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solar load through windows


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What is the solar energy that enters interior spaces through transparent wall parts (i.e. windows) and what is its distribution on the different inside wall surfaces (i.e. solar spot due to direct solar radiation plus background irradiation due to diffuse solar radiation) ?


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A transparent material is defined by a solar transmission factor > 0 in the Colours window

The transmission factor is applied on the incident solar radiation on a material surface after deduction of solar reflection

Consider a room (volume: 4.20 m x 3.00 m x 2.40 m) behind a south oriented external wall with a window (area: 1.80 m x 1.80 m)
The other walls and ceiling (clipped off in the figure below) are not exposed to the sun

The absorbed solar radiation on the window surface is lower (due to the loss of transmitted radiation) than the absorbed solar radiation on the non-transparent wall surface (Uccle climate, 10 June 12:30 am, cf. figure below: wall 287 W/mē, window 45 W/mē)

The transmitted solar radiation (through the window) originating from the direct solar radiation is projected through the window on the floor (and other room wall surfaces) to create a sunlit spot
The figure below (top view of room after cutting off the ceiling) shows the sunlit spot at 10:30 am

The transmitted solar radiation (through the window) originating from the diffuse solar radiation is projected through the window on the room wall surfaces proportional to the view factors from the inside window surface to the other wall surfaces

The view factors are calculated (without significant loss of accuracy) using a less fine grid mesh than the final grid mesh of the solar radiation fluxes
Otherwise the number of view factors could be unacceptably high: both memory space and calculation time are proportional to the square of the number of view factors

View factor grid mesh (largest mesh size = 0.30 m)

Final calculation grid mesh (largest mesh size = 0.10 m)

The inside wall surfaces have a reflection factor of 0.50 (defined in the Colours window)
The reflected radiation is diffuse and redistributed based on the view factors
The final distribution of absorbed radiation (due to all reflections) is calculated using a radiosity method

Absorbed solar radiation at 10:30 am (full flux scale 0 - 300 W/mē)

Absorbed solar radiation at 10:30 am (lower flux scale 0 - 60 W/mē, which shows better the distribution of background diffuse solar irradiation)

The chart below shows the distribution of average solar irradiation on the different walls, floor and ceiling throughout the day (Uccle, 10 June)
The right wall has its peak value after noon, because it receives mainly the afternoon sun

Inside walls may also be transparent
The reflected solar radiation from direct solar radiation is automatically converted to diffuse solar radiation at interior wall surfaces
The transmitted solar radiation to the outside environment is lost (in the same way as the reflected solar radiation on an exterior wall surface)
All other reflected and transmitted solar radiation is solved using the radiosity method

The figures below show the low scale absorbed solar radiation in the room, divided by a double glazing wall (parallel to the side walls)

Next page: temperatures due to solar load