Heat can be stored in the structural materials of the building to reduce the indoor temperature, reduce the cooling load peaks, and shift the time that maximum load occurs. The storage material is referred to as the thermal mass. In winter, during periods of high solar gain, energy is stored in the thermal mass, avoiding overheating. In the late afternoon and evening hours, when energy is needed, heat is released into the building, satisfying part of the heating load. In summer, the thermal mass acts in a similar way as in winter, reducing the cooling load peaks.
Heat gain in a solar house can be direct or indirect. Direct gain is the solar radiation passing through a window to heat the building interior, whereas indirect gain is the heating of a building element by solar radiation and the use of this heat, which is transmitted inside the building, to reduce the heating load.
Indirect gain solar houses use the south-facing wall surface of the structure to absorb solar radiation, which causes a rise in temperature that, in turn, conveys heat into the building in several ways. Glass has led to modern adaptations of the indirect gain principle (Trombe et al., 1977).
By glazing a large south-facing, massive masonry wall, solar energy can be absorbed during the day and conduction of heat to the inner surface provides radiant heating at night. The mass of the wall and its relatively low thermal diffusivity delay the arrival of the heat at the indoor surface until it is needed. The glazing reduces the loss of heat from the wall back to the atmosphere and increases the collection efficiency of the system during the day.
Openings in the wall, near the floor, and near the ceiling allow convection to transfer heat to the room. The air in the space between the glass and the wall warms as soon as the sun heats the outer surface of the wall. The heated air rises and enters the building through the upper openings. Cool air flows through the lower openings, and convective heat gain can be established as long as the sun is shining (see Figure 6.4, later in the chapter). This design is often called the Trombe wall, from the name of the engineer Felix Trombe, who applied the idea in France.
In most passive systems, control is accomplished by moving a shading device that regulates the amount of solar radiation admitted into the structure. Manually operated window shades or Venetian blinds are the most widely used because of their simple control.
The thermal storage capabilities inherent in building mass can have a significant effect on the temperature within the space as well as on the performance and operation of heating, ventilating, and air-conditioning (HVAC) systems.
Effective use of structural mass for thermal storage has been shown to reduce building energy consumption, reduce and delay peak heating and cooling loads (Braun, 1990), and in some cases, improve comfort (Simmonds, 1991). Perhaps the best-known use of thermal mass to reduce energy consumption is in buildings that include passive solar techniques (Balcomb, 1983).
The effective use of thermal mass can be considered incidental and allowed for in the heating or cooling design, or it may be considered intentional and form an integral part of the system design.
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