It was in 1967 that the first Trombe wall was implemented in France. The system, named after engineer Felix Trombe, combines glass and a dark, heat-absorbing material to conduct heat slowly into the house. The standard Trombe wall places glass approximately 1 to 2 inches from a 4 to 16 inch thick dark masonry wall, often made of bricks, stone, or concrete. Solar heat passes through the glass, is absorbed by the thermal mass wall, and then slowly releases into the home.
Direct solar radiation has a shorter wavelength and is therefore easily conducted through glass, but the re-emitted heat from the thermal mass takes the form of longer-wavelength radiation, which cannot pass through glass as easily. This property of solar radiation traps heat between the glass panel and masonry wall, allowing the Trombe wall to effectively absorb heat while limiting its release into the environment. Because the glass is only on the exterior of the wall, heat can pass uninhibited into the interior of the home, a process that typically takes around 8 to 10 hours for an 8 inch thick Trombe wall. Usually this means that the wall absorbs heat during the day and slowly re-emits it into the home at night, drastically reducing the need for conventional heating.
To maximize solar gain, the glazing typically faces toward the Equator, which allows the wall to collect more sun during the day, especially during the winter. Different materials, dimensions, colors can affect the efficiency of the Trombe wall system. Trombe walls can also serve load-bearing functions.
A ventilated Trombe wall will supplement the natural conduction of the thermal mass with convection. Vents are placed at the top and bottom of the space between the glass panel and masonry wall. As the air in this space is heated, it rises into the top vent, which redirects it into the home. At the same time, cold air from inside the home passes through the lower vent into this space, where it is heated and later redirected back into the home through the upper vent. This creates a circulating heater effect.
Besides masonry materials it is possible to use water as a thermal mass. Darkened containers, like oil drums, are filled with water and stacked behind the glass panel. Because water has a greater heat capacity than masonry, this system theoretically can beb more efficient than the standard Trombe wall.
Other alterations can also improve the effectiveness of the Trombe wall. If you apply a radiant barrier, like a sheet of metal foil, on the outer surface of the masonry wall, you can improve the performance. Foil has high absorbency, which allows it to absorb high amounts of sunlight and turn it into heat, but it also has low emittance, which prevents this heat from being re-emitted back toward the glass. If the foil is a roll-down radiant barrier, it can be used to reduce nighttime heat loss and summertime heat gain. Combined with a shading device like a roof overhang, overheating during warmer seasons can be reduced drastically.
The thickness of the masonry wall should vary with the precise material used: more conductive materials will transfer heat more quickly, which can be offset by designing thicker walls. You can paint the masonry wall black to increase its heat absorption, or use high transmission glass to maximize solar gain. It is possible to use patterned glass to obscure the thermal mass.
The Trombe wall is also a highly climate-dependent system, so that location and weather variations can impact the effectiveness of the wall. However, if these concerns are adequately addressed, this system can drastically improve a structure’s energy efficiency and lower heating costs dramatically.
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