direct gain
The simplest
of approaches is a direct gain design. Sunlight is admitted to the space (by
south facing glass) and virtually all of it is converted to thermal energy. The
walls and floor are used for solar collection and thermal storage by
intercepting radiation directly, and/or by absorbing reflected or reradiated
energy (Fig. 4). As long as the room temperature remains high in the interior
space storage mass (walls, floors) will conduct heat to their cores. At night,
when outside temperatures drop and the interior space cools, the heat flow into
the storage masses is reversed and heat is given up to the interior space in
order to reach equilibrium. This re-radiation of collected daytime heat can
maintain a comfortable temperature during cool/cold nights and can extend
through several cloudy days without "recharging".
Direct gain design is simple in concept and can employ a wide variety of materials and combinations of ideas that will depend greatly upon the site and topography; building location and orientation; building shape (depth, length, and volume); and space use. A direct gain design requires about one-half to two-thirds of the total interior surface area (RT) to be constructed of thermal storage materials. These can include floor, ceiling and wall elements, and the materials can range from masonry (concrete, adobe, brick, etc.) to water (Fig. 5). Water contained within plastic or metal containment and placed in the direct path of the sun's rays has the advantage of heating more quickly and more evenly than masonry walls during the convection process. The convection process also prevents surface temperatures from becoming too extreme as they sometimes do when dark coloured masonry surfaces receive direct sunlight. The masonry heating problem can be alleviated by using a glazing material that scatters sunlight so that it is more evenly distributed over walls, ceiling, and floor storage masses (Fig. 6). This decreases the intensity of rays reaching any single surface but does not reduce the amount of solar energy entering the space. |
Figure 4.
Direct gain design - A direct gain design collects and stores heat during the
day. At night stored heat is radiated into the living spaces.
Figure 5.
Direct gain interior - A direct gain design with an interior water wall for
heat storage. Heat stored in the water wall is radiated into the living space
at night.
Figure
6. Diffusing glazing materials. Translucent glazing scatters sunlight to all
storage surfaces.
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indirect gain |
Figure
7. Indirect gain Trombe wall stores heat during the day. Excess heat is vented
to the interior space. At night Trombe wall vents are closed and the storage
wall radiates heat into the interior space.
Figure 8. Indirect gain water wall collects and stores heat during the day. Heat stored in indirect gain water wall is radiated into the living space at night.
Figure 9. Attached greenhouse with vented storage wall. Heat is stored in the wall during the day - excess heat is vented to the interior space. At night the wall vents are closed and stored heat is radiated to both the greenhouse and the interior space.
Figure 10. Attached greenhouse with water storage wall.
Figure 11. Heating cycle - Roof pond collects and stores heat during the day. At night roof ponds are covered and stored heat is radiated into the space below.
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This passive
solar design approach uses the basic elements of collection and storage of heat
in combination with the convection process. In this approach, thermal storage
materials are placed between the interior habitable space and the sun so there
is no direct heating. Instead a dark coloured thermal storage wall is placed
just behind a south facing glazing (windows). Sunlight enters through the glass
and is immediately absorbed at the surface of the storage wall where it is
either stored or eventually conducted through the material mass to the inside
space. In most cases the masonry thermal storage mass cannot absorb solar
energy as fast as it enters the space between the mass and the window area.
Temperatures in this space can easily exceed 100°F. This build-up of heat can
be utilized to warm a space by providing heat-distributing vents the top of the
wall (where the heated air, rising upward due to less density, can flow into
the interior space (Fig. 7). Vents at the bottom of the wall allow cool air to
be drawn into the heating space thereby replacing the outflowing hot air, and
picking up heat itself. The top and bottom vents continue to circulate air as
long as the air entering the bottom vent is cooler than the air leaving the top
vent. This is known as a natural convective loop. At night the vents can be
closed to keep cold air out and the interior space is then heated by the
storage mass, which gives up its heat by radiation as the room cools. A
variation of the vented masonry wall design is one that employs a water wall
between the sun and the interior space (Fig. 8). Water walls used in this way
need not be vented at top and bottom and can be constructed in many ways - even
55-gallon drums filled with water, or specially constructed plastic or sealed
concrete containers. Again, as the water is heated, the convection process
quickly distributes the heat throughout the mass and the interior space is
warmed by heat radiated from the wall.
Another design approach (Fig. 9) takes advantage of the greenhouse effect as well as the direct gain storage wall. A south facing "greenhouse space" is constructed in front of a thermal storage wall exposed to the direct rays of the sun. This wall would be at the rear of the greenhouse and the front of the primary structure. The thermal wall absorbs heat at the same time the interior space of the greenhouse is being heated. If a vented masonry wall is used as storage, heat can also be released into the living space by convection. This combination also works with an unvented water wall. The greenhouse, then, is heated by direct gain while the living space is heated by indirect gain (Fig. 10). The advantage is that a tempered greenhouse condition can be maintained through days of no sun, with heating from both sides of the thermal storage wall. An indirect gain design which provides both heating and cooling is the thermal pond approach, which uses water encased in ultraviolet ray inhibiting plastic beds underlined with a dark colour, that are placed on a roof. In warm and temperate climates with low precipitation, the flat roof structure also serves directly as a ceiling for the living spaces below (Fig. 11) thereby facilitating direct transfer of heating and cooling for the spaces below. In colder climes, where heating is more desirable, attic ponds under pitched roof glazing are effective. Winter heating occurs when sunlight heats the water, which then radiates energy into the living space as well as absorbs heat within the water thermal mass for night-time distribution. During the summer, a reverse process, described later, occurs. For best effect, roof ponds must be insulated (movable) so that heat will not radiate and be lost to the outside. One of the major advantages of this approach is that it allows all rooms to have their own radiant energy source with little concern about the orientation of the structure or optimal building form. |
isolated gain |
Figure 12. Water or air convection loop.
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Finally, the isolated gain design approach uses a fluid
(liquid or air) to collect heat in a flat plate solar collector attached to the
structure. Heat is transferred through ducts or pipes by natural convection to
a storage area - comprised of a bin (for air) or a tank (for liquid), where the
collected cooler air or water is displaced and forced back to the collector
(Fig. 12).
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If air is used as the transfer medium in a convection loop, heated air coming from the collector is usually directed into a rock (or other masonry mass material) bin where heat is absorbed by the rocks from the air. As the air passes its heat to the rocks it cools, falls to the bottom of the bin and is returned to the collector completing the cycle. At night the interior space of the structure is heated by convection of the collected radiant energy from the rock bin. If water is the transfer medium, the process works in much the same way except that heat is stored in a tank, and as hot water is introduced, cooler water is circulated to the collector. In naturally occurring convection systems (non-mechanically assisted) collectors must be lower than storage units, which must be lower than the spaces to be heated (RT). Of course, the addition of distribution assisting equipment can allow for placement of system elements anywhere, but that would then be an Active Solar System.