Solar Air Heater

Way back in the seventies a friend built a solar heated house. He had six inch walls with good insulation, and the doors and windows sealed really good. Most of his South facing roof was corrugated plastic over a solar collector in the attic area. In his basement was a 6X6X8 foot room full of round stones the size of grapefruits. He had a sensor controlled fan moving air from the attic to the rock room, and a sensor controlled fan moving air from the rock room to his house. There was a back-up electric heater in the duct. He had a wood burning stove in the house, don't know what he used for cooking.

The solar unit worked fine, he only had to burn wood occasionally, used maybe a cord or so in the whole winter. He did have to supply electricity to the fans, but at the time it was not too expensive. He was very pleased with the whole project.

The only thing I would be concerned about is air quality. Fresh air is important to me, and I would want to have an air exchanger in the system. A good one will heat the incoming air with heat from the outgoing air, or cool it in the summer.

Thanks for the info. I have 10 inch walls, no insulation and have to check the sealing for doors and windows. The heat exchanger is a very good idea, especially when some people don't want to open the windows for fresh air.

I think your term is a little bit of a misnomer. The solar energy transmits quite easily undisturbed through the air itself. So you will not be heating your air directly with the sun, you will be heating something fairly massive instead with direct sunlight and then selectively drawing heat at night from that heated mass. But I'm sure you're already aware of this. _{(I'm just being a little anal again.)}

A friend many years back lived in such a house on the south side of a slope in the Catskills of New York. The days that I visited, the house was quite comfortable and warm. The only drawback my friend pointed out to me was that since the system relied on how long it took for thermal transfer to happen, once the thermal mass became to hot or cool to be of help and the backup was turned on, it took quite awhile for solar thermal control to be useful again. Oh, I forgot to mention that the system also worked to help cool in the summer.

The major flaw I see with passive solar heating/cooling is that everything hinges on location. You cannot take any piece of land and place a developer's standardized building in and expect a passive heating system to work. But when and where it works it seems like a great idea.

Yes, for house warming. BEHIND GLASS I had a accumulator. It is very important that you can store the heat for the evening. I used a stone mass in which air is canalized for convection. Is heavy and takes a lot of space. But it works. If there is sun.

Another source is to research Traum Walls (French designer, I believe)----I help build a 2500 sq ft house in the Paso Robles area of California, and the idea was basically south exposure , through a 2 story glass wall, heating up water in black drums, that fed a floor heating system, all through passive pressure (heat expands water, forcing it up, and through the house cavity, (primarily floors) circulates it through the day, and at night, the mass of the house retains the heat, and it starts up again the next day) It was augmented by a regular heating and cooling system for those cloud filled months, (Winter) and those super hot 110 degrees days(Summer , Also that could go for a month or two, or three)--Just another approach--depends where you live, and what the dynamics are that you have to work with.

Usually the house is heated using accumulation wood burners. For every winter it uses about 2-3m³ of wood per room per year. What I intend is to reduce the amount of wood needed. Solar heating would be substantial during early spring and late autumn when the sun still shines quite a bit. The heating system designed is an active system, that takes the air out of the room, heats it using the solar panel and sends it back into the house. I didn't included a thermal mass because I thought a could use the house walls(10 inch brick and mortar) as one. The air would be circulated using a fan, maybe solar powered.

The house is located in a village called "Gheorghe Doja" in România, in a plain area called "Bărăgan"(you can google earth it).

For Romania, sunny days are not many, in the winter. Besides, in Baragan there is the north predominant wind that cools down, the house. A 10 cm brick is not much for having a large resistance to the heat transfer. As Andy said (and did) a supplementary insulation is the most important to improve your house isothermal properties. You mentioned blowing some foam. Other solutions would be the foam panels. I think that you should look to the traditional building materials. In that area, bricks made out of "chirpici" (clay mixed with straws, some organic liant and sun burned) present a better thermal resistance. What I have in my (close to) Houston house is a "solarium" on the south face of the house, 12x4 meters. All the wall is windows (seven of them) plus four 1.5m x 1.5m skylights. They are NOT double pane!. During the winter days (this year was an unusual cold winter) the temperature outside is between -2C and 6C. In my solarium the temperature at 14:00, on a sunny day, is over 26C. I open the doors to the rest of the house, and I don't use any other heating till 5 PM. Use triple pane windows (they are very effective in Moscow and Siberia!).

My unfinished project is to heat water in a solar collector, collect it in a water boiler and recirculate the warm water in my two west wing bedrooms (last year was a "dry" year for a self-employed like me, so I stopped working on it ). The collector is made by 3/4 in copper tubing that I have soldered on a 0.021 in copper sheet. I want to blacken it or paint it black. Andy gave a link to a great idea - to use copper pennies.

This system will be backed-up by the gas burner of the water boiler. In fact I have mentioned this at: http://cr4.globalspec.com/thread/39083/Solar-Heating-Redivivus. If you have access to natural gas or even propane tanks, I think that it has some merit.

As for the quality of the air in the house, I can tell that the air heaters (electric or gas) use recirculated air, in all the house. Still, I noticed, in the attic, a small conduit that brings fresh air from above the roof to the heated air in the burner, so the air is fresh, even in an air-tight house.

10 inch is about 25 cm, (and add 2 cm of finishing on each side). As for chirpici, who would put dirt over a nice finished brick wall ?

And anyway first I just want to test a 2 sqm collector, to measure the just how much power I can get out of it during autumn, winter and spring months. After that I should be able to fairly decide if it works and how much.

The thermal insulation for the house is a MUST, I am aware of that.

1) The foam such as Dow XPS (a rigid closed cell type) have fantastically better insulating value than 'chirpici' - İ would definitely go with the foam. 2) For the solar thermal water heater - calculate the volume of hot water you need to provide support to the fired house heating system - İ have and it isn't worth the effort. İt is OK for hot water for the sinks and baths but that is all.

I don't know where you get your data about thermal resistance of different materials, but adobe (chirpici) has a R value of 7. Your Dow XPS has a value R=5. The regular brick has a R=0.2. From the point of view of thermal "inertia", both adobe and brick are better than XPS.

Adobe (chirpici) is used all over the world, in rural areas. The price is much less than XPS and, for some, much more available. As for esthetics, I don't see that a nice brick wall covered with Styrofoam panels looks better than a nice brick wall covered with chirpici. I would use, on both some mortar/stucco protection layer (like calciovechio), but that's only me.

I would say that American way of building a house, a cement slab with a wooden frame on it and with 1in of Styrofoam (it could be XPS), 3in of wall filling, a 0.75in Sheetrock and whatever external siding, to reach an equivalent R=18 is the best, but one must look for all the possibilities.

R values per inch of thickness count. The insulating capacity of a material generally depends upon the material density. Adobe does not have a lower density (and thereby higher R value than polystyrene foam. New math apparently you work with. When 0.2 (brick) has a better insulating value than 5 (XPS foam) İ don't know. Both materials have the capacity to hold heat or cold better than foam if that is what you intend to say by 'inertia'. That is a different thing. You do nornally not use styrofoam for insulation - XPS insulation is extruded polystyrene - a different ball game. Normal styrofoam (think coffee cup) is not closed cell. İt should never be used for insulation though it is and some sell it for the application.

Wait a minute, I've got my knowledge from graduating Home Depot's 5 minute seminar on house insulating. So let's split all the sentences to make me understand

1)We were talking about using, for insulating purposes, of brick, adobe or XPS. If the latest one is a foam or extruded polystyrene is not very relevant so we will call it XPS (you brought into discussion, it is the first time that I heard about it). R values are 0.2, 5, 7 for brick, XPS and adobe. The thickness is the standard one, used in buildings (I bet that 10 in XPS has higher R value than 1 in, but not linearly increasing!). Higher the R value, the better for insulating the house.

2)In labels, fact sheets, ads, or other promotional materials, they do not give the R-value for one inch or the "R-value per inch" of the product.

3)What density of the materials has to do with our discussion. We are not talking structural analysis, where the weight counts. If, you state that the insulating properties of a material are increasing as their density decreases, that is something new for me and I thank you for the info.

4)What a closed cell is? If is a honeycomb structure I would "feel" that is has better insulating properties.

5)"When 0.2 (brick) has a better insulating value than 5 (XPS foam) İ don't know." I have mention thermal "inertia", meaning thermal capacitance, and, indeed, brick is better that foam from this point of view, of accumulating heat.

I described a solution that might be cheaper than others.

And who knows, maybe after 12/21/12 we will all need to build adobe ...shelters.

İ can believe the Home Depot qualifications: Do what you choose and pick a cave out if desired. İ do not agree with your statements on insulation in general - you are wrong in many places. İnsulating value and heat holding capacity are at the opposite ends of things. Adobe as a good insulator? What are you smoking? R value definition: Definition of R value of insulation a commercial unit used to measure the effectiveness of thermal insulation. A thermal insulator is a material, manufactured in sheets, that resists conducting heat energy. Its thermal conductance is measured, in traditional units, in Btu's of energy conducted times inches of thickness per hour of time per square foot of area per Fahrenheit degree of temperature difference between the two sides of the material. The R value of the insulator is defined to be 1 divided by the thermal conductance per inch. This means R is an abbreviation for the complex unit combination hr•ft2•°F/Btu. In SI units, an R value of 1 equals 0.17611 square meter kelvins per watt (m2•K/W).

If two inches of foam insulation is applied to the outside walls at this point it will increase the ability of the adobe walls to maintain an even temperature. This will bring a 14" adobe up to an R-value of 22. A screen is applied after that to help the plaster tack to the walls. Plaster is applied in three coats after the foundation, walls, ceiling and roof are completed.

The first article about the use of adobe in a google search: adobe

To continue from this point is meaningless. I love you, man

I think the two of you are mixing applications of materials for different aspects in an efficient thermal design. Adobe has much higher mass density and a relatively slow thermal transfer rate so it will make a better solar energy absorbing mass than high or low density XPS, but not a very good thermal insulator for the exact same reason.

That is what İ wanted to say but you did it better!

With the 10 inch wall thickness Nikolay all ready has the thermal mass and you do not want that on the outside of the insulation - defeats the purpose.

well put. .

In the first place a just wanted to get some impressions from people that have installed solar heaters (especially solar air heaters, because that is what I have in plan to test) or have experienced this technology for real.

Suddenly the subject changed from solar heating to thermal insulation.

So in the end, with or without "insulation, 10 inch, 15 inch, chirpici" or other, how does a solar heating system "feels like" , What do you all feel about it if you have experienced one?

Regards

@Nıkolay

İn two posts İ provided links where such systems have been built and the guys İ know from other forums are quite happy with the air heaters they have built. Hope İ have some funds in the future so İ can do the same.

Yes, saw them. Very interesting designs.

thanks

It is code in a lot of places that combustion heaters must use outside air for the combustion. This goes for hot water heaters, furnaces, and wood burning devices, including fireplaces. This is not going to bring fresh air into the living space, just the combustion area. Then that air and the combustion gasses are vented outside.

Solar air heaters are sold commercially though very expensive. İ have read of several people making them as DİY projects that are very happy with the results and they were built at very reasonable costs. İt is a super use of solar as the relatively low temps and delta T involved mean that solar thermal air heating (like solar thermal water heating) is quite efficient. When İ have some funds available İ intend to follow this route for my home in İzmir, Turkey. We built in a HRV (heat recovery ventilation) system with filtering so İ am not worried about that aspect. İ suggest it is one of the better ideas around.

Good day, nikolay, where I live in South West Africa / Namibia, it gets hot hot hot in summer and even though the sun shines in winter, it gets COLD. The general roofing material here is corrugated galvanized sheet metal & even in winter I have +42deg. Celsius of free heat under the roof. 12Vdc solar panel on the roof powering 12V computer fans in the gable, vented through flexible ducting to a grille in the passage ceiling. the fans are switched by a thermostat in the roof under the gable, set to switch on @ +35deg celsius and switches itself off when the temperature drops below that again. I can walk around shirtless in the house on winter evenings. OK, we don't have snow, actually I've never seen snow in my life before, but here the system works. Good luck on your project. Regards, Leon

To get the house warm and keep it warm, you also need to insulate the walls and roof, otherwise you may find that your solar heating is simply not enough.....

Apply a layer of Dow XPS rigid foam or equivalent to the exterior. İ used a 50mm layer.

Our house was OK heating cost wise, but in 2008 we put a 10cm (almost 4 inches) layer of insulation on, it has dramatically dropped our heating bills, in spite of a long and very hard winter, our gas usage now means that instead of filling once a year, we only need to fill every 18 months or so.

We have friends who only put 6cm on and they have not had the return on investment that we have and the difference in the cost of materials was minimal. The cost of the work was identical....

So don't go for thin insulation, if I knew then what I know now I would have put on 15cm.....

We also installed a pellets burner (for warm air only) in the kitchen in 2006 and that also dropped the amount of gas used significantly since then...also having two sources of heat is good if one of them needs a repair, though that has not happened as of yet.

We have no solar heating as of now, though I am considering making a few units for testing......

Did you do any calculation for the necessary insulation in the first place, or it was just trial and error? And what is the insulating material, because I am thinking to use spray polyurethane foam especially on the north side from where the winter wind blows fiercely.

İt will be far easier and equally good to use the board. İt is adhered to the building surface with mortar type adhesive and completely sealed from the outside. Cost of labor for installing two or three layers vs one would increase with the thickness (the work increases) but not proportionatly. The thickness you need depends on the location - Germany would certainly need more than we do here. We had one day below freezing this year at -1,7 degrees. There are various programs around for calculating the thickness - our architect took care of that for us. Search should find one. The roof is the most important part - that is where the main heat loss is (outside of the windows).

You seem to forget that it also keeps the house REALLY cool on a hot sommer's day also......in summer, some people think that we have AC, we don't.

I did not need to use our portable AC unit last year at all......also a saving on energy!!!

The foam board is adhered to the wall with a mortar type of adhesive plus anchors into the existing wall. Here the architect could not understand why İ wanted exterior insulation of more than 25mm and the 150 in the roof drove him wild - different than normal. The extra in the roof was worth it as the attic stays comfortable on the hottest days. We didn't use the AC even once last summer.

Some "professionals" simply aren't!!!

the winter temperatures are below -15 deg C so the insulation layer should be considerable, maybe 15 cm as Andy said he should have installed.

The company that offered it gave us a choice of 3 thicknesses, I picked the middle one, it was as simple as that!!! Not very scientific I am afraid.....

If energy prices remain the same (unlikely) it will take us 15 - 20 years to get our money back, but as gas prices are more than 5 x what they were twenty years ago, I feel that it could repay much earlier......we will see!!

Although that side of your house where the wind comes from is the coldest side, until you have insulated all outside walls and the ceiling upstairs/roof, you will not see many gains.

I have little experience of foam except as a "glue" for doors and windows, that what I used "pushes" very hard against both surfaces.....that might be dangerous to the house construction if used over large areas, so be careful......

I concur with Russ; don't use spray-on foam, use foam insulation board. It can be easily finished with an appropriate exterior top-coat application, such as stucco. In addition to being extremely expensive, spray-on foam thickness is not easily controllable unless you are spraying into a wall cavity that can be "trimmed to fit" later. On an exterior wall such as you are talking about, it would just create a big, expensive and ugly mess. Rayzer

We did in plain city (Antwerp) a 4 story building in a row of houses, between 2 higher houses- roof invisible from the street (too high) a PU foam insulation on top of a Clay tile (Model Boom - with plenty of holes between) roof of 200 years old. Twenty- five years ago.

About 3" thick. Then coated it with a silver (aluminum) thick finish compound paint.

Still functional and not bad looking. There is no stagnant water remaining. The foam guys did a good job.

Of course here it could be, because the view came second place.

The owner made 3 extra rooms underneath the roof now. It saved taking down the roof and gives it more strength too. Note: always cover with a finish because birds like to pick in PU.

Two sites where air heater projects have been built are as follow: http://www.mountainelectric.ca/hotbox/#HB_junior http://www.builditsolar.com/Projects/SolarHomes/Doug/DougsProjects.htm

Here is another concept - don't know that İ agree with some of the fellows assumptions or calculations but it is different: http://www.particlepanels.com/p/particle-panels.html

Solar energy and good insulation are one and the same theme, they go "hand in hand", having one without the other is wrong.......

Even fresh air can be drawn into a house and in winter the incoming air takes the heat from the outgoing air, in summer, the opposite happems.....if you have the right design!!

About adobe insulation value see this web site: http://www.greenhomebuilding.com/adobe.htm Adobe is a good thermal mass material, holding heat and cool well. It does not insulate very well, so walls made of adobe need some means of providing insulation to maintain comfort in the building. Sometimes this is accomplished by creating a double wall, with an air space, or some other insulation in between. Another approach is placing insulating materials on the outside.

I think I'll just insulate the walls on the outside, without adding more thermal mass.

We did some studies on solar heat collection in the early 70s. At 13ºN in South India the peak insolation is about 0.8KW/sq.mtr.

The maximum we could collect was 4 to 5 KWHr/sq.mtr. on a clear sunny day in summer.

The issue is to have adequate thermal mass to store the heat. Even if a suitably designed absorber is used, quite often the absorber itself has no thermal mass. Some form of double glazing can help. We did try one design with the absorber being the outer surfaceof a thin rectangular water tank. The high specific heat of water gives excellent thermal mass in a reasonable size. Evaporation is not a major problem if the temperatures are kept below 95ºC. The surface of this heat sink can have longitudinal direction (parallel to air flow) fins to improve heat transfer to the flowing air.

The nuisance of a liquid heat sink can perhaps be avoided if black granite slabs are used as collectors. They have quite good thermal mass and can be heated to any temperature without the problems of evaporation.

Have you considered the option of making the wall itself the heat sink?

Nikolay,

Solar air heating is one of the essential green technologies that humanity must adopt to replace fossil fuel use. Solar air heating is more cost effective than solar water heating and solar electric. Even with the cost of heat storage, air diverting dampers and control electronics, it is already the most cost effective use of your money after insulation. I live in Pennsylvania, and even in this partly cloudy area, it pays to solar heat. Only social inertia/ignorance is holding it back from wide adoption here.

The first U.S. patent for solar air heating used dyed wool as the collector. In my opinion this is still a great way to go. Various open-cell or reticulated foam materials can be modified to be good solar air heaters. By drawing air in from the sunlit side, the air in the collector is coolest near the glazing (glass or plastic layer), reducing conduction losses. The air comes in intimate contact with the heated element (wool) so the element stays at the lowest possible temperature, minimizing infrared emission.

The solar element can be made of various materials such as natural wool, fiberglass, aluminum wool, stainless steel wool, mineral wool, polyester wool, etc. The fibers can be loosely held on a backing or pressed into a felt. Reticulated urethane foam with UV stabilizers and solar absorbent coating can last a decade or so.

CAUTION: Natural (animal) wool is highly flammable once ignited, so care must be taken to eliminate stagnant collector conditions! Do not use fiberglass or mineral wool insulation as an absorber because the fibers are too fine and are a health hazard. Any fiber used in air filters is thick enough.

The fiber can be enhanced with dyes or coated with a thin layer of solar absorbent. There is no need for selective solar absorbents or infrared trapping glazing since you are usually working at low (room) temperatures, but they are OK. For example, both nickel and stainless steel are excellent selective (a/e=11) absorbers if you heat treat it until it turns blue. Plastic glazing does not block infrared, but here it is not needed. Dyes may bleach in sunlight, some more that others, but you may reapply it if you can remove the wool from the panel.

The solar absorber should be thick enough to block more than 90% of the light striking it. The light that passes through should be reflected back with a white reflective coating (cheap) or mirror (expensive). That should bring blocking efficiency above 97%.

You should design the panels to be serviceable, so the absorber can be vacuumed and/or washed, and the glazing can be replaced. The absorber should be mounted diagonally (front to back) across the panel to enhance air flow. The entrance and exit air passages should be large like room air ducts. They should have thin film air dampers to prevent back flow.

If the panels do not thermo-siphon, you should supply their fan(s) with a battery backup or (preferably) solar electric power.

The number of glazing layers depends on the outside temperature. In your case, you need at least two and maybe three glazing layers. If you can mount the panels against the building insulation (remove siding) then the panel back will not need insulation.

Diverted heat storage is essential for warmth at night and moderate temperatures during both sunny and cloudy days. If the consumer has many partly cloudy and rainy days like I do, you will need more heat storage. Diverted heat storage requires intelligent adjustment of air dampers based on room temperature, collector temperature and store temperature. The more collectors and storage you have, the less fuel you will consume.

Water is very cheap, but the water containers and storage shelves cost money i.e. $1-$2/gallon. However, think about collecting used plastic carbonated drink bottles (2 and 3 liter bottles), including the caps. They are designed for stacking. You can choose to alternate upside-down and rightside-up layers. The bottom layer of bottles is upside-down for cross ventilation. (The bottom and top layer of the store must pass air sideways to the air ducts.)

Adobe (dried mud brick) is cheap, as is stone in various coarse sizes, cinder block and concrete blocks. The bottom layer of these stores is usually cinder block turned on it's side for cross ventilation, and separated with gaps and/or chipped to ventilate the stacks.

You can also install closed-foam rigid insulation in a sub-basement and add alternate layers of soil and heat transfer plumbing (air ducts or water pipes).

Did you build it yet? Maybe you could pipe the heated air to one room and insulate THAT one room only, as an experiment. My bet is that it will be a popular room in the house. If it is possible, you could probably get the same heating effect for that one room by installing very large windows in the room on the south side - if the entire south wall is glass that would be best. Insulate ALL the other room surfaces as well as you can. If you use foam on the inside of the room, you need to be careful that you cover it with something that will not burn so you don't create a fire hazard! At night, use moveable pieces of foam to cover the windows to prevent as much heat loss as possible (watch for condensation on inside of windows). Next morning when the sun comes up, store the moveable window covers somewhere and let the sun shine in. This is called passive solar heating. Google it. I'm sure you are aware of passive solar. This is the system I want but have not done it yet. (Our home has a lot of south facing windows and on sunny days here in western Oregon we get a good amount of heat - if we had more south facing windows we'd get more heat.) Let us know about your system and how it works. Good luck.

try www.backwoodssolar.com

www.mrsolar.com

Google "passive solar homes" etc.

This is my first attempt to collect solar power. Is only experimental. Cardboard box, double walled. I painted it black inside and added some thin metal sheets made from beer cans. the air passes under and over the sheets. The panel was covered with a polyethilene transparent foil. The colecting area is about 0.5 m2. All testing was done inside with the panel facing a south window. The sun struck the panel between 10 am and 2 pm. That was on the third and the fourth of november, 2010.

I measured the airflow of the fan mounted at the air input and the temperature difference between input and output.

The maximum temperature difference on a clear sunny day was about 30 deg C. The maximm ammount of energy calculated was about 2 kWh-2.5kWh/m2, extrapolated for a full sunny day. From 10am to 2 pm I calculated about 1.5kWh/m2. The actual heat wes about 0,7 kWh. The collecting efficiency went up as I increased the airflow in the panel but still it was not enough. the fan was a 12V, sleeve bearing, 80x80x25 mm. The pressure drop was quite significant so the airflow dropped by a factor of 3 compared to the airflow measured before installing the fan to the collector (yes I measured the airflow for the fan both with and without the collector).

I have to mention that between the collecting surface and sun there were 3 transparent surfaces and that lowered the overall collecting efficiency. Frst two surfaces were glass, and that was not even cleaned before (something to remenber for the future ). The third was the foil.

Given the simplicity of construction and the ammount of heat collected the collector for direct air heating is awesome.

Hope to get a 3 sq m one soon.

There is somewhere on the web information to use aluminium cans, missing top and bottom, stacked one on top of the other, painted black, that keep a workshop warm for free on sunny winter days.......

.......saves having to flatten them first!!!!

Didn't have enough of them at the moment and couldn't wait . For the next one I thought to use blackened wire mesh - better for heat transfer.

No need for metal screen. Waste of money. Use cheap plastic window screen and black paint. The heat transfer and solar absorbtion is all at the surface, so the conductivity of the wire interior is irrelevant.

I share with you the working version of the solar air heater that has been doing its job since 2012. It is around 5+ sqm.

The experience with it until now encourages me to go for a second improved version on the entire south facing wall, shown in the photo.

Along with this I also started the insulation process; first on the list was the attick.

The panel is quite efficient that in sunny autumn days it heats the wall all the way from outside to inside surface.

The drawback is the efficiency for air circulation. The flow cross section is small (40 mm diameter tube) which limited also the fan used. I tried a high flow centrifucal fan but it was too noisy so I settled with a 40mm axial fan. It starts at 10 degree temperature difference between the room and the collector.

Thanks again for the great ideas.

Nicolae

Congratulations!

Glad to hear you are adding insulation. Insulation and sealing all air leaks should be the highest priority.

I notice that you were concerned about the low air flow rate. The best improvement would be to increase the air duct cross-section. If you add penetrations between every wall stud, both at top and bottom, you achieve maximum cross-section. This modification would require that you remove all baffles, so the air moves straight up in the collector. This also eliminates the use of fans inside the ducts. One-way air flaps are essential.

A screen absorber sounds great. Lightly spray both sides of the screen with carbon black paint. Use a blower to keep the pores open. Painted screen typically blocks around 85% of the light on the first pass, passing 15% to the back wall.

No need to paint the back wall. Even if the back wall was a perfect reflector: 85% + 85% = 97.75% ( really ).

Good luck!

Larger duct cross-section is almost impossible. The wall has no studs since it is a european house, all brick. I have indeed noticed that in the american style houses you can poke a hole almost wherever you like. Not here. I have to try some other solution.

I am thinking to try something else. In the middle of the house there is a hallway with yet unfinished flooring. It also previously had a diesel/oil stove, which is now out. I want to use the chimney hole in the ceiling to bring the airduct so as to allow me to store the heat under the house. I need to tinker a bit more on this one.

I'll come back with updates as soon as I put plans into practice.

Thanks for the ideas.

I'm sorry I didn't notice this back in 2016! From your description, you have created a Trombe Wall !

By installing a double glazed surface outside your non-insulated south wall, you have directly heated the stone wall from the outside-in. Of course, the black paint has improved the solar absorption. If your construction has a wood panel behind the black paint, then the wood is an insulator and is actually less effective than black paint directly on stone. If so, you should omit the back surface of your construction.

(Many people leave the stone unpainted for aesthetic reasons. You may use a wood dye to alter the solar absorption without loosing the appeal of natural stone features.)

If the wall gets cold on cloudy days, you may want a third layer of glazing. During the hot summer you will need to shade the solar collector and/or open the top-and-bottom of the construction to cool the wall. If doing the latter, you will also have to remove the horizontal baffles.

Anyway, congratulations on the Trombe wall !

My friends Trombe wall system had a simple overhang roof eave so that for spring and summer very little, high angle direct sun light landed on the rock. In fall and winter the low angle sunlight fell under the eave onto the rocks.

You can use the Trombe wall during the summer to pull warm air out of your house.