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gbwillson

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Reply with quote  #11 
A lot of good questions Carlton-

You are correct in that a collector that sits on the drawing board gives you no heat. So an empty black box is better than nothing.

There are several ways in which to measure how well a collector is performing. Basically, you have BTU output and efficiency. BTU is the actual amount of heat a collector is producing, while efficiency percentage is an indicator of how well it is operating. I like to focus on the BTU's produced as it is more helpful to me for choosing the size/type of collector for the areas I wish to heat. Pretty much any well built solar air collector should be 50% efficient or better. 

The depth of a collector front to back depends on the type of collector. A backpass tends to be one of the deeper designs as the air has to pass both in front and in back of the absorber. The gap behind the absorber is ideally twice as deep as in front. This allows the air behind the absorber plate to slow somewhat to pick up more heat as it moves behind the absorber. I don't know if there IS an optimal gap, but in any case, be careful not to make it too small as air gets restricted and slows, especially at the turn. 

I don't know about using screws or nails to act as spacers. I suppose they could, but I agree the amount of turbulence/heat transfer would be minimal. Fins attached to the underside of the absorber plate have shown to be quite effective. 
http://simplysolar.supporttopics.com/post/experimental-investigation-of-solar-air-heater-with-free-and-fixed-fins-efficiency-and-exergy-loss-6746940?highlight=fins&pid=1281455241


The manifold/plenum are basically the same thing for our purposes. It is a way to distribute air as desired though the collector. So in your case, the air would enter the collector at the bottom, into a section of the collector box(manifold/plenum, which in turn allows air to enter the heat chamber evenly from side to side. This can be done with a narrow slot, or many holes. It is more difficult to get an even flow if the air is moving very slowly, or if the collector is very wide. The M/P doesn't have to be much larger than the ducts that are entering or exiting the collector.

"To push or pull the air, that is the question". Both have merit, as it depends on the size, shape, design of a collector. Air pulled tends take the shortest, most direct path from entry to exit. This can lead to hot and cold spots inside a collector. Pushing air tends to create more internal turbulence, which in turn reduces areas with low airflow, but it also tends to increase flow resistance. Two fans, one at each opening is also an option. Often, it comes down to where the fan fits with the space you have. You can make it work in either case.

Hopefully, some of your questions have been answered, but I'm sure there will be plenty more before you are done. So don't hesitate to ask...


Greg in MN

littlecreek

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Reply with quote  #12 
Thanks Greg.

 Showing how little I know, my project apparently is not a backpass design lol.

I drew a picture of what I have planned.
solar.gif 

gbwillson

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Reply with quote  #13 
Looking at the drawing I would still refer to your collector as a backpass. This is due to the fact that the air passes BEHIND the only heated surface. 

And looking at both the intake and the exhaust, is there any reason they both couldn't pass through the wall into the house? Or are they to be ducted to an interior room? 

Greg in MN
littlecreek

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Reply with quote  #14 
Thanks Greg,
Part aesthetics, part practicality. I've got lots of room to work in the floor joist and attic, without needing it to be compact and meet. The exhaust really isn't meant to be more than maybe 8' into the room. Room btw is open living/dining/ kitchen, roughly 75% of the house,so it uses that % of the heat.

Also, if this all goes terribly awry mid winter, I wanted to be able to seal everything up nice and tight instead of stuffing insulation into the wall. LoL
gbwillson

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Reply with quote  #15 
Having an open floor plan will certain make things easier once you start pumping heat into your home. My home is roughly 900sf on the main floor with the same amount down in the basement. The basement is unfinished and roughly ¾ of it is my workshop. My solar heaters draw air from the basement floor, heat it and return it to the basement, which in turn warms the main floor while active and for several hours thereafter. A couple of years ago I experimented with the warm air being blown directly into a window on the main floor. The result was the main floor was about 90˚F while the heater was running, and the house seemed to cool down much faster in the evening. It was great if you want to hang out in the middle of winter in your Bermuda shorts. In any case it was a nice experiment, and actually wasn't hard to redirect the air back into the basement. Having too much warm air is a nice problem to have during a cold winter. 

What do you expect the dimension of your glass surface once you have finished?

Greg in MN
littlecreek

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Reply with quote  #16 
Greg, it's going to be 100sq ft, 12.5' wide by 8'high.  We are all-electric for utilities here, on what's called an 'equal payment plan' with Manitoba Hydro. Today we just got our yearly adjustment. We went up 3$ from last year, so this year we will be paying 302$/month. I'll be checking in & out temperatures but not planning on throwing a bunch of sensors in my unit, That adjustment next year will be what matters the most to me, how much $$ did I save. 



gbwillson

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Reply with quote  #17 
Yikes! All electric in such a cold climate is a rather expensive way to go. I expect that with the large size of the unit you are building you should see a some nice savings. Even if the savings are less than ideal you will likely be more comfortable. I forgot to ask, is the crawlspace insulated, specifically between joist below the main floor? The reason I ask is that if it is, it may be possible to store some heat down there during the day and slowly release it on the main floor well into the evening or overnight. This is especially effective if you don't have carpeting. My natural gas furnace rarely runs in the evening or overnight if the previous day was sunny due to my solar air heat being stored in the basement ceiling.

The use of temperature sensors, even simple ones, are useful in that they can help measure if your collector is running efficiently, or if there is a problem. For example, if the output temps are too high, it is an indication that the airflow is too low and you are wasting heat. x

Greg in MN


littlecreek

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Reply with quote  #18 
Haven't completed the finishing touches inside or out, but the unit is hung and sealed. It's a u shaped design, inlet up in one corner with outlet up in the other. 200 CFM centrifugal fan that I manually turn on right now but snap disc is ordered & on the way. The back wall of the unit is corrugated metal and there is a maximum airgap of 2" between that and the flat metal front.

I had a real hard time with building it all one unit and all in place. Definitely see the value of making smaller units and joining them together.
Also had difficulty using materials that I had vs buying to what I needed. Saved money but wasted time.

Rain and cold fall prevented me from finishing paintjob on frame but we are in store for nice weather next week so hopeful on getting it done.
Wife likes the look so I guess that's a win

Attached Images
jpeg IMG_20181112_121958.jpg (3.16 MB, 21 views)
png Screenshot_20181112-122105.png (81.69 KB, 21 views)

littlecreek

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Reply with quote  #19 
Haven't completed the finishing touches inside or out, but the unit is hung and sealed. It's a u shaped design, inlet up in one corner with outlet up in the other. 200 CFM centrifugal fan that I manually turn on right now but snap disc is ordered & on the way. The back wall of the unit is corrugated metal and there is a maximum airgap of 2" between that and the flat metal front.

I had a real hard time with building it all one unit and all in place. Definitely see the value of making smaller units and joining them together.
Also had difficulty using materials that I had vs buying to what I needed. Saved money but wasted time.

Rain and cold fall prevented me from finishing paintjob on frame but we are in store for nice weather next week so hopeful on getting it done.
Wife likes the look so I guess that's a win

Attached Images
jpeg IMG_20181028_152428.jpg (2.53 MB, 20 views)

littlecreek

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Reply with quote  #20 
Guess I should have said, 70 degree F rise in temperature

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