Solar Heater

by robbtoberfest

646K888298

Intro: Solar Heater

This is a solar air convection heater for my garage that is powered by the low angle winter sun. It's easy to build and it works very well. With enough building insulation, this can be a primary heat source with a secondary needed for cloudy days. The idea and design is from Mother Earth News December/January 2007

Ingredients:
2x8 lumber
2x6 lumber
2x4 lumber
2x2 lumber
glass, plexiglass, or some kind of clear material.
black aluminum window screen
caulking, paint, screws, lag screws, staple gun + other tools

I have some results on temperature differences from the first weeks of operation. Just like when you put up your first wind generator, the wind won't blow for days/weeks, I had overcast and mostly cloudy weather the first few days operating this thing. The heater puts out 87 to 104 degrees F at the vents on partly cloudy days to sunny days. The uninsulated garage hits about 17 to 30 degrees F above outside temps, very comfortable for me. It seems the colder it is outside, the bigger the temperature difference between the garage and outside. If you want more heat in an uninsulated area I would recommend possibly 1/3 more collector area than the ratio I give in the instructions. Seal up any drafts, it will do a lot to hold the heat. This thing works great.

STEP 1: Frame and Fit

Make a box to mount on the wall of building (A south wall, SE and SW also can work.) This should be mounted vertically on a wall; the higher in the sky summer sun won't hit it as directly as the low winter sun. The walls of the box are 2x6 lumber; the angled "roof" of the box is a 2x8. I found it good to prebuild the box on the ground and then mount it to the wall. Make the box based on stud spacing in the walls; I have 24" studs, so my box is 8' wide with vents cut into four sections.

I sized the solar collector based on the numbers from the Mother Earth article where the guy built 160sq ft. of collector for his 700 sq ft building; using that ratio I built a 48 sq ft collector for my 200 sq ft garage.

STEP 2: Paint and Add Screen Mounting Boards

Paint the wood and then mount the boards that the aluminum window screen will mount to. Insert a 2x4 horizontally near the bottom leaving room for vent holes, insert 2x2 horizontally at the top; make it even with the 2x4 (look at the drawing to understand this better.)

I notched the middle board for a 2x2 that will later support the glazing.

STEP 3: Mount to Wall and Seal

Use a level, floor jack and support wood to set it straight. Have a neighbor hold it while predrilling holes from the inside of the building for the lag screws. I used six 4" lag screws.

Seal the box from the inside with window and door foam. Also seal the bottom 2x4 that holds the screen.

STEP 4: Cut Vents Into Wall

Cut the holes in the wall like in the diagram, a vent on the bottom and on top for each stud section. Watch out for electrical wiring, etc.
My vents are 4" x 16"
The cuts are not pretty, use foil tape to trim the inside edge of the vents

This was the toughest part because I didn't have a good saw.

STEP 5: Install Metal Window Screen

Cut and staple black metal window screen (two layers) to the wood inserts in the box.
Note, this needs to be metal window screen for good heat exchanging; fiberglass screen won't work well.

I used 48" wide screen and had to trim the edge some with a utility knife.

STEP 6: Install Glazing and Vent Valves

Install glazing and seal it. I used corrugated PVC; it comes in 8' x 24" sheets for about $12/sheet (2007 pricing.)

On the top vents in the building you'll need to put a flapper valve made from plastic sheeting or a trash bag, this keeps the warm air from leaving at night as the heater would work in reverse. Also put some screen on the vents to keep most of the critters out and to keep the plastic valves from sucking into the vent.
Extra notes
Using recycled wood and some on-hand hardware, I spent around $100 for this; now I'm enjoying free heat.

Ducting with a fan and a thermostat switch is what they use on commercial versions installed on homes. That works well for precise heat control. I like convection, no moving parts; and I will just close the vents when the weather gets warmer with some cardboard stapled to the wall.

284 Comments

bdarnold 15 years ago

I aplogize if this has been covered but what about hanging the screen in a corragated / wavy fashion to increase the surface area and heat output. All I have is enough room for a 4x8 unit but my garage is 800 sq ft. If I hand 8 feet of screen in the 4 foot space won't I double the output?

fzbw9br 6 years ago

buy those filler strips for corrugated plexi, and sandwich a tight screen between two on either side of the screen. This will create a horizontal wave in the screen.

Also paint the backing flat black, and perhaps make it from corrugated material as well... turbulence is what helps with the heat exchange.

avid0g 14 years ago

You can see the wall behind the screen in the photo, so it is certainly true that the screen is not collecting every bit of light.  Corrugating the screen tightly enough would capture more light and improve efficiency.  How would you get the pleats to stay in the screen?  You might want to experiment with painting other filter media with flat black paint.  

I would love to hear of other experiments with 1" or 2" air filters that have been painted with flat black paint, especially fiberglass, open-cell sponge and cheap pleated filters.  

Although it is expensive, 1/4" thick white Melamine foam (like in Mr Clean Magic Sponge and thicker sound dampers) has outstanding properties and might be close to 98% efficient in absorbing solar energy, once it is painted.  

I suspect that the material does not have to be a good thermal conductor, if it is in intimate contact with the air stream.  The earliest american patents on solar air heaters refer to prior art using wool!  

Greencrafter 13 years ago

I agree with you avid0g. When I saw the wood behind the screen I knew there was an opportunity to improve efficiency. We know that if an object appears anything but black, it is because it is reflecting photons (heat, light). Since the point is to collect as much as possible within a given area the best solution is a solid surface. I have had excellent results using corrugated R-panel painted flat black. There are many trade names, R-Loc, Panel-Loc. The thinner panels are 29 gauge sheet metal. I paid $11.60 for a 39" X 8' panel last week. I don't know how much the metal window screen costs now but I would believe that the R-Panel would be in the same price range per area. Any heat absorbed in the panel is radiated to the air on the front and back of the panel.

avid0g 13 years ago

I have been giving this a Lot of thought, and I think Non-expanded micro-louvre-punched staggered hole stainless steel sheet metal foil would be ideal. Here are my reasons:
1) Air flow through the panel is much better than laminar air flow along the panel.
2) A louver-shaped punched will admit very little light through the panel to the wall surface.
3) A Micro-punched array will minimize laminar flow and reduce the surface temperature of the sheet. Foil, or very thin sheet metal, is easy to micro-punch.
4) Using stainless steel, which has a high nickel content, means that the surface can be heated until it oxidizes the nickel content, creating a dark blue metal surface. Many studies show that this is an excellent selective solar absorber. This should increase the performance of the absorber by a factor of 2 or 3 over any available paint.

Spiraling Homesteader 13 years ago

I have another question/wonderment...
Say you have south facing windows - what's stopping us from using the screen for each window, pitching it correctly and having it placed so air can pass between it and the window easily (since most screen blocks 70% of air flow)?
Wouldn't this work in the same manner - just inside the structure, rather than outside the structure?
I will be trying this as I have many windows appropriately placed in the house and 1 large 1 in the work space I am hoping to heat.
Any suggestions on measuring efficacy since it will be in a heated space but a cloudy location of the US?

robbtoberfest 13 years ago

Are you just reusing the existing windows as heaters? Unless they are modern heat reflecting glass windows, using the windows to capture more heat like this heater design isn't going to make the room any warmer than having just the window. Basically for more heat, you need more area that captures the heat energy.

wobbler 13 years ago

Sorry, but you won't double the output because you are still getting the same amount of energy from the sun falling on the actual area used, which would still be 4foot of space. All you could do is to increase the efficiency of the energy collection, but that wouldn't gain much if the collector is black and there is little loss through the surfaces to the outside world.

To double the output, you would need to double the area of sunlight you are collecting from, not the area of the collector surface.

You can however collect more energy by angling the collector to point towards the sun so that the collector is making best use of its area, but the angle and gains would depend on your latitude and I suspect not be worth that much in terms of construction problems compared to just making it slightly bigger.

EagleEye 14 years ago

Adding more screen in a wavy fashion will allow the heat to flow from screen to air more quicker because you have more metal in contact with air. This does not mean you'll get more heat. The heat comes from sunlight striking the dark wires in the screen. So unless you have more wires hit by sunlight you won't get any more heat (since your collection area is fixed.)

Dudeyowuzup 13 years ago

ok, so I installed the heater on what I thought was the south-facing wall. Turns out it's the south-east facing wall, and though I don't have a thermometer to measure, just from sticking my hand in the top vent, the air only feels lukewarm, like maybe 70 degrees. Can I install 2 mirrors on the side to curve around a little and reflect the light into the heater, similar to the concept of solar ovens? My husband claims it would be pointless because it would have to move as the sun moves. Couldn't I just put it in a general angle to catch the sun during peak times (like between 10 & 2)? Does anyone know how I would figure out what angles I would need to place the mirrors for them to reflect the heat in at those times?
Please, I'm open to suggestions. Until then, I feel like my solar heater project was a waste. Thanks for your help.

gfry 13 years ago

I built one of these as well. But I made some changes to the original design. Mine is only 3'x7' and is on a south facing wall. I am heating a shop area of about 800 sqft with it...well, supplimenting the heat is a better way of putting it. The solar heater keeps the room at about 65 degrees without the electric heater coming on while it is getting hit by the sun...the electric heater stays off for about 5 hours through the late morning and early afternoon. I threw some foil backed styrofoam insulation on the ground in front of my heater to reflect more sunlight at the unit. I didn't position it in any specific way...it just reflects the light that was hitting the ground up at the heater...seems to work. I also used a smaller input and output hole in my wall...mine are only 8" in diameter. It gives the air more time to heat up? I am getting about 90 degrees out of it when the temp outside is 0 Fahrenheit. i also used a thermal pane from a patio door, not a single piece of Plexiglas for my front cover...my unit gets far less heat loss than the one in the Instructable. Hope this helps

fzbw9br 6 years ago

I have read it is important to "Unseal" a sealed thermopane window, as the pressure can build up in them from heat that they will blow out the glass.

four small holes, one at the bottom of each corner, and one at the top of each corner is all you need, and it vents the moisture also. Also, ensure you have upward holes drilled through the frame somewhere to allow the air to move through, but water will not track up. stuff with "ScotchBrite" pad material to keep out small bugs

As windows have negligible R value, you don't lose much by venting with air.

fzbw9br 6 years ago

I agree with the author, build your box to include the angle. , or put a back on it, move it away for the wall, build channels to feed the air, and insulate the heck out of the back, and channels.

as for the angle of mirrors, just take one and a carpenter's protractor, and when the sun goes past the point where the rays are starting to miss, set up your mirror and measure the angle you need.
A small mirror would work for this to get this info, before spending $ on big ones.

You should be able to see if it will be effective or not.

Good Luck.

robbtoberfest 13 years ago

I don't know how to help here. Maybe you can rebuilt the heater box to face the right direction, angle it more southerly. Add more depth on one side and shorten the other. The mirror idea sounds reasonable, let me know how it works out.

andrefierens 7 years ago

Nice ! Genius ! Great Job ! I like it ! Super !

GGinn 7 years ago

Great detailed design. I was wondering, having some vents at the top on the outside solar, do you think you could have the system work in reverse, an almost solar cooler in the summer. So as that as the solar heats up as standard, the natural convection is to flow out of the top outside while while creating a vacuum and sucking air from that house via the base inlets?

katabillups 8 years ago

WOWWWW! Thank YOU!!!! a million times!!!

nddell 12 years ago

Hello All,

I have had success with a design similar to this. I used black spray-painted aluminum sheets, polystyrene (i think) insulation, scrap 2x4's, and a ton of caulk. It was by no means perfect and some of the glass is even cracked, yet just after peak sunlight today (about 60 degrees in January, weird) it was pushing around 130 degrees consistently. I was surprised. It is a small unit, between 6 and 7 ft tall and 1 and 1/2 ft wide. I was expecting less, honestly.

My question is, then, how would it perform if the box was taller or wider and if it was perfectly sealed. Would either of these modifications make a large difference? I would like to build a non-prototype model of this for actual use, but was wondering if anyone would be able to assist me by answering these couple of questions.

Thanks

ND Dell.

robbtoberfest 12 years ago

Yup, I felt exactly the same way. How can something solar and so simple put out so much heat? I'm amazed these aren't installed on most cold weather region buildings.

Yes, more box area gathers more energy, so bigger box = more heat energy collected. Perfectly sealed helps with heat loss of the building itself, like leaving the window cracked open a bit; this hurts most on windy days.

Good luck Nddell
More Comments