Indirect Gain Designs

Indirect gain heating places a thermal storage mass between the sun and the occupied space. A sheet of glass covers an opaque wall 20 to 30 cm (8-12 in.) thick. The sun strikes the mass, where its energy is stored and slowly transferred to the interior space. The absorbed solar energy moves through the wall by conduction and then to the space by radiation and convection. The interior side of the wall must be kept free of hangings and large furniture so that radiant heat can transfer into the space. Indirect gain systems admit less daylight than direct gain systems, and offer little or no view to the south. Radiant heat continues to flow into the space in the evenings after sunny days.

Thermal storage walls are painted black on the outside surface, which is then covered with a sheet of glazing. Due to thermal lag, heat takes hours to conduct through the wall. During the hottest part of the day, some of the heat flows back out through the glass before it can be passed into the building. There may be vents at the top and bottom of the wall to circulate room air, thus delivering warmer air sooner, but this feature also adds dust and dirt to the space between the mass and the glass.

The first example of a thermal storage wall was constructed in a house built by Felix Trombe and Jacques Michel in Odeillo, France, in 1967. Openings in this 61-cm (2-in.) thick concrete Trombe wall were double-glazed. Trombe walls are a classic element in indirect gain passive solar design. Trombe walls can have large openings for daylight and view. It can be difficult to clean the airspace between the Trombe wall and the glass, and placing objects near the interior surface must be minimized.

Water walls are made of corrugated galvanized steel culverts, steel drums, or fiberglass-reinforced plastic tubes. An air space is provided for expansion of the water when heated. Water walls are opaque, and can be fitted below windows. The use of water allows convection, which moves the heat more rapidly through the wall. Steve Baer constructed a wall of 55-gallon drums filled with water and stacked horizontally at his residence in Corrales, New Mexico. The home includes adobe walls and a concrete floor. A rigid insulation panel is hinged at the bottom with its reflective surface on the interior side. It can be opened flat on the floor in front of the collector, reflecting additional sun on the system, and is folded up out of the way when not in use.

Greenhouses and sunspaces (Fig. 23-4) combine direct and indirect gain systems. When a greenhouse is used as part of a solar heating system, a masonry or water thermal wall is used between the greenhouse and the occupied space. Solar greenhouse efficiency can be as high as 60 to 75 percent. Only 10 to 30 percent of the energy entering the greenhouse is supplied to the occupied space unless an active heat storage system is used. The remainder of the heat is used to heat the greenhouse itself.

Sunspaces are glass-enclosed porches or rooms adjoining another living space, and oriented to admit large

Thermal mass

Figure 23-4 Passive solar greenhouse/sunspace.

Thermal mass

Figure 23-4 Passive solar greenhouse/sunspace.

amounts of sunlight. They are used in passive solar designs with thermal mass. Where sunspaces don't include thermal mass for night heating, and are not heated at night, they are only used as weather permits. If an insulated, lightweight frame is the common wall with the occupied space, the sunspace should contain a row of water containers along the full east-west width of the space. These containers, two times as wide as they are tall and sitting adjacent to the floor and common wall, take up much of the sunspace's floor area. Heat is transferred into the occupied space for use as it builds up in the sunspace. Greenhouses and sunspaces are easy to retrofit onto the south side of an existing building. The floor should be a thermally massive slab-on-grade, with perimeter insulation. The greenhouse or sunspace needs shading and ventilation through windows or with fans to prevent overheating.

Sunspaces, solariums, and greenhouses are available as manufactured systems with wood or metal frames, complete with glazing and flashing. A ventilating fan can be mounted on the roof or in either of the gable ends. Insulated shades and blinds that follow the slope of the roof can be operated manually or by remote control. Awning and casement sashes for ventilation, ventilated roof sashes, and doors are available from manufacturers.

Indirect gain passive solar systems are fairly versatile for retrofitting existing buildings, where they can be added to the south wall of buildings with a clear southern exposure. Their appearance may be more difficult to integrate into the building's architecture. The overall efficiency of indirect gain systems runs about 30 to 45 percent, with water walls being slightly more efficient than masonry.

Roof ponds expose water to the sun during the day. Water in large plastic bags, typically 15 to 30 cm (6-12 in.) deep, is usually supported by a metal deck roof structure, which also serves as the finished ceiling of the room below. The metal deck conducts heat from the water storage and radiates it to the space below. An insulating panel is moved mechanically with an electric motor over the roof pond at night, so that stored heat radiates downward into the space rather than up into the sky.

The roof structure must be able to support the 160-to 320-kg per square meter (32- to 65-lb per square ft) dead load of the water. Indoor masonry partitions moderate the indoor temperature fluctuations, and help support the weight of the roof. Roof ponds are limited to one-story buildings. The indoor temperatures remain very stable, in the range of 5°C to 8°C (9°F-14°F) fluctuations when using lightweight materials, and 3°C to 4°C (5°F-8°F) when masonry is used.

In summer, reversing the process allows internal heat to be absorbed during the day to be radiated to the sky at night. The outside of the water bags can be sprayed or flooded for added evaporative cooling in summer, increasing the cooling effect fourfold. The original roof pond system was built by Harold Hay in Atascadero, California, where it has provided all of the heating and cooling needs of the building since 1973.

Most glass and plastics do not pass long-wave thermal radiation, so closed windows and glass- or plastic-covered solar collectors do not radiate much heat to the night sky. Open windows work better, but if warm trees, buildings, or earth are within the radiation's path, rather than colder open sky, less heat will be removed.

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