The Trombe Wall: Heating Buildings with Sunlight

On a hillside at Odeillo, in the French Pyrenees, stands a row of houses whose southern faces are almost entirely dark glass. They were built in the 1960s by the engineer Félix Trombe and the architect Jacques Michel, and they heat themselves, in large part, with nothing more elaborate than a wall. The idea was not strictly new — a patent for a glazed, sun-warmed wall had been granted to the American inventor Edward Morse as early as 1881, and promptly forgotten — but it was Trombe and Michel who turned the curiosity into a working system, and it is Trombe's name the design now bears. The Trombe wall belongs to the family of 'passive' solar designs, which are distinguished from solar panels and heat pumps by having no moving parts, no fluids to circulate and nothing that draws electricity: the building itself is the machine.

The construction is disarmingly simple. On the side of the building that faces the winter sun — the south side, in the northern hemisphere — a massive wall is built of brick, stone or concrete, materials chosen for their capacity to store large quantities of heat. Its outer face is coated with matt black paint so that it absorbs as much sunlight as possible, and a sheet of glazing, of single or double glass, is fixed a few centimetres in front of it, sealing off a narrow cavity of trapped air. Rows of vents pierce the wall near the ceiling and near the floor, connecting this cavity to the room behind.

During a winter day the wall works as a slow engine of circulation. Sunlight passes through the glazing and strikes the black surface, which absorbs the radiation; the glass, meanwhile, prevents the resulting warmth from escaping — the same trapping of heat that makes the inside of a parked car so uncomfortable in summer. The air in the cavity can reach sixty degrees Celsius. Being hot, it rises, and streams into the room through the upper vents, while cooler air near the floor of the room is drawn out through the lower vents into the bottom of the cavity to be heated in its turn. The result is a continuous convective loop — engineers call it a thermosiphon — that carries warmth into the building all afternoon without a single fan.

The wall's second act begins at sunset. Masonry conducts heat slowly: warmth absorbed at the outer face takes several hours to creep through to the inner one, and this time lag is precisely what makes the design valuable. Heat collected at midday reaches the interior surface in the evening, just as the outside temperature is falling and the rooms most need it, and the wall then radiates gently into the room for much of the night. Two precautions are essential. The vents must be closed after dark; otherwise the circulation reverses, and air chilled against the cold glass sinks and pours back into the room. In careful designs an insulated shutter is also lowered over the glazing at night, cutting the heat that would otherwise be lost back through the glass. Summer brings the opposite problem, and the classic remedy is architectural rather than mechanical: a projecting roof overhang above the glazing shades the wall from the high summer sun while leaving it exposed to the low sun of winter, and additional vents at the top of the cavity can be opened to the outside so that unwanted hot air is dumped there rather than indoors.

How well does it all work? The honest answer is that it depends where you build. In regions where winter skies are reliably clear — mountain plateaux, high deserts, much of the Mediterranean — a well-proportioned Trombe wall can supply a substantial share of a house's winter heating, and studies of the Odeillo houses themselves reported savings of that order. In persistently cloudy regions the case is far weaker, for a wall that receives little sun spends much of the winter as nothing more than a poorly insulated slab of cold masonry. Nor is money the design's real attraction: a Trombe wall is not especially cheap to build, and its appeal lies less in economy than in simplicity and endurance — there is nothing to break down, nothing to service and nothing to replace. It will never match the glamour of the photovoltaic roof, but as a means of letting a building heat itself, quietly and for decades, it has few rivals.