After walking into our new workshop one December morning and finding the inside temperature to be a bone-chilling 10°F (-12°C), I decided that it was time for a heating system! Given the rising costs of propane and our environmental concerns about using nonrenewable fossil fuels, a solar solution seemed fitting. I reviewed many solar collector concepts, and finally decided to install a thermosiphon air collector on the south wall of the building. The concept is elegant and simple. A thermosiphon design uses only the buoyancy of heated air to circulate air through the collector, eliminating the cost, maintenance, and energy consumption of fans, sensors, and controllers commonly used in other collector designs. On a sunny day, in a cold climate like ours here in Bozeman, Montana, this simple system can produce the heat equivalent of burning about 2 gallons (8 l) of propane. To minimize material use, I integrated the collector within the building’s structure. I also tried to make the collector easy to construct using readily available materials. In fact, making this collector should only take one trip to the hardware store and US$350. Set aside two or three days to complete the project.
How It Works
The thermosiphon collector consists of clear, corrugated poly carbonate panels fastened to vertical 2 by 6s. The clear panels, on the building’s south face, admit sunlight. An absorber—in this case, two layers of black metal window screen—suspended inside the collector captures the sun’s
heat energy. The air around the mesh expands and rises as it warms, creating a convection current. Vents located at the top and bottom of the collector allow air to circulate and become heated. Cool air enters the lower vent, is heated by the absorber, and rises through to the upper vents that exit into the building’s interior. This circulation of air continues as long as the sun shines on the collector. At night, as air in the collector cools to outside temperatures, airflow tries to reverse. Air in the collector sinks through the bottom vents and attempts to pull the warmed air from the building through the top vents. Use of flapper valves on the top vents helps prevent this reverse circulation and keeps the heat inside.
Nuts & BoltsThe thermosiphon collector consists of clear, corrugated poly carbonate panels fastened to vertical 2 by 6s. The clear panels, on the building’s south face, admit sunlight. An absorber—in this case, two layers of black metal window screen—suspended inside the collector captures the sun’s
heat energy. The air around the mesh expands and rises as it warms, creating a convection current. Vents located at the top and bottom of the collector allow air to circulate and become heated. Cool air enters the lower vent, is heated by the absorber, and rises through to the upper vents that exit into the building’s interior. This circulation of air continues as long as the sun shines on the collector. At night, as air in the collector cools to outside temperatures, airflow tries to reverse. Air in the collector sinks through the bottom vents and attempts to pull the warmed air from the building through the top vents. Use of flapper valves on the top vents helps prevent this reverse circulation and keeps the heat inside.
The collector is 20 feet wide by 8 feet high (6.1 x 2.4 m) for an overall area of 160 square feet (15 m2). The collector is 6 inches (15 cm) deep. In most cases, make the collector as large as your south wall allows (see sizing solar collector). The top vent and bottom vent areas should each be at least 50 percent of the collector’s horizontal cross-sectional area (again, more is better). The collector frame is constructed from wood, and consists of six vertical members, a bottom sill, and a top sill. The six vertical 2 by 6s divide the collector into five, 4-footwide (1.2 m) bays. A 2 by 6 is used for the bottom sill. A 2 by 8 is used for the top sill, which should be sloped at about 10 degrees to shed rain. The collector frame attaches to the building by lag bolts from the inside. The collector is glazed with clear Suntuf corrugated polycarbonate panels. These panels have an ultraviolet light-resistant coating on their sun-facing side to extend their life. Each panel is 26 inches (66 cm) wide by 96 inches (244 cm) high. There are ten panels. Pairs of 26-inch-wide panels are joined over a 1- by 1-inch (2.5 x 2.5 cm) vertical wood strip to make the 4-foot-wide panels for each bay. Two, 1- by 1-inch horizontal members provide additional support for the glazing. The absorber is installed on battens placed about halfway between the glazing and siding. After measuring the thermal performance with one, two, and three layers of window screening, I found that two layers work best.
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