Direct gain systems are most commonly used in passive solar architecture. The roof, walls and floor are insulated to a high level. Solar radiation enters through the windows and is absorbed by the heavy material of the building. The whole building structure gradually collects and stores solar energy during the day. Heavy building materials provide thermal storage. The collected solar energy is gradually released at night when there is no solar gain.
Direct gain systems commonly utilize windows or skylights to allow solar radiation to directly enter zones to be heated. If the building is constructed of lightweight materials, mass may need to be added to the building interior to increase its heat storage capacity. The proportion of a building's heating needs that can be met by solar energy increases as the area of sun-facing glazing increases. Additional mass must therefore be used to reduce interior temperature swings and delay the release of solar energy into occupied spaces. While the mass that is directly illuminated by the incident energy, sunshine, is the most effective for energy storage, long-wave radiation exchanges and convective air currents
Passive solar systems: A, a typical passive solar system; B, mass wall system; C, Trombe wall system; D, water wall system; E, Barra Constantini system; F, semi-direct gain sunspace; G, indirect gain sunspace; H, thermosyphon system; J, thermosyphon system with rock bed; K, underfloor rock bed actively charged from a sunspace during the day; L, underfloor rock bed passively discharged by radiation and convection at night.
in the solar heated rooms allow non-illuminated mass to also provide effective energy storage.
Thermal storage mass plays a vital role in the effective performance of direct gain buildings. Thermal mass is needed to store the solar energy for subsequent release into the interior space as heat. It is necessary because the sun's energy is not always emitted in phase with heating requirements. This is why the heat gained must be stored for future use, rather as a brick in a night storage radiator is heated at night using cheap energy and re-radiated as heat out into the space during the day. In passive solar systems the cheap energy comes from the sun.
Thermal mass in a building does two things, it reduces the peaks of the temperature swings (decrement factor) and shifts them to a later time than the air temperature peaks (time lag). The more mass there is, the lower and later the indoor air temperature peaks (see Figure 3.4).
As a general rule, buildings that contain very little thermal mass are unable to store heat for night time use; thus, only the daytime portion of the heat load can be met by solar gains, and overheating can be a serious problem if the solar gains are excessive. Furthermore, heat losses through an aperture whose effectiveness is impaired by insufficient thermal mass can exceed the useful solar gain. Lightly constructed stud-wall buildings with no floor slab (or a carpeted floor slab) are the worst offenders in this category.
The effectiveness of thermal storage mass in direct gain buildings depends on its thickness, surface area and thermal properties (volumetric heat capacity and thermal conductivity). The best materials are those that are capable of storing large quantities of heat (high volumetric heat capacity) and that can readily transport heat from the mass surface to the mass interior for storage and back again to the surface to meet the building's heat load (high thermal conductivity). As a general rule, mass surfaces should be relatively dark in colour compared with non-massive surfaces in order to promote preferential absorption of solar radiation by the thermal storage medium and, for optimal effectiveness, should be located in building zones that experience direct solar gains. Mass located in zones not directly illuminated by solar radiation entering through south-facing windows will be ineffective unless care is taken to ensure adequate free convective exchanges with directly heated zones or unless a forced-air distribution system is employed. In either case, remote thermal storage mass will be less effective than that located in direct gain zones.
Some rules that apply to high-density masonry (roughly
2000-2500 kg m-3:
• Performance variations for mass thickness between 10 and 20 cm are small. The thickness may be reduced to 10 cm without incurring significant performance penalties. This generalization is independent of location, configuration and mass surface area.
• The range of mass thickness between 5 and 10 cm can be considered a transition region. In this region, performance penalties for reduced thickness are becoming significant but, in some cases, may be considered acceptable as design cost trade-offs.
• For mass thickness below 5 cm, performance falls off much more rapidly than in the transition region. Under most conditions it is not advisable to employ mass thickness of less than 5 cm for passive solar building assemblages.
• Lower-density masonry has a lower thermal conductivity and, therefore, has a smaller effective thickness for diurnal heat storage. The same heat storage capacity must, therefore, be achieved with material spread over a larger area.
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