About Solar Radiation

Explains the concept of solar radiation and its computation in the cooling load calculation.

Solar radiation makes a key contribution to the generation of a cooling load in a building: Via solar radiation through transparent areas and because of the heating of enveloping surfaces, heat inputs occur, increasing the cooling load.

Direct and diffuse radiation

On entry into the earth’s atmosphere, solar radiation is split into a direct component and a diffuse component. In each case, both components are reduced by a clouding factor, which stems from the meteorological data and is time- and site-dependent. The diffuse component from the atmosphere is independent of the positioning of the structure relative to the sky.

Reflection factor

The direct component from the atmosphere is also split into a diffuse component reflected from the ground and the direct component, which is reduced by the angle of incidence θ. The component reflected from the ground is dependent on the properties of the latter.

Solar radiation is consequently made up of the direct component from the atmosphere, the diffuse component from the atmosphere and the diffuse component from ground reflection. If this radiation comes into contact with transparent surfaces such as windows, it is reduced by
  • external shading,
  • Energy transmittance of panes of glass,
  • internal shading.

Solar Calculation Models in LINEAR Building

Depending on the selected method of calculation, various solar calculation models are available for selection. In VDI 2078, the VDI 6007 solar calculation model is used. With ASHRAE, you have a choice between the ASHRAE 2005 (less direct, more diffuse radiation) and ASHRAE 2013 (closer to the VDI 6007-3 values) solar calculation models. If the atmospheric clouding factors τd and τb are absent from the climatic data, only the calculation according to ASHRAE 2005 is possible.