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.