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A Trombe wall is a passive solar building technique where a wall is built on the winter sun side of a building with a glass external layer and a high heat capacity internal layer separated by a layer of air. Heat in close to UV spectrum passes through the glass almost unhindered then is absorbed by the wall that then re-radiates in the far infrared spectrum which does not pass back through the glass easily, hence heating the inside of the building. Trombe walls are commonly used to absorb heat during sunlit hours of winter then slowly release the heat over night. The essential idea was first explored by Edward S. Morse and patented by him in 1881. In the 1960s it was fully developed as an architectural element by French engineer Félix Trombe and architect Jacques Michel.
Trombe walls work on the basic greenhouse principle that heat from the sun in the form of near-visible shorter-wavelength higher-energy ultraviolet radiation passes through glass largely unimpeded. When this radiation strikes objects the energy is absorbed and then re-emitted in the form of longer-wavelength infra-red radiation that does not pass through glass as readily. Hence heat becomes trapped and builds up in an enclosed structure with high internal heat capacity and glass surfaces that face the sun
How effectively objects absorb and shed radiant heat depends on a number of factors; how dark they are in colour, how directly the surface of the object is opposed to the angle of the radiation striking it, how matt or reflective its surface is, the heat capacity of the object, the surface conductivity of the object. For Trombe walls to work effectively they are made from high heat capacity materials such as concrete or water, whose surface is dark and matt in colour and placed in direct opposition to the sun striking them.
The clearer the glass in front of a Trombe wall appears in the UV spectrum the more short wave radiation will penetrate and the more reflective or non transparent the glass appears in the infra-red spectrum the less re-emitted heat will be escape. 
Trombe walls may be constructed with or without internal vents. Non-vented walls rely on conduction through the wall to heat the space behind the wall, while vented walls allow the user to actively or passively circulate room air past the heated side of the wall for more immediate heating. Vented Trombe walls may use passively or actively controllable flaps to prevent convection in the undesired direction, as when the wall cools at night in winter or heats during the day in summer. In climates that have higher summer temperatures Trombe walls may also be designed with external vents to improve the shedding of heat at night. 
Vented walls offer the advantage of being able to shed more heat earlier in the evening when it is more commonly required while higher heat capacity non-vented walls offer the advantage of improved overall diurnal stability. Views differ among the passive solar community as to which is more advantageous.
A simplistic rule of thumb that is often used when designing dense masonry walls, is that heat will be absorbed and lost at around an inch per 2 hours. i.e a 4inch (100mm) thick masonry wall will absorb and shed its heat load in around 8 hours and a 8inch (200mm) thick wall in around 16 hours.
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. At the same time, it protects people and furnishings from ultraviolet radiation more than do windows with high ultraviolet transmittance.
- Electric blowers controlled by thermostats, to improve air and heat flow.
- Fixed or movable shades, which can reduce night-time 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 added heat capacity.
- Using a selective surface to increase the absorption of solar radiation by the wall.
Half height Trombe walls
There is a misconception that Trombe walls must be full height, completely blocking light and direct solar gain into the adjacent living space rather converting it into absorbed radiant heat that is then re-emitted as the room cools. In reality Trombe walls can be built to whatever height suits the needs of the home owner, their reduced height simply reducing the solar absorption area and increasing direct light and heat gain area. Half height Trombe walls are a relatively simple solution that can greatly enhance the solar storage capacity of a passive solar home, whilst still allowing for views to the sun's winter direction.
Half height Trombe walls function in the same way as a full height wall. They are commonly constructed around 4-6" 100-150mm from the inner window surface, allowing a gap large enough for curtains or blinds to reduce heat loss on winter nights and heat gain on summer days.
Application in developing regions
In Ladakh, India, the Ladakh Project is designing Trombe walls that complement Ladakh's 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. The traditional fuel, dung, 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. Ladakh receives about 320 days of sun annually, and the traditional building materials — stone and mud brick — provide the heat capacity needed for heat storage in a Trombe wall.
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