Global solar transmittance of vertical glazings oriented towards the Equator

Abstract

The thermal gain of glazings in buildings is usually modelled using concepts such as the solar factor (SF), the sum of the normal-incidence solar transmittance plus the ratio of the thermal energy emitted indoors to the incident solar irradiance. An important part of the SF of a vertical glazing is the visible solar transmittance which depends on the sky conditions and the relative amounts of beam and diffuse components of incident solar radiation. The directional transmittance of both components can be very different for large incidence angles.Using three years of 1-min measurements under outdoor conditions, this dependence is investigated and three global solar transmittance models are locally adjusted and evaluated. When the diffuse fraction of global solar radiation is measured, all models show good performance, with relative RMSD under 3% of the average transmittance and negligible bias. However, if the diffuse irradiance measurements are not available and it is estimated from locally adjusted diffuse fraction models, it is found that the rRMSD increases to 10% . This increase in accuracy is associated mostly with cloudy conditions.