By Bud Coburn
A typical Trombe wall is a south-facing structure (in the Northern hemisphere), 4 to 16 inches (10 to 41 cm) thick, made of stone, concrete or adobe, with a dark, heat-absorbing material on the exterior surface that absorbs scant rays from the southerly winter sun. Heat loss to the outdoors is minimized through the implementation of a single or double layer of glass raised ¾-inch to 2 inches (2 to 5 cm) from the masonry wall, which creates an air space from which warm air gradually conducts inward through the masonry. Since this diffusion of heat is slow, the interior wall does not begin radiating heat into the interior until late afternoon or early evening, when ordinary windows are no longer capable of allowing direct solar heating. Specifically, heat travels through a masonry wall at an average rate of 1 inch per hour, which means that the heat absorbed on the outside of an 8-inch-thick concrete wall at noon will enter the interior living space by around 8 p.m. This time lag, combined with a reduction of temperature variations, allow the use of variable daytime solar energy as a more consistent night-time source of heat.
Advantages of Trombe Walls
- They provide comfortable heat. Rooms heated by Trombe walls often “feel more comfortable than those heated by forced air because of the large, warm surface providing radiant comfort,” according to the National Renewable Energy Laboratory.
- The system is passive. It has no moving parts and requires almost no maintenance.
- They are based on simple and inexpensive construction. Trombe walls are relatively easy to incorporate into an existing building structure or into new construction. Materials include masonry and glass, which are inexpensive.
- They can significantly reduce heating bills, especially as conventional heating becomes less used or needed.
The only real complication caused by a Trombe wall is that it can become a source of heat loss during extended overcast days. Insulation can be added between the collector space and the wall to address this problem.
Common modifications to the Trombe wall include the following:
- An exhaust vent near the top can be installed to vent to the outside during the summer. This venting increases indoor air quality by pumping fresh air through the house during the day, even if there is no breeze.
- Windows installed in a Trombe wall may reduce the wall’s efficiency, but they may be installed for lighting or aesthetic reasons. Electric blowers controlled by thermostats can improve air and heat flow.
- Fixed or movable shades or insulative covering can be installed to reduce night-time heat loss.
- Trellises are sometimes installed to shade the solar collector during summer months.
- Tubes or water tanks can be integrated into the wall as part of a solar hot-water system.
- The application of eutectic salts to the wall can add significantly to its stored energy.
- Exterior glass can be patterned to limit the exterior visibility of the dark masonry wall without sacrificing transmissivity.