General model and definitions

According to the applicable standards (see section 3.4), the total solar energy transmittance (or solar factor) 𝑔𝑡𝑜𝑡 or (solar heat gain coefficient SHGC) describes the ratio of the total heat gains to the incident solar radiation. It can be written as the sum of two components:

The first component 𝜏𝑒 , referred to as solar direct transmittance, reflects the fraction of solar radiation which is directly, i.e. “optically”, transmitted into the interior of a building in the form of shortwave radiation.
The second component 𝑞𝑖 , referred to as secondary heat transfer factor represents the fraction of solar radiation which is absorbed by the individual layers of the fenestration system, transmitted to the innermost surface of the system and subsequently released to the interior by convection and thermal radiation. Just like 𝜏𝑒 , the quantity 𝑞𝑖 is dimensionless and not a unit of energy, as indicated by using the letter q.
While not explicitly stated in the formulation presented below, the relevant international standards (ISO 15099, 2003; ISO 9050, 2003; EN 410, 2011) implicitly define the secondary heat transfer factor 𝑞𝑖 as:

Where 𝐸𝑆 is the incident solar radiation flux density, i.e. irradiance, while 𝛷𝑞,𝑖𝑛𝑡(𝐸𝑆) and 𝛷𝑞,𝑖𝑛𝑡(0) represent the heat flux densities to the interior with and without solar irradiance. The considered heat fluxes comprise components of thermal radiation, convection and conduction. While there are some differences regarding the components’ definitions and calculation methods, the applicable standards define secondary heat transfer as the amount of additional heat flux induced by solar irradiance. This also comprises non-linear effects, such as the change of thermal conductivity of gas layers due to an increase in absolute temperature and temperature gradients.
The relevant standards (discussed in section 3.4) provide detailed information regarding the thermal modelling of the multi-layer system (e.g. consideration of ventilation, buoyancy, change of gas viscosity etc.). However, the major determining factor for all secondary effects is the absorption of solar radiation in the individual layers of the fenestration system. Reliable thermal performance values can, therefore, only be obtained based on a model that is able to provide accurate layerwise absorption and transmission values for any given incident solar radiation profile.