Brief description

A solar chimney is a passive ventilation device that uses the sun’s energy to heat the air inside a vertical duct, increasing the thermal buoyancy (stack effect) and forcing the extraction of air from the building.

Use / Function

Its primary function is to improve natural ventilation on hot, sunny days when the temperature difference between the interior and exterior may not be sufficient for conventional ventilation.

• Primary use: Forced extraction of hot air from homes or greenhouses.
• Secondary uses: Can be integrated with heating systems in winter (by reversing the flow or redirecting the hot air).
• Scale: Residential and agricultural buildings.

Operating principle

1. Solar Collection: The chimney has one face oriented towards the sun (generally south in the northern hemisphere) covered with glass or transparent material.
2. Heating: Inside, an absorbent surface (painted matte black) is heated by solar radiation.
3. Greenhouse Effect: The air inside the chimney becomes much hotter than the ambient air.
4. Thermal Buoyancy: The hot air becomes less dense and rises rapidly, creating a strong suction at the base of the chimney that extracts stale air from inside the building.

How to create it

1. Vertical Duct: Build a vertical shaft that is as tall as possible to increase the draft effect.
2. Absorbent Surface: The back wall of the duct should be made of a material with high thermal mass or conductivity (metal, dark brick) painted black.
3. Transparent Cover: Install glass or transparent plastic on the face exposed to the sun to allow light in and retain heat (greenhouse effect).
4. Insulation: Ensure that the walls that are not glass are well insulated to avoid losing the generated heat to the interior or exterior.

Technical Level Required

• Intermediate: Requires knowledge of construction and precise solar orientation.

Materials needed

• Glass: For the front face (essential for the greenhouse effect).
• Metal (Black Iron or Steel sheet): Excellent for absorbing heat quickly and transferring it to the air.
• Brick / Concrete: For structure and thermal mass.
• Bitumen / Black Paint: To coat the absorbent surface.

Variants and improvements

• Trombe Wall: Similar to a solar chimney but primarily designed for winter heating; it has upper and lower openings to recirculate warm air back inside.
• Solar Roof: Using the roof slope as a solar collector for ventilation.

Limits and risks

• Solar Dependency: Only works effectively when the sun is shining. At night, the flow stops or can reverse (drawing in cold air) if dampers are not closed.
• Overheating: If not well-insulated from the interior, it can radiate unwanted heat into the house.
• Maintenance: The glass must be kept clean to remain efficient.