the rod to the mesh with more nuts and washers.

yep, mesh screen! I was thinking along the same lines, but I want to give the mirrors a little more strength by glueing them to a piece of plywood or something, and then attaching the hardware to that. I think I'm going to try 1/4" plywood with a couple coats of barn paint to seal it.

Thanks for the input!

If you could get your mirrors fixed and focused, then you could lay the dish mirror-side down and coat with low-expansion foam over everything. This will help spread the load.

And as I'm thinking about this I thought "What if a mirror broke? How would one replace it?" An option would be to spray or brush cooking oil or something similar on the mirror backs. This would prevent the sticking and the foam would be right at the mirror back still supporting it.