The principle of thermosiphoning - the natural buoyancy
of heated air or water is used to circulate heat - in a Trombe-wall is
an example of a soft-technology approach to solar heating. With this technique,
developed in the sixties under the direction of Professor Felix Trombe
at the Centre National de la Recherche Scientifique in Odeillo (France),
sunlight is absorbed on an external dark-coloured surface protected by
a glazing system. The technique provides winter space heating and can
be used to draw cool air through the house in the hot season.
 Illustration of a Trombe-wall (from: Bruce Anderson - The Solar Home Book)
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The classic 'mass Trombe-wall' makes use of a massive wall which can store several hours of sun energy falling on the wall. During daytime heating is provided by circulating air through openings at the top and the bottom of the wall. This hot air, collected between the glazing and the wall, rises by natural convection, drawing cool room air in through the bottom vent. Thus the system circulates warm air around the room, passing its heat on to the interior. A reverse flow valve is used to prevent reverse flow after dark when it is cold.
The heat collecting capacity of the heavy wall itself is used for heating during the night-time. As it takes several hours to heat up this wall and to transmit its energy through the room, by the time of night it functions as a radiator.
During hot weather the Trombe-wall can be used to prevent heat entering the room by bringing the air moving up the wall directly to the exterior. In that case the storage wall functions as a natural convection cooler, drawing cooler air from the north side into the space.
A number variations of the mass Trombe-wall are developed to improve its performance. They include insulation, improved absorber surfaces, fans to regulate the flow of air and other capacitors like water, like in the water Trombe-wall. The thermal storage capacity (heat capacity) of water is greater than that of concrete or bricks. The use of water also ensures a lower surface temperature of the collector and so enhances the performance of the system.
A disadvantage of the system is its inflexibility. Trombe-walls are often in conflict with the requirements for view and access and may therefore be difficult to implement in a design.
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