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.
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.
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.