Separation of the optical and thermal domain

Any attempt to provide precomputed static directional solar heat gain coefficients (DSHGC) to calculate the total solar energy transmitted through a building component is linked to systematic errors. The total energy induced by solar radiation comprises optically transmitted energy and thermally transmitted secondary components. Deriving DSHGC values, therefore, implies solving a thermal balance equation with predefined thermal boundary conditions and thermal material properties. This means that in order to derive the commonly applied DSHGC values, it is necessary to anticipate the results and boundary conditions of the dynamic thermal simulations to which they will be applied. This actually constitutes a circular reference.
The DSHGC approach emerged from gradually adapting the constant total transmittance coefficient approach (g-value). It does, however, not seem reasonable to try to adapt this concept to assess the dynamic performance of glazings. The total transmittance coefficient was defined to provide a method Main methodological approaches RadiCal, D. Rüdisser 12 for simple performance characterisation of windows and glazing systems and relies on standardized and stationary boundary conditions. The method is not designed to accurately model dynamic effects related to thermal mass in the glazing or façade under real-world irradiance. While some errors occurring on dynamic simulations will cancel out when integrated heating or cooling energy demands are analysed, others will not, as they will effectively change the heating and cooling load profiles. Beyond that, dynamic effects significantly affect interior surface temperature profiles, which is relevant for thermal comfort assessment. In order to overcome these issues, the novel approach developed here follows a strict separation into solar radiation modelling (optical domain) and thermal modelling (thermal domain) (see Figure 4). The results provided by the RadiCal method allow for accurate calculation of all relevant absorbed and transmitted solar radiation components during the dynamic thermal simulation process, overcoming the shortcomings of the DSHGC approach and making its use obsolete.