On second thought...

Sometimes its good not to rush into things.  My last post outlined my plan for the next panel using double wall acrylic sheets.  As you recall, actually getting these panels was problematic.  I could not find a local distributor and the manufacture imposed a $300 custom crating charge on orders less then 20 panels.  I was not able to find a distributor willing to cut the panels down, which means shipping is expensive.  All said I was looking at quite a lot of money just to get some panels to prototype.  One of my goals is to find a design that can be built by anybody at low cost.  I think many people will want to build these panels just for fun and help improve the design, but this in not going to happen if it costs over $1,000 just to get the parts.  I have tried calling the company a few times with no answer, left some emails with no answer, and posted a message to their forum that had a less then helpful response.  I got the feeling I was more of a problem to them then a guy with an idea that could open up a whole new application area for their product.  Its their loss.  The result is that I have re-thought things and come up with a design that solves the anoying shipping problems by using single wall panels, which are available all over the place, from multiple manufacturers and distributors, are cheap, and will be cut down to size for little or no cost.   The result is that getting the materials is no longer a problem and shipping is no longer an issue. 

Let me just cut straight to the new design.  The idea is to form a single diamond-shaped flow channel where water enters on the bottom and exits the top.

The diamond flow chamber

The idea here is that as water enters the bottom input port and flows to the top exit port, the particles will be pushed up.  However, only a portion of the water will exit the top, with the remaining water circling back and setting up eddies.  As particles sink on the periphery they move back to the inlet port where they are taken back up.  The result should be a constant mixing of the particles throughout the panel while it is on. When it turns off, the particles sink down to the bottom corner, right at the mid-line of the input port.

There are a number of advantages of this design.

1. The one large flow chamber will greatly reduced the internal drag and thus help to lower the pressure drop across the panel. This will let me use larger particles, which are both cheaper and also will sink faster. The result is a more efficient panel because the pressure drop can decrease substantially.   Remember, the goal of a solar thermal panel is to capture the heat and get it into the outlet pipe as fast as possible.  Since the particles are in the liquid they instantly release their absorbed heat into the water, which rapidly exits the output port.  The faster the water is flowing, the faster that heat will be captured. 
2. The design can be easily built from single-panel acrylic sheets.  This is great, because these panels can be ordered for low cost and shipped standard mail.
3. The design allows for freeze-protection with a build-in particle by-pass.  I you recall from the last post, the idea is to have the input port be such that the water can get by the particles if they are frozen.  This lets the incoming water melt the particles to get it going in the morning.
4. The use of single-pane sheets resolves a problem that the double-layer panels are not actually perfectly flat on the surface.  The internal ribbing causes a slight dip in the surface of the panel. As I plan to bond the panels with solvent (its so easy and cheap), this could be problematic because the bond may not be solid.  As long as I can build the panel by solvent welding flat pieces together I am confident the joint will be solid.
5. The flow chamber can be built by heat-forming a piece of acrylic.  I just need to put it in the oven to soften it up. If this is confusing, just search youTube for "acrylic" and "oven".  The "problem" that plastics melt if exposed to high temperatures is actually a  huge advantage because it allows us to easily thermo-form the flow channel.
6. The total material cost for this design hits my target of $100 per sq. meter, which is about 20% the cost of commercial flat panels.  A Do-It-Yourself person could build them for 20% of the cost, and a company could sell them with a 100% mark up and still come in at half the cost of the competition.

Well, if you have been following this blog then I hope the above all makes sense.  As you can likely tell from my blog, I spend a great deal of time thinking and researching, so its possible I'll have a new and better design next week.  However, this one is the most promising yet, and as it turns out I have all the parts I need to build a test system.  All I need now is time.

The Next Steps

I think its time I review what I know now and what I plan on doing.  I have received some emails from readers.  Some are attempting to build their own panels, others just have some suggestion, and generally this is all just great!  I think I have failed to write about all the ideas I have had and the paths I have explored.  Many of the reader comments related to issues I have already put some thought into, so let me try to map out what I now know and the reasons for why im choosing the plan I will outline.

The Big Picture

The main problem that I am trying to solve with these panels is solar panel cost.  The second main problem I am trying to solve is space heat.  I want inexpensive solar panels I can use for space heat in the winter and water heat in the summer. My plan is to use plastic because its low-cost.  The problem is that plastic melts if it gets to hot, hence the reason for particle panels.  If I cant build a low-cost panel, I may as well go buy whats out there now.  ALL my decisions for how to build the panel are funneled through my cost filter.  If the material is too expensive, forget about it.

Key Lessons Learned

When I started this I was totally naive about plastics and solar thermal in general. I have a physics background, so the concept of solar heat capture is pretty straight forward, despite what some people/experts may have you believe.  I am still very naive, but I have learned a few things.

• Polycarbonate and hot water is not a good mix.  Hydrolysis will occur and degrade the panels considerably.  The chemical compatibility charts show that polycarbonate is ok with glycol, and glycol is used as a circulating fluid in solar systems to avoid freezing.  This is a definite route to take, however:

• There are two types of glycol.  The first kind is poisonous and cheap.  The other is not poisonous but expensive.  Those are not good choices in my mind.  If I can make the system a drain-back with water, that's the route I am going to take.  If I cant use water, then I may have to use polycarbonate and the poisonous form of glycol (ethylene glycol).

• Clear Coroplast is not actually clear.  Its more white then clear.  Although it will definitely work with long-term exposure to water, unless I can find a source of it that is actually clear, the efficiency drop due to reflection is going to be too great.
• Acrylic is, so far, a good candidate.  However, it comes in two types: cast and extruded.  Extruded is cheap but not as stable.  Cast is expensive but much more stable.  Read my blog article about it here.  
• I don't really have much info about how long I can expect extruded acrylic to last.  Since cast is too expensive, I have decided to try extruded acrylic.  
• I found some fabulous double-wall acrylic panels with exceptional clarity.  In fact acrylic can be more transparent then glass.
• You apparently cannot heat-weld acrylic like you can with the other plastics.  Rather, they recommend solvent welding.

My Next Steps

My box of Deglass Acrylite Acrylic double-wall samples finally arrived, and the stuff just looks ideal.

  The Deglass samples.  From left to right: 8mm, 16mm, 16mm HIGHLUX, 32mm 4-wall

  The 8mm is closest to the prototype I made from polycarbonate.  Its the least expensive.  I really like the 16mm because of its thickness, and the reason for this will become clear shortly.  The HIGHLUX is also very nice, with really big channels.

Side view of the 16mm Deglass double-skinned panel.  I really like the thickness of the walls, about 1/16''.  

I think I am going to go with the 16mm panels.  The extra channel width and depth will cause a few things to happen.

1. Lower flow resistance.  This will cause a lower pressure drop across the system and less stress on the panel.  However, the internal ribs provide a great deal of support.  I am worries about the HIGHLUX panels having too few rib supports.  
2. Higher water volume.  The more water can flow through the panel panel in a given amount of time, the more efficient the panels will be at extracting heat.
3. Less particles needed.  As the depth (16mm versus 8mm) increases, the particle density required to provide adequate absorption drops.  Stated another way, a water/particle mixture that is half as concentrated but twice as deep will absorb equivalent amount of light.

Freeze protection...the last hurdle?

The standard way to protect against freezing in solar thermal systems is to drain the water back into the house at night or use glycol.  I will drain the water back.  However, the particles in the system will likely stay wet and freeze over night.  The result will be a hard frozen lump of particles clogging the panels in the morning.  To prevent this I have come up with a simple design that should get the panels started by allowing the water to flow by and thaw them.

My new, slightly modified design.  See text for details.

The basics of this design is simple.  The panel is cut at an angle at the bottom, and the wire mesh is heat-pressed into the ribbing.  The panel is deep enough (and not too tall), so that the required volume of particles fit in cut section with a little extra mesh on top.  This extra gap lets the water flow around the particles in the event that the particles are frozen.  As the water flows by, the particles melt and the panels turns on.  This is better seen in the figure below:

Other then cutting the panel at an angle, and using a slightly thicker panel, the design is very similar to my first prototype. 

My only real obsticle right now is that the manufacturer of the panels charge a $300 custom crating charge (on top of the regular shipping cost) if you order less then 20 panels.  This is a bit hard to stomach, but fortunately I have a use for panels in enclosing an outside patio.  Since I was just wiped out with taxes, I'll have to wait for the next paycheck in a couple weeks before I can order the panels.

Until then...

The Vegatable Oil Insight

Last night, just after I got in bed, my mind was still racing with my "Craze and Haze" problem.  As I mentioned in my last post, the problem is that I should expect the inside of the panels to wear down over time.  Extruded acrylic (rather then cast) has a lower molecular weight and consequently is more prone to "crazing".  Crazing is basically a bunch of really tiny cracks and makes the panels look white, since the light becomes scattered.  Beyond this, the particle panels concept requires a particle slurry made out of abrasive micro-particles constantly wearing at the inside of the panel.  This, more then anything else, is going to cause scratching. 

My first thought about this problem is that the water would fill the scratches, and since water has a high(er) index of refraction, it should lessen the effect.  I tested this last night, and the results were not quite as good as I wanted.  I took a piece of left over acrylic I had lying around and I roughed it up really good with sandpaper.  I then sprayed some water on the surface.  To my credit, it does make a difference, as you can see below, but it still leaved something to be desired.

Just the piece of acrylic with half of it sanded to simulate heavy crazing:

Now I sprayed water on the surface. You can see it helps a little.

 My insight last night was that acrylic is hydrophobic.  I think that the water is not filling the cracks because of its surface tension.  So rather then water I smeared a little vegetable oil on the surface.  As you can see, the totally scratched surface is completely repaired.  I left a little bit of the edge of the acrylic piece without oil so you can see that this is not just another piece.

So here is my crazy idea.  What if I put some vegetable oil (or some other oil) in the recirculation tank and pulled water from the surface of the tank?  Since oil floats, this would cause the oil to be pumped up to the panels first.  I have designed a particle by-pass system (I still need to blog about that), which should cause the oil to bypass the particles and sit on top.  (I have a hard time imagining the oil pushing through the particles, but perhaps that is not a problem)  When the water arrives, it pushes the oil up the panel and out the top, coating the inside surface.  Since both the oil and the panels are hydrophobic, I should end up with oil sticking to the inside of the panel.  This should have the effect of (1) protecting the panel and (2) filling in any scratches or damage that develop.  At night, the panels are drained back and the process starts over again.  In essence, the circulating fluid contains a "repair and protect" agent. 

Well, that's the idea.

I have over spent my time budget on particle panels and its time to get back to my other work.  It may be awhile before my next post, as I plan on building a new particle panel design and I just ordered the materials.