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Irishvoyageur

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Reply with quote  #1 
I am planning building a long narrow SAH this fall that will be placed under my porch windows. It will be about 2.5 ft x 18 ft. I am leaning towards some type of screen absorber or aluminum downspouts design. However, I recently came across a research study by P. Velmurugan & R Kalaivanan in India from 2015 where they studied four different double pass SAH design. They reported very high efficiencie (max approaching 80%!) with the unit that passed air through a metal fin backpass then returned the air in the through coarse wire mesh screening (see image). They had a double glass glazing. I would not use glass due to the weight and saftey issue. Has anyone attempted a similar SAH design? Itching to get started. Thanks all.

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SunFun

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Reply with quote  #2 
Interesting idea. I imagine (based on no knowledge at all) that a lot would depend on the actual materials used, particularly the wire mesh gap size and the spacing & thickness of the fins. On a practical level, do you have a source for, or can you easily make, the fins?

It would be interesting to know the how much benefit the double glazing provided.
Bruce

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Reply with quote  #3 
I like the design, but the 2.5 cm height of the duct areas seem very tight to me...for construction and air flow volume reasons. 
stmbtwle

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Reply with quote  #4 
Looks like a good collector as it combines many proven designs. However I think it may be overly complicated (read expensive). You can spend a lot of money trying to get a few percent in "efficiency". There's also COST efficiency to consider. In the end, it's usually about the money.
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Irishvoyageur

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Reply with quote  #5 
Here is the research paper for those interested: 
https://www.ias.ac.in/article/fulltext/sadh/041/03/0369-0376

gbwillson

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Reply with quote  #6 
Nice find! As this drawing is more of a concept at this point, the exact specs will likely change. The drawings in post #1 has a few items worth discussing. 

This design concept comes from India, from which come a lot of great solar ideas, projects, and research papers. But almost of the ideas are geared towards drying out food, and not maximizing BTU's. So while the solar heater may work great as a food dehydrator, it's far less efficient as a solar heater. But that doesn't mean it can't be altered to make it more efficient for space heating.

1.   In what I'll call drawing #1 above, the air looks like it is designed to pass through many layers of zig-zagged wire mesh or screen. This would add an incredible amount of resistance. Yes, the air would get very hot from the time the air entered the collector to when it exited, but even with fans rated for high resistance, much heat would be lost before the air exited the heater. Over 18', that's an awful lot of screen layers to pass through. And the 180˚ return at the far end of the collector would also add a great deal of resistance. So I would guess the volume of air at the exhaust would be rather low, but very hot.

2.   The 2.5cm spacing above and below the absorber plate may be something to experiment with. In a similar design, I had fast moving air entering the collector above the collector plate between two screen layers. At this point, the collector was essentially a ZeroPass configuration. Once the air moved to the return end, the size of the gap where the fins are doubled in size compared to the upper gap. This had the effect of slowing the air passing the fins to slow down a great deal, allowing more contact time with the fins, and also prevented the return gap from forming a bottleneck for the collector. Experimenting with the collector air entering either above or below the absorber plate might be worth experimenting with. Testing the size of the gaps above and below would also be worth exploring. 

The biggest issue with the design above as I see it is the extremely restricted airflow. Eliminating the zig-zag screens would be a big improvement. Once that is done, the 180˚ return becomes the biggest bottleneck for proper airflow. And if you make the return big enough, you should have much better airflow.

Remember that the most efficient collector designs absorb the most heat, with enough airflow to move the heated air with minimal waste.

Here is my similar design from a few years ago:
http://simplysolar.supporttopics.com/post/zpdp-7740121?&trail=10

Greg in MN
Irishvoyageur

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Reply with quote  #7 
They used about 1/8 inch mesh steel screen in the second pass. That would provide much better air flow than typical window screen. If my calculations are correct, they had about an 80F temp rise at a flow rate of 70 cfm. This yielded an apparent efficiency of about 80% (measured and provided in paper) which is very high. The double glazing is probably a big factor in the high efficiency and likely the reason they made their second pass at the top of the collector plate. This is an impressive design but one that would be a bit difficult (and costly) to build.
gbwillson

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Reply with quote  #8 
Irishvoyager-

Good point about the mesh. Many countries call window screen mesh. A typical window screen, which is very cheap, has normally about 16x18 fibers per square inch and the fibers are quite fine. 1/8" steel would not transfer heat as well as a finer gauge mesh. Their mesh is more open, thicker, and rougher in texture than window screen, so airflow would be better, especially at an angle. Rougher textures will help break up the laminar flow better than a flatter screen. But the wider openings and thicker material may not transfer heat as well.

Every heater design is different, and as is the size and other variables. An 80˚ temp rise at 70CFM is not going to be efficient. Much of the heat will be lost through the glazing. High internal temps may damage a collector internally. A temp rise of 30˚-50˚ seems pretty typical. That's the nice part of have more airflow available than needed. That way you can tweak the output temp a few degrees to be both efficient and comfortable. Nobody wants a lukewarm breeze blowing on them, but that lukewarm breeze is likely far more efficient than a warm output. So finding a balance between output temp and efficiency is important. If the output air blows directly into a space where the breeze can be felt, you may decide to lose a bit of efficiency to increase comfort.

The use of double glazing is most important is very cold climates since the air outside the collector will often get below 0˚F. Single pane glass or plastic would have a greater affect on air passing nearby. Although testing has shown that fast moving air between two layers of screen isolates the moving air from the cold glazing. Single layer glazing would be more cost effective in moderate climates. 

Greg in MN

Irishvoyageur

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Reply with quote  #9 
They measured the efficiency in their testing. They reported an efficiency of about 80% with this collector. I don't think I've seen any higher than this.
stmbtwle

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Reply with quote  #10 
I think 80% is probably about the highest possible.

Each sheet of glass costs you about 10%, so two panes would only leave you with about 81%. Twinwall is also about 80%.

https://www.shimadzu.com/an/industry/ceramicsmetalsmining/chem0501005.htm
https://www.greenhousemegastore.com/easy-ship-2-ft-wide-twinwall-polycarbonate?gclid=EAIaIQobChMIibWA8s_94wIVjq_ICh2KRw-gEAQYASABEgI0w_D_BwE

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Solar is like the wind. It may be free, but putting it to work isn't!
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