Editing Passive solar building design (section)

====Thermal Storage (Trombe) Walls====
{{Main articles|Trombe wall}}
In a '''''thermal storage wall''''' system, often called a '''''Trombe wall''''', a massive wall is located directly behind south-facing glass, which absorbs solar energy and releases it selectively towards the building interior at night. The wall can be constructed of cast-in-place concrete, brick, adobe, stone, or solid (or filled) concrete masonry units. Sunlight enters through the glass and is immediately absorbed at the surface of the mass wall and either stored or conducted through the material mass to the inside space. The thermal mass cannot absorb solar energy as fast as it enters the space between the mass and the window area. Temperatures of the air in this space can easily exceed 120&nbsp;°F (49&nbsp;°C). This hot air can be introduced into interior spaces behind the wall by incorporating heat-distributing vents at the top of the wall. This wall system was first envisioned and patented in 1881 by its inventor, Edward Morse. Felix Trombe, for whom this system is sometimes named, was a French engineer who built several homes using this design in the French Pyrenees in the 1960s.

A thermal storage wall typically consists of a 4 to 16 in (100 to 400&nbsp;mm) thick masonry wall coated with a dark, heat-absorbing finish (or a selective surface) and covered with a single or double layer of high transmissivity glass. The glass is typically placed from {{frac|3|4}} in to 2 in from the wall to create a small airspace. In some designs, the mass is located 1 to 2&nbsp;ft (0.6 m) away from the glass, but the space is still not usable. The surface of the thermal mass absorbs the solar radiation that strikes it and stores it for nighttime use. Unlike a direct gain system, the thermal storage wall system provides passive solar heating without excessive window area and glare in interior spaces. However, the ability to take advantage of views and daylighting are eliminated. The performance of Trombe walls is diminished if the wall interior is not open to the interior spaces. Furniture, bookshelves and wall cabinets installed on the interior surface of the wall will reduce its performance.

A classical '''''Trombe wall''''', also generically called a '''''vented thermal storage wall''''', has operable vents near the ceiling and floor levels of the mass wall that allow indoor air to flow through them by natural convection. As solar radiation heats the air trapped between the glass and wall and it begins to rise. Air is drawn into the lower vent, then into the space between the glass and wall to get heated by solar radiation, increasing its temperature and causing it to rise, and then exit through the top (ceiling) vent back into the indoor space. This allows the wall to directly introduce heated air into the space; usually at a temperature of about 90&nbsp;°F (32&nbsp;°C).

If vents are left open at night (or on cloudy days), a reversal of convective airflow will occur, wasting heat by dissipating it outdoors. Vents must be closed at night so radiant heat from the interior surface of the storage wall heats the indoor space. Generally, vents are also closed during summer months when heat gain is not needed. During the summer, an exterior exhaust vent installed at the top of the wall can be opened to vent to the outside. Such venting makes the system act as a solar chimney driving air through the building during the day.

Vented thermal storage walls vented to the interior have proven somewhat ineffective, mostly because they deliver too much heat during the day in mild weather and during summer months; they simply overheat and create comfort issues. Most solar experts recommended that thermal storage walls should not be vented to the interior.

There are many variations of the Trombe wall system. An '''''unvented thermal storage wall''''' (technically not a Trombe wall) captures solar energy on the exterior surface, heats up, and conducts heat to the interior surface, where it radiates from the interior wall surface to the indoor space later in the day. A '''''water wall''''' uses a type of thermal mass that consists of tanks or tubes of water used as thermal mass.

A typical unvented thermal storage wall consists of a south facing masonry or concrete wall with a dark, heat-absorbing material on the exterior surface and faced with a single or double layer of glass. High transmission glass maximizes solar gains to the mass wall. The glass is placed from {{frac|3|4}} to 6 in. (20 to 150&nbsp;mm) from the wall to create a small airspace. Glass framing is typically metal (e.g., aluminum) because vinyl will soften and wood will become super dried at the 180&nbsp;°F (82&nbsp;°C) temperature that can exist behind the glass in the wall. Heat from sunlight passing through the glass is absorbed by the dark surface, stored in the wall, and conducted slowly inward through the masonry. As an architectural detail, patterned glass can limit the exterior visibility of the wall without sacrificing solar transmissivity.

A water wall uses containers of water for thermal mass instead of a solid mass wall. Water walls are typically slightly more efficient than solid mass walls because they absorb heat more efficiently due to the development of convective currents in the liquid water as it is heated. These currents cause rapid mixing and quicker transfer of heat into the building than can be provided by the solid mass walls.

Temperature variations between the exterior and interior wall surfaces drive heat through the mass wall. Inside the building, however, daytime heat gain is delayed, only becoming available at the interior surface of the thermal mass during the evening when it is needed because the sun has set. The time lag is the time difference between when sunlight first strikes the wall and when the heat enters the building interior. Time lag is contingent upon the type of material used in the wall and the wall thickness; a greater thickness yields a greater time lag. The time lag characteristic of thermal mass, combined with dampening of temperature fluctuations, allows the use of varying daytime solar energy as a more uniform night-time heat source. Windows can be placed in the wall for natural lighting or aesthetic reasons, but this tends to lower the efficiency somewhat.

The thickness of a thermal storage wall should be approximately 10 to 14 in (250 to 350&nbsp;mm) for brick, 12 to 18 in (300 to 450&nbsp;mm) for concrete, 8 to 12 in (200 to 300&nbsp;mm) for earth/adobe, and at least 6 in (150&nbsp;mm) for water. These thicknesses delay movement of heat such that indoor surface temperatures peak during late evening hours. Heat will take about 8 to 10 hours to reach the interior of the building (heat travels through a concrete wall at rate of about one inch per hour). A good thermal connection between the inside wall finishes (e.g., drywall) and the thermal mass wall is necessary to maximize heat transfer to the interior space.

Although the position of a thermal storage wall minimizes daytime overheating of the indoor space, a well-insulated building should be limited to approximately 0.2 to 0.3&nbsp;ft<sup>2</sup> of thermal mass wall surface per ft<sup>2</sup> of floor area being heated (0.2 to 0.3 m<sup>2</sup> per m<sup>2</sup> of floor area), depending upon climate. A water wall should have about 0.15 to 0.2&nbsp;ft<sup>2</sup> of water wall surface per ft<sup>2</sup> (0.15 to 0.2 m<sup>2</sup> per m<sup>2</sup>) of floor area.

Thermal mass walls are best-suited to sunny winter climates that have high diurnal (day-night) temperature swings (e.g., southwest, mountain-west). They do not perform as well in cloudy or extremely cold climates or in climates where there is not a large diurnal temperature swing. Nighttime thermal losses through the thermal mass of the wall can still be significant in cloudy and cold climates; the wall loses stored heat in less than a day, and then leak heat, which dramatically raises backup heating requirements. Covering the glazing with tight-fitting, moveable insulation panels during lengthy cloudy periods and nighttime hours will enhance performance of a thermal storage system.

The main drawback of thermal storage walls is their heat loss to the outside. Double glass (glass or any of the plastics) is necessary for reducing heat loss in most climates. In mild climates, single glass is acceptable. A selective surface (high-absorbing/low-emitting surface) applied to the exterior surface of the thermal storage wall improves performance by reducing the amount of infrared energy radiated back through the glass; typically, it achieves a similar improvement in performance without the need for daily installation and removal of insulating panels. A 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.{{Sfn|Wujek|2010}}