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G3OCR

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23-Feb-2013

Remember that project that I started last May?   Well, it's been on-going throughout the year . . . . . .

First problem was getting an 18" slab of concrete poured with pipes running from end to end to warm the slab with hot air . . . . .  I had to wait several days between pours because of the rain.  We've had "the wettest Summer since whenever" in UK this year .

I eventually got 9 pipes installed in each direction (with a plenum at the end farthest from the collector to turn the air around and send it back to the collector input.  Then I completed the slab and built the 10-foot by 6.5 foot stud-wall building above it, with 3" of fibreglass insulation within the OSB walls, and  10mm twin-wall PVC cladding on the outside of the outer OSB.

THEN someone pointed out that the 9 runs each way would have too much air resistance to get a decent flow of air to collect the heat in the concrete to store for the night, and it was suggested that I dig out the10mm pipes and install some much bigger ones .  As the concrete is very thick and well hardened by this time, that was a real no-no, so I took another suggestion which was to open up the plenum at the back to the inside of the structure so that both sets of pipes would run through the floor (18 in total) and the air would return through the structure.  Having done this, I now get a flow of between 5 and 10 cfm with a couple of 3" diameter computer fans pulling the air from the far end.

Now that the roof was installed I made the next mistake which was to start installing the benches and some of the shelves - I wanted to get my ham radio gear inside before the colder weather (and dampness at night) was upon me.  Of course, this meant that subsequent work (like constructing the collector) couldn't be done in the dry and it rained . . . . .and rained . . . and then it rained

So the last week has been dry, and sometimes the sun has even shone!  The collector shell was completed and the back lined with Polyiso.  Two 3" diameter PVC pipes are installed in the shack wall and another couple of 3" computer fans installed in the bottom of the collector to draw the "cooled-down air" from within the structure.  It was nice to see that they would also pull about 10 cfm into the (open) collector, but then, and hour ago . . . . . . it started raining .  This was actually a "blessing in disguise" as while I was clearing my tools, I realised that I could fit the fans inside the shack wall instead of inside the collector, which would make construction of a plenum (where the air enters the collector) a lot easier.  Tomorrow's job - remove the fans and re-install inside, then try to get the screen in and the twin-wall glazing installed!.

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solardan1959

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Stu,
   Have you thought about pushing air as well as pulling?  10-15 cfm is going to be very low for your collector.  If you could double that it would still be low but in a way that may work out as the much higher temps will heat up the concrete more.  I expect your temps are going to be 140+  It's not to late to add a small fan and blow some of that air directly into the radio room to warm it up faster.  You could put a higher level snap/fan switch so it would only come on at 135 or so.
   I've always thought that as long as you heated the room the concrete would warm also.
Dan
G3OCR

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Hi Dan, thanks for the encouragement .

Yes, the return (from the floor slab) now comes back through the building, so any heat left in the air will be brought inside and will heat the building too (and the floor slab, from above downwards).  I also have a pair of similar fans drawing the air from inside of the building back into the input of the collector as you suggest. 

So far this Winter, the insulation of the structure has kept the temps inside a few degrees warmer than outside with little heating (from a radio that's running 24/7, but only consuming about 25 watts).  That's with the South wall not insulated, where the collector is being installed, so I'm hopeful that the final construction won't need too much heat to protect my equipment from moisture forming from condensation at night (this is the main priority) and anything more is a bonus!  With outdoor temperatures going down to about freezing at night, the inside has not dropped below about 40F.

Rain again yesterday afternoon - only light, but enough to make screw-and-glue construction dubious!  Hoping for a full working day today if the rain holds off (and work doesn't interfere too much as I work from home).  Still, I'm learning a lot as this project goes on, and I'm encouraged by the results of your own 20' x 3' collector and already looking at a similar area on my own home where there is a 45-degree tiled slope between the ground floor and the floor above - thinking of running air through the ground floor crawl space (but with  MUCH bigger ducting!)

Stu

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G3OCR

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Some Pictures


Gluing the polyiso to the back of the collector (constructed of OSB)


 . . . and screw



Exit vents from inside

(more to follow . . . . .)

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G3OCR

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More from yesterday . . . . . . .



Testing a very simple backflow damper



Which seems to work - at least with forward flow



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solardan1959

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Stu,
  Sounds good, I think you only had a goal of about 45 degrees anyways, (hope you did not mean C).
That holey pegboard stuff by the inlet, is that going to go all the way across?  What is the purpose of it?
To me it just seems way to restrictive.  Will not comment more about it till I know it's purpose or your intent.

  The 20X3 is fine but I feel the 12X1.5 solar screen has a lot more potential.
Solar Web Page > Forums > DIY Solar Experimentation & Performance > Experimenting with a screen collector
A longer version of this say 20 by 2 would generate a lot of heat.  I would stay away from the metal backing I put on the 20X3 as I don't think it helped much and just made it more complicated.
Thanks for the pictures,
Dan
G3OCR

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Hi Dan,

The "holey stuff" is a thin MDF board and the holes are about 1/2" diameter (a lot more total cross-section than the pipes through the concrete) so it shouldn't restrict more than the pipes.  The purpose of it is to try to spread the air-flow right across the collector as evenly as I can  (note the pattern of slightly fewer holes near the centre of the collector) which I hope will defeat any tendency (or at least reduce it) for the air to come through the closest place to the two inlets.  Yes Dan, it does now cover both sides at the bottom.  It's all very experimental and I've built it so that these can be removed, and the screen can be easily taken out if I want to try out painting the inside matt black instead of the reflective silver finish, or changing the screen arrangement.

Further progress today - I've made the frame for the screen (not installed yet) a bit more rigid.  I found that with my wife and I moving it around it easily distorted (just 4 timbers screwed in a square at the corners - I never learned to make these fancy cabinet-makers joints) so I've added some fillets in the corners and re-glued, clamping the joints hard instead of just placing it in the box.  The fans have now been removed from inside the bottom of the collector and installed on the inside wall of the shack, with temporary wiring from a bench power supply.  Eventually the plan is to use a 1 ft square PV panel, via a snap-switch, which should provide enough current to run both pairs of fans.

I've also got the pipes from the concrete sub-floor connected to the top of the collector - that was a job and a half, trying to align them with the holes in the collector!  I'm hoping tomorrow to finish the insulation on the collector sides and get the screen installed on its frame - and maybe get the glazing fitted to try it out - the BBC is forecasting sun all day tomorrow

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solardan1959

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Stu,
  I was afraid you would say that though they looked a lot smaller than a 1/2 inch.  For each 3 inch pipe with an area of 7.06 square inches you need 36 1/2 inch holes (they have .196 square inches area).  Do you really have that many holes?  Even if you do it would be better to do all the restricting with the pipes and not add to it with more resistance.  I would have closer to double the amount of holes required to equal the pipe sizes. I do not know how you will be hooking up your screen, but I would try and use it more as a deflector.  If you raised the top part and put some spacers under it you could release more air that way also and better direct it towards the top of the collector.
 Sorry, I know it's your collector and you are closer to it and the issues but just trying to help.
 Circle calculator here: http://www.calculatorsoup.com/calculators/geometry-plane/circle.php
Dan
G3OCR

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Reply with quote  #9 
Point taken on the holes Dan - I'll check them in the morning when it gets light here.  

Can you explain a bit more about what you mean about a deflector?Stu

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G3OCR

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I checked - There's 90 holes across the full width.  Even with both "push" and "pull" pairs of fans, there's still a place that I can check airflow, so if I find there is any extra restriction there will be no problems modifying the design to
add more/larger holes.

But elaboration of your suggestion about a "deflector" would be of interest.

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