Acrylic to the Rescue

After the disappointment with the "clear" Coroplast not actually being clear, I have taken this morning to look in a few more directions and I have made what I think is a significant discovery.  Let me review the problem first.

For the particle panel concept to work I need to find a plastic that is highly transparent that will serve as the container for the particle "absorption flow chambers".  I built the first prototype out of polycarbonate, as it was available at a local distributor and its "double wall" configuration makes it easy to construct the particle flow chambers.  The problem with polycarbonate, as I learned later, is that it degrades in the presence of hot water.  Since water will be pumped through it, this is obviously not good.  I went looking for another plastic and thought I found it with Coroplast, which is actually polypropylene.  Polypropylene is highly hydrophobic and consequently highly resistance to water, so this appeared to solve the problem.  Since I am trying to build a particle panel out of off-the-self components, it helps tremendously if I can get it pre-formed into channels or tubes.

When the shipment of "clear" Coroplast arrived, it was not actually clear.  I have not bothered to measure exactly how much light is passing, but just by looking at it I would guess at least 15-20% of the visible light is reflected.  The result is that the panel looks white.  Unless I can find polypropylene in a double-wall configuration, this is another dead end.

Ideally I need a double-wall plastic sheet that is both transparent and can withstand long exposure to water.  I think I may have found it, and wouldn't you know, this stuff seems to have been around for awhile. 

I started by checking out Tap Plastics, to see what sort of plastics they had available.  I was looking for a solid sheet of acrylic that I could bend to form a channel, having not had any luck searching for "double wall" acrylic.  Acrylic is used for aquariums, so my thought it that its OK around water.  I actually just visited the Monterrey Bay Aquarium with my girl friend after a business trip and while she was marveling at the sharks swimming by, I was marveling at the weld between the two 6 inch think panels and how clear the Acrylic was despite its thickness.  I have later learned that acrylic is more transparent then glass.  I have not yet found much information about long term-exposure to hot/warm water.  I seriously hope this OK.  The price on Tap Plastics for a solid sheet was not all that great, so I searched for what I could get in bulk.  I noticed the brand Acrylite came up multiple times, so I thought I would check out their website.  I took a look at their product listing, and came across a "double skinned" configuration.  Well wouldn't you know...all this time I was searching for "double wall acrylic" and got nothing.  On the product specs, they claim "increased light transmission of 91%, compared to 72% for 16mm polycarbonate".  Well that, my friends, is exactly what I was looking for!

Looking at the product specs I was pleasantly surprised to find that the channel widths are actually much wider then the polycarbonate panels.  This will have a wonderful effect of increasing efficiency of the particle panel because it will lessen the internal resistance to flow, which will allow the water to flow faster (increases absorption efficiency), which also lets me use larger particles (cheaper) and larger wire mesh (cheaper).  They also have a "NO DRIP" coating that is applied to the outside and inside of the panels.   I don't yet know much about this coating, but it sounds nice because it lets the panels "self-clean" after a rain which would help to keep the panels clear over time.  As far as the coating on the inside, i think the particles would wear it down after while.

The panels are marketed under the brand Deglass.  A look at their product listing indicates that the "DEGLASS IMPACT" product is the best bet for the absorption channel and possibly the glazing as well. They state it is "the most optically clear multi-skinned sheet available".  Its rib spacing is 2.5'', which will substantially lower the resistance to flow.  They make a 4-wall (oops, I mean quadruple-skinned) sheet that has an IR reflective "Heatstop" coating.  This answers my question about if a transparent IR reflective coating existed.

If I glazed the panel with the same material as the absorption channel and joined them with acrylic resin, I could get exception transmittance onto the particles.  Most of the light is reflected at the air/plastic boundary.  Acrylic has an index of refraction of 1.48 and air is about 1.  Whenever two substance differ in their index of refraction, light will be reflected at their interface.  Check out Fresnels equations to learn all about this.  The inside of the absorption channel is water, not air.  Using Fresnel's equations at a normal incidence, I get 3.61% reflectance at an air/water boundary and .28% at the acrylic/water boundary.  This means that the absorption plate should actually reflect less then 3.89% of the light (accounting for internal reflections).  With a design where I join the panels with resin, I can eliminate the reflectance at the air/acrylic boundary and create a structurally sound solar panel by simply laminating panels together. I'll bet I can get the transmittance up to ~92%.  Combine this with the inherent efficiency of the particle panel absorption and we just might have the worlds cheapest and most efficient flat solar thermal panel...and it can be built from off the shelf components with cheap tools. Well, that remains to be seen, but I have a plan...and so far, so good!

I still need to look at the cost of these panels, but from what I have read PMMA (another name for acrylic) is cheaper then polycarbonate.

Now I just need to contact a distributor of Deglass acrylic "double skinned" panels and order a few sheets and get my workspace in order.  I should also enquirer about the reflective IR coating.  If I could get that on a double-wall panel, that would be the icing on the cake.