Trombe Wall

Trombe Walls

A Trombe wall is a passive solar energy capture device built into the side of the building which faces the winter sun. The Trombe Wall has a glass outside surface and a high mass layer inside the wall, separated by a layer of air.

Trombe walls

Trombe walls

UV Spectrum heat passes through the outside glass of the Trombe Wall and is absorbed by the interior mass of the wall which goes on to re-radiate it in the infra red spectrum. Infrared heat does not pass so easily back through the glass of the Trombe Wall and dissipates instead through the inside surface into the building. Trombe Wall constructions are usually used to absorb sunlight in the winter and then to release the heat overnight into the building. A Trombe Wall is therefore similar to a storage heaterbut the Trombe Wall uses free sunlight to heat the mass rather than electricity. Edward S Morse identified the basic idea that makes a Trombe Wall work in 1881. But it took until the 1960s before French architect Jacques Michel and engineer Felix Trombe arrived at a fully functional design. It works on the greenhouse principle. Short wave high energy ultraviolet radiation passes through the Trombe Wall glass easily. But when the light reaches the dense mass in the Trombe Wall the energy is absorbed and re-emitted in the form of long wavelength infra red energy which can’t easily pass through the air layer and the glass. The energy is therefore trapped in the Trombe Wall and eventually dissipates into the building. The warm air in the wall rises and escapes through vents at the top and draws in cooler air from the vents at the bottom to replace it. In the evening the one way vent at the bottom closes to prevent backflow and the heat in the wall radiates into the living area. Readers can imagine for themselves how much energy a Trombe Wall might be able to trap in this way by thinking about how hot the inside of a car can become when left parked on a sunny day, even in the winter and a car is not even designed for the heat collection purposes.

Design

Trombe walls

Trombe walls

The success of a Trombe Wall in capturing heat depends on the same factors as heat capture in anything else (including the leather seats of a car). Trombe Wall heat capture depends on, colour, the angle of the Trombe Wall to the sun. How reflective the surface of the Trombe Wall is, the thermal mass and the conductivity of the materials used to build the Trombe Wall. For a Trombe Wall to work best it needs to be made from high thermal mass materials (for example water or concrete), and the Trombe Wall has to face the sun. Clear but non-reflective glass at the front of the Trombe Wall allows for more heat to enter and a dark matt surface exposed on the part of the Trombe Wall, which faces the sun, will absorb the most heat.

The type of wall we refer to has a Trombe Wall however has been in existence in a more rudimentary form for millennia. Any thick dense wall acts like a Trombe Wall. We are all aware that thick, dense walls keep us cool in the summer and warm in the winter. Modern Trombe Wall design however is more sophisticated and fits in with modern integrated energy efficiency practices.

A Trombe wall may be constructed with or without internal vents. A non vented Trombe Wall uses all the heat trapped between the glass and the internal wall to heat up the thermal mass for release later. A vented Trombe Wall allows the occupant to release the heat as required and use it for any purpose. A vented Trombe Wall uses controllable flaps to prevent convection of the heat into directions where it is not required. A Trombe Wall can even be designed to allow for heat to be diverted to the outside on hot summer nights. There is some debate amongst the ‘passive solar community’ as to which type is best. A vented Trombe Wall has considerable advantages over a non vented one. They are able to shed heat early in the evening when the household needs it. A non vented Trombe Wall does however have the advantage of maintaining improved diurnal stability overall but much of that benefit can be obtained with a vented wall. And a vented Trombe Wall is usually more practicable for use as a usable source of energy in one form or another. A general guide to be applied when designing dense masonry walls is the rule that heat is lost and absorbed at about an inch per 2 hours. So a six inch thick wall will absorb and lose its heat in twelve hours.

Trombe walls A Trombe Wall can incorporate many features. Typically a Trombe Wall can include an exhaust vent near the top that can be opened during the summer. This type of venting makes the Trombe Wall act as a solar chimney pumping fresh air around the house during the days when there isn’t a breeze. Windows in a Trombe Wall lowers its efficiency but may need to be present for lighting and to improve the appearance. But the different properties of glass can be applied to different parts of the Trombe Wall. If the outer glazing of the Trombe Wall is made of glass designed for high Ultra Violet transmittance whilst the windows are made of standard glass, high heat absorbtion can be achieved whilst still protecting the furnishings in the room from excessive ultraviolet radiation from the sun and heat. A Trombe Wall can incorporate electric thermostatically controlled blowers to improve heat and air flow, movable shades to reduce heat loss at night and trellises to shade the solar collector in the summer. A Trombe Wall can also incorporate insulation coverings to be used at night on the outside surfaces, water tanks and tubes to power the hot water system, and even fish tanks can be used to provide the thermal mass.

Half Height Trombe Wall

A Trombe Wall does not have to be full height. They can be built to whatever height is required and to suit the residence. Building a Trombe Wall at half height is popular because they greatly enhance the passive storage capacity of the home whilst being very easy to construct. A Half height Trombe Wall is also reasonably easy to incorporate into local architecture. A Trombe Wall is particularly suited to parts of the Third World where availability of any heating at all is regarded as a luxury. In parts of India the traditional fuel is dung. Yet they have 320 days of sunshine a year.  Burning dung in the living room is not the ideal first choice as a sustainable heating option so a Trombe Wall made from the local mud and brick offers better relief from the smoke and the cold.