A Trombe wall is a massive equator-facing wall that is painted a dark color in order to absorb thermal energy from incident sunlight and covered with a glass on the outside with an insulating air-gap between the wall and the glaze. A Trombe wall is a passive solar building design strategy that adopts the concept of indirect-gain, where sunlight first strikes a solar energy collection surface which covers thermal mass located between the Sun and the space. The sunlight absorbed by the mass is converted to thermal energy (heat) and then transferred into the living space.
Trombe walls are also named mass walls, solar wall, or thermal storage wall. However, due to the extensive work of professor Félix Trombe and architect Jacques Michel in the design of passively heated and cooled solar structure, they are often called Trombe Walls.
This system is similar to the air heater (as a simple glazed box on the south wall with a dark absorber, air space, and two sets of vents at top and bottom) created by professor Edward S. Morse a hundred years ago.
In 1920s, the idea of solar heating started in Europe. In Germany, housing projects were designed to take advantage of the sun. The research and accumulated experience with solar design then spread across the Atlantic by architects like Walter Gropius and Marcel Breuer. Besides from these early examples, the heating of homes with the sun made slow progress until the 1930s, when several different American architects started to explore the potential of solar heating. The pioneering work of these American architects, the influence of the immigrant Europeans, and the memory of the wartime fuel shortages made solar heating very popular during the initial housing boom at the end of World War II.
Later in 1970s, before and after the international oil crisis of 1973, some European architectural periodicals were critical of standard construction methods and the architecture of the time. They described how architects and engineers reacted to the crisis, proposing new techniques and projects in order to intervene innovatively in the built environment, using energy and natural resources more efficiently. Moreover, the depletion of natural sources generated interest in renewable energy sources, such as the sun. Also, parallel to the global population growth, energy consumption and environmental issues become a global concern especially while the building sector is consuming the highest energy in the world and most of the energy is used for heating, ventilation and air condition systems. For these reasons, the buildings today are expected to achieve both energy efficient and environmental-friendly design by using renewable energy partly or completely instead of fossil energy for heating and cooling and solar energy that utilize solar radiation from the sun. In this direction, the integration of passive solar systems in buildings is one strategy for sustainable development and increasingly encouraged by international regulations.
Today's low-energy buildings with Trombe walls often improve on the ancient technique that incorporates a thermal storage and delivery system people have used: thick walls of adobe or stone to trap the sun's heat during the day and release it slowly and evenly at night to heat their building. Today, the Trombe wall continues to serve as an effective strategy of passive solar design. The well-known example of the Trombe wall system is first used in the Trombe house in Odeillo, France in 1967. The black painted wall is constructed of approximately 2 feet thick concrete with an air space and a double glazing on its exterior side. The house is primarily heated by radiation and convection from the inner surface of the concrete wall and the results from studies show that 70% of this building’s yearly heating needs are supplied by solar energy. Therefore, the efficiency of the system is comparable to a good active solar heating system.
Then another passive collector-distributor Trombe Wall system is built in 1970, in Montmedy, France. The house with 280 m³ living space required 7000 kWh for space heating annually. At Montmedy-between 49° and 50° North latitude-5400 kWh were supplied by solar heating and the remainder from an auxiliary electrical system. The annual heating cost for electricity was approximately $225 when compared to an estimated $750 for a home entirely heated by electricity in the same area. This yields to a 77% reduction in heating load and a 70% reduction in the cost for winter heating requirement.
In 1974, the first example of Trombe wall system is used in the Kelbaugh House in Princeton, New Jersey. The house is located along the northern boundary of the site to maximize the unshaded access to available sunlight. The two-story building has 600 ft² of thermal storage wall which is constructed of concrete and painted with a selective black paint over a masonry sealer. Although the main heating is accomplished by radiation and convection from the inner face of the wall, two vents in the wall also allow daytime heating by the natural convection loop. According to data collected in the winters of 1975-1976 and 1976-1977, the Trombe wall system reduced the heating costs respectively by 76% and 84%.
Unlike an active solar system that employs hardware and mechanical equipment to collect or transport heat, Trombe wall is a passive solar-heating system where the thermal energy flows in the system by natural means such as radiation, conduction, and natural convection. As a consequence, the wall works by absorbing sunlight on its outer face and then transferring this heat through the wall by conduction. Heat conducted through the wall is then distributed to the living space by radiation, and to some degree by convection, from the wall’s inner surface.
The greenhouse effect helps this system by trapping the solar radiation between the glazing and the thermal mass. Heat from the sun, in the form of shorter-wavelength radiation, passes through the glazing largely unimpeded. When this radiation strikes the dark colored surface of the thermal mass facing the sun, the energy is absorbed and then re-emitted in the form of longer-wavelength radiation that cannot pass through the glazing as readily. Hence heat becomes trapped and builds up in the air space between the high heat capacity thermal mass and the glazing that faces the sun.
Another phenomenon that plays role in the Trombe wall’s operation is the time lag caused by the heat capacity of the materials. Since Trombe walls are quite thick and made of high heat capacity materials, the heat-flow from the warmer outer surface to inner surface is slower than other materials with less heat capacity. This delayed heat-flow phenomenon is known as time lag and it causes the heat gained during the day to reach the interior surface of the thermal mass later. This property of the mass helps to heat the living space at the evenings as well. So, if there is enough mass, the wall can act as a radiant heater all night long. On the other hand, if the mass is too thick, it takes too long to transmit the thermal energy it collects, thus, the living space does not receive enough heat during the evening hours when it is needed the most. Likewise, if the thermal mass is too thin, it transmits the heat fast, resulting in overheating of the living space during the day and little energy left for the evening. Also, the Trombe walls using water as a thermal mass collect and distribute heat to a space in the same way, but they transfer the heat through the wall components (tubes, bottles, barrels, drums, etc.) by convection rather than by conduction and the convection performance of the water walls differs according to their different heat capacities. Larger storage volumes provide a greater and long-term heat storage capacity, while smaller contained volumes provide greater heat exchange surfaces and thus faster distribution.
Trombe walls are often designed to serve as a load-bearing function as well as to collect and store the sun’s energy and to help enclose the building’s interior spaces. The requirements of a Trombe Wall are glazing areas faced toward the equator for maximum winter solar gain and a thermal mass, located 4 inches or more directly behind the glass, which serves for heat storage and distribution. Also, there are many factors, such as color, thickness, or additional thermal control devices that have an impact on the design and the effectiveness of Trombe Walls. Equatorial, which is Southward in the Northern Hemisphere and Northward in the Southern Hemisphere, is the best rotation for passive solar strategies because they collect much more sun during the day than they lose during the night, and collect much more sun in the winter than in the summer.
The first design strategy to increase the effectiveness of Trombe Walls is painting the outside surface of the wall to black (or a dark color) for the best possible absorption of sunlight. Moreover, a selective coating to a Trombe wall improves its performance by reducing the amount of infrared energy radiated back through the glass. The selective surface consists of a sheet of metal foil glued to the outside surface of the wall and it absorbs almost all the radiation in the visible portion of the solar spectrum and emits very little in the infrared range. High absorbency turns the sunlight into heat at the wall's surface, and low emittance prevents the heat from radiating back towards the glass.
Although the Trombe walls are usually made of solid materials, such as concrete, brick, stone, or adobe, they can also be made of water. The advantage of using water as a thermal mass is that water stores considerably more heat per volume (has a greater heat capacity) than masonry. The developer of this water wall, Steve Bare, names this system “Drum Wall”. He painted the steel containers similar to oil drums and filled them almost full of water, leaving some room for the thermal expansion. Then stacked the containers horizontally behind an equator-facing double glazing with the blackened bottoms facing outside. This water wall involves the same principles as the Trombe walls but employs a different storage material and different methods of containing that material. Like the dark colored thermal mass of the Trombe walls, the containers that store the water are also frequently painted with dark colors to increase their absorptivity, but it is also common to leave them transparent or translucent to allow some daylight to pass through.
Another critical part of Trombe wall design is choosing the proper thermal mass material and thickness. The optimum thickness of the thermal mass is dependent on the heat capacity and the thermal conductivity of the material used. There are some rules to follow while sizing the thermal mass.
- The optimum thickness of a masonry wall increases as the thermal conductivity of the wall material increases. For instance, to compensate a rapid heat transfer through a highly conductive material, the wall needs to be thicker.
Accordingly, since the thicker wall absorbs and stores more heat to use at night, the efficiency of the wall increases as the conductivity and thickness of the wall increase.
There is an optimum thickness range for the masonry materials.
The efficiency of the water wall increases as the thickness of the wall increases. However, it is hard to notice a considerable performance increase as the walls get thicker than 6 inches. Likely, a water wall thinner than 6 inches is also not enough to act as a proper thermal mass that stores the heat during the day.
In the early Trombe wall design, there are vents on the walls to distribute the heat by natural convection (thermocirculation) from the exterior face of the wall, but only during the daytime and early evening. Solar radiation passing through the glass is absorbed by the wall heating its surface to temperature as high as 150 °F. This heat is transferred to the air in the air space between the wall and the glass. Through openings or vents located at the top of the wall, warm air rising in the air space enters the room while simultaneously drawing cool room air through the low vents in the wall. In this way additional heat can be supplied to the living space during periods of sunny weather. However, it is now clear that the vents do not work well in either summer or winter. It becomes more common to design a half Trombe Wall then combine it with a direct gain system. The direct gain part delivers heat early in the day while the Trombe wall stores heat for the nighttime use. Moreover, unlike a full Trombe wall, the direct gain part allows views and the delight of winter sunshine.
To minimize the possible drawbacks of the Trombe wall system, there are additional thermal control strategies to employ to the wall design. For instance, the minimum 4-inch distance between the glass and the mass allows cleaning the glazing and the insertion of a roll-down radiant barrier as needed. Adding a radiant barrier or night insulation between the glazing and the thermal mass reduces nighttime heat losses and summer daytime heat gains. However, to prevent overheating in summers, combining this strategy with an outdoor shading device like shutter, a roof overhang, or an interior shading to block excessive solar radiation from heating the Trombe wall would be the best. Another strategy helps to benefit from the solar collection without some of the drawbacks of the Trombe walls is to use exterior mirror-like reflectors. The additional reflected area helps Trombe walls to benefit more from the sunlight with the flexibility of removing or rotating the reflector device if the solar collection is undesired.
When three different Trombe wall facades with single glass, double glass, and an integrated semi-transparent PV module are compared in hot and humid climate, the single glass provides the highest solar radiation gain due to its higher solar heat gain efficiency. However, it is recommended to use the single glass with a shutter for the evening and night times, to offset its heat losses. High transmission glazing maximizes solar gains of the Trombe wall while allowing to recognize the dark brick, natural stones, water containers, or another attractive thermal mass system behind the glazing as well. However, from an aesthetics perspective, sometimes it is not desirable to distinguish the black thermal mass. As an architectural detail, patterned glass can be used to limit the exterior visibility of the dark wall without sacrificing transmissivity.