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This page contains pictures of my solar boxes and information explaining how they work and how to make them as well as data recorded about their performance. Since my main objective is to GET OFF OIL, I am first looking for ways to make heat so I don't use as much oil. Solar cells are only about 15% efficient so it makes no sense to try to heat your house from electricity generated from solar cells. However, heating air directly from the sun is many times more efficient. Also, if you heat air instead of having a liquid based system, you don't have to worry about it freezing or have to use toxic antifreeze to stop it from freezing or worry about leaks or the weight of a liquid based system. It's just air that becomes hot air and it doesn't matter if it gets to freezing temperature at night when the sun goes away because it's just air. This may never be complete or totally up to date as I am always trying to improve!

Here is a side view diagram of a solar window box. The theory is simple. This type is meant to clamp in a standard window. The box is divided down the center. Sun shines through the glass and strikes the heat collector. In this example the heat collector is a plate of metal that is painted black. The plate is warmed and warms the air around the plate. Warm air rises and exits the box at the top into the room. As the warm air rises it draws cold air in the bottom which is then drawn towards the plate and the cycle continues. Efficiency can be improved by suspending the plate above the center divider by 1" or more allowing air to flow on both sides of the plate. Still there will always be a film of air about 1/2" thick directly against the plate held in place by friction and insulating the plate from the moving air thus reducing efficiency. In my boxes I use black polyester felt as the absorber and draw the air through the felt eliminating the insulating layer of air and increasing the heat transfer efficiency many times.




This is my attempt to document the build of my latest solar air heater. I decided to put the new build at the top of the page instead of the bottom.
The picture on the top is a piece of plywood with a pair of 2x3s mounted to the back to stiffen it and 1/4x20 rod coupling nuts mounted to the top. This is the backboard and serves as the frame for the box. The picture on the bottom shows the foam sides installed on the back board over the rod coupling nuts.

Top shows the duct hole cut in the foam with a "drill" I made out of a piece of 4" thin wall PVC pipe which I sharpened on a belt sander. This worked amazingly well and left a clean accurate hole. Bottom shows a duct nipple installed in the hole. I coated the inside of the hole and the outside of the nipple with caulk.

Top shows the fiberboard insulation installed on the back board. Bottom shows bottom of the absorber frame. I made a piece sticking out the back which forms a divider which creates an area that is 4"X 4" the width of the box. Cold air enters the intake at the bottom of the box into this area and is forced through the felt to the front of the box. The air then rises up the front of the box between the polycarbonate and the felt before it is sucked back through the large area of the felt picking up the heat from the felt on it's way. The hot air is then removed through the upper nipple from the most insulated area of the box so it losses the least heat on the way.

Top shows the top of the absorber frame in the box with the spacers I made to keep it 4" from the back of the box. Bottom shows the frame laying on the felt on my living room floor where I layed it out for assembly because it was the cleanest largest place I had.

Top shows the completed absorber. Bottom shows the absorber in the box.

Top shows the box with the polycarbonate glazing on. Bottom shows the interface board I made to get the air into my computer room along with the fan on the floor and the fan control box on the window sill. It also shows the dryer vent draft blocker on the cold air side to stop reverse thermosyphoning. I put a tube on that goes down close to the floor to suck the coldest air off the floor.

Last picture shows the box outside and the flexible insulated heating duct hooked between the box and the interface board.

Here are 4 pictures of a box I made for the garage as a test in about 4 hours. It is about 2'high by 3'wide by 6"deep, has no insulation, single pane glass and not even enough felt to occupy the entire exposed area behind the glass. Still I see a 15 degree rise in temperature between input and output on a sunny day. Sometimes it is even more than that. The last picture is of the intake and output in the garage. I just stuffed the 6" flexible insulated duct through a cat door in the wall of the garage and put a coffee can with the top and bottom cut out in each duct to make the ends rigid and easier to work with. The only addition not pictured is an inline dryer vent draft blocker installed like a check valve on the intake to prevent reverse thermosiphoning. Reverse thermosiphoning is when the box gets cold after the sun goes away and begins to cool the air inside. The cold air falls and pushes out the bottom and draws warm air into the top and so that cycle continues cooling all the air you just spent all day warming. I also painted the box flat black for protection from the weather and hopefully some solar defrosting. This seems to work OK. The fan draws the air through the box. It is a 12 VDC fan powered by a solar cell. Since these pictures were taken I have made a little circuit that uses thermistors as temperature sensors and compares the temperature inside the box to the temperature in the room to be warmed and only turns on the fan when the box is warmer. I spent maybe 3-4 hours coming up with the circuit and another hour and $10.00 worth of parts making it and then found almost the exact same circuit on another website. Wish I had found that website first. If I get enough interest I may decide to make these circuits to sell.

Here is the big box I built for the basement. This is the one pictured on the home page. It is much like the smaller box above but with some improvements. It is about six times bigger, has R9 fiberboard insulation inside with the shiny side facing in to reflect any sunlight that gets through the felt back onto the back side of the felt so no sunlight is wasted and Industrial double pane glass I got from a store down the street they were tearing down. Recently I have had temperatures of 92 degrees on the output while sucking 57 degree air off the basement floor. This is a 35 degree gain. Air is pulled through the box by 2 fans controlled by the same kind of circuit as the box above at a rate of about 100 CFM. Originally the fans had no control and were just hooked directly to the solar cell but the solar cell made the minimal electricity required to run the fans long after the box stopped making heat so I was making cold air.

This is a graph of the output high and low temperature between 12/13/04 when the box was installed to 2/18/05. The extreme low points on the low temperature (blue) line are times when I came home from work and found the box reverse thermosiphoning. As stated above, a dryer vent draft blocker installed on the input at about the number 50 point on the graph finally solved the problem. I tried several things that didn't work at all or worked poorly. I will not elaborate on the failed methods. As you can see there are times when significant warming is taking place even though the temerature outside is very cold.

This is a graph of the 05/06 season between 10/30/05 and 05/27/06. The extreme low points on all lines are days I missed taking readings.

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© 2005-2008 B. McGowan