Highly Reflective and Easily Sourced Film for Solar Concentrator?

Cooking foil, if bonded with something with a bit of compliance (Evostick?) might do the job. Try to find the heavy gauge stuff - not the cheap'n'nasty. I've used it for indoor reflectors - don't know about weathering, tho'.

Cooking foil is only about 70% reflective and tricky to work with. Also for my design I need ideally one large sheet.

Is that 70% in the IR? Wikipedia has "The reflectivity of bright aluminium foil is 88% while dull embossed foil is about 80%.^[4]" , but the main text doesn't specify the wavelength. The reference may.

Take your point about it being tricky to work with. Maybe you could bond it to a sheet of heavy grade polythene? The join lines in the foil would lose you very little.

I'm trying to find a wavelength reflectivity graph for aluminium foil, but so far can't.

There's a lot of different figures for it's overall reflectivity, anything from 55-95%

Aluminium itself looks promising - bit of a wriggle in the near IR, but good past 1μm.

I Googled "bendable plastic mirrors" and got some interesting results.

Another possibility, an "array" of smaller glass mirrors fixed to a mastic of some sort?

Good luck!

Should stress that it has to be very, very cheap and easily sourceable. As in available from most hardware stores or supermarkets.

I don't know about "ease of source", but small glass mirrors 1"x1" can be purchase in bulk at about 12.5 cents apiece. and attached to a flexible backing.

Try some thin acrylic, polycarbonate or polyester sheet film; thin in the range of a 2 mm thickness. Polyester probably has the best environmental endurance if you can find it, next I think is acrylic and third would be polycarbonate (which I think turns yellow after a long exposure to sunlight).

Bend the plastic to the shape you want and then paint the rear side with a shiny metallic spray paint like a chrome or aluminum (aluminium) or silver*, then overcoat that with a protective layer of gloss white paint. This give you a durable, second-surface curved mirror with good reflectivity.

If you can find it, a large, thin plastic mirror would save you some time. Cut it to the size you want and use a heat gun to bend it to shape.

*Silver and gold have the highest reflectivity (>95%) above 500 nm; Aluminum's reflectivity is better in the visible range down to 300 nm, but dips a bit around 850 nm (~ 82%) and is otherwise around 85%.

That's kind of what I'm doing, except using a wire screen instead of a clear film, and a reflective foil instead of paint.

The advantages of stretching the reflective surface across the back of the shape rather than adhering it to the front are enormous.

Aluminium foil is starting to look more attractive, Al itself is clearly reflective enough, it's just a matter of how much is lost making it into kitchen foil.

Well, it would be easy enough to check. You could temporarily cover a section of your array with the AL foil and check the reflectivity directly.

Just to throw my $0.02 in, I'm with some of the folks above. Given your requirements the best bet may be to go with bending some thin plastic and coating the inside with something shiny. Whether that's paint or aluminum foil or stainless foil or gold leaf is up to you.

FWIW, I found this site which basically tells you what you can't use:

http://www.fsec.ucf.edu/en/research/solarthermal/solar_cooker/documents/reflectivematerialsreport.pdf

And here's a sort of official-seeming relflectance spectrum chart from the NPL. Although like you I saw others sources stating around 85%.

http://www.npl.co.uk/science-+-technology/optical-radiation-and-photonics/optical-characterisation-of-materials/aluminium-reflectance-spectrum

Although do note that many plastics tend to be somewhat opaque to UV. So going the above route I guess your reflector will loose much of that part of the spectrum. I don't know how much energy loss may impact your application.

I have photos of my setup here:

http://solarflower.blogspot.com/2010/07/collector-improved.html

Which works perfectly, minus the delicacy of the mylar.

Sticking something to the front of a surface is a hassle and encourages bubbling, paint won't give sufficient specular tightness, using a clear plastic will absorb too much energy on the way through (twice).

Al foil will be fine as long as it reflects enough. I can stretch lengths of it in the same way as I have the space blanket, and reinforce the edges with tape. It'll be easier to clean and is very cheap and available.

It's just the reflectance...

(which I don't really have a good way of measuring.)

"It's just the reflectance..."

Very true, but you've got to draw a balance somewhere. As you said, it has to be cheap & easy to get.

For sticking it to a substrate (polythene sheet or whatever), you could try photo-mount spray - but it's a bit pricey. May need to experiment a bit.

I'd strongly advise using some kind of backing material - foil on it's own would be very vulnerable, especially when it comes to cleaning. (Number of times I've poked my finger through when wrapping up the chicken/turkey/whatever for the oven ).

I think paint can be be much shinier than you seem to think. It depends on the surface painted, so if you are painting shiny plastic with a metallic paint, you should get a mirror-like surface. Remember than ordinary mirrors (like in a bathroom) are a type of paint on glass. There are many types of shiny metallic paint, but here's a link to one listed on amazon.

http://www.amazon.com/Duplicolor-Dupli-Color-Instant-Chrome-Enamel/dp/B000E4HWKS/ref=sr_1_fkmr0_1?ie=UTF8&qid=1282003939&sr=8-1-fkmr0

Anther option, if you can find sheets large enough, is the shiny silver trim used on automobiles. It wears very well with sun exposure, automatic car washes, etc. Try 3M's website.

Daniel,

I was really impressed by the panoramic shots.. I enjoyed the submarine.

Please tell me more or provide links to each.

If something occurs to me for the solar unit, I will let you know.

ps.. what part of the world are you in, if you don't mind?

Chris

This fellow has built a nice looking system and as I remember he provided information on all the materials.

http://georgesworkshop.blogspot.com/search?updated-min=2009-01-01T00:00:00-05:00&updated-max=2010-01-01T00:00:00-05:00&max-results=9

Excellent link, russ123...

mylar is used in the hydroponic industry but tends to peel if it get wet. there are two alternatives 1 white plastic or 2 chemicaly brightened aluminum better look up ALZAK www.finishing.com/1400-1599/1496.shtml the alzak process is owned by the aluminum company of america and is used to brighten aluminum street light reflectors as well as almost all HID lighting fixtures.

http://reflectivemylar.com/

the mylar just mentioned is the stuff used in the hydroponic industry and as i mentioned it will peel if it gets wet -- Chemicaly Brightened Aluminum is the way to Go.

> For sticking it to a substrate (polythene sheet or whatever)

I'm just stretching the foil across the back of a wire mesh. Keeps it tight, avoids buckling, and is very simple and accurate.

> I think paint can be be much shinier than you seem to think.

Haven't found reflectivity data, but it's described as 'more a dull alloy finish than mirror.'

> the shiny silver trim used on automobiles

Chromium isn't actually all that reflective, also not sure if this stuff comes in a form I can really use.

> This fellow has built a nice looking system

Yeah I stumbled across that recently. It's a very nice setup, I'll have to have a closer look at what he's doing. Seems he's using proper acrylic flexible mirrors, which is nice if you can get your hands on them.

> mylar is used in the hydroponic industry but tends to peel if it get wet

Yeah, Mylar would be perfect if it was a bit more durable. Or a lot more.

> chemicaly brightened aluminum better look up ALZAK

This looks interesting, but is it available in foil/film?

Re: the substrate - just a flexible polythene sheet to make the foil more durable. It shouldn't make any difference to the way you're applying it to the wire mesh frame.

Ah right, I get you now.

I'm using gaffer tape to reinforce the edges, if wind etc tear the foil I can stretch a bed sheet or similar under it.

I'm trying to avoid use of adhesives if I can, but they're always an option if I can't.

I built something similar to http://georgesworkshop.blogspot.com/search?updated-min=2009-01-01T00:00:00-05:00&updated-max=2010-01-01T00:00:00-05:00&max-results=9

around 15 years ago - and went through all the same trials.

In the end, I fitted .4 mm mirror finish stainless steel sheet.

It is not as reflective as foil, or mirrored acrylic sheet or a raft of 'bright ideas' - but it has not decayed cracked or warped.

ALZAC may be available in a thin aluminum foil I'm not sure because I've only seen it used in HID Lighting.

I like to add the following suggestions to existing comments:

• Least surface roughness is preffered which is very important for reflection
• Use fluorinated like PVF or PVDF films which will have better environmental stability. Typically most fluorinated films are stable in UV and Ozone.
• Look for films that are highly stable for UV and Ozone. This will affect reflectance
• Not just initial results but also durability should be considered

Thin aluminum sheet, acid etched and polished will give a high reflectivity. You can also buy reflective aluminum sheet - "bright one side", that is specifically made for reflectors in lighting (I don't know what "cheap" means here). The best by far is glass mirror

does your trough need to be open to the elements. if you use glass or somthing similar across the opening, the materials inside would be weatherproofed, and the inside temp would get higher

Needs to be open - all parabolic troughs I have ever seen or read about are open. The focus point is only point where you really have heat. If you heat up the entire interior of the parabola materials considerations would be more important with no real gain.

The guy at Georges workshop seemed to have solved the problem.

Solved which problem how?

I thought of covering the parabola front, but my main concern would be energy reflected and absorbed by the medium. Also glass would be heavy, hard to source, and a hassle. Plastic would likely degrade.

> Thin aluminum sheet, acid etched and polished

would be perfect, yes. Hard to get without paying mint tho. I'm looking for something in the order of dollars, rather than hundreds of dollars. This is an open source project, largely for the developing world, and people need to be able to easily and cheaply (if not for free) construct the things themselves.

Bodepudi; all you say is correct.

Still looking into the chemically brightened alu, but haven't found anything so far.

Cheers guys for all the input, very helpful.

"bright dipping" is a cleaning process - it will not stay 'bright' outdoors.

To get aluminum to last exposed, you need to anodize it - that means a loss on focus in this application.

It seems you might be a design student in degree show mode, looking for what cannot be, both 'cheap' and 'sustainable' in performance.

If you can make the amortization argument, use mirror finish stainless. 26

If your lectures are the usual "techno absentia stylists" - tell them whatever you like on materials - they have no idea and will swallow anything if you throw in a colorful pamphlet from a plastics company.

You could cover the top with a lexan type material, plastic, strong, lighter than glass and available in a wide variety of thicknesses. This would keep the business parts cleaner, (less dust and other outside delights) and protect the finish from unwanted damage from foreign objects. If plastic gets dirty it can be cleaned easily with a hose. This, at leat in my mind, would be simlar to PV cells that are encased in glass for protection. If you are concerned about heat build up in the trough ( warping due to expansion) you could easily vent it from both ends.

Makes no sense at all - the amount of additional heat would probably not be measurable.

There are very different reasons to cover PV cells. Also for a standard solar thermal collector - very different reasons.

Here you are only worried about concentrating light on the collector tube above the parabola.

Read Georges description of this - his experiences don't indicate a problem. I have looked at many sites using a concentrator over the past couple of years and I have yet to see one covered.

Heat buildup wouldn't be an issue either way; the reflector isn't backed, so would lose heat rapidly. My copper receiver will have a clear housing to keep the heat from dissipating from it too much.

A clear plastic front would stop dust and rain from the reflector tho, which would mean I could use mylar...

hmm.

It'd need to be at least 90% percent transmissive, super light weight and easily available by the square meter...

> It seems you might be a design student in degree show mode

Nope, personal project I've been working on for the last year.

> looking for what cannot be, both 'cheap' and 'sustainable' in performance.

As long as it's cheap and effective, it can be replaced every couple months if necessary.

I just placed a length of kitchen foil in the assembly. It's not as good as the thermal blanket (quite reflective but a little scattery, few more wrinkles) but kinda seems good enough. Seeing as so far I have no other choice, it's enough to go forward with.

As to why to cover and perhaps do things a bit different than the rest of the world, from the OP

After leaving my prototype collector trough:but in weather for ten days, I discovered that the mylar thermal blanket I'm using for the reflective surface is not at all robust enough. It seems that wherever dust settles on the surface (and there was a lot of that) it eats through the silver layer, leaving only the clear plastic.

A solution to a non-problem - wonderful thing.

I guess you could make the whole apparatus larger to make up for the loss of light to the collector - or you could wash it once a week.

How about an "inflatable" reflector, which could double as a tanning bed?

Turn me over I'm done!

We've finally had a bit of sun this morning so I tried out the aluminium foil test strip.

It's hard to tell exactly, but the reflectance seems comparable with the mylar thermal blanket; with my highly scientific measuring technique being putting my hand in it and seeing how bright and hot it seems.

The problem tho is focus, it's about a fifth as tight as with the blanket. I think that can be mostly resolved by attaching the lengths of foil together longways so that I can draw the edges tight, as I was with the mylar.

So: kitchen foil might be the go. It's not a lot more work than the mylar, possibly reflects better, and will certainly be more durable and easy to clean.

Some concepts will never be revealed because of proprietary nature

A good source of potential reflective materials and analysis.

http://www.fsec.ucf.edu/en/research/solarthermal/solar_cooker/documents/reflectivematerialsreport.pdf

You've probably already thought of this, but I'll throw it out anyway.

http://sci-toys.com/scitoys/scitoys/light/solar_hotdog_cooker.html

I'm thinking once this wore out, you could just apply reflective film for windows, without having to remake everything. This could be done several times a year and would be inexpensive.

Well, this is interesting.

I taped three lengths of foil together to make a 0.9 X 1.2 meter surface with wire reinforcement on each edge for tensioning, and placed it in my parabolic cradle. A bit of work, but nothing too challenging.

The focus is about twice as wide (10-12 mm) as the mylar blanket, but even at half the concentration the energy feels (still just sticking my arm in) waaay more. Like 50% more. And it's not a particularly hot sun today.

I'm starting to suspect the blanket isn't actually that reflective..

So. This whole rumour that aluminium foil isn't very reflective? Lies.

I think the main problem is that it really likes to crinkle, gluing it to a surface just isn't the way forward. You've got to stretch it tight. It doesn't tear (touch wood) if you tape the edges and any holes. It remains to be seen how quickly and badly it oxidises, but given it's apparent high reflectivity (especially to IR), it's durability and ease of cleaning, and the fact that it's effectively free:

_aluminium foil is a very effective reflector material for solar applications._

Myth: busted.

I have seen food service size rolls of foil, would this be of any advantage?

I was going to get 45 cm wide, which would've meant joining two sheets rather than three, but the 30 cm looked slightly more reflective.

If you can the stuff in 90-100 cm, that'd be awesome.

You can polish aluminum sheet metal to a mirror finish. Seems like it would be more durable, and if it starts to oxidize, just give a fresh polish.

Just thought I'd let you know I'm reading this thread with interest.
the raster scan toaster is till on my to do list.
Unfortunately Mrs Cat has put 'decorate the bedroom' at the top....
Del

does experience matter?

get a good glass cutter, go to your typical home and lumber store, and get glass mirrors they sell there. usually they're 12"x12", and learn how to cut glass.

the mirrors cost about $10 for six square feet

you may break a few, but so what.

I did this on a ten foot solar dish, cut every piece to fit, and it took me a month.

glued them in with some good carpenters glue, and I can set a piece of cardboard on fire in about thirty seconds.

accidently left the dish in the wrong position one day, and now I have half a semi-circle where the grass used to be

Sweet.

I'm looking for something which can be done easily be pretty much anyone in less than a day's work. Not counting sourcing materials. Which would ideally be as close to free as possible.

Hi , This is why the old school is the best school. Copper is 99.9 pure and is still one of the best elements about solar collection. The flat plate collectors are tried and proven but the retro fit industry lags behind. A well built collector system should come out of the ground with the home. The south side of the house should be used to capture as much energy as possible, and along with a simple low cost green house/solarium as the collector , food and starter plants can utilize the water and the sun energy. This puts the collectors down low enough so they can be maintained properly by a simple washing. Also, the whole system is user friendly, because it has so few moving parts, and can even be a manual system, where when you want hot water, you make it yourself, using a timer instead of an automatic system.(cuts costs). kEEP IT SIMPLE.

"The south side of the house should be used..."

?

only if you live north of the equator.

Like just north?

here's a collector/radiator all in one that can be hand built using simple materials.

I know this is off mylar topic, o well.

" Ineed something that will withstand rain and dust"

OSB sub floor over TGI floor joist on 12" centers over a 4 1/2' Minni basement.

A good greenhouse/solarium built on the south side . The roof of this now becomes the "collector" roof, for flat plate collectors. (which can be added later to reap the tax credits). The south wall of it is dbl hung windows with screens, so overheating is not an issue.

If you build all of this first and get it working, then you can move in to it (camp out)

while the house is built over an already working system. Then maybe you might see how the light comes into the site, and build according to that. East light into bedroom or kitchen is good.

aluminum foil shinny side up (radiates energy up ) over entire sub-floor, then 2x4 sleepers 24" o/c, then 1/2" H-Pex Tubing lightly stapled to sub floor and poured and screeded with good sand. A new sub floor can now be put down on the sleepers and walls built, (take pictures with measurements of all the in-floor tubing so your framers won't put nails in it) The tubing is routed to a good mechanical room and hooked up to a heat source. 450 gallon "solar" tank is good. but a 50 gallon water heater will do for about 600 to 700 sq. ft.

Solar collectors heat the tank automatically or manually, 50 gallon water heater does the same. Thermostat turns on in-floor pump. Now the heated water (125 to 145 degree) passing through the tubes (200' ft. loops) heats the mass of the floor and radiates it's energy to the house. It does not heat the air, it heats what is touching it

Your floor becomes a "collector" and radiates most of the energy upward into the home/garage. If it takes X amount of time to heat the floor, then it takes exactly X amount of time to lose it. (during power failures, it takes a long time before your house freezes .

This initial process costs a bit more, but is well worth it because any structure will flourish upon this " good foundation" .

A "good foundation" usually takes one contractor to bring this out of the ground,( this is not done, but should), then it is handed over to another contractor who "finishes" the home.