Anybody build a solar air heater?

[Corey]

I have 4 sheets of tempered glass approximately 36" x 72" (18sf each, 72sf total) that are laying around screaming "do something with me". I've kicked several ideas around and most recently thought about building a solar collector. Especially this time of year when we have fairly decent sun and day highs in the mid 60's but the house has a little chill due to still being in shade from all the trees. I'm hoping that 72sf of homebuilt collector might be able to keep a couple of rooms close to 70F and maybe give me a little head start on heating from the evening fire. I'm not planning on any storage and don't have room for a thermo siphon system. Just a couple of solar fans to move a little hot air when the sun shines.

Trouble is, searching the internet, I've found a ton of different designs with no real way to quantify what works best. Some have flat absorber plates, some are fluted or ribbed or rough, some have the absorber just under the glass with the air circulating behind, some have the absorber at the back with the air circulating above it, some put it in the middle and pump air down the front and back. I found a "rules of thumb" list, but it is going on 30 years old.

http://www.mobilehomerepair.com/solarrules.htm

I know there have been some discussion on hot water, but just curious if anyone has built a hot air collector and any input on the best design?

 

[Eric Johnson]

Not knowing anything about the topic at hand has never stopped me from jumping in and shooting my mouth off.

Water is a lot better conductor and transporter of heat. I think if you want to provide heat from solar, you would be ahead to heat the water in the collector, pump it into your house, then recover it with a heat exchanger and a fan. You might be able to heat your hot water, too.

But if you're trying to heat lots of air, why not get some galvanized, corrugated steel roofing and paint it black? It has lots of surface area and you could cut it into manageable pieces and build some sort of a multi-layer hx that the air could pass through. If you mount it in a glass-covered box with a fan, I bet you'd get lots-a-hot.

 

[Burn-1]

Cozy I am in the process of doing the same thing.

Here is the one I am using for inspiration.

Solar air heater

(broken link removed) and Solarsheat are off-the-shelf products and have PV fan options to move the heat around but passive works too. This person sort of made their own based on the Cansolair.

 

[Corey]

Thanks Eric - I had considered water - may still look into it, as I think it would have even more benefit in the summer. On a pure cost standpoint, (ROI type stuff) I would be better to offset the gas used for hot water in the summer (to the tune of about $40/ month) rather than a little bit of relatively free firewood in the winter.

But I was hoping for something simple, cheap, and effective. I was hoping not to mess with all the valves, piping, heat exchangers and worry with either draining it or running antifreeze in the winter. The other downside is that about 30 years ago, someone had their thinking hat on and planted several trees all around the house. They are now close to full grown and shade the house and most every location of my property except for a few choice locations for a few hours a day. In the winter, they loose their leaves and the solar gain is a little better.

Burn - thanks for the links I'll definitely check them out.

 

[rhetoric]

I have a couple big windows I got for free (not white glass of course, but it will do) and someday I'll get out of my treestand and make a thermosyphon collector. The best recommendations I've heard for building materials is the foil covered foam insulation board. The box has to be insulated anyway (you don't want your interior air getting colder before it gets warmed) so while you may decide to build a wooden frame for strength (and line it w/ the foam), some people actually just build the box out of foam. Is this making any sense? Then you insert the thing in a window -- it pulls cool air from the room in as the heated top layer pulls the warmed air in.

I figured I'd do this as an experiment and if I think it works and is fun, then I'll start thinking about heating water. One thing at a time.

 

[Mike Wilson]

I thought this through, and to make it cost efficient, it has to be inexpensive. So, what about using a roll up window shade, in black. The material could be black vinyl, or black mylar. Mount it inside the "box" of your window, like an inside mount shade. Mount the top of the shade (the roller), about 1" down from the top, and only pull the blind down to within 1" of the bottom. You would have the sun heating the black material, drawing cool air in from the bottom and exhausting it on the top. The idea is to use the window box that you already have as a frame, and an inexpensive 20 dollar shade as your collector. Roll up shades are very inexpensive... so while they may not be the most efficient, they may be very cost efficient. Thoughts?

-- Mike

 

[Corey]

Mike - that may be a thought. It does seem pretty cheap. From what I gather, the absorber plate needs to be good at conducting heat. I don't know how the plastic would fare in that regard. Most of the designs I have seen use aluminum roof flashing - not free, but it probably won't break the bank. I noticed one design that used soda cans - not a bad thought, but I just don't have that many cans around.

From what I gather, the whole idea is to move a lot of air and actually keep the solar collector fairly cool. As it heats up, it begins to radiate in the long-wave IR which means a loss in efficiency. Sadly, this house isn't set up for solar gain at all - It's a 1970's ranch that is long in the north/south direction and skinny in the east was dimension with lots of trees on the east/west sides. Great for the summer, but not too much heat in the winter. The only place to catch south sun is up on the roof.

Thanks for all the input.

 

[Mike Wilson]

Is the main motivation for using a metallic surface to conduct heat into the room, or to re-radiate it into the enclosed space, or both? Take a look at this product, seems like a similar concept to what I mentioned above, using fairly inexpensive fabric. www.cetsolar.com/solardrapes.htm On that note, does anyone make a mylar roller shade? Another reason I like the roller shade idea is that you can wasily retract it when you want, thus increasing the wife tolerance factor.

FYI, we purchased a new product this year for 39 windows in the house. Its called Hunter Douglas Architella, and its a redesign of their triple honeycomb shades. The honeycomb is no longer stacked, instead, each one is enclosed within the other. I had them installed on about 2/3 of the windows in the house, and the difference in my home's heating and cooling load is noticeably better. According to Hunter Douglas' propaganda, "A product with an R-value of 4.00 stops about 75 percent of heat loss through the window. Duette Architella boasts R-values of up to 7.73."

Also, I tried an experiment using my window box idea with them. As they are all mounted inside the window frames, and all of them have the "top down / bottom up" feature, I tried lowering the tops 1 inch and raising the bottoms 1 inch when the sun was on them. There is a definite heat flow coming out the top. Its enough that if I lower the bottom of the shade all the way to the bottom of the window, and close off the air inlet, the lower 1/3 of the shade gets sucked into the window frame about 1.5 inches, as a result of the negative pressure in there. Note, the outside facing material of these shades is white... so it would probably work much better if it were say... black. That's where I came up with the black solar roller shade idea. Cheap, convenient, and effective.

Here's a link to some more projects.

What about using foil backed foamboard inside the window frame?

I have central atrium in this house, with a 34' high ceiling on the south side, with 12 six foot by three foot Andersen windows facing south, and a stone tile floor inside. The room acts like a giant solar sink in the winter, and holds heat quite well, but I am thinking of using the window area to generate more solar heat... plus, its conveniently located for experimenting.

Just thinking aloud...

-- Mike

 

[Sandor]

Why not use the panels for a batch solar water heater?

http://www.byexample.com/projects/current/batch_collector/

 

[Rhone]

Is the main motivation for using a metallic surface to conduct heat into the room, or to re-radiate it into the enclosed space, or both?

I'm not sure about using metal to radiate it into the room, but the collector should be metal at least. The reason for metal, two identical cars are sitting in the sun in a parking lot for four hours in the summer. One car has a black bumper, the other, a chrome bumper. Which one is hotter? Black? Makes sense, chrome reflects heat and black absorbs heat, everyone will agree on this. Now, think about touching the chrome seat belt buckle in your car, the ignition switch, or leaving a toolbox open in the sun and trying to pick up a chrome tool. Want to change your answer? Reality: The chrome bumper, tools and seat belt buckles are MUCH hotter (over time), than an identical surface painted black. This site goes on to explain why. The above was just a cut & paste. Wonder how a black chrome air collector does....

 

[rhetoric]

Is the main motivation for using a metallic surface to conduct heat into the room, or to re-radiate it into the enclosed space, or both?

I'm not sure about using metal to radiate it into the room, but the collector should be metal at least. The reason for metal, two identical cars are sitting in the sun in a parking lot for four hours in the summer. One car has a black bumper, the other, a chrome bumper. Which one is hotter? Black? Makes sense, chrome reflects heat and black absorbs heat, everyone will agree on this. Now, think about touching the chrome seat belt buckle in your car, the ignition switch, or leaving a toolbox open in the sun and trying to pick up a chrome tool. Want to change your answer? Reality: The chrome bumper, tools and seat belt buckles are MUCH hotter (over time), than an identical surface painted black. This site goes on to explain why. The above was just a cut & paste. Wonder how a black chrome air collector does....

And that's why the foil backed foam works so well. Cheap and metallic.

The only problem with the "collector in the window" method (I thought of suspending some inside my 5 skylights) is that IT PLUGS UP YOUR WINDOWS! Look, I'm as cheap as anybody on this site, but I want to enjoy, especially on a sunny day in Rochester (we don't get many), looking out the window! If you drop the collector beneath the window, you get heat and a view.

 

[Rhone]

I don't get that "collector" in the window thing. Not having anything after the window and letting the sun shine in will net you 100% of the energy that comes through the window. Putting something in the window will create a warmer environment between the window and said object which increases the amount of conductive heat loss to the outside, that's how curtains in summer reduce your cooling costs (along with reflecting radiant energy back outside).

Putting something behind a window to collect solar energy should always be a net energy loser compared to just letting the sun shine through said window into the living space.

 

[rhetoric]

I don't get that "collector" in the window thing. Not having anything after the window and letting the sun shine in will net you 100% of the energy that comes through the window. Putting something in the window will create a warmer environment between the window and said object which increases the amount of conductive heat loss to the outside, that's how curtains in summer reduce your cooling costs (along with reflecting radiant energy back outside).

Putting something behind a window to collect solar energy should always be a net energy loser compared to just letting the sun shine through said window into the living space.

Not if the thing in the window absorbs more heat than whatever it's hitting, right? So if you have a black tile floor you'll get more heat gain than if the sun shines in on a white floor? Am I kidding myself?

 

[Soopah 27]

This one may be a little big, but it's pretty current. Not sure why he used the screen instead of a metal plate. But, it seems to work.

(broken link removed)

 

[KeithO]

The purpose of the screen is to disrupt the flow of air over the absorber to prevent it becoming laminar. Turbulent flow will improve heat transfer dramatically. Its all a trade off, since the efficiency of the absorber itself goes down. I think that the fact that it is cheap was also an important consideration.

Not sure why he used the screen instead of a metal plate. But, it seems to work.

 

[rhetoric]

Here's mine, finished today...

 

[rhetoric]

and here's the rear...

 

[rhetoric]

The two small inlets on the bottom...

 

[rhetoric]

and finally the results...

Construction:
1 double pane Anderson window (free)
1/4 sheet celotex mylar coated foam (free w/ purchase of 160K home, I guess)
12 inches of Duct tape ($00.03)
160 inches of packing tape (ran out of duct tape) ($00.08)
a few squirts of Rustoleum Stove Paint ($00.31)
labor at $500,000.00 per hour (in case anybody is interested in buying it) ($250,000.00)
Total Cost Before Labor ($00.42)
Total Cost After Labor ($250,000.42)
Total Cost After Labor Including home purchase price ($410,000.42)

Results:
Clear sun, mid afternoon (between 2:30-3pm) between 44-46 degrees outside in Rochester NY.
I got about 130 with the meat thermometer, 118 with the wireless home thermometer. That's a boost of between 70-85 degrees. If you hold your hand in front of the hole on top, it's actually uncomfortable.

Application:
None for this, but I have a bigger window and I'd like to install a wall on the lower level of my split to help the rooms that don't enjoy much of the woodstove's heat.

 

[Rhone]

I don't get that "collector" in the window thing. Not having anything after the window and letting the sun shine in will net you 100% of the energy that comes through the window. Putting something in the window will create a warmer environment between the window and said object which increases the amount of conductive heat loss to the outside, that's how curtains in summer reduce your cooling costs (along with reflecting radiant energy back outside).

Putting something behind a window to collect solar energy should always be a net energy loser compared to just letting the sun shine through said window into the living space.

Not if the thing in the window absorbs more heat than whatever it's hitting, right? So if you have a black tile floor you'll get more heat gain than if the sun shines in on a white floor? Am I kidding myself?

Well, energy can't be created or destroyed it can only change forms. If the same amount of energy is let into a room hitting a floor regardless of color it has to go or transform into another form. In the above example with it striking white, it would disperse around the room and be absorbed by the objects & walls warming them up. Having it strike a black tile floor, that floor will absorb the brunt of it, and very little will disperse around the room. So you either warm up a smaller area to larger temps, or warm up a larger area to smaller temps. The end energy should be about the same. The common way of phrasing it is... does a kettle full of steaming water have more energy than a bucket full of warm? Black is certainly the way to go as it's inevitable that white will reflect a fraction back out windows but once let into a room that loss won't be significant. So, that's why I believe even a black thing in a window to collect solar energy will be a net loser as, it will inevitably create a hot space between it and said window, and increase the radiant & conductive energy loss vs. just letting it in and dealing with the slight loss reflected back. I'm thinking of a typical house, as if you have a lot of windows that slight loss reflected back can add up.

The biggest difference in color would be dealing with it before it enters the house. A white roof deflecting the energy to the sky and objects outside (like trees, plants, etc.) or a black roof absorbing it, and through conduction & radiant some will end up on the inside which is good in cooling dominated areas.

 

[ekubec]

Ditto on the window heater idea. Some guy on ebay sells these things. But the point of having windows is to let LIGHT and SOLAR in.

My wife and I just bought a house in thornton, co. It is situated with regard to the compass like a baseball diamond: the front of the house faces northwest and the back of the house faces southeast. The south east has vaulted ceilings two stories high with LOTS of windows. our bedroom's window also faces the southeast. So we are getting great solar gain in the morning. If it sunny, the house quickly heats up to the upper 60 and beyond.

our walls in the house are white and our carpet is a grey white -- not the best for thermal gain, but you can litterally feel the heat. I am guessing the light and the solar radiation that is bouncing off the light interior is bouncing on to another part of the interior, thus the thermal gain.

Ideally, having a pure south facing wall in the backyard with lots of windows is the preferred way to go for solar gain needed in the winter. However, I am sure my house heats up quicker as the sun rises in the south east this time of year it goes directly into our home -- so it is a better time to be heating the house--the soonest possible after the cold night and also when my wife and I are using the house before we go to work.

Having window heaters would not benefit any heat gain (although probably heat loss at night would be reduced) and the house would be dark.

Plus they would look really ugly.

I am looking to install solar air heaters on the southwest side of the house. Granted, they would only get warm from midday on, however, they would be providing maximum heat gain right before it got dark and cold again.

 

[Nofossil]

If you were to mount this between or below your existing windows (with appropriate ducting) you could get some additional gain.

 

[ekubec]

If you were to mount this between or below your existing windows (with appropriate ducting) you could get some additional gain.

Yeah that would create additional gain.

I have two HOAs to deal with....

 

[ekubec]

HOA #1 = Home owners association
HOA #2 = Husband Owners Association = wife

The solar panels need to look good externally.

My plan is to mount them on the southwest wall near the ground. The air ducts then I will send through a basement window that is below ground level. More acurately, I would remove the window and build some kind of airduct junction to go in the space.

From the unfinished basement I could do a lot of things, the simplest would be just to heat the basement. Since heat rises and the ducting for the gas heater goes through this space, I would think the thermal gain would be passed on to the living area over time, providing heat in the evening.

I could also install 2nd water tank and a device that would allow heat from the hot air to be transferred to the hot water supply.

My questions are how thick of glass should I use for the collectors and do I have to worry about these things getting so hot in the summer that they combust ?

-e

 

[slim62]

Yes we have built one of these wall heaters . with an old storm window 36"x56"and scraps of lumber . we installed it onto my fathers small travel 26' trailer and it works unbeleavably well . it dose not have a fan . but kicks out heat at 150f and above , he has to shut it off quite often as it works too well.. we were suprised when something so simple actually worked !! we are about to take 3 more old patio doors and build 3 more to heat the garage , whose back wall happens to face south as well . it should keep it comfortable during the day . its about 150-200 sq ft. We did not build it using cans but lined ply-wood channels with heavy duty aluminum foil and painted it all black inside.