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...