Acrylic Glass versus Polycarbonate Panel

[MattM]

I'm looking into the practicality of heating water with a flat plate solar collector.  Money wise its the only RE project that is practical in my part of the country; the eastern extreme of Nebraska.  At my parents house we've used recovered glass from doors to build solar air heating panels, but for this project I'm more keen on heating water.  In my own personal location glass is not going to be practical due to the accessibility by the kids.  Polycarbonate and acrylic materials both seem to offer a comparable solution.

From what I've read about polycarbonate materials it is not transparent to the infrared wavelengths.  I have access to 16 ounce copper sheeting as a backplate, and infrared wavelengths are readily absorbed by copper.  (Real world experience tells me its an excellent solar collector all by itself!)  It sounds like blocking infrared wavelengths would thwart the use of copper in this respect, meaning it then would offer no benefit to me over a matte black aluminum backing.  I am also concerned with the corrugated shape of affordable polycarbonate panels.  Is it difficult to seal the innards off from outside air to keep them from getting dirty inside?

Acrylic on the other hand is transparent to infrared wavelengths of light.  The price for .040 thickness sheets is very competitive with polycarbonate.  From what I've heard online people seem to worry about the ability of acrylics to withstand the temperatures in a solar collector.  The acrylic materials will handle plenty of heat (over 250ºF) before it begins to deform, so I'm not too concerned with that aspect of acrylic.  I have no experience with manipulating acrylic material and was wondering how drillable and cuttable this material is, whereas I already know polycarbonate is very simple to work.

A copper backplate, while expensive, would appear to be ideal for flush mounting copper line with soft solder.  I haven't read about anybody exceeding the melting point of 50/50 with their flat plate solar collectors.  Is this going to be overkill over the simpler pex/aluminum fin arrangement?

With the lack of people using acrylics it makes me wonder if I'm missing something obvious with this material.  Otherwise it looks like a pretty decent material that can be used around the kids.

[willib]

Feedback OK

short version

Polycarbonate is made for outdoors , acrylic not so much because it starts to get hazy when in direct sun?

if you can afford copper tubing with copper fins go for it.

Gary at' build it solar' tests showed copper on copper worked slightly better than Cu tubing and Al fins , which makes sense because of the thermal characteristics of Cu.

thats my OPINION

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Bill

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[ubud]

I'm going to use .0125 lexan film for my glazing (430 sqft) for my heat/dhw system.

Mothernature just willnot cooperate with me, I'm 2-2 1/2 months behind schedule.

1 section of pex-al-pex (212') almost completed, 112 sqft with salvaged aluminum siding absorber plates.

Holding tank is 4200 Gal. reinforced concrete with R-50 Ish. insulation to the outside of the tank. 9 yd. of concrete for thermal mass.

I think it was Nick pine that said about lexan film for glazing.

Gary From Do-it solar has been my inspiration.

Frank

[MattM]

I haven't noticed any haze in the plexiglass panels on the old garage that have been in and out of the sun for 30 years.  Those old car headlights that used acrylic glass had haze from abrasions; polishers take the haze right out.  Acrylic paints have been used by the auto industry for decades and the haze they get is from abrasions, too.  I'm not aware of any circumstances where acrylic glass gets discolored in the sun other than from abrasions.  Polycarbonate lenses and windows have the same problems with abrasions, whereas real glass is hard enough that you don't normally notice it.  That was one of the other reasons I was leaning towards acrylic glass.  That and people are always discarding huge panels of it...

[GaryGary]

Hi,

Polycarbonate transmits near IR, but absorbs far IR.  This is just like glass, and it works fine for collector glazing.  The solar spectrum  has near IR, but does not go into the far IR -- so it passes sunlight fine.  The collector absorber radiates in the far IR, and the polycarbonate absorbs this, which reduces some of the heat loss from the absorber.

Polycarbonate is better on temperature capability -- its good up to about 270F.  Acrylic's maximum usable temperature is a good deal lower -- I'm skeptical of the 250F you mention -- I'd have another go at looking that up.  You are looking for something like "maximum service temperature".

Polycarbonate will tend to yellow over time if it does not have a treatment applied to it that protects it from UV.  But, most polycarbonate products intended for outdoor use have this.

I've used the SunTuf corrugated polycarbonate sheet and the a couple brands of the twindwall polycarbonate (as are used a lot in greenhouses).  Both have done well.  The SunTuf one is about 5 years old, and looks just like the day it went up:

http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm

The polycarbonate is also much more impact resistant than Acrylic.

Some compartive info on collector glazing:

http://www.builditsolar.com/Projects/SpaceHeating/Glazing.htm

I'd strongly recommend that you look further into the idea of using Acrylic for collector glazing -- my 2 cents is that its not a good choice for collectors.

Gary

[willib]

Hi Gary,

When i'm ready,where can i buy that double wall polycarb stuff.i did some searching and the stuff i would like is only sold in high quantities?

I found this at sundance supply but their minimum order is high, does anyone sell it by the sheet?

I thought it looks so cool i had to share it.

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Bill

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[dnix71]

Drum shields are sold by the sheet.

http://www.pennzonidisplay.com/drumshields.aspx

Those are either acrylic or lexan. Lexan is a lot more money, but impact resistant. Locally you can buy Lexan in the shape of hurricane panels that meets wind code for people who care about looks rather than money (stupid rich people in condos). It's obscenely expensive.

[wdyasq]

WOW!

The bands where I used to hang out and listen played behind chicken wire .....

BTW, I am NOT advocating using chicken wire as a glazing material on solar collectors.

Ron

[GaryGary]

Hi Bill,

Nice greenhouse.

There are a lot places that will sell small quantities.

I bought mine at EnviroCept:

http://www.greenhouses-etc.net/glazing/twinwall.htm

They did a good job on the order, but may not be best for shipping if they are a long way from you.  Shipping can be pretty high, so be sure to check what any place you are thinking about gets for shipping.  I liked EnviroCept because they ship in cardboard and don't charge a separate crating fee.

You might find that a local glass show or greenhouse supply place can get it for you without shipping -- I tried this in Bozeman, but found that while the shipping was free, the sheet price was very high.

This is another one:

http://www.greenhousemegastore.com/Multiwall-Polycarbonate-Panels/products/1233/

Gary

[Volvo farmer]

Hehe, Silly. You know it's for sound isolation and not for deflecting beer bottles right?

[MattM]

260º is about the lowest melting point for any acrylic.  After digging a little big I found this site:

http://www.machinist-materials.com/comparison_table_for_plastics.htm

The maximum Continuous Service Temperature - in Air (max.) of acrylic is 180ºF, while for Lexan it is 475ºF.  Not all polycarbonates are the same as PALTUF® has a maximum of only 248ºF.  The table wasn't complete so its not a perfect place to find comparisons.