Simple Hot Air Solar Collectors

The angle should be 60% and corrugated steel painted with high temperature flat black and off set from the back of the box by intermittent spacers. This is so you get ventilation on both sides if you leave a slot at the bottom and top. Insulate the box and ducting with at least 2" of building foam and bubble wrap (for round ducts).

You should get about 160°F at optimum, and a small fan will greatly increase efficiency.

The tubes you see in others are probably for hot water.

For winter heating your angle should be your Latitude plus 15 degrees.

As Tippycanoe said about 60 degrees or calculated by your approximation method: 53 TO 55 degrees.

If you can get the metal annodized black it would be better than paint which can peel and outgas noxious fumes.

Are the door lites insulated glass units? If so they might have a low e coating and or a tinted glass if so it would be important to orient that coating to reflect the heat back inside of the collector instead of bouncing it away.Many glazing companies have a hand held coating detectors or a window salesman might have a device that shines UV,Visible and IR energy thru the glass and measures the transmission.It would be best to use a glass that does not have a coating or tint. A low iron glass like PPG Starphire would work best.

I don't know if they have a low e coating, Iwill look into finding that out. If they do have a low e coating can it be removed with some sort of solvent, or can they be turned around, inside-out??

this is benbenben...i still cannot comment when logged in..

spending some time choosing your suface treatment for your absorber (corregated steel) can make a significant difference in the performance. not all black surface treatments are the same. while it is true that black paint typically absorbs a high percentage of the incoming radiation, most black paints also radiate at a very high rate. additionally paint is typically a poor conductor of heat and acts as an insulator, retarding heat transfer between the absorber plate and air. When dealing with high temperatures and cooling only with air, spray painting dull black may actually decrease efficiency. google 'selective surfaces', and you will find lists of surface treatments with high ratios of absorptivity to emmisivity. off the top of my head, black chrome (places that chrome plate, will probably be able to plate in black chrome as well) and black nickel oxide (heating 300 or 400 series stainless in the presence of oxygen until the surface nickel oxidixes black, will likely work on nickel plate material as well) have very high ratios of absoptivity to emmisivity. there are also some treatments for copper, and some more exotic treatments.

Dear,

The kind of application is being discussed here require that we should have a good absorber as well a good radiator paint. You are correct to say that good absorbers are generally good radiators of heat and that is exactly what is required here, as air heating will be possible only with metal coated with paints that are not selective coatings type, ohter wise all the radiation will be only absorbed and will not be re-radiated to make air warm. We do not require selective coating her ein this application a normal matt finish black paint will do the job of heating the air.

Regards-

Dr N P Singh

''Dr. Singh'' . . first, allow me to thank you for shedding some light on the reason i am probably banned from commenting as Benbenben: my short tollerance for those with plenty of titles and advice; advice held in high self-regaurd, but of little utility. i think i previously angered an advertiser for pointing out their advice/knowledge mismatch. . . .. (With any respect you feel is due to someone proclaiming your title) Your comments suggest you have some fundamental misunderstanding about thermodynamics. . . . Specifically, your statement that the absorber must radiate heat back out to the air conveys a lack of understanding of the basic principals of heat transfer is a system such as this. . . . . Air is not easily heated by radiation, so designing the absorber radiate heat back out to heat the air is, well, dumb. if air was heated easily by radiation, then there would be no need for the absorber. infact, the radiation wouldn't have made it through the atmosphere for the most part. the fundamental idea your suggestion is missing is that, far and away, the most significant mode of heat transfer to the air in a system such as this convection of heat from the absorber to the air. i would wager with a high degree of confidence that less than 1% of heat transfered to air in any reasonably efficient existing solar air heater is a result of radiation directly heating the air. . . . . The problem with your approach is that very little of the energy radiated from the absorber is captured by the air. most of it continues back out of the system, especially through the glazing. yes, some glazings like low iron glass allow a larger percentage of ultraviolet and visible light to pass, than the amount of infrared, but much of what is radiated from the absorber will still be lost. Convection in this case is so much more efficient at transfering heat to the air, it is foolish to design with the intent of heating with radiation in a soalr air heater. . . . . With much of the black paint that is widely available actually being better emmiters than absorbers, in combination with the fact that most are poor conductors of heat (retarding the heat transfer from absorber to air), choosing even a slightly selective surface can result in significantly better performance. . . . . . if you feel i am incorrect, doctor, then i look forward to hearing your response.

Your comments about thermodynamics and heat transfer were just plain dumb. A radiator on your auto transfers heat from the block to the coolant and from there to the radiator and then into the air. All of your air cooled engines and air compressors rely on thermal transfer of heat to the air in order to work.

Rich Hurd

although it is called a 'radiator', that term is somewhat misleading, since most of the heat is exchanged by forced conduction.

Just so you are on the same page, or at least in the same library, heat transfer can be generally described as occuring on one of three modes

conduction

convection

radiation

radiation is the heat you might feel on your face (even though the air could be cold) when gathered around a camp fire on a cold winter night.

conduction is why the handle on a skillet might get hot even though only the cooking surface was on the heat source.

convection is why blowing on a hot spoonfull of soup cools it faster.... faster than it might cool from conducting heat to the handle of the spoon, ....faster than it might cool from radiating heat to your face.

in this example, as in many, dominates as the most effercive mode of heat transfer. this is almost always the case when transfering heat to a gas.

in this case, air is not nearly as easily heated by radiation as it is by conduction. the most efficient solar hot air heaters are likely to be designed to capture as much radiation as possible and radiate out as little as possible....this makes the absorber hot, allowing the air to be heated by convection.

the problem with trying to use radiation to heat the air is that most of the radiation will not be absorbed by the air, and it will leave the collector unused.

go ahead, check my facts. Benbenben

'forced CONVECTION'

(damn my proofreading, or apparent lack thereof)

Richard Hurd....

so....how many of your radiators and air cooled engines are spray painted black?

perhaps you have enough conviction in youyour judgement.....go grab a can of flat black spray paint and paint the radiator on your car black, so that it might radiate heat better.

that's a hot idea

'it is better to keep your mouth shut and be thought a fool, than to open it and remove all doubt'

bbb

As a matter of fact the older radiators as well as the air cooled fins on aviation engines were painted black. The radiators were made of brass and the cylinders were made of cast iron instead of present day aluminum because of thermal transfer. The more surface area the better for cooling. Look at your lessons of history and you will find that there were good reasons for the way things were done. The cost and weight is the only reason that readiators are not now still made of brass and painted black. The old ones were far superior to the ones used now. And FYI I just replaced my radiator on my "89" Ford F-250 and before I installed it I painted it flat black.\

And your saying is very approprate, you might try listening to yourself sometimes. Even a blind cat catches a mouse sometimes.

Rich Hurd

rich Hurd:

no one is disputing that black paint typically radiates at a high rate. in fact, that is what i brought up, in what you felt qualified to call dumb.

to assist you with your reading comprehension, the summary of my argument against painting the absorber black is summarized below:

1. most black paint on the market is not a very good conductor of heat, so conduction and convection will be retarded.

2. most black paint on the market radiates at a similar or sometimes greater rate than it absorbs

3. a low percentage of the radiation coming from the absorber will be 'caught' by the air in the solar heater, meaning most of the re radiated energy will exit the system.

4. convection provides a much more efficient way to heat the air.

just for my own enjoyment, what did you use to paint your radiator black? was it spray paint? what type of paint was it? acrylic? laytex?

unless you are either one of those people who likes to back up arguments with claims of experiences that never actually happened....or if you live in a climate that rarely gets above freezing, you should soon discover what a bad idea it is to insulate your radiator with black paint.

(i'm fairly certain you will claim that your radiator performs better than some fictitious control radiator...but perhaos you intend to thrill me with some creative anticdote)

Black absorbs most all of the light. That's why it's black. You want to absorb all the rays you can. Note that I said LIGHT. The absorber converts these light rays to heat. The radiator in a car is dealing only with heat, no light. The glazing lets most of the light through but will pass very little heat. Heat waves are very different than light waves. Thats the way it worky. Don't let the heat escape except to warm the air going into the room.

O.K. Guys now I am somewhat confused, paint or no paint or high temp paint. First, I checked the glass and it is tinted. I am thinking of using it anyway unless I'm told it will be a waste of my time, in which case I will recycle it again. I went on Mother Earth's web-site and down loaded some plan examples. It seems to me that there is energy losses in all types of glazing to a varing degree. This is why I am still thinking of continuing with my project. On the absorber,radiater, whatever, one fellow used two layers of black metal window screen. This would be cheap and easy. Can I have your thoughts??

this link has tables showing absorb/emmit ratios for various common materials. galvanized metal, both new and when weathered, is listed as being reasonably selective with higher absorbtivity than emmisivity. galvanized corrugated, mesh, and perforated metal is widely available at home improvement stores.

my original link did not show up...so here is the original, plus another good one for DIY solar projects:

http://www.redrok.com/concept.htm#absorptivity

http://www.builditsolar.com/index.htm

hope this helps. Benbenben

Maybe black is a poor conductor but dang, my black truck sure gets hot! And all this time I thought it was the black paint.