Trombe wall

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Introduction

Since ancient times, 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 buildings. Today's low-energy buildings often improve on this ancient technique by incorporating a thermal storage and delivery system called a Trombe wall. It is a wall with high thermal mass used to store solar energy passively in a solar home. It is named after the French inventor, Felix Trombe, who popularized the design in 1964 although Edward Morse had patented it back in 1881.


Brief Description

A Trombe wall consists of a vertical wall, built of a material such as stone, concrete, or adobe, which is covered on the outside with glazing. Sunlight passing through the glazing generates heat which conducts through the wall. Warm air between the glazing and the Trombe wall surface can also be channeled by natural convection into the building interior or to the outside, depending on the building's heating or cooling needs.

During the day, sunlight shines through the glazing and hits the surface of the thermal mass, warming it by absorption. The air between the glazing and the thermal mass warms (via heat conduction) and rises, taking heat with it (convection). The warmer air moves through vents at the top of the wall and into the living area while cool air from the living area enters at vents near the bottom of the wall.

At night, a one-way flap on the bottom vent prevents backflow, which could act to cool the living area, and heat stored in the thermal mass radiates into the living area. These vents are an addition to the original Trombe wall design, which relied entirely on conduction through the thermal mass to transport heat to the living area. In the original design, the majority of the heat collected radiates back through the glazing at night or on an overcast day. This problem is best addressed by adding insulation between the collector space and the thermal mass, and arranging for the thermal mass to be heated by the air circulating through the collector space via the one-way flaps. This change avoids the massive loss of heat at night or on overcast days. Modern passive solar design emphasizes the separation of collectors and thermal masses.


Trombe Wall Design and Construction

A typical unvented Trombe wall consists of a 4- to 16-in (10- to 41-cm)-thick, south facing masonry wall with a dark, heat-absorbing material on the exterior surface and faced with a single or double layer of glass. The glass is placed from ¾ to 2 in. (2 to 5 cm) from the masonry wall to create a small airspace. Heat from sunlight passing through the glass is absorbed by the dark surface, stored in the wall, and conducted slowly inward through the masonry. High transmission glass maximizes solar gains to the masonry wall. As an architectural detail, patterned glass can limit the exterior visibility of the dark concrete wall without sacrificing transmissivity.

Applying a selective surface 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. 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 light into heat at the wall's surface, and low emittance prevents the heat from radiating back towards the glass.

For an 8-in-thick (20-cm) Trombe wall, heat will take about 8 to 10 hours to reach the interior of the building. This means that rooms receive slow, even heating for many hours after the sun sets, greatly reducing the need for conventional heating. Rooms heated by a Trombe wall often feel more comfortable than those heated by forced air because of the large warm surface providing radiant comfort.

Architects can use Trombe walls in conjunction with windows, eaves, and other building design elements to balance solar heat delivery. Strategically placed windows allow the sun's heat and light to enter a building during the day to help heat the building with direct solar gains. At the same time, the Trombe wall absorbs and stores heat for evening use. Properly sized roof overhangs shade the Trombe wall during the summer when the sun is high in the sky. Shading the Trombe wall can prevent the wall from getting hot during the time of the year when the heat is not needed.


Trombe Wall Efficiency

Trombe walls are thermal disasters during long strings of cloudy days. When the sun goes in for a week or two, they lose their stored heat in less than a day, and then leak house heat badly, dramatically raising backup heat or other solar thermal storage requirements. A lot of people are apparently still willing to settle for high-cost, low-performance passive solar house heating techniques, that get them a 30% yearly savings in backup space heating costs over a 20 year payback period, vs. warm-stores, solar closets, sunspaces and transparent siding, which really can save close to 100% of the space heating energy needed for a house and provide close to 100% of the hot water needed for a house, as well, also, too, over the top, with a payback period of a year or two.


Advantages

o Comfortable Heat: Radiates in the infra red, which is more penetrating and pleasant than traditional convective forced air heating systems.

o Passive: No moving parts and essentially no maintenance.

o Simple Construction: Relatively easy to incorporate into building structure as an internal or external wall. Materials (masonry, concrete) are relatively inexpensive.

o Effective: Can reduce heating bills by large amounts.


Disadvantages

o Extended Overcast: Exterior walls become a heat loss source during extended overcast days. Not a problem for interior Trombe wall.


References:

http://www.nrel.gov/docs/fy04osti/36277.pdf

http://www.daviddarling.info/encyclopedia/T/AE_trombe_wall.html

http://www.allanstime.com/SolarHome/Trombe_Wall/index.html

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