As regards indirect gain systems, solar radiation does not directly enter the room that has to be heated up, but it falls on a thermal mass which is placed between the Sun and the living space. The solar energy absorbed by that mass is converted into thermal energy and then distributed inside the room in different ways. By the position of the thermal mass, we can distinguish two kinds of indirect systems: solar walls where the thermal mass is contained inside a wall and roof-ponds where the thermal mass is put on the roof of the room which is to be heated up. Indirect gain systems need a wide glazed surface oriented southward and the ther­mal mass used for storing the absorbed energy is placed behind it at a distance of at least 10 cm.

The storage is normally made of brickwork or water (with the latter put inside metallic, plastic or concrete waterproof containers) [1, 3, 4].

2.3.2.1 Brickwork solar walls Solar radiation received by a solar wall which is painted with a dark colour is absorbed causing a superficial heating up (Fig. 70). This heat, which is like a temperature damped wave, is transferred by con­duction to the wall’s inner surface and from there it spreads all over the room by radiation and convection. The delay and the temperature wave damping depend on the storage material and thickness. Dispersion of stored heat towards the outside is resticted by the insulation created by the air space between the glazed surface and the solar wall.

Trombe’s wall (Fig. 71) is different from the solar wall owing to the pres­ence of air holes in both the lower and the upper parts of the wall. In this way, the activation of a mechanism for natural circulation of air through the heated area is favoured. The warm air volume between the glazed surface and the thermal mass can reach high temperatures (about 65°C). The air holes in the upper part of the storage mass allow warm air to move up and enter the room, while the colder air which is inside the room is recalled inside the collector through the holes in the lower part of the storage mass. The openings should be located by means of dampers to prevent the reverse movement during the night [1, 3, 4].

2.3.2.2 Water wall The processing by a water wall (Fig. 72) is based on the same principle which regulates the processing by a solar wall, excepting that heat transmission through the wall also depends on thermal convection and not only on conduction. Because of the high thermal capacity of water and the inner convec­tive currents, which make it an almost isothermal heat accumulation, the system can work with a higher efficiency compared with brickwork solar walls. One of the most important problems is where to confine the liquid. Until now, bottles, tubes, watertight tanks, barrels, drums and cement walls filled with water have been used as containers (see Fig. 72) [1, 3, 4].

2.3.2.3 Roof-pond As regards roof-pond passive systems, the thermal mass is placed horizontally on the building’s roof (Fig. 73). The storage medium is water which is enclosed in small bags similar to little mattresses. They com­pletely or partially cover the roof which works as the ceiling of the rooms that are to be heated up. Water containers have to be placed in direct contact with the ceiling which sustains them to make the heat exchange between the inner room and the storage easier. During the hot season, the storage is exposed to solar radiation during the day; the intercepted energy is then transferred by conduction through the roof structure and directly exchanged by radiation from the ceiling of the room to be heated. During the night or during cloudy days, a mobile insulation mechanism covers the hot water and restricts its heat dispersion. In contrast to the systems with solar walls, systems with water walls are not always provided with a transparent cover to put on water. The use of translucent containers or glazed surfaces put on the water mirror is an efficient solution to reduce sensitive and latent (evaporation) heat losses when climate is very cold.

In places where there are high thermal ranges between day and night and where humidity is very low, the water storage on the roof can also be also for summer refresh­ing by insulating it during the day and exposing it during the night. The utilization of the water wall also presents numerous problems: besides the extra structural costs, the system does not guarantee sufficient advantages at high latitudes because of the reduced solar radiation intercepted by the horizontal plane; moreover, the stored heat can be spread by radiation only over the floor below the roof [1, 3, 4].