Solar Wall (or Trombe Wall)

This page provides plans for a simple solar wall collector for space heating. These walls are also known as Trombe walls. The diagram shows how the collector operates. The solar radiation heats the outside of the solar wall. The heat slowly passes through the massive wall, and arrives at the inside surface of the wall several hours later to provide heat in the evening. The glazed covering on the outside of the wall reduces heat loss, and allows more heat transfer to the inside space. The vents shown at the top and bottom of the wall in this diagram are optional -- they allow heat to be transferred to the living space earlier in the day than occurs with an unvented solar wall.

These walls can be used in new construction or as a retrofit. Some existing homes have wall construction that can be converted to a solar wall easily.

The characteristics of a Solar Wall as compared to a direct gain window:

Its efficiency in collecting solar heat is not as high as a direct gain window of the same size.
Night heat losses are less than for a direct gain windows.
Very simple -- no fans, no ducts, no controllers.
Does not provide daylighting or views as a direct gain window would -- this can be an advantage or disadvantage.
The inside surface of the wall can be used to some extent, but should not be covered with anything that reduces heat transfer from the wall to the living space.
Depending on the current wall construction, it may be easier to retrofit a solar wall than to retrofit a direct gain window, since no wall structural members are cut.

The collectors can be used in new construction, or as a retrofit.

These plans are excerpted from the book "Passive Solar Energy" by Bruce Anderson and Malcolm Wells. The full book is available for free download here. Solar walls are covered in chapter 5.

From "Passive Solar Energy", B. Anderson, M. Wells

Solar Walls -- Introduction

Solar windows let sunlight directly into the house. The heat is usually stored in a heavy floor or in interior walls. Thermal storage walls, as solar walls are often called, are exactly what their name implies- walls built primarily to store heat. The most effective place to build them is directly inside the windows, so that the sunlight strikes the wall instead of directly heating the house. The directly sun-heated wall gets much hotter, and thereby stores more energy, than thermal mass placed elsewhere.

These "solar walls" conduct heat from their solar hot side to their interior cooler side, where the heat then radiates to the house. But this process takes a while. In a well-insulated house, a normal number of windows in the south wall will admit enough sun to heat the house during the day. Thermal storage walls will then pick up where the windows leave off and provide heat until morning.

South-facing windows with an area of less than 10 percent of the floor area of the house are probably not large enough to provide enough heat during the day. If this is the case, vents could be added at both the base and the top of a solar wall. The wall can then provide heat to the house during the day just as solar chimneys do. Although the vents need be only 10 to 12 square inches for each

lineal foot of wall, they can add cost and complication. Therefore, it is best not to use them unless heat is needed during daylight hours. Thermal storage walls with vents are normally called Trombe Walls, after Dr. Felix Trombe who, in the early 1960s, built several homes with this design in the French Pyrenees. 1

One type of thermal storage wall uses poured concrete, brick, adobe, stone, or solid (or filled) concrete blocks. Walls are usually one foot thick, but slightly thinner walls will do, and walls up to 18 inches thick will supply the most heat. Further thicknesses save no additional energy. Containers of water are often used instead of concrete. They tend to be slightly more efficient than solid walls because they absorb the heat faster, due to convective currents of water inside the container as it is heated. This causes immediate mixing and quicker transfer of heat into the house than solid walls can provide. One-half cubic foot of water (about 4 gallons) per square foot of wall area is adequate, but unlike solid walls, the more water in the wall, the more energy it saves.

The main drawback of solar walls is their heat loss to the outside. Double glazing (glass or any of the plastics) is

adequate for cutting this down in most climates where winter is not too severe (less than 5000 degree days:

Boston, New York, Kansas City, San Francisco). Triple glazing or movable insulation is required in colder climates.

Plans for a Solar Wall

This glazed thermal storage wall is comprised of glazing frame members milled from cedar 4x4's bolted to an eight-inch thick structural brick wall. The bricks are dense paving bricks-a dark umber color on the outside, standard terra cotta color on the inside-and are laid up with all cavities filled with mortar. The triple glazed panels, designed for use in the northeast, reduce heat losses to the outside from the warm wall. Standard operable triple-glazed casement windows are incorporated into the wall to provide direct gain heating, light, views and ventilation. Double glazing is suitable for use in milder climates. (Construction details, the Brookhaven House.)

Other Consideration:

The inside surface of the solar wall should not be populated with bookcases or the like that will reduce the heat transfer from the wall to the living space.
There should be a good thermal connection between any inside wall finishes (e.g. sheet rock) and the masonry or concrete of the wall -- this will maximize the heat transfer to the living space.
If possible, the foundation area should be insulated in the usual way with rigid insulation board that goes down a couple feet to reduce heat loss from the solar wall to the foundation. This insulation is shown in the "Sill At Grade" illustration.
Some form of summer overheat protection is a good idea to prevent the solar wall from collecting heat and transferring it to the living space during the summer. An overhang can be used to block the summer sun, but still allow the lower winter sun to hit the wall. You can find an overhang design tool here.
Solar walls do not have as high a collection efficiency as direct gain windows or Thermosyphoning wall collectors, but can be a good choice, particularly if heat is wanted later in the day, or if a window or Thermosyphoning wall collector would be difficult to build or undesirable.
The triple glazing shown in the plan is, perhaps, a bit over-the-top -- most Trombe walls use double glazing.

Gary 05/14/2006

Trombe Wall and Attached Sunspace

A Trombe wall is a system for indirect solar heat gain and, although not extremely common, is a good example of thermal mass, solar gain, and glazing properties used together to achieve human comfort goals passively.
It consists of a dark colored wall of high thermal mass facing the sun, with glazing spaced in front to leave a small air space. The glazing traps solar radiation like a small greenhouse. An attached sunspace is essentially a Trombe wall where the air space is so big it is habitable.

A Trombe wall (left) and attached sunspace (right).

Trombe walls are a very useful passive heating system. They require little or no effort to operate, and are ideal for spaces where silence and privacy are desirable. Sunspaces are equally simple and silent, and can allow views. Rooms heated by a Trombe wall or sunspace often feel more comfortable than those heated by forced-air systems, even at lower air temperatures, because of the radiantly warm surface of the wall.
A successful Trombe wall or attached sunspace optimizes heat gain and minimizes heat loss during cold times, and avoids excess heat gain in hot times.

Trombe Walls

Trombe walls are thermal storage walls, named after the French inventor Felix Trombe. A typical Trombe wall consists of a 20 - 40cm (8" - 16") thick masonry wall painted a dark, heat-absorbing color and faced with a single or double layer of glass. The glass is placed between 2 - 15cm (1" - 6") away 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.
The glass prevents the escape of radiant heat from the warm surface of the storage wall. The heat radiated by the wall is therefore trapped within the air gap, further heating the wall surface. For a 40cm (16") thick Trombe wall, heat will take about 8 to 10 hours to reach the interior of the building. This means that the room behind remains comfortable through the day and receives slow, even heating for many hours after the sun sets. Such designs are ideal for use in residential living areas and bedrooms.
In addition to radiant heat, you can also configure Trombe walls to heat air within the internal space. Including upper and lower air vents in the wall allows convection currents, as cooler air from the room enters at the bottom and air heated in the Trombe wall escapes into the room at the top. These vents must be operable to prevent reverse convention currents occurring at night, which would cool the occupied space. Operable vents also allow the occupants control over instantaneous heating.

A vented Trombe wall heats air convectively as well as heating the space radiatively.

Vents can be shut at night to keep the convection loop moving the right direction.

Avoiding Losses

Using low-E glazing can prevent heat from re-radiating out through the glass of a Trombe wall and greatly reduce the amount of heat lost. Applying a spectrally selective surface or low-E coating to the wall itself can also improve performance by reducing the amount of infrared energy radiated towards the glass.

Low-E glazing reflecting heat back into the Trombe wall

Adapting to Day & Season

To avoid overheating at hot times of day or hot seasons of the year, architects can use Trombe walls in conjunction with overhangs, eaves, and other building design elements to evenly balance solar heat delivery.

A Trombe wall with overhang to shade from summer sun

Ideally, the glazing should have exterior insulating shutters for nighttime use in order to prevent the heat gained from being returned back to the outside.
While even seasonally-adapting Trombe walls can have no moving parts, you should provide for some method of cleaning the internal area between the glazing and the storage portion of a Trombe wall.

Sunspaces

Attached sunspaces (also called "conservatories") work much like vented Trombe walls. They can heat spaces both through radiation and convection. The difference is that the space between the glass and the thermal mass creates a habitable space.

A sunspace with vents for convective heating as well as radiative heating

The same sunspace at night, with vents closed, to keep convection going the proper direction

Sunspaces are primarily used for indirect solar heat gain and generally have more glazing area than floor area. Nighttime heat loss is not as critical in a sunspace as in direct gain systems, since the room can be closed off from the rest of the building. However, night insulation or double-glazing is recommended if the sunspace serves as living space after sundown.

Designing Sunspaces

Important considerations for sunspace design are:

In very cold climates, double glazing reduces conductive losses through the glass to the outside.
Insulated panels, shades, or blinds are more important for sunspaces than for Trombe walls, as sunspaces are sometimes occupied.
As with Trombe walls, the darker the internal surfaces of the sunspace, the more effectively the thermal mass can store heat during the day.
Do not overpopulate conservatories with vegetation, as foliage can reduce the system's heat capture by significantly shading the floor and wall.
For all climates except those with very cool summers, operable or mechanized windows should be considered at top and bottom. These allow the sun space to avoid overheating by passively venting hot air out the top of the glazing and pulling cool air in through the bottom of the glazing.

Variations

Water Walls

Instead of using masonry, water can be used as the thermal mass of a Trombe wall or sunspace. Due to convection currents within the water itself, heat is transferred through the entire thermal mass much quicker than a masonry wall that relies solely on conduction. This can be useful when a shorter delay in heat delivery is required.
Such systems can not only bring heat into a space, they can be translucent to bring light in as well. When using a water Trombe wall, it is better to seal the air between the glass and wall, to further increase the surface temperature of the wall.

Barrels of water for thermal mass

Solar chimney

During hot seasons, a Trombe wall or sunspace with vents through it can be used as a thermosiphon. If vents are placed at the top of the glazing, then air from the room will be pulled out by convection in the air gap between glazing and mass wall. This form of passive ventilation is called a solar chimney.

A Trombe wall acting as a solar chimney

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Trombe wall