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SteveGerber

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Reply with quote  #1 
The recent thread about building a rotating sun tracking hot air collector got me thinking in a different direction. How about building separate panels that are oriented towards southeast, south and south west in order to produce hot air over a longer portion of the day? Right now, January 26, the sun rises at 7:26 a.m. at an azimuth of 120 degrees at my latitude (38.5) and I don't get much out of my south facing panel until around 9:30 a.m. when the azimuth is around 135 degrees. (My panel actually points slightly SW at about a 190 degree azimuth because the south side of my house doesn't point directly south.)

So I wonder how useful it might be to have another panel and oriented for best morning heat production, maybe pointed at about a 130 degree azimuth. Would sure be nice to have a panel firing up as early as possible on a cold clear morning. Has anyone experimented with this at all?

gbwillson

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Reply with quote  #2 
Steve-

One advantage of living in the city is my lot is too small to even consider such things.

The thing to remember is how much will it cost in time and money to build two or more collectors versus a single unit. In winter, the sun is not only very low in the sky, but the sun's path across the sky is very short. So you might have  8 hours of daylight, but an hour at each end of the day the sun is likely too low, below the tree line, or has too much atmosphere to pass through to be useful for a solar heater. Something else to consider is where you live as frigid mornings can and will delay solar heat collection for an hour or more AFTER the sun hits the collector until it warms up enough to activate.

So if you were to add up all the time and materials for three collectors, you would have to more than simply offset the benefit of an extra hour at the beginning or end of each day. Another way to look at it is to compare the average BTU's collected by a single, large collector facing south to those of three, smaller collectors oriented SE, S, and SW. Early in the day the SW collector is useless, as is the SE collector late in the day. 

Don't get me wrong, if I had no room for a collector to face south I would certainly consider SE and SW facing collector, even if they only gave me a few hours of free heat each day. Another thought is even if the heat gain was only a few hours, this might be a great place for experimental collector designs.

Greg in MN

There is a great little phone program called Sun Finder. It will tell you the exact location of the sun in the sky at any given time of day, on any day of the year. This makes it very useful for helping locate future solar collectors, garden, patios, etc.
stmbtwle

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Reply with quote  #3 
Interesting concept, I've wondered about that too. It would work, but the question is how well, bearing in mind that one or another panel might be shaded part of the day. You'd need independent fans and controls so the shaded collector wouldn't be pumping cold air into the system.

Would it be any better than the same three panels all facing south? The only way to know for sure would be to build three more panels (total 6) and do a side by side test. That's a lot of work/expense.

If you build them small and moveable, when the testing is over you could reconfigure to whichever is best.

I addressed the problem with a water-based collector, and hot water storage. The heat is available in the morning even before sunrise. But it's complex, and more expensive.



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Willie, Tampa Bay
SteveGerber

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Reply with quote  #4 
Yeah, I don't know if it would be worth the effort or not.  It definitely wouldn't be worth doing in a region where you typically have cloudy or foggy mornings that clear off later in the day.  My region isn't too bad, it's technically a semi-arid climate and we often have clear mornings, though not as often as the desert southwest would have.

I'm thinking of this in terms of the bigger picture of trying to provide heat for more hours of the day rather than just trying to produce the maximum amount of heat during the peak hours.  It could be more useful to be able to produce heat over a wider range of the daytime hours even if it means a lower peak production of BTUs.  In the early morning, you really need heat as soon as possible because the house temperature has been falling all night (assuming the thermostat for your backup heat isn't set very high.)  Unless you have some kind of heat storage system it might not even be helpful to have a surplus of BTUs in the middle of the day if all your panels point south.  You would end up overheating the house or just shutting off the collector fan.

Another perspective is to consider the entire heating season not just the very coldest months.  In my climate, Shenandoah Valley of Virginia - z6b - latitude 38.5, our last frost date is around the middle of May.  I even remember one tomato killing hard frost on May 17th several years ago so I still need heat late into spring.  Consider May 15th when sunrise is a few minutes after 5 a.m. and the suns azimuth is at 64 degrees.  That's 26 angular degrees north of due east.  By 6:30 a.m. the suns azimuth will be at 78 degrees, still not even due east yet, and with the sun at an elevation of 15 degrees it should be possible to generate heat if a panel was pointing the right direction. 
stmbtwle

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Reply with quote  #5 
"Unless you have some kind of heat storage system it might not even be helpful to have a surplus of BTUs in the middle of the day if all your panels point south.  You would end up overheating the house or just shutting off the collector fan."

Which is exactly why I went with storage.  Here in FL I can overheat the house in January with just the windows, yet it still gets cold at night. We also get up early and it's nice to be able to sit and drink one's coffee in the Florida room without freezing one's butt off. My system is nowhere near big enough, but it DOES work for the few hours between 6 am and the time the sun comes in the windows.  Currently it faces south, but I think more to the west would help with storage.  East (as you mentioned) would be better for direct morning heating.  

A problem arises with easterly or westerly facing collectors though in the summer.  They're still exposed to the sun but you don't need the heat so they need to be covered.  A south facing collector is often shaded by the roof overhang.  A moveable mount might be a good idea.

"just shutting off the collector fan" doesn't really work.  Without the fan to "cool" the collector, it will stagnate, overheat, and possibly be damaged.  You need to find a way to store or at least dump that excess heat.

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Solar is like the wind. It may be free, but putting it to work isn't!
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mclark999

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Reply with quote  #6 
As I started the topic on a sun tracking panel, I thought I'd chime in with my thoughts. I was thinking in terms of options and judging criteria of cost, simplicity, aesthetics. My goal was heat earlier in the morning. I suppose it could also be more even heat throughout the 24 hour cycle. 

OPTIONS
1) Build multiple collectors facing southeast, south, and southwest
2) Build a single collector that tracks the sun
3) Build a single collector that stores heat


#1 (multiple collectors) might seem the most simple at first, but you would need multiple entry and exit points in your building, multiple control systems, ducts and fans. Certainly not cheap.  Depending on your site, it may not be the most aesthetic.

#2 (sun tracking panel) may at first seem the most complicated, but in this thread, forum members came up with some pretty simple ducting options and the suntracking modules are readily available from electric solar systems. Only one entry exit point in the building and one fan/controller system. IMHO, I think this would be the most simple option.  

#3 (panel with heat storage)  If you could store heat passively in the building materials themselves (floor, walls, etc) this could be the simple option.  I don't think passive storage would really help a whole lot in the morning, especially in cold weather areas, unless the building was super insulated, and then it may be too hot in the middle of the day.  Active storage through a water heated system is more complicated and more expensive, but I believe is more efficient than hot air solar.  

Because of my building parameters on my house, I am leaning towards option 2 or maybe 3 using a large hot water system.  My south wall is far from the spaces in the house that need heat so I lose a lot mechanically trying to blow hot air from one end of the house to the other. A hot water system makes it easier to move the heat from the collector to the rooms that need heat. It also provides a fairly easy way to store heat in a home like mine that has a basement. 

Thanks everyone for your thoughts.  I love the comraderie here. 

Michael




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Double screen hot air collector
gbwillson

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Reply with quote  #7 
Michael-

#3 is the simplest, most cost effective way to go. Added reflectors will make up any difference in heat gain, but are simple and cheap to make. As far as storing heat, ducting into the basement will collector plenty of heat during the day, and slowly giving it back during the evening and overnight. 

If you want to experiment with other, more complicated designs such as a pivoting collector, you can always build test units while the large collector warms your home. 

Greg in MN
stmbtwle

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Reply with quote  #8 
One could add tracking reflectors to a fixed collector, too. It would be simpler than trying to move the entire collector.  They could double as covers in the summer.
reflector.jpg 


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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
mclark999

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Reply with quote  #9 
Quote:
Originally Posted by stmbtwle
One could add tracking reflectors to a fixed collector, too. It would be simpler than trying to move the entire collector.  They could double as covers in the summer.
reflector.jpg 


Yes, good idea. Add that in as option 4.

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Denver, CO

Double screen hot air collector
stmbtwle

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Reply with quote  #10 
Looks neat but probably not as easy as it looks. Getting the angles and the sensor location right would be tricky. At least only one reflector would need to be driven, the other would be slaved to it.
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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
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