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solardan1959

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Reply with quote  #21 
Done, Thanks!

AntonioDB

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Reply with quote  #22 
I'm thinking to build a big horizontal screen collector (9 meters x 2 meters).
Which is the best placement for the screens in your opinion?
solardan1959

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Reply with quote  #23 
Antonio,
   You mentioned screen was hard to find in Italy but you found this:

You did not mention any sizes,  That is a huge collector your talking about about 162 square feet, (54 square meters).  I am not sure the best way I would approach it.  Can you get the screen in 2 meter widths?  If so I would make an angled line from one end of the length collector to the other, then probably use a small strips of wood and use it hold down one edge of the screen to the frame the whole length.  Then stretch the screen across the width of the collector and take another strip of wood and while stretching the screen screw in the second piece of wood stretching it tight.  Do this the entire length in one layer.  Take the second layer and just staple the screen to the top of the wood strip.  Hopefully I was clear I would just staple the ends to the end board and the back board, or use screws to get though the polyiso or whatever you use for insulation.
Dan

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Scott Davis

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Reply with quote  #24 
Hi Antonio,

Whether you flow air from bottom to top or side to side, with a screen collector, the import part of the design is that air starts out above the screen and then flow through it, picking up heat and being directed away from the colder glazing simultaneously.  Dan's drawings area a good illustration.

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solardan1959

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Reply with quote  #25 
Antonio,
   Scott brings up a good point, you never mentioned how you wanted air to flow.  I made some assumptions based on what I would do.  If you could have many vents along the top and bottom it could then just run naturally, (without fans), or you could still use some.  Screen frames would be easier to manage also.
   Downside is it takes a lot of holes though what your trying to heat. Large plenums could be used at the top and bottom but you will have probles trying to get equalized flow out of the collector, though it can be done.
Dan

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AntonioDB

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Reply with quote  #26 
@Scott
Scott Davis has wrote:
...the import part of the design is that air starts out above the screen and then flow through it...

I understand very well the physic principle, but normally a countercurrent heat exchanger has a better efficiency. I suppose that the screen nearest the glazing is hotter than the other due to better sun exposition, so may be that inverting the screen we can get a better exchange efficiency. Have you done or can you do some tests about ?

@solardan
I have a vertical wall measuring about 9.3x1.8 meters (about 16 square meters) where I can place the collectors.
My idea is this one


Green arrows are inlets (for inside home to the collector) and orange ones are outlets (from collector to inside home).
The black diagonal area is the floor between 1st and 2nd level of my home.
The levels are connected by an internal stair, so I think that is better to have inlets and outlets at the lower level.
Internally I will do a duct till to the floor of the lower level in order to suck the coldest air and push hot air as lower as possibile.
I will put some fans in order to get a good air circulation inside the collectors.

Garage_Hermit

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Reply with quote  #27 
Antonio, have you considered picking up your intake air on the UPPER floor, and injecting your hot return air, into your LOWER floor... You could use one big pump, on the lower floor, dragging air through the system and just dumping air en masse in some dark isolated corner as far as possible from your south facade.

The advantage in this, is , looking at your drawing, you have a window available at each level, to allow ducting direct into the collector, so (perhaps...) no wall penetrations...

This system would allow better heat transfer into thermal mass. Then the exhaust air rises to upper floor by natural convection via the stair well, to be dragged back out and in to the collector.  You might have to "help" it in places with a small fan... or break out some ventilation grilles (at ceiling and/or floor levels), for best circulation.

This might give you better aeraulics.  It might also save you having to install a duct. Also, you would certainly be backflowing your air through your collector, like you said, since you would be forcing the air stream to work "backwards" against thermosiphon. This would "scrub" the air against your screen.

Also, you could consider configuring your screen as "thin shelves" or "trays", installed horizontally across your collector at, say, height 60 and height 120. Each tray is a timber screen like SolarDan drew, with 2 layers of screen wrapped around it. This means your air is traversing 4 x thicknesses of screen.  You could perhaps place the odd pack of steel wool or metal swarf in between the two layers of screen...

Hope this makes sense, if not, I can send you a rough drawing !

Garage_Hermit

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solardan1959

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Reply with quote  #28 
Antonio,
   Thanks for the details,  one concern would be the lower vents.  A lot of hot air will be at the top since it is 2 meters high.  Not sure of your plans for fans but if you just have external fans it may be hard to draw the hot air from the top of the collector.  With the length of the collector the middle outputs are probably a good idea.  If it was half as high I would think it's a great design but I do worry it will get very hot at the top.  Another good option would be to take two outputs directly into the upper floor also at the top right above the outputs you show.  Variable speed fans could direct where you focus the heat at different times.
   Also keep in mind that this is a big collector and can generate lots of heat.  You are going to need good flow to try and extract it.
  Nice drawing, that really helps. Dan

edited,
  Also like a lot of what Garage Hermit said, vents on opposite corners could improve flow, I was envisioned a collector like this, (below), you could make two like this at 4.5 meters each an 2 meters high or one long one with center vents.
Dan

Garage_Hermit

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Reply with quote  #29 
Antonio, I did a *VERY* rough drawing !

Like SolarDan says, this is a big collector, I reckon you will need a circulating fan in the middle somewhere, as it will get very hot at the top (right-hand side per my suggested configuration).

Also, please check my arithmetic (it is Sunday!).

G_H

 
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pdf Antonio_Backscrub_Air_Collector.pdf (665.01 KB, 170 views)


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(1)  "Heat goes from hot to cold, there is no directional bias"
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AntonioDB

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Reply with quote  #30 
@Garage Hermit
A drawing better explain what you mean. Thanks.

@solardan
The "middle" outputs are at not in the middle, but the left one is about 3 meters from the left inlet and the right one is about 5 meters from the right inlet.
In my project.
I'm thinking:
- to put a deflector inside the collector in order to suck the air from the top of the collector and drive it to any outlet, but I have not a very clear idea on how to do it
- to make some ducts inside the home in order to suck air as lower as possible and also do push hot air as lower as possible. More cold is the air that comes inside the collector, more efficient is the collector (due to higher delta temperature between air and the absorber) and, for sure, the coldest air inside the house is at lower floor level. Moreover, hot air naturally as an ascendent flux so, probably, it's better to push it as low as possible. In order to have a good circulation inlets and outlets have to been not near each other (in my design the distances at 3 and 5 meters).

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