Effect of Dew Point Temperature and Sky Patch Discretization on Annual Daylight Simulations

Climate-based daylight modeling is now the established state of the art in daylight planning practice. The validated matrix-based methods such as the three-phase or five-phase method as extensions to the daylight coefficient approach, enable time-efficient annual daylight simulations and form the basis in most common software tools. In addition to the room, the façade system and the external situation, the description of the sky model is an essential input variable for these methods. Common practice is to use the Perez sky model, which predicts a sky distribution from direct and diffuse irradiance values. The continuous model is then discretized into sky regions to satisfy the based matrix approach.In this paper we first investigate the influence of the consideration of the dew point temperature in the Perez model both on the sky model itself and on the resulting daylight performance metrics in an example building. Based on a study for 3130 locations worldwide, we show for which climates a correct mapping is of major importance. Secondly, we analyze the effects of the different distribution of skylight and sunlight on discrete sky patches and discuss the differences with regard to resulting interior daylight values as well as the challenges for daylight simulation methods.