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Passive solar

Solar energy can be used to heat buildings and is known as passive solar heating. As the Portuguese architect Francisco Moita points out in his report on passive solar energy, it can be exploited through direct, indirect and greenhouse systems.

Direct gain system

Direct gain system consists of capturing solar radiation through the windows of the dwelling. The diagram opposite shows the variation in the intensity of solar radiation through windows orientated differently on a winter day (inverno) or a summer day (verão). It is clear that the window is a fundamental element in the thermal comfort of a building. In other word, depending on the orientation of the windows, the radiation will be more or less intense.


Indirect gain system

Indirect gain system collects and traps sun’s heat through a collector wall in heavy materials. The latter one is located between the space to be heated and the glass that protects from the outside. The heat produced on the outside during the sun exposure is stored by the mass of the wall. It is then transported to its inner volume. In there, it  will propagate to the space by convection and radiation.

Heavy building materials such as concrete, brick or stone have a high capacity to store heat. Indeed they absorb and accumulate the solar radiation radiating from their surface.

Two phenomena explain how the heat returns to the environment:

  • By convection: this phenomenon appears when the temperature of the environment is lower than the temperature of the surface of the material

  • By radiation: this phenomenon appears when the surface of a nearby object is lower than that of the material that stored the heat

Two parameters define thermal inertia:

  • The diffusivity of a material: it represents the rate at which the temperature of a material will change as a function of external thermal stresses.

  • The effusivity of a material: it is the ability of a material to exchange thermal energy with its environment.

The figure opposite shows an indirect gain system, also known as a trombe wall. It was implemented by Professor Félix Trombe and architect Jacques Michel in the 1950s and 1970s. To maximize the wall’s ability to absorb solar radiation, the surface of the wall exposed to the sun should be painted in dark.


passive-solar-greenhouseAnother passive solar heating system consists of a glass enclosed space, a greenhouse and a thermal storage mass (wall and floor). On cold days or at night, the greenhouse acts as a buffer zone, thus reducing energy losses. Heat captured inside the greenhouse (direct gains) can be transmitted directly by air circulation inside the compartment. It can also be accumulated in heavy materials and later delivered by radiation (indirect gains). Southwest facing greenhouse can lead to excess heat in summer. To avoid this situation it is necessary to install shading systems on the west side of the greenhouse that will not affect solar gains in winter. It can be for instance a wall or deciduous leaf vines.