While a thermosiphoning solar collector may sound complicated, in fact it’s a simple concept. A panel attached to the south facing wall of a structure absorbs sunlight. A vent at the bottom of the collector allows cool air from the interior of the building to flow into the collector using natural convection. The suns rays heat the ambient air inside the collector causing it to rise. Vents at the top of the collector allow the warm air to flow out of the top and circulate inside of the building, heating the area without the use of fans or controllers, typical of expensive convection solar heaters.
The basics of building a thermosiphoning solar collector are simple. The size of your collector can be just about any size you want, but the bigger the collector panel, the better chance you’ll have of collecting enough sunlight. I recommend building the collector panels in 4×8 sections, since plywood and most other building materials typically come in four and eight foot sections. However, you can build it to any size you want, so long as you keep the depth of the collector 1/15th of the height or more.
The larger the ventilation openings you cut, the better the system will work. Minimizing air flow resistance will lower overall temperatures inside the collector, but it will prevent heat loss and allow the collector to work more efficiently. A minimum vent opening size should be at least half the width of the collector and at least 12″ tall.
Building the Collector
A 2×8 pressure treated lumber is used to create a bottom sill and sides. They should be cut to length and attached together using your choice of fasteners. The top sill needs to be at a pitch to allow water to run away from the structure and off of the solar collector. A minimum of a 4/12 pitch is recommended for proper water displacement. Flashing is used to prevent rainwater from entering the collector and building. Attach the frame to the structure, leveling, squaring and centering the collector as needed onto the dwelling. Seal all cracks, gaps and openings with caulk. Cut out the vents inside the building into the solar collector.
Inside the panel’s center, 1×1 furring strips should be attached to the sides. This will allow you to attach the absorber screen. An absorber screen is basically a black metal screen-in fact you can use any metal screen and paint it black. I would recommend using two screens with about a ½-1″ gap between them for optimal collection capabilities. Attach the screens but leave the area where your vents will be, screen free.
Next, attach an acrylic, glass or other clear ½” thick panel to the outside of the solar collector, sealing them best you can. You can inset the clear panels by attaching another 1×1 furring strip inside of the collector. Trim can help cover any gaps or airleaks as needed.
Finish the project by attaching opening and closing doors over the vents, inside the building. Since the solar collector works both ways, this will prevent backflow which occurs during sunless times and at night.
Bracing can be added inside the solar collector as needed to support the use of additional acrylic panels and absorber screens. But keep in mind that air flow resistance can lead to an incorrectly working thermosiphoning solar collector. Minimize the supports, but without compromising the integrity of the collector.