Trombe wall

A Trombe wall is a sun-facing wall built from material that can act as a thermal mass (such as stone, metal, concrete, adobe or water tanks), combined with an air space, insulated glazing and vents to form a large solar thermal collector.

Edward Morse patented the design in 1881 (US Patent 246626), but it wasn't until 1964 that the design was popularized by the engineer Felix Trombe and architect Jacques Michel following the construction of a passive solar house using the principle in Odeillo, France.

The Trombe wall concept

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.

In the original design, very little of the received heat ends up in the interior and most is lost to the environment at night, because resistance to heat flow between the collector surface and the interior is the same in both directions.

Current basic design

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.

Nighttime thermal losses through the thermal mass can still be significant. The modern design can be still further improved by insulating the thermal mass from the collection surface. The insulation greatly reduces nighttime heat losses at the cost of small reductions in daytime heat gain.

Common variations

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 act as a solar chimney pumping 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 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.

Application in developing regions

In Ladakh, India, the Ladakh Project is designing Trombe walls that complement [http://www.trekearth.com/gallery/Asia/India/photo226032.htm Ladakh's beautiful traditional architecture] and has promoted building them in Ladakhi homes. This has shown Ladhakis a clean, reliable alternative to fire as a source of heat. Dung, the traditional fuel, burns poorly and offers poor relief from the bitter winter temperatures. The smoldering dung produces significant amounts of smoke that fouls the air and causes many health problems. Trombe walls offer relief from both the cold and the smoke. [ [http://www.cs.org/publications/csq/csq-article.cfm?id=538 Trombe walls in Ladakh] ] Ladakh, India receives about 320 days of sun annually, and the traditional building materials - stone and mud brick - provide the thermal mass needed for heat collection in a Trombe wall.

The Druk White Lotus School uses Trombe walls and as part of "a model of appropriate design and development" [ [http://www.dwls.org/Overview-Of-Awards.html Overview of Awards page"] ] in Ladakh.

ee also

*List of pioneering solar buildings
*Barra system
*Solar chimney

External links

* [http://www.dwls.org Druk White Lotus School website] including [http://www.dwls.org/Sustainable-Design-Examples.html Trombe wall example] .
* [http://www.nrel.gov/docs/fy04osti/36277.pdf Trombe Walls] -- NREL page extolling Trombe walls, with no reference to heat loss issues.

References