Passive Solar Heating
The passive solar heating is based on the use of solar energy to naturally increase the indoor temperature of a dwelling, without resorting to mechanical systems.
As explained by the Portuguese architect Francisco Moita, this energy can be harnessed through three types of systems: direct, indirect, and greenhouse.
Direct Gain System
The direct gain occurs when solar radiation enters through the windows, warming the air and the interior surfaces.
The intensity of the radiation depends on the orientation of the windows — for example, a south-facing window captures more solar energy in winter, while a west-facing window can cause overheating in the summer.
The windows are therefore key elements in the thermal comfort of a passive house.
Indirect Gain System
In this system, heat is stored in a collector wall, placed between the glazed exterior and the interior space.
This massive wall (made of concrete, brick, or stone) absorbs solar radiation during the day and releases heat gradually at night.
This process occurs through:
- Convection, when the ambient air temperature is lower than the surface temperature of the wall;
- Radiation, when nearby surfaces are cooler than the heated wall.
Thermal inertia — that is, the ability of a material to store and release heat — depends on two factors:
- Thermal diffusivity → how quickly the material reacts to temperature changes;
- Thermal effusivity → the ability of the material to exchange heat with the environment.
A classic example of this system is the Trombe wall, created by Professor Félix Trombe and architect Jacques Michel in the 1950s and 1970s.
To increase solar absorption, the exterior surface of the wall should be painted dark.
Greenhouse System
The passive solar greenhouse system consists of a closed glazed space, associated with walls and floors of high thermal mass.
During the day, the greenhouse captures and stores heat, acting as a buffer zone that reduces energy losses on cold days or at night.
The heat can be transferred:
- Directly, through air circulation between the greenhouse and the interior;
- Indirectly, through the radiation released by the materials that stored heat.
To avoid overheating in the summer, especially in southwest-facing greenhouses, seasonal shading should be installed, such as walls or deciduous vegetation that block the summer sun, but let in the winter sun.