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
Step 1: How the Vented Aluminum Soffit Collector Works
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.