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Author Topic: Solar Air Heater  (Read 1677 times)

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Offline Brian_Weekley

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Solar Air Heater
« on: November 10, 2016, 03:43:05 pm »
Has anyone here actually tried building a solar air heater?







I know a solar air heater is not very efficient compared to solar water heaters, but I have a small, insulated, off-grid barn/cabin with a south facing wall and I was thinking this would be a low-tech, inexpensive, low-maintenance option that might help keep it above freezing during the winter.  Other than being kind of ugly, do you think itís worth a try?

e aho laula

Offline DMcCoy

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Re: Solar Air Heater
« Reply #1 on: November 18, 2016, 07:24:39 am »
I have started but it is on the back burner for the time being.  I think it is a great idea.  I purchased a small fan to connect to a small solar panel to circulate the air.
I found a detailed and technical you-tube by an engineer who tested different panels.  Beware the people who show amazingly high temps - while they can be achieved, what is more important for testing real performance is in/out temperature difference over time. Sorry I don't have a link but it should still be out there.
Good luck on your project!

Offline Gearbox

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Re: Solar Air Heater
« Reply #2 on: November 18, 2016, 02:01:06 pm »
Some where I saw one built out of beer cans painted black with a small fan on a thermostat inside to turn the fan on and off .
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Offline Brucer

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Re: Solar Air Heater
« Reply #3 on: November 19, 2016, 01:09:00 am »
I saw a few examples years ago. Instead of being mounted on a wall, they were usually sloping away from the house on the South side. The top of the panel would be connected to the bottom of a window opening. There was no need for fans with this design ... natural convection would cause the air to circulate through them and into the house.

They were pretty simple. Usually a wooden frame, a plywood back, a pieced of sheet metal in the centre (painted black), and a piece of window glass on top. The back and sides would be insulated with rigid foam insulation. I saw a few with heavy clear plastic film instead of glass tops. Ideally the top would be double-glazed.

Typically cool air from the house would flow down the panel between the back and the metal sheet. The it would curl around the bottom of the metal sheet and rise up between the metal and the glazing. This would enter the house at the bottom of the window.

These would work well anywhere you could get sufficient height on the south side of the building.
Bruce    LT40HDG28 bandsaw with two 6' extensions.
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Offline square1

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Re: Solar Air Heater
« Reply #4 on: November 19, 2016, 04:21:19 am »
I've dabbled, building several renditions of passive & active solar air heating panels.  Only once did I go all out with new materials, usually they were built from pieces and parts picked up here and there.

Two things you will immediately notice after you install one. First, how little the sun shines in the winter followed closely by how long it takes the sun to get into a useful position in the morning and how quickly it exits stage west in the afternoon in the winter.  You can download historical data for your area from websites like weatherunderground that when studied will demonstrate just how little time the sun is available to provide heat when you need it. 

Here are my findings from October 2007 - May 2008 using approximately 2100 data points per month. Each data point was awarded a score of 0 to 4 in increments of 1/2, i.e 0, 0.5, 1, 1.5, 2, 2.5 etc... A clear sky, after sun-up (plus a little delay for the sun to get in position to produce heat), before sundown (minus an hour during which no heat would be gained due to position of the sun) and below 60 degrees scored a 4.  Cloudy, raining, before or after heat producing sun position, or outside air temps above 60 scored a zero.

Percent of time heat is needed and sun is able to produce heat:
October - 6.7%
November - 7.7
December - 10.3
January - 5.8
February - 9.6
March - 11.4
April - 6.3
May 2.8

Another issue you will have to deal with is reverse convection taking the warm air from your home, cooling it and returning it to your living space during the many hours your panel is not producing heat.

Offline DMcCoy

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Re: Solar Air Heater
« Reply #5 on: November 19, 2016, 06:37:12 am »
square 1-

Just to make sure.  Those are negative numbers until May or was there a typo?

Offline square1

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Re: Solar Air Heater
« Reply #6 on: November 19, 2016, 10:38:02 am »
I should have used a ":" instead of "-" for a separator / spacer. I missed putting in the separator for May.

The numbers are positive and represent the percentage of those ~2100 monthly data points when the sun was in a position to provide heat and outside temp was below 60 degrees f. (the random number I chose to determine a need for heat), AND the sky was mostly clear to clear.  The 2.8% in May might have had many hours of sunshine, but few hours of needing heat thus the low number.

Example: The shortest day of the year here has ~9 hours between sunrise and sunset.  However, maybe 6 of those hours (that's generous IMO) the sun shines on the panels directly enough to produce heat.  Right from the start only 25 % of the hours in a day have potential to produce heat.  The sky has to be pretty clear to get heat out of a panel.  Clouds (even scattered), overcast, rain, drizzle, snow all will greatly, and negatively, affect the panels output so if it snows 3 of the six hours the day your down to 12.5%.  It's usually overcast before & after a snow...more deductions.

You also have to factor the panel may not be big enough to produce all the heat you need.  Can it produce 75%, 66%, 50%, etc.. of the requirement? Again you have to reduce the return on your cost by the inverse of that percentage.

Offline Brian_Weekley

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Re: Solar Air Heater
« Reply #7 on: November 23, 2016, 08:47:58 am »
Square1óbut you live in Michigan where the winters are always overcast (I know--used to live there)!  :D

If I did build one, I really think a passive system is the way to go.  This is the most intriguing design Iíve found so far:

http://www.iedu.com/Solar/Panels/

It includes an ingenious design to prevent back-draft at night.  Unfortunately, the posts and braces in my timber frame do not allow me to place the necessary horizontal inlets along the top of the wall.  I really want to try building one, but Iím leaning against it at this point.
e aho laula

Offline square1

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Re: Solar Air Heater
« Reply #8 on: November 23, 2016, 12:34:29 pm »
Square1óbut you live in Michigan where the winters are always overcast (I know--used to live there)!  :D
I hear that!
My next experiment will be year round water heating with the ability to extract heat for space heating when conditions permit.. I think that's a better system, at least for Michigan..

Offline Brucer

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Re: Solar Air Heater
« Reply #9 on: November 24, 2016, 12:32:18 am »
Another issue you will have to deal with is reverse convection taking the warm air from your home, cooling it and returning it to your living space during the many hours your panel is not producing heat.

This won't be an issue if you have the space to use the design I suggested. The cool air intake is just below the hot air outlet and the entire panel sits below these openings, acting as a trap. At night cold air will simply settle to the bottom of the panel and sit there. Also, by keeping the intake and outlet low on the wall, more of the air will be heated then if the outlet was near the top of the wall.

Height of the wall and geography will determine weather this design is practical.

It's always interesting to build a cheap mockup using scavenged materials, even if you don't connect it to your house.
Bruce    LT40HDG28 bandsaw with two 6' extensions.
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Offline square1

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Re: Solar Air Heater
« Reply #10 on: November 24, 2016, 06:28:16 am »
No doubt a good design is of critical importance. Scavenged materials can considerably save on the cost to implement but you have to be pretty selective.  Solar panels are a torture device for construction materials and methods with their inherent rapid and extreme temperature cycles.  The more complicated the system the greater the cost to build it to last. Air leaks and/or moisture collection, which raises another type of issue (mold), sharply reduce efficiency.

Not included in the info I shared above are the calculated savings. IF (and that's a big IF) a system were capable of producing 75% of heat requirement during Oct 2007 - May 2008 while the sun was available for solar gain the savings (natural gas at the time was $0.95 - $1.01 per CCF) would have been $40.53.

The last passive collector I built was 5' x 10', not near big enough to provide 75% of the heat for my 1400 sq. ft. 1 1/2 story home on the best solar days.  It cost ~$200 or $4 sq. ft. (you will be amazed how much good sealers cost and in how many areas it is needed. Sealers are not an item to skimp on quality) to build not including the glazing that I had salvaged from an earlier project at no cost.

The math says my break even point is 5 years IF (there's that big IF again) the panel produces that unobtainable 75%. Remember that's 5 years of torturous conditions during which construction materials will be broken down and minor flaws introduced during the build will be exasperated!

Don't get me wrong, I've been romanticized by alternative energy / energy conservation for most of my adult life having built my first system in the early 1980s.  If your goal is cost savings there are many things you can do with $40 (or $200) that will pay back much more quickly than, and many times over, passive solar air heaters.  If, like me you like jousting windmills in Quixotic endeavors, solar air heaters are a noble quest by all means :)

Offline Brian_Weekley

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Re: Solar Air Heater
« Reply #11 on: November 24, 2016, 08:26:16 pm »
I was considering a solar heater for an unheated workshop-barn (20x20).  The cost/payback time isn't really a concern to me.  Since this is a non-heated building, I'm really just looking for a simple way to help warm it during the day when I might be using it without a lot of effort on my part.  Maybe so the woodstove doesn't need to be used except when it's very cold.  My expectations aren't terribly high--just looking for some free, easy heat when the sun is out to make the space more comfortable.  If it had the capacity to keep the building above freezing, that would be success to me.
e aho laula

Offline square1

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Re: Solar Air Heater
« Reply #12 on: November 25, 2016, 06:03:45 am »
Is the workshop well insulated? Has the majority of air infiltration been stopped?

It's said if all the air leaks in the average home, most of which have had some attention given to energy savings, were to be concentrated in one spot it would create a 2' x 2' square opening. Imagine a passive solar panel with a 6" round exit trying to overcome that.

Rodale used to publish New Shelter magazine. It was the Mother Earth News of energy efficient construction and design with a strong bent toward green energy. I used to camp by my mailbox waiting for each new edition to arrive.  The magazine always insisted insulation and infiltration be addressed before any thought went to how a structure would be heated / cooled.  I hated that, there is no "cool factor" in air sealing and insulation. That's boring stuff!  One month they featured a new build (~2000 sq. ft. IIRC) in Colorado that had no heating system at all. The house was so well insulated & sealed that body heat from it's occupants and heat generated by everyday electrical items (light bulbs, refrigerator, TV, etc..) supplied the entire heating requirement for the home.

Somewhere over the past 35 years I've lost my "cool factor" and agree with Rodale's boring conclusion.  My suggestion for the best result in your workshop is first to seal up every crack & crevice (there are many methods from wrapping the structure, to caulk & expanding foam, to elastomer barriers,etc...), 2nd install one or two good quality south facing windows, 3rd insulate, insulate, insulate. THEN if you still feel the need, build & install a solar air heater.

I know, not cool and no wow factor, but it is sound advice. Best wishes!

Offline Brian_Weekley

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Re: Solar Air Heater
« Reply #13 on: November 25, 2016, 09:28:51 pm »
My shed does have insulation, but not as much as you would prefer for a house.  The one inch of wood siding is covered with 2 inches of polyiso under the floor and on the walls and 4 inches of polyiso on the roof.  After wrapping with Tyvek, there are air spaces within the lattice that is used to attach the shingles and between the roof insulation and the sheathing/metal.  Stopping air infiltration is always important and worth the effort.  This building is still a work in progress.  In the photo below, you can see the true-south facing wall with the location of two windows ready for shingles to be attached to the lattice.



Iíve also provided a drawing below showing where I could install four solar heaters if I decide to try this.  However, if I do, I would want to attach the panels before I shingle the south wall.


e aho laula

Offline square1

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Re: Solar Air Heater
« Reply #14 on: November 26, 2016, 06:12:06 am »
Nice workshop! Did you do the design?
I would of course put solar heat on it! But that is not sound advice ;)

Offline r.man

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Re: Solar Air Heater
« Reply #15 on: December 02, 2016, 06:16:25 pm »
I can only offer an opinion and some second hand results to this discussion. I know a fellow who built a solar heater from scrap, time only invested, and when asked about his results he was happy with the added heat to a work/storage area and thought it was time well spent. As to whether you should build one, I say yes if not much money is involved. Do one on the cheap, try it for a year and if it seems worth it build more. Nothing like first hand experience to decide.
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Offline 21incher

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Re: Solar Air Heater
« Reply #16 on: December 02, 2016, 06:52:34 pm »
the way that I understand it is it works best if you have some kind of thermal mass to help store the heat and release it overtime. You may need to fill the shop with some big old cast iron machines.  ;D
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