Good evening welcome to my entry for the "Off the Grid Contest". I present to you a product aimed at lowering your winter heating bill and carbon footprint by generating heat for free using the power of the sun!
Projects that involve warming air for space heating using the sun are plentiful. However, most of them involve permantly installed flat plate collectors made out of soda cans or aluminum downspout. Installing a permanant collector usually means drilling two large holes through the side of your house in order to route the ductwork. My collector mounts just outside a window and can be taken down when the heating season is over. The most invasive part of the installation is the removal of the window's flyscreen. Furthermore, the aluminum soffit based absorber plate is much more efficient than soda pop cans or aluminum downspout; you can harvest more heat for a given size of collector. The aluminum soffit based collector is more expensive than a soda can collector but less expensive than an aluminum downspout collector.
As an added bonus, this collector does not require electricity or fans or forced air of any kind. The current of air through the collector is driven solely by natural convection. As the sun heats the air in the collector, it rises and escapes through the output vent. As a consequence, cold air is drawn into the collector through the input vent to replace the warmed air. The whole loop continues without the need for fans.
If you want to make one of these I suggest you use some better quality (more expensive) materials than I did but for this particular project, the cost was about $60
Projects that involve warming air for space heating using the sun are plentiful. However, most of them involve permantly installed flat plate collectors made out of soda cans or aluminum downspout. Installing a permanant collector usually means drilling two large holes through the side of your house in order to route the ductwork. My collector mounts just outside a window and can be taken down when the heating season is over. The most invasive part of the installation is the removal of the window's flyscreen. Furthermore, the aluminum soffit based absorber plate is much more efficient than soda pop cans or aluminum downspout; you can harvest more heat for a given size of collector. The aluminum soffit based collector is more expensive than a soda can collector but less expensive than an aluminum downspout collector.
As an added bonus, this collector does not require electricity or fans or forced air of any kind. The current of air through the collector is driven solely by natural convection. As the sun heats the air in the collector, it rises and escapes through the output vent. As a consequence, cold air is drawn into the collector through the input vent to replace the warmed air. The whole loop continues without the need for fans.
If you want to make one of these I suggest you use some better quality (more expensive) materials than I did but for this particular project, the cost was about $60
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Signing UpStep 1: How the Vented Aluminum Soffit Collector Works
I've drawn a diagram for you that I hope is self explanitory but nevertheless I will go through it starting from the cold air intake.
1) Colder air from the room is drawn in from the intake vent at the bottom of the unit. From there it travels inside the cold air plenum until it reaches the chamber at the front of the unit that is exposed to the sun's rays.
2) Inside the chamber behind the polycarbonate glazing the sun's rays shine on the black aluminum soffit and heat it up. When the cool air encounters the soffit, it is warmed as it rises through the perforations. This rising air is continually replaced by cooler air being drawn into the cool air intake.
3)The warmed air travels through the hot air plenum until it is released out into the room through the hot ait outlet.
The collector unit is ment to hang on a windowsill with the intake and exhaust vents inside the house with the rest of it on the outside of the house. The window must be the single hung type; where the pane of glass slides vertically to open and close the window. The collector is built to be the same width as the window opening. When the collector is hung on the windowsill, the window can be closed down onto the collector to sort of "clamp" it into place. After some weatherstripping is added to seal up the small gaps, installation is complete.
1) Colder air from the room is drawn in from the intake vent at the bottom of the unit. From there it travels inside the cold air plenum until it reaches the chamber at the front of the unit that is exposed to the sun's rays.
2) Inside the chamber behind the polycarbonate glazing the sun's rays shine on the black aluminum soffit and heat it up. When the cool air encounters the soffit, it is warmed as it rises through the perforations. This rising air is continually replaced by cooler air being drawn into the cool air intake.
3)The warmed air travels through the hot air plenum until it is released out into the room through the hot ait outlet.
The collector unit is ment to hang on a windowsill with the intake and exhaust vents inside the house with the rest of it on the outside of the house. The window must be the single hung type; where the pane of glass slides vertically to open and close the window. The collector is built to be the same width as the window opening. When the collector is hung on the windowsill, the window can be closed down onto the collector to sort of "clamp" it into place. After some weatherstripping is added to seal up the small gaps, installation is complete.
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The second diagram shows the cold air is forced through a piece of solid soffit, then through the main panel of solid soffit to then lastly, pass between the glass and the solid soffit.
Green = solid soffit
Grey = glass
Black = walls and partitions
i wonder if it will work for solar water heater
with suitable modifications of course!
can anybody help
Th SS group engages in a lot of discussion of various aspects of D.I.Y. solar. Check it out at:
http://tech.groups.yahoo.com/group/SimplySolar/
Simple and smart. Thats like an idea has to be! :)
A few suggestions. If the box is well insulated, what is the point of painting it black?
Would it be less obtrusive if it were painted to match the house?
Could it also be detailed to look like a part of the house, instead of a window air conditioner?
Could the design be altered to fit in the wall below the window?
Would it make more sense to reveres the slope of the vented collector plate so that it is more nearly at a 90 degree angle with the sun?
Would triple wall or greater glazing provide better efficiency?
I dont want to fit it in the wall below the window because the whole point of this perticular design is to do no permanant change to the house. This will make this instructable useful to anyone who rents their home. The rest of your inquiry is taken care of by "StuNutt" below. Thanks for your questions.
The polycarbonate is transparent to the solar spectrum (enabling the collector to work). A white absorber will reflect about 90% of the incoming solar energy. The insulation does not factor into this loss. For a shallow cavity, such as your collector, nearly all this reflected energy will leave via the polycarbonate. A black absorber will only reflect about 10% of the incoming solar energy. Thus, a black absorber is about 9 times as effective as a white absorber. This is why all commercial collectors (insulated or uninsulated) But I encourage you to experiment. Just post the data =) !
If you wanted to keep the collector more perpendicular to the sun's rays, the answer would be to angle the whole assembly away from the wall at the bottom, but remember that, when we need heating most is when the sun is low in the winter.
Even though the sun is lower in winter, the optimal angle from vertical for a solar panel in St. John's, NF, is 20° in December, 27° in January and 35° in February. So if the collector is 10° past vertical, the sun will be hitting it at a 45° angle in February. It would be neat to do a test of three different versions--one as is, one with the collector vertical and a third with the collector angled backward--to see which one works best in real-world usage.
Props to the OP for a job well done, even if it is only a "beta" version. I'll be looking forward to seeing your updates during the winter.
I am, however, a bit unclear as to why you oriented the collector panel tilted out at the top rather than tilted out at the bottom. The systems I'm familiar with absorb the most energy when the surface is normal to the incident rays. Maybe the difference isn't material in this case and it results in easier fabrication?
When considering the alternatives for improving V2, be sure to include using closed-cell insulation with a reflective foil surface. It performs significantly better than a bare foam surface."blue foam board" is a possible candidate although my familiarity is in refrigeration applications and don't know the high-temp behavior without further research. Marine supply outlets are a good source to pursue for foams and such.
I do like the idea of constructing the entire structure of rigid foam with the overall structural stiffness provided by an external plywood skin uniformly bonded to the foam. In boats this is done using epoxy resin bonding a marine plywood skin to a foam core. This results in a strong and stiff structure which is also very light.
Again, this is a great project and wish you success in the competition.
The idea of tilting the absorber such that its closer to the glazing at the top of the collector is that at the bottom, all the flow is on the glazing side of the absorber, and then as the air rises and passes through the absorber more and more of the flow is on the back side of the screen -- so tilting the absorber just makes for a larger flow passage where the flow is large.
Another alternative for the absorber is to use two layers of metal insect screen separated by a half inch or so. In our tests, the vented soffit and the 2 screen collectors were tied for performance.
Gary
I was planning on building the "heat grabber" with the corrugated metal roofing absorber exactly as it showed in the plans until I seen the results of your aluminum soffit absorber testing and decided to put the two together. I'm glad I did because I think it worked out really well.
I learned everything I know on your site. Thank you for providing all of the free information and thank you for your comment.
however, you need the "sofit" material to be solid as in no holesYou could just get a lenght of aluminum trim metal or from a metal place the right size. By the sheet being non perforated, you will get a better air flow no fan needed, especially if you bring the input side down to floor level (much much more complicated) you do need to physicaly block them at night/at end of sunlight hour, or as posted by another they will go in reverse and cool your house. If you can find temperature activated louvres you need not be there to do it.
color simple fact darker colors absorb more energy, and black is better followed closely by red and blue.
I looked over their website and could not find the article (solar heat grabbers or heat grabbers) but did find this:
http ://www.motherearthnews.com/do-it-yourself/storm-windows-solar-collectors.aspx
http://en.wikipedia.org/wiki/Trombe_wall
Regarding the "angle towards the sun" comments you can easily work out the gains to be made and decide if it's worth it. To work out the gain from angling the collector towards the sun at the winter equinox, get the angle of the sun to the horizon from your location. In the northern hemisphere, you can calculate the lowest angle the sun gets to from 66.5 – Latitude.. (If this goes negative, it means you're above the arctic circle. Good luck in getting any heat from this in the middle of winter! there)
Then use a scientific calculator to get the cosine of this angle. That then gives you the ratio of placing the collector vertical to pointing towards the sun.
e.g. I live at 53 degrees N, so the sun drops down to 66.5-53=13.5 degrees in winter. Cos(13.5)=0.97. This means the pointing the collector toward the sun would make about 3% difference. However, if the sun was higher, the difference would be greater. Taking the cos of the angle to the horizon, with the sun at 30 degrees to the horizon with the collector vertical, the difference is approximately 14%.
Whether these would make a significant difference in a real situation, I'm not sure. Bothering about the angle in higher latitudes may not be worth it and in aesthetics and simplicity, I'd probably find it easier to make the collector 14% larger (as this is an area increase, it's approx 7% larger in each direction).
There's a more comprehensive analysis of the effects of angle here:
http://www.macslab.com/optsolar.html
Looking forward to your updates!
How about the absorber panel sloping the other way.... Out from the house at the bottom and towards the house at the top..... to allow a better sun to plate angle..
Just a thought.
Keep up the good work.
Dan
Excellent design though.
Since you have the aluminum make a shell to cover the OSB and caulk all seams. I have tried to use OSB in several outdoor projects and could not get the paint to seal it good enough to keep moister out, the OSB ends up flaking apart and rotting. This will add many years to the useful life of your heater.
People that have built these pass though (screen, perforated) type collectors, later find that a air filter for the incoming air would have been a good idea. As dust in the air will collect on the front of the collector and block some of the sun from reaching the collector plate.
Where did you find the double wall glazing?
A web site you may like is http://www.builditsolar.com
The double wall glazing is actually left over from my greenhouse; three sheets of glazing (each 6 feet by 14 feet) arrived damaged when I ordered them so I got to keep them and get free replacements. they came from Toronto. You can easily find suppliers online. Search " 8mm twin wall polycarbonate".
I have been visiting builditsolar.com for many months now gathering information to try and heat my home from the sun. I got some plans for my window mounted collector there only I modified it to have an aluminum soffit absorber plate rather than a sheet of metal roofing. Thanks for your comment.