Solar air heaters - horizontal flow - opinions??

[dtompsett]

Hi Guys.  First-time poster, long-time reader.  I live in Northern Ontario... cold climate.  Typical winter temps are -25°C (-13F), with dips to -35 to -40C at times.

I'm planning on building a solar air heater, to heat the air in my basement, using two of the existing window openings as the inlet/outlet points.  

I got lucky with this house... that is the southern exposure.  My plan is to go between the basement window on the far right, and the basement window in the middle.  Only drawback with this position, I have to extend out past the downspout, and I will block the electrical outlet on that exterior wall.  

I've been reading up on various designs, especially those listed on Gary's website.  (Gary, you're lucky... I've held back sending you several long emails to discuss design concepts!)

Two designs that I'm thinking to emulate are:

A) http://h1.ripway.com/glensolar/GlensSolarHeater.htm

B) http://www.atlanticenergy.ca/projects/WayneLangilleSolarAirCollectors/tabid/72/Default.aspx

Questions I have with these designs (this is where I want opinions)...

1. Both designs use flexible aluminium duct.  Design A is one continuous length, which the builder mentions results in excessive backpressure in the system. Design B is several straight lengths, with headers at the inlet/outlet.  I'm figuring that several lengths of 3" or 4" flexible duct with 6" headers is ideal to keep air resistance to a minimum.  
   
2. Both designs use flexible aluminium duct.  Is this more efficient than straight lengths of rigid aluminium duct? (rigid duct would have less air resistance, but less surface area for the air to capture heat from... flow through flexible duct would be more turbulant, which mixes the air...)
   
3. Glass or Polycarbonate (Lexan)?  Not sure if the window panes I have will be the size I want to use, but we replaced a bunch of windows during our renovations, and I have 4 panels of ~3x3ft double-paned glass (one with a bad seal).  Do I risk thermal shock and cracked glass?  I'm assuming single-paned glass (from some old glider windows we replaced) is no good for this due to lack of insulative value.
   
   

So... give me some opinions!  

[scottsAI]

dtompsett,

You have seen the $1000 low cost design on Gary's web.

With that in mind the surface area is not the issue.

Air flow is, your much better off to use straight pipes.

Glass looking at the edge, if greenish not the best to use. It blocks solar through the glass. Polycarbonate must have UV protection.

Green house Polycarbonate is available at building stores, made for this kind of thing. Handles 200F+ just fine.

Single layer glass/Polycarbonate works fine with your proposed design. It creates a dead air space between your air tubes and the outside air. 5/8 inch all you need. If the air beneath the glass moved then a second layer would be needed to get the insulation benefit.

Your web pages bandwidth is exceeded, only 23 looked at your post when I wrote this.

Have fun,

Scott.

[Ungrounded Lightning Rod]

Our company's firewall blocks your A) link as an alleged phishing / illegal-activity-hosting site.

[GaryGary]

Hi,

Here are some other designs you might want to take a look at -- they are all on this page:

http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm

The ones I think might be interest for your situation are these -- just search for the quoted text:

"Model-TEA", and the next one down.  This is a great design --very well thought out.  You would have to change the design somewhat for your situation, but its worth a look.  The wall version has the kind of vertical manifolds you want.  The entry right after it is a recent TEA wall version.

"Solar Air Heating Systems"  -- this is a great book on solar air heating collectors, that is now back in print.

"Wayne's Solar Air"  -- I think this is one of the designs you are already considering.

"Ganged Forced Air Solar Collectors" -- this is a nicely made collector for exactly the situation you have.  

"Mother's Mobile Home"  -- kind of fits your geometry.

Some of the other ones on the page might also work out for you, so you might scan the whole page.

Single glazing will still produce quite well on sunny days -- my climate is about the same as yours (SW Montana), and I have a couple of single glazed collectors that do quite well even when the high for the day is -10F.  Good sun makes more difference than outside temp.  That said, double glazing would help some in your climate if you can swing it easily.  You might consider the twin wall polycarbonate glazing -- the stuff that greenhouses use.  Its very nice to work with, looks nice, and give you double glazing.  The cheapest good choice is the single wall corrugated polycarbonate that Home Depot sells as SunTuf -- its about $1 per sf down here, but more in CA.

You do run some risk if you use window glass of cracking.  I'd also be a bit concerned with the kids playing and that big glass surface. You would also limit your collector size and shape by the glass panels you have on hand -- maybe not a good thing to do?

The best thing you can do with one of these homemade air heating collectors is to make it BIG.  There is no substitute for area if you want a lot of heat output.  You might gain 10% by having an efficient design over a run of the mill design, but that's not going to get you anything like doubling the area -- these collectors are cheap to build, so make them big -- at least that's my 2 cents

I think that the tube or beer can style of collectors are a good and easy way to get good uniform airflow over the full absorber, and that's what makes an air collector efficient, but that does not mean that a good back flow, sheet metal absorber collector will not also do as well -- for example the TEA design mentioned above.  Its just easier to screw up the regular sheet metal absorber collectors, because they need a good baffle system to distribute the air over the full absorber.

I don't think you would see much difference in performance between the smooth duct and corrugated duct versions -- they should both provide good uniform air flow over the full absorber surface.

For forced air heating collectors to work well, they need about 2 cfm per sqft of collector area.  The blower needs to put out this level of airflow with the pressure drop from all the duct friction.  If you are getting a temperature rise of more than 60F, then the collector is not getting enough airflow, and there will be an efficiency penalty (higher losses out the glazing).

There are a lot of claims made for the various air collector designs, but precious little actual test data -- so take the claims with a grain of salt.  

Gary

[spinningmagnets]

Gary, when using window glass, what causes the cracking? Heat expansion with not enough room to expand?

When working as a dump truck driver I have pulled several large windows from structures slated for demo, hard to beat free. More glass than I can use. I haven't built an air-heater yet mostly because I live where its hot, but I am moving to "snow in the winter" Kansas.

If I build a glass-glazed solar heat collector, and then decide to add a second layer of glazing, should polycarbonate be on the inside layer (less cracking from higher heat than glass), or on the outside? (less breakage due to wind-blown branches?)

[GaryGary]

Hi,

I think that you can get cracking even if the glass is mounted so that its free to expand.  I think that most glass panes fail because of an imperfection on the edge, and the more you heat the glass the more likely that one of the imperfections shows up -- I've been told that uneven heating, like sun on part of the glass and shade on the rest is the worst.  All that said, people do use regular glass on collectors, and only some of them have a problem.

If you can get tempered glass, its pretty bombproof, but you have to take it in the size it comes, as it can't be cut.

Our local glass place usually has a stack of tempered glass panes that were cut wrong -- they sell these cheap.

On double glazing with one layer of polycarbonate and one of glass, if you put the polycarbonate inside, it may run hotter than you really want it to.  Its good for about 270F, but the inside of stagnated double glazed collectors can get that hot.  So, I'd say put it on the outside.

I don't know of anyone who has tried this, so you might want to do a test panel to see how it works out.

Gary

[dtompsett]

Oh great... I just realized that my reply yesterday didn't get posted (must have hit preview... and not followed up with Post).  Arrggg!   (and luckily I hit copy-paste on this one... cause I checked back and it hadn't posted either! post failed... subject too long.  wtf!)

So... the local lumber yard on my drive home, carries SunTuf panels... and at a decent price.  Yay.  (roughly $1.50/sq ft for the 8ft long panel)   They also carry the polyisocyawhatchamawhosit insulation!  

Still debating about 4" ducts, or 3" ducts.  3" gives more surface area for metal-air heat transfer, but will cost more, and may be harder to adapt to a 6" header.  And still debating about straight rigid aluminium duct, or flexible corrugated aluminium duct.  The flexible stuff would also provide more surface area, and create a more turbulent airflow (which improves heat transfer by eliminating the boundary layer of air around the edge of flow inside the pipe... I do still remember some stuff from my college Thermodynamics course!)

Also need to convince the wife to let me make something big... since bigger is better.  I'm at latitude 46°29' (Sudbury, Ontario)... so ~72 degrees from horizontal for winter heating.   If I make this thing from 8ft lengths of SunTuf, then when at it's angle, it sits 7.6ft high.  

Perhaps I use 12ft lengths of SunTuf cut in half... 6ft = 5.7ft high. If I can make it 15ft long, that's 90sq. ft.  But if I can convince her to let me do 8ft lengths (which optimizes most materials used), then 8x15 = 120sq. ft.  

At some point I would also like to do a solar water heater, similar to Gary's $1000 setup.  But I want to put it in my crawlspace, since that takes up ~2/3 of my basement area... and the access points to the crawlspace aren't very big... and the crawlspace isn't very tall in some areas... so it may have to live in the larger basement area.  We're on a drilled well, and that water can get cold.  Pre-heating the water with a solar setup, and using our existing electric hot water tank, means we only have the standby losses to deal with, not the cost of heating the water.  

Any thoughts on straight duct vs. flexible corrugated duct for the surface area and turbulant flow?

Doug.

[GaryGary]

Hi,

You might think about a vertical collector if that works well in your situation.  If you have snow on the ground in front of the collector, vertical will do very well because it takes full advantage of the snow reflection.  The steeper you make it, the less its stagnation temperature in the summer will be, and that is good.

If you are going to do the solar water heating later, you might just want to just include and extra bay in the heating collector now.  This way you will end up with one big collector, which will look better.

Not sure about the ducts.  

If you stick to the 2 cfm per sqft of collector, the 3 inch will give you higher air velocity, which is good for heat transfer.  But, it will also mean more duct loss, which means a fan with more pressure drop capability, and more fan power.

You might just look for an online duct loss calculator and see what the pressure drops come to.  Then see if you can find a reasonable fan to make it work -- Grainger Supply is a good place to start looking for fans.  You can also calculate the Reynolds number -- if you can get it up to 2000 or more, you will have turbulent rater than laminar flow, and that will help heat transfer.   If I get a chance, I'll do some calcs, but things are a bit hectic.

This is just a random idea, but I've always wondered why people building the round duct collectors don't squish them into an oval shape to get more absorber area out of each duct?

If you look at the good backflow designs, they tend to use pretty low height air passages to keep the air velocity up -- maybe there is a clue there.

Gary

[xboxman]

hi

i'm also thing of making the heater in this link http://h1.ripway.com/glensolar/GlensSolarHeater.htm

what if i use 6 inch pipe inside this heater would that give me more surface area to transfer more heat ??

with 6 inch pipe it would make it easy to hook up a 6 inch duct fan... maybe solar powered fan

thanks

[GaryGary]

Hi,

I'm not really sure if the bigger pipes would improve performance -- they do give more heat transfer area, which is good, but they also result in slower airflow, which is not good.  This is the kind of thing where you either have to do a whole lot of book work or build a couple prototypes to resolve.  The building a couple small prototypes, goes fast,is more fun and more reliable -- let us know how it comes out if you do.  You need two prototypes so that one can be a baseline that you measure the other against.  If they are the same size, then the product of (temperature rise)*(airflow rate) if a direct indication of heat output -- anything that improves this product improves heat output.  

Gary