Solar heating has existed in architecture since ancient times, when people used adobe and stone walls to trap heat during the day and slowly release it at night. In its modern form, however, solar heating first developed in the 1920s, when European architects began experimenting with passive solar methods in mass housing. In Germany, Otto Haesler, Walter Gropius, and others designed schematic Zeilenbau flats that optimized sunlight, and following the import of “heliotropic housing” to the U.S., wartime fuel shortages during World War II quickly popularized passive solar heating. Variations of this system then proliferated around the world, but it was not until 1967 that the first Trombe wall was implemented by architect Jacques Michel in Odeillo, France. Named after engineer Felix Trombe, the system combines glass and a dark, heat-absorbing material to conduct heat slowly into the house.
The standard Trombe wall places a glass panel approximately 2- to 5-centimeters from a 10- to 41-centimeter-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. Whereas direct solar radiation has a shorter wavelength and is therefore easily conducted through glass, 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, described by Wien’s displacement law, traps heat between the glass panel and masonry wall, allowing the Trombe wall to effectively absorb heat while limiting its re-emission into the environment. Moreover, because the glass panel 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 a 20-centimeter 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.
Trombe walls often serve load-bearing functions alongside their passive heating roles. To maximize solar gain, the glazed side of the wall typically faces toward the Equator, which allows the wall to collect more sun during the day and during the winter. Different materials, dimensions, colors, and other alterations can also affect the efficiency of the Trombe wall system.
A common variation is the ventilated Trombe wall, which supplements the natural conduction of the thermal mass with vent-facilitated 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.
Another example is the “Drum Wall,” developed by Steve Bare, which uses water as a thermal mass rather than concrete or stone. Darkened steel 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 absorbs heat more efficiently than the standard Trombe wall.
Smaller scale alterations can also improve the effectiveness of the Trombe wall. For example, architects often apply a radiant barrier or selective surface – usually a sheet of metal foil placed on the outer surface of the masonry wall – for better results. 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 specifically. Combined with a shading device like a roof overhang, overheating during warmer seasons could be reduced drastically.
Finally, careful specification of color, dimension, and material could optimize the efficiency of the Trombe wall as well. 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. Architects can also paint the masonry wall black to increase its absorptivity, or use high transmission glass to maximize solar gains. However, clients may want the masonry wall to be less opaque to allow daylight into the home, requiring designers to balance aesthetic appeal and efficiency. Architects may also use patterned glass to obscure the thermal mass, though this choice will not sacrifice transmissivity.
Though early innovators of heliotropic housing likely were not considering climate change, passive solar heating systems like the Trombe wall are highly attractive today for their low energy use and relative sustainability. A study by the National Renewable Energy Laboratory of the Zion National Park Visitor Center found that 20% of the building’s annual heating was supplied by its Trombe wall. Of course, architects designing with a Trombe wall must overcome certain aesthetic disadvantages, especially lighting. Dimness from the opaque equator-facing wall can be offset by skylights, adjacent windows, and adequate artificial lighting. The Trombe wall is also a highly climate-dependent system, meaning location and weather variations could negatively impact the effectiveness of the wall. However, if these concerns are adequately addressed, this system can drastically improve a structure’s energy efficiency – and even lower heating costs dramatically.