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gbwillson

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Reply with quote  #181 
JC-

Here are a couple of example of small tests and discussions about reflective versus black interiors.

http://simplysolar.supporttopics.com/post/black-absorbent-interior-vs-reflecitve-radiant-interior-6756917?highlight=reflective&trail=10

Here's another discussion...
http://simplysolar.supporttopics.com/post/black-box-vs-reflective-box-when-using-black-screen-8211084?highlight=silver+back&trail=10

For my ZP collectors I always use two screen layers closest to the back of the collector and a single layer of screen closest to the glazing. I wanted more of the heat from the sun to stay in the screen gap where it can be heated by the screens instead of the back surface of the collector. With three layers, I really can't see the back of the collector, and if I could, the light would have to bounce back through three layers to escape the collector box. 

If you want to test on your own, you would need to build two identical test units with the only difference being the reflective surface. Simply testing a white surface and then painting it black and testing again wouldn't be a fair test since the conditions wouldn't be the same. 

Greg in MN


gbwillson

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Reply with quote  #182 
Quote:
Originally Posted by JC77
The way I'm understanding a zero pass, the air between the top screen and the glazing is mildly stagnated along with the air between the rear screen and the back of the collector. The two layers of screen basically act as hot walls the air passes between/around? It obviously has to interact with the screen to pick up heat. Anyway, I read a lot of discussion on low mass collectors being better at getting heat out of the absorber material and wondered of doubling up the screen on each layer and painting everything else white would have a reflective effect to put more energy in the absorbing material (screens) and less into the collector.


JC-

On a ZP collector, the air outside the screen gap is not actually stagnated as there is minimal movement of air in and out of the gap via the openings in the screen. At higher air velocities, which the ZP needs, most of the fast moving air stays between the screen gap. This also helps keep the bulk of the airflow somewhat isolated from the cold glazing. You DO want the air to "mingle" with the surface of the screens to break up the laminar flow. So a rough surface mesh or screen with many surfaces works very well at transferring heat to the air. I use a double layer of screen for the back of the collector, and only single layer on the front which still allows much of the sunlight to hit the back screen layers and keep the heated screen on top and bottom of the moving air.

Greg in MN
JC77

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Reply with quote  #183 
Thanks Greg. I think that settles the argument for white or reflective. I'm kind of surprised it's only a ten percent difference. I understand I'd need two identical collectors to do a side by side test. I've since switched my plans on my hydronic system to closed loop so now the locations that were going to get solar air heaters could have solar water heating panels instead. I can use hot water year round so I'm giving up on solar air heating panels for now.
larrypac

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Reply with quote  #184 
I have been reading several ZP collector threads which mostly are talking about efficiency of different tweaks and comparisons to double screen collectors and I appreciate all of the work people have done in researching all of this.  What I am wondering, and have yet to find, is a detailed design for a ZP collector.  I understand that ZP design is being constantly improved and whatever I build might be surpassed by another design, but I have the time to work on a collector now and might not later in the fall.  I would like to find something with enough drawings and material specs that a competent builder (I built custom houses for 30 years) could go out and build one that would work reasonably well.  I am hoping that this exists but have not yet found it.  Thanks for any help.  Larry
gbwillson

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Reply with quote  #185 
Larry-

The closest thing to an actual drawing is this:

ZeroPass Solar Heater.jpg

This is obviously a very basic drawing, but there are only a few things that are key elements.

1. The air enters what we call the intake manifold from either the end or as in this drawing. Your collector would use the back entry.

2. The air entering the manifold swirls wildly upon entering the manifold. The only place for the air to exit is through the narrow screen gap. The manifold must prevent air from passing above or below this screen gap. So if your collector is deeper, you must seal off everywhere so that the air is forced to pass between the screen layers. The screen frames themselves sealed off any airflow as the collector box was very shallow(3⅝").

3. While both manifolds are much the same, the intake manifold should be overpressured a bit so the only place for the air to go is through the screen gap. This is easily accomplished by pushing the air into the intake. It also insures that the air flowing through the screen gap is widely dispersed from side to side. Otherwise you can have hot and cold spots. If you end up needing two fans, just make sure the pushing fan is doing most of the work. 

4. Lastly, the size of the screen gap is very important, so make sure the method you use to make your screen gap allows you to tighten the screen enough so they don't sag. Screen gaps variances as little as ⅛" can make a big difference in performance. Frames can be made from wood or metal, or any way you can think of to allow for tight screens.

As you said, the ZP is a newer design, so there are always ways to improve. With your experience, you may have thoughts on how to build that are different from anything that has been tried far. So please share any ideas with the group. It may have already been tried, or it is just as likely that it is new and a great idea. Even a half-baked idea can turn into a gem with a bit of brainstorming.

Greg in MN

Bruce

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Reply with quote  #186 
Larry...my plans are shown on post #163 of this tread.  Or go to look at the Member Projects threads for more details on what others have done.
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