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GARVEY MN

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Posts: 10
Reply with quote  #21 
I found my first few design flaws. First, the lower 2 exhausts I now have closed off. The air coming out of those 2 were only slightly warmer than the air going into the collector. Second, since blocking off the lower 2 exhausts, I should close off a few of the intakes directly below the exhaust that is open. That would allow the air to travel further, therfore warming up more, before it gets exhausted into the shop.

I did install a small 12 inch box style fan inside the collector to not only force air through the collector more efficiently, but also circulate the air throughout my shop. I have that fan on a timer. It turns on a half hour after the sun hits the collector and shuts off an hour before the sun gets too low in the sky. I realize I will have to play with the timer as the seasons change, but so far it is working well for me.

I am excited to see how well the collector works in the dead of winter when the outside temps are well below freezing.

I've included a picture of my fan install.

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gbwillson

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Posts: 2,286
Reply with quote  #22 
Garvey-

You may want to reconsider the placement of the fan directly in the only output duct. It's likely not made to handle the heat and will fail prematurely. 

 I like your idea of closing off the two lower output openings, but this may create a hot spot in the upper right corner. Yes, the air will eventually move to the highest opening, but much heat could be lost through the glazing before that happens. Maybe only a partial blockage is needed. Since the amount of output duct area has been reduced, consider decreasing the intake a bit too. Or have the fan help move air through ALL openings, not just one.  

Passive heaters will run hotter than an active heater. But this wastes heat through the glazing. So adding a fan can and will make a sizable difference in the amount of air collected and not wasted. With so many intake and exhaust openings this is a great time to experiment with these openings to achieve the best airflow and heat output. Simply block openings with cardboard until the desired results are achieved. 

Greg in MN
JC77

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Posts: 15
Reply with quote  #23 
Air expands and becomes less dense as it's heated. A cubic foot of air that is 60 degrees F will occupy more than a cubic foot of space after it's heated to 90 degrees F.  Fill a balloon with room temperature air and place it in your freezer and check it an hour later. Generally speaking, it's better to have more outlet opening than inlet opening to get good natural draft thru a passive collector. Looks like you have plenty of room for the air to get in but limited room for it to get out. If it's not too much trouble, I'd try experimenting with inlet and outlet openings. Maybe start with at least equal amount of inlet and outlet and see what happened (you could cut the same sized openings all the way across the top to match the bottom) Also, since you have a fan, you could place it at the inlet. Cooler/denser air is easier to move than hot(maybe not "easier" but you'll get more CFM at the same fan speed). Another thing you could try is like you already mentioned; inlet openings in the lower right and outlet openings in the upper left (or whatever place on your wall is the highest corner as it appears your side wall is shorter on one end)  This will create a longer flow path but I'd probably use your fan if you tried that... I'd be tempted to reconfigure the screens to a horizontal zero pass as that would greatly reduce the resistance thru the collector and keep the inlet/outlet configuration somewhat cross flow with the inlet being on the lower right and the outlet being in the upper left. You could use the holes you already have to control where entered/exited the plenums and create openings in the plenum to control where air entered/exited the absorber area... I'd definitely find a fan if I went that route. Lots of experimenting. But those are some ideas that came to mind looking at your pics... 
     Like other posts have said, you'll get better results with a fan. Since your collector is kind of an asymmetrical shape and already configured for passive flow, you should be able to achieve good results without a fan by experimenting with vent openings. I wouldn't worry about the lower output temps on the short end of the wall. That's less heat loss thru the glazing. As Greg mentioned, a large amount of warm air is better than a small amount of hot air. You'd have to fill your collector full of politicians to get a large volume of hot air. I'd create equal openings all the way across the top to match the inlets and see how it did. Maybe place your fan up near the ceiling in the highest point of your shop to push heated air down towards the floor kind of like a ceiling fan would do. If you decided to try a fan, then I'd do it kind of like you have mentioned before and close up vents so you have an inlet in the lower right and outlet in the upper left.
    I also agree with twinwall glazing. The glazing is by and far the number one spot for heat loss in a collector.You probably already know that corrugated single wall not only adds surface area but also a rough surface for both the cold outside air and the warm inside air to pass against, enhancing heat loss. The 10 mm twinwall is a good choice for glazing as it's pretty rigid and easy to cut but 6mm will work too. The little bit of sunlight loss thru twinwall will be more than offset by the smooth surface and dual wall insulation. On my collectors that had corrugated polycarbonate on them, I noticed a pretty decent drop in performance once the outside temps got down in the teens. They would do pretty good around 25 to 30 degrees and above. Twinwall got me performance down around zero and below. I'd definitely recommend switching to twinwall glazing at some point in the future. Good job on your collector and good luck tweaking it. 

Jason in IL
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