  indirect
gain systems
One of the most important
indirect-gain passive solar building type is the water wall, in which the
sun`s rays are intercepted beyond the collector glazing by a water storage
mass, then converted into heat and distributed by convection and radiation
to the living space . The water wall involves the same principles as the
mass wall, but employs a different storage material and different methods
of containing that material.
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The
requirements for the water wall are again a large glazed area and an adjacent
massive heat storage. However , the storage is now water, or another liquid,
contained in a variety of containers, each representing different heat
exchange surface-to-storage mass ratios. Larger storage volumes provide
greater and longer-term heat storage capacity , while smaller container
volumes provide greater heat exchange surfaces and thus faster distribution
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| 2. TROMBE
WALL |
The second indirect
gain solar building type is the mass wall, in which the sun`s rays are
intercepted directly behind the collector glazing by a massive wall that
serves as heat storage. A Trombe wall is a masonry or concrete wall covered
externally with a glass skin. A small air space of 4-8 inches is left between
the wall and the glazing. Solar radiation passes through the glass and
is absorbed by the mass wall. The glazing should have exterior insulating
shutters for nighttime use in order to prevent the heat gained from being
returned back to the outside. The mass is heated during the day and releases
its warmth to the interior during the evening and night hours. Vents may
also be placed in the wall to permit heat to flow directly into the room
during the day.
The required elements of
the mass wall building type involve only a large glazed collector area
and a storage mass directly behind it. The material property to consider
in deciding on storage construction is the method of distribution inherent
in massing materials with different heat storage capacities and emission
properties. Radiant distribution from a storage mass to an occupies space
can be almost immediate, or it can be delayed up to 12 hours, depending
on the depth and time lag property of the storage material chosen. Distribution
of air by natural convection is also viable with the mass wall system since
the volume of air in the interventing space between glazing and storage
mass is being heated to high temperatures and seeks constant means of escape. |
The third indirect gain building
type is the roof pond. In the roof pond building type, the passive collector
storage mass has been relocated, from the floor and wall of the building,
into the roof , for radiant heat distribution to the occupied space. The
roof pond systems requires a body of water to be located in the roof, protected
and controlled by exterior movable insulation. This body of water is exposed
to direct solar gain, which it absorbs and stores. Since thermal storage
is the ceiling of the building, it will radiate uniform low-temperature
heat to the entire layout in both sunny and cloudy conditions. Distribution
of solar heat from the roof pond is by radiation only, so proximity of
the ceiling to the individual being warmed is important since radiation
density drops off with distance.
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The
roof pond is also suited for natural summer cooling in regions where day-light
temperature swings exist. |
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They
are used to capture solar radiation and store it at temperatures of nearly
100°C. Natural or man-made ponds can be made into solar ponds by creating
a salt-concentration gradient made up of layers of increasing concentrations
of salt. |
After
having cooled down on summer evenings by exposure to the night air, the
ceiling water mass can then draw unwanted heat from the living and working
spaces during the day, taking advantage of the temperature stratification
to provide passive cooling.These layers prevent natural convection from
occurring in the pond and enable heat collected from solar radiation to
be trapped in the bottom brine.Approximately 4000 ha of solar ponds
(40 ponds of 100 ha) and a set of evaporation ponds that cover a combined
1200 ha are needed for the production of 1 billion kWh of electricity needed
by 100,000 people in one year (Table 2). Therefore, a family of three would
require approximately 0.2 ha (22,000 sq ft) of solar ponds for its electricity
needs. Although the required land area is relatively large, solar ponds
have the capacity to store heat energy for days, thus eliminating the need
for back-up energy sources from conventional fossil plants.
The efficiency of solar ponds
in converting solar radiation into heat is estimated to be approximately
1:5. Assuming a 30-year life for a solar pond, the energy input/output
ratio is calculated to be 1:4 (Table 2). A 100 hectare (1 km2) solar pond
is calculated to produce electricity at a rate of approximately 14¢
per kWh. According to Folchitto (1991), this cost could be reduced in the
future.
Advantages Disadvantages
-
Energy delivery to the space
is more controllable Heat losses are increased
-
Less impact on the overall building
design To minimized overheating during summer, a mass wall must be shaded
and vented to the outdoors.
-
Glare, ultraviolet degradation
and reduction of night privacy are not problems with indirect gain system.
-
Indoor temperature variation
is less than direct gain system
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