Passive Solar Heating | Trombe Wall
Written by: Atharva Modi
The Trombe wall is a passive solar heating method which uses direct solar radiation to supply the buildings heating needs. It is one of the cheapest and most Eco-friendly ways to reduce your building heating loads. Its particularly useful for remote locations with an abundance of sunlight. To put it bluntly, it involves trapping the solar radiation using dark colored walls and external glazing, the heat from which is transferred to the space within. But as lack-lustrous it might seem, there is a little more than meets the eye. Lets introduce ourselves to a bit of history here.
Th first concept of a Trombe wall was patented by an American engineer, Edward Morse in 1887. But the system as we know it today was devised by Professor Felix Trombe and Architect Jacques Michel in 1960s, hence the name. Passive solar heating methods had two major phases of innovation — during the housing boom at the end of WWII (due to wartime fuel shortages) and during the oil crisis in the 1970s. The first modern use of the Trombe wall was done in 1967 at Trombe House in Odeillo, France.
The structure of the Trombe wall includes a dark colored concrete(mainly, but brick, stone or adobe is used also) wall placed about 4 inches from an external glazing (a thin glass layer). The dark color is used to improve the efficiency or ‘blackness’ of the wall body. As the wall is the body that absorbs and conducts the solar radiation, one might the doubt the value of the glazing. Once the wall is heated it radiates significant energy back to surroundings but courtesy to the Wiens Displacement law, this back-radiation occurs at longer wavelengths (Remember the Greenhouse effect?). The glass glazing does not allow these longer wavelengths to pass through and traps the heat. Glass transmits the lower wavelength incident radiation effectively but blocks the longer wavelengths. This same concept is used in solar thermal power generation as well, to retain the concentrated radiation. Thus the absorbed heat conducts through the wall to heat the space inside. As the concrete wall acts a thermal mass, the heat stored in the walls during daytime is used for heating during the night. To put it into perspective, a 35 cm dark painted concrete wall absorbs about half the energy incident on it and conducts the rest. Hence, one might deduce correctly that the material, thickness and colors can significantly affect the efficiency of the Trombe wall. A more efficient way to get hold of the trapped heat is to have vents at the top and bottom of the concrete wall. The cooler air from the living space enters the enclosed space between the glazing and wall through the bottom vent and heats up to move back into the living space through the upper vent, thus establishing a natural convection current.
Throughout all this, remember that the sun doesn’t remain in the same place throughout the day nor does it maintain the same cycle throughout seasons. Unless you are filthy rich, an automated tilting wall is safely out of the question. As a general rule, again from solar panels, buildings in the Northern hemisphere have their Trombe walls facing South, towards the equator. This ensure maximum incidence of solar radiation. If you visualize the tilted earth revolving around the sun, you can confirm this makes sense. To prevent overheating during the summer, one can use roof overhangs, deciduous trees in order to block excess solar radiation. Some cooling effects may also be generated by slightly modifying the vent system mentioned earlier.
Since its use in 1967, there have been numerous variations of the Trombe wall. ‘Drum wall’ developed by Steve Bare, involves storing water in dark painted barrels and using that as the thermal mass as water stores higher density of thermal energy. A metal foil can be placed over the wall to improve the absorptivity and also prevents the heat from going back to the glass. But this barrier needs to be removable according to the seasons. Architects can also use decorative glass glazing as it does not affect its transmittance. Trombe walls can reduce heating loads up to 30 percent in optimum conditions. A study by the National Renewable Energy Laboratory of the Zion National Park revealed that the Trombe walls reduced their heating loads by about 20 percent.
Trombe wall and similar passive solar heating methods are no strangers to northern parts of India. Organizations like the Geres association and the Ladakh Ecological Development Corporation have been working for years in those regions promoting and assisting with building passively heated structures. If you are interested in delving in this deeper, check out some of the work by Dr. Marwa Dabaieh on passive heating and cooling methods.
References
Medici Piero, 2018. ‘The Trombe Wall during the 1970s: technological device or architectural space?’. SPOOL, Vol. 04, Issue. 02
https://en.wikipedia.org/wiki/Trombe_wall
https://energyeducation.ca/encyclopedia/Trombe_wall#cite_ref-Article1_3-0
https://www.archdaily.com/946732/how-does-a-trombe-wall-work