A Trombe wall is a sun-facing wall built from material that can act as a thermal mass (such as stone, concrete, adobe or water tanks), combined with an air space, insulated glazing and vents to form a large solar thermal collector.
The idea popularized in the 1960s was just the glazed, heavy wall. During the day, sunlight would shine through the insulated glazing and warm the surface of the thermal mass. At night, heat would escape from the thermal mass, primarily to the outside. Because of the insulating glazing, the average temperature of the thermal mass can be significantly above the average outdoor temperature. If the glazing insulates well enough, and outdoor temperatures are not too low, the average temperature of the thermal mass will be significantly higher than room temperature, and heat will flow into the house interior.
Modern Trombe walls have vents added to the top and bottom of the air gap between the glazing and the thermal mass. Heated air flows via convection into the building interior. The vents have one-way flaps which prevent convection at night, thereby making heat flow strongly directional. This kind of design is an isolated passive thermal collector. By moving the heat away from the collection surface, it greatly reduces thermal losses at night and improves overall heat gain. Generally, the vents to the interior are closed in summer months when heat gain is not wanted.
Common modifications to the Trombe wall include:
- Exhaust vent near the top that is opened to vent to the outside during the summer. Such venting makes the Trombe wall pump fresh air through the house during the day, even if there is no breeze.
- Windows in the trombe wall. This lowers the efficiency but may be done for natural lighting or aesthetic reasons. If the outer glazing has high ultraviolet transmittance and the window in the trombe wall is normal glass this allows efficient use of the ultraviolet light for heating while protecting people and furnishings from ultraviolet radiation when compared to using windows with high ultraviolet transmittance.
- Electric blowers controlled by thermostats, to improve air and heat flow.
- Fixed or movable shades, which can reduce nighttime heat losses.
- Trellises to shade the solar collector during summer months.
- Insulating covering used at night on the glazing surface.
- Tubes, pipes or water tanks as part of a solar hot water system.
- Fish tanks as thermal mass.
- Using a selective surface to increase the absorption of solar radiation by the thermal mass.
