Transpired Solar Collectors are solar air heating systems made of pre-finished perforated steel skins. Treated to enhance its absorbance of solar energy, the steel skins are installed onto south-facing walls or roofs creating a cavity between the metal skin and the walls or roofs.
Transpired Solar Collectors’ Air-Heating Process
- The steel skin absorbs the sun’s energy and heats up the boundary layer of air on its surface.
- A fan unit delivers the air in the cavity into the building, which subsequently draws the outside air through the perforations into the cavity, including the boundary layer of heated air around the perforations.
- As it passes through the perforations, the warmed air continues to be heated by the edges of the perforations.
- When it moves upwards in the cavity, the warmed air is further heated by the internal side of the steel skin. 
- The heated air can then either be distributed into the building or ducted to a HVAC (Heating, Ventilation, Air Conditioning) unit to reduce the energy required by the main heating system.
Transpired Solar Collectors’ Other Potential Benefits
- Low maintenance and operation costs with a long life span
- Bring back the heat lost through building fabric
- Circulate hot air to the floor level, which reduces heating requirements
- Provide an extra layer of insulation
 The immediate areas surrounding perforations on the exterior of the steel skin heat up the air before it is drawn through the perforations: see C.F. Kutscher, “Heat Exchange Effectiveness and Pressure Drop for Air Flow Through Perforated Plates with and without Crosswind”, Journal of Heat Transfer, 1994, vol. 116, pp. 391-399 (p. 397, fig. 9). Most of the heat gain comes from the surrounding areas of perforations on the external side of the steel skin. The edges of the perforation, which heat up the air when it passes through, also contribute to some heat gain. Finally, there is a small contribution to the heat gain from the cavity-facing side of the steel skin: see G.W.E. Van Decker, K.G.T. Hollands and A.P. Brunger, “Heat-Exchange Relations for Unglazed Transpired Solar Collectors with Circular Holes on a Square or Triangular Pitch”, Solar Energy, 2001, vol. 71, No. 1, pp. 33-45.