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Solar air heating involves using the sun either to heat outdoor air and bring it into a building or to heat the indoor air without introducing new, fresh outdoor air. The latter is the most frequent case, probably due to the fact that it's easier to start with the already room temperature indoor air.
A few pictures of an installation of the Cansolair done by the author follow. After that are various scenarios of how it works, hopefully helping you either to choose which type you'd like to buy or to make yourself.
Work done by the author
Built a mini can solar air heater prototype - Just to see if I could make a solar air heater that would actually heat air and to try out the necessary electronics, I built a mini can solar air heater.
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Window solar air heater - Since my mini can solar air heater was a success, I designed one that I could hang out my window during the winter. I made two versions, the first was for outdoor to indoor air, which I then modified to be an indoor to indoor air one.
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Installed a Cansolair for a customer (with Isolara Energy Services)
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How it works (and construction tips)
In its most simple form, you need a material with a dark colored surface that the sun can shine on. This will cause the material to get hot. You then have flowing air make contact with this hot material and take some of the heat from it. Arrange it so that this heated air ends up indoors and you're done! The rest are details of different ways to do it based on efficiency, convenience, patent rights (if you're going to manufacture and sell them), materials at hand, ...
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Figure 1 is an example of a very efficient one with all the bells and whistles. It sucks in relatively cold indoor air using the fan at the bottom. The air is pushed up the inside of an air flow chamber. The dark colored skin of the air flow chamber is what the sun heats (outdoors is on the left in the figure.). So while the air moves up the inside of the air flow chamber, it takes heat from the walls of the chamber. The now hot air exits the solar air heater at the top, reentering the building.
The skin of the air flow chamber in Figure 1 is called an absorber. The absorber is the part that absorbs the sun's heat and transfers it to the air. You can get rid of everything else except the absorber.
As long as the absorber is a dark color then it will absorb the sun's energy and turn it into heat energy. The measure of how much it absorbs is called its absorptivity. So you want high absorptivity. If you don't take away the heat from the absorber fast enough, then it will heat up. If it gets hot enough, it will begin reradiating the energy back out again. The measure of this effect is called its emissivity. So a high absorptivity and a low emissivity is best, but if the heat is being taken from the absorber by your flowing air then it will not get hot enough for emissivity to matter much.
For homemade solar air heaters people usually use flat black paint. A good choice for this that is available to the average person and budget is the flat black paint for painting barbeques and radiators since it can handle the heat without getting damaged and can be found in a spray can at your local hardware store. Its absorptivity can be as high as 94%1
Manufactured solar air heaters typically use a more expensive and hard to do coating but one that has high absorpivity and low emissivity (absorbs plenty and reradiates back little.) An example of this is black chrome, part of the process of which includes electroplating (a more difficult process than simply painting.) Its absorptivity ranges from .92 to .97 and its emissivity ranges from .8 to .252.
One feature of the one in Figure 1 is the use of glazing, which is a fancy word for the glass or plastic material at the front. Why not skip the glazing and leave the air chamber in contact with outside air? A big reason is that this glazing keeps any moving, cold outdoor air from contacting the air flow chamber and stealing its heat. With the glazing you have a box that contains trapped air. This trapped air heats up and stays heated as long as the sun is shining and perhaps a little while afterwards. If the surface of the air flow chamber gets hotter than the trapped air then it will give up some of its heat to the trapped air. But at least it's hot trapped air and won't give up as much heat as it would to cold air.
The glazing can be tempered glass or plastic or something else. Tempered glass doesn't break as easily as regular glass and if it does break, it shatters into a million tiny pieces, so it's safer than regular glass. The side windows in cars are made of tempered glass. Also, tempered glass allows about 10% more light to shine through it than regular glass. This is called its transmissivity, a measure of how much light is transmited through it. This means more of the light will get to the dark colored absorber's surface to be heated. If you're making your own solar air heater, tempered glass is more expensive than regular glass and comes in a limited number of sizes (4 feet x 8 feet and that's about it.) You also can't cut or drill tempered glass. So unless you're out for maximum efficiency and have a lot of money, you might go with regular glass or plastic instead.
According to Cansolair's website, they use LexanTM, a type of plastic for their glazing. LexanTM is very strong and also flexible. Rather than being flat, the glazing on the Cansolair is rounded so that it wraps around the unit a bit, catching the sun for more hours of the day. You can see this roundness in the pictures above.
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Making the air contact the absorber
Another interesting feature of the Cansolair is that they make the air flow inside the air flow chamber in a turbulent manner. Instead of being nice straight flowing air, the air actually spirals or vortexes. The reason for this is to make ALL of the air contact the surface of the air flow chamber as much as possible, thereby heating all of the air and making the air spend a longer time in the chamber. Cansolair is so named because as of this writing, the air flow chamber is made up of pop/soda cans stacked one on top of the other. Where one can sits on top of another, one of the cans has been cut such that it has vanes. So when the air passes from one can to the next in the stack, the vanes make the air vortex.
More details of the Cansolair design can be found by examining it's Canadian patent number 2393273. I highly recommend reading this if you will be building your own solar air heater.
Insulation (or not)
Notice in Figure 1 that there is insulation at the back of the panel as well as the top and the sides. This is to prevent heat loss.
However, you must be careful in selecting the insulation. My first homemade solar air heaters used pink solid extruded polystyrene. According to my research though, if in contact with direct sunlight then it would offgass formaldehyde, not something I would want to be mixing with my indoor air. If you have a separate air flow chamber, which prevents the air being heated from mixing with the air behind the glazing, this wouldn't be an issue except that the heated insulation offgassed something that fogged up the inside of my glazing, reducing the amount of sunlight hitting the absorber. The insulation I used isn't made for the temperatures encountered (around 50C (122F)).
A good insulation to use is one made with a low-binder glass fibre. It is made with a high temperature glue that holds the fibres together. This glue will not offgass at the temperatures encountered. Unfortunately I don't know of a brand name but if you do, please let me know (my email address is at bottom of this page.) Another is a rigid board foil faced closed-cell isocyarurate insulation. It can handle temperatures up to 120C (250F) which should be just fine for homemade solar air heaters.
Notice in Figure 2 that there is no insulation at the back. The one in Figure 2 must be installed firmly against the building wall and will use some of the heat on the other side to heat the inside of the unit. Note, however, that if it is hotter inside the unit than it is inside the building then heat will flow from the unit directly through the wall and into the building, rather than heating the flowing air. If the wall around is cold too then heat may be lost to there too. This is a tradeoff.
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No glazing
Notice in Figure 2 that there is no glazing and so the absorber is in direct contact with the outdoors. A cold wind blowing over the surface of the outside of the absorber will cause heat to be pulled from the absorber surface.
A mixed version
Figure 3 illustrates a mix of features from the solar air heater in Figures 1 and 2. It has glazing so the absorber doesn't loose heat to the outdoors directly. It doesn't have a separate air flow chamber as in Figure 1 but instead uses an absorber with holes for the air to flow through as in Figure 2. Personally I like the separate air flow chamber for homemade systems because I'm always worried about offgassing from the various materials, especially since those materials are being heated. With a separate air flow chamber you can isolate the air that is going into the house from the air that might be getting contaminated by materials. I might be overreacting though as many people use homemade solar air heaters without worrying about this issue.
Figure 3 is actually a design by Gary Reysa that came from his article Build a Solar Heater in Home Power magazine, issuse 109, October/November 2005. It's made of simple to find materials. The absorber is actually black window screen. The glazing is made of clear Suntuf corrugated polycarbonate corrugated panels (http://www.suntuf.com). The frame is attached to the exterior of the bullding wall and is made of wood.
The solar air heater in Figure 3 doesn't use any fan to move the air but instead it uses something called thermsiphoning. This uses the fact that hotter air rises above cooler air. It works as follows. The unit in Figure 3 has a hole at the top and a hole at the bottom. Ignoring the holes, when the sun hits the absorber, the air will start taking the heat from the absorber and become hot. As you now know, hotter air rises above cooler air. In Figure 3 the only place that hot air has to go when it rises is through the top hole and into the building. But when that hot air exits through the top hole, new air has to take its place. So new air is automatically pulled in through the bottom hole. This new air is cooler than the hot air already in the unit. But now that it's there it too will heat up, rise and exit through the top therebye pulling new cool air at the bottom and so on... This process is called thermosiphoning.
Fan/blower versus thermosiphoning
So why use a fan at all? You don't have to if you don't want to since you can use thermosiphoning. But with a fan you can move more air per unit time through the solar air heater, blowing more hot air into your building heating it up more. But it isn't as easy as saying "I want more hot air so I'll add a fan."
When the air flowing through the solar air heater pulls heat from the absorber, it is literally pulling heat from the absorber! i.e. it is cooling the absorber. If the flowing air has just pulled so much heat from the absorber such that the absorber is now cool, the new air coming in won't have any heat to pull off and so will remain cool air. This is what could happen is you have a fan since a fan doesn't care if the air is hot or cold, it will just continue blowing/sucking air.
The problem is that the absorber is hot being reheated by the sun fast enough to keep up with the air flow. One reason could be that your solar air heater is not efficient enough and could be due to any number of things:
On the other hand everything may be just fine except that your solar air heater just isn't big enough for your needs. You need a bigger absorber to capture more energy from the sun and produce more heat. It's like having the most efficient air conditioner in the world but if it's too small for the room it's in then your room just won't get cool. Same problem.
Using temperature sensors
Let's say you do have a solar air heater that can heat enough air for your needs, and perhaps sometimes too much. You need a way to stop heating the air if your room is cool enough. In that case you need some temperature sensors and some electrical experience. You really need at least two temperature sensors.
One of them would be a regular thermostat available at hardware stores. This would monitor the temperature in your room and turn the solar air heater's fan on when the room is too cold and turn it off when the temperature is just right.
But what if the room is cold and it's cloudy (or nighttime!) ourdoors. The solar air heater may not have any heat for you. This isn't like a natural gas or electric heater where the necessary energy is always available. This is where the second temperature sensor comes in. The second temperature sensor is inside the solar air heater and monitors the temperature of the air there. If the room thermometer says that the room is too cold, it will ask the temperature sensor in the solar air heater if there is any hot air available. If there is, the temperature sensor will allow the fan to turn on. If there is not, it will prevent the fan from turning on. The temperature sensor in the solar air heat basically has a veto over the room thermometer.
A good type of sensor to use inside the solar air heater if you are using the above mentioned circuit algorithm is a bimetallic snap disk (or disc) type thermal switch. Get the kind that closes the switch then the temperature is above a certain amount (e.g. 30C (86F)) and opens when it goes below a lower temperature. I found some cheap ones in a local electronics store for $5CDN each. A higher quality one would cost more of course. See here for a circuit that uses a bimetallic snap disk controller.
When building your solar air heater, you may want to do some testing and fine tuning. For this you would want to embed thermometers in your solar air heater. Sometimes the thermometer can be in a location that's visible from the outside but sometimes it's embedded deep in the bowls of your solar air heater making it hard to know what the temperature is. A remote thermometer can be handy for this. These come as two separate pieces, an electronic thermometer and a display piece. Some products have these two pieces communicate via radio and some use a long connecting wire.
Some manufacturers
An example of one similiar to Figure 1 is the Cansolair (http://www.cansolair.com). The Solarwall® (http://www.solarwall.com/sw/solarwall.html) is similar to the one in Figure 2. The Solarsheat (http://www.yoursolarhome.com) is similar in principles to the one in Figure 3 though much different in construction.
References:
1. STT 100: Solar Domestic Hot Water Installation - Fundamentals.
Canadian Solar Industries Association,
Version 1.4, October 2005, pg. 15.
2. Ibid, pg. 27.
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