indirect gain systems
In the indirect gain, a storage mass collects and stores heat directly from the sun and then transfers heat to the interior space. The sun rays do not travel through the occupied space to reach the storage mass. There are several indirect gain passive solar 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.

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 .


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.

The roof pond is also suited for natural summer cooling in regions where day-light temperature swings exist. 
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
Indirect gain system rules of thumb for thermal storage walls
  • The exterior of the mass wall (toward the sun) should be a dark color.
  • Use a minimum space of 4 inches between the thermal mass wall and the glass.
  • Vents used in a thermal mass wall must be closed at night.
  • A well insulated home (7-9 BTU/day-sq. ft.-degree F) will require approximately 0.20 square feet of thermal mass wall per square foot of floor area or 0.15 square foot of water wall.
  • If movable night insulation will be used in the thermal wall system, reduce the thermal mass wall area by 15%.
  • Thermal wall thickness should be approximately 10-14 inches for brick, 12-18 inches for concrete, 8-12 inches for adobe or other earth material and at least 6 inches for water.