Calculation of solar factor, g-value, of glazing

Method 1: The equivalent value of the thermal conductivity coefficient for all air cavities of the window frame determined according to EN ISO 10077-2: 2012 Thermal performance of windows, doors and shutters - Calculation of the heat transfer coefficient - Part 2: Numerical method for frames,

Method 2: The radiative and convective heat transfer in the air cavities is taken into account using computer-aided numerical method, calculating the equivalent convective component of the heat transfer coefficient hc for the air cavities depending on the dimensions of the cavities and the temperature difference across them. The radiative heat transfer coefficient in the air cavities is calculated using the view factors model, where the emissivity of the adjacent material surfaces is set to = 0.9.

Calculation of solar factor, g-value, of glazing

In contrast to opaque structures, characterised mainly by the thermal conductivity coefficient, the properties related to solar transmittance are also important for glazing. There are two main types of properties - solar and luminous. Solar refers to more or less the whole spectrum of solar radiation as an integrated radiation including both spectral and directional radiation, luminous refers only to its visible part - light and its direction of incidence and reflection. The "betrayal" is that the symbols for both solar and luminous characteristics, i.e. transmissivity (direct transmittance), , reflectance, , and absorptivity, , are the same. It is therefore a good idea to add the subscripts 'sol' (solar) and 'opt' (optical/luminous) respectively to avoid confusion. In addition to the above properties, the total solar energy transmittance factor, the so-called solar factor or also g-value, is given as a global characteristic of the solar properties. The solar factor, g, is defined according to EN 410:2011 as the sum of the direct solar transmittance, sol, and the secondary heat transfer factor, qi, through the glazing towards the inside. The secondary heat transfer factor is due to the longwave infrared radiation (emission) of that part of the incident solar radiation that has been absorbed by the glazing, and also due to the heat conduction and convection induced by it. The corresponding equation for the g-value is then

g = sol + q

i

The direct transmittance of solar radiation, sol, is a property of glazing. It is the fraction of incident solar radiation that passes through the glazing and can be described as the primary heat gain, g1, divided by the total incident solar heat flux (radiation intensity), e (some standards, such as ISO 15099:2003, use the symbol  for the total incident solar radiation intensity instead of e). The secondary heat transfer coefficient, qi, depends on the absorption coefficients, , of the individual layers of the glazing, their emissivity, , and thermal conductance,  , including cavities and the transfer of heat through surfaces. It is, as already mentioned, the absorbed fraction of incident solar radiation converted to heat flux by radiation, convection and inward conduction, which can be described as the secondary heat