## 5 Conclusions  
Daylight/sunlight modeling is needed in diverse renewable energy applications from PV, through solar concentrators to daylight harvesting, including the illumination of interior spaces. The latter is important to evaluate the potential of using natural daylight for electric lighting energy savings in buildings. The system of the hollow light pipes is a useful concept to deliver a considerable portion of daylight to interiors. Correct predictions of daylight illuminance levels under different meteorological conditions can be decisive in designing electric illumination systems for various building zones. However, most of the models currently in use are based on a simple concept of the homogeneous skies that on one hand allow for rapid numerical modeling, but on the other hand cannot mimic the real sky states that occur in nature, especially under partly cloudy conditions. Isolated clouds reflect sunlight and also diffuse light in a complex manner, while making the sky luminance a non-monotonic function of the zenith and azimuth angles. Computers are now efficient enough to be used to model sky  
Figure 14: The same as in Figure 13, but for [PERSON]’s all-weather model. luminance under unstable broken cloud arrays, so there is no real reason for relying on oversimplified empirical tools. We have developed a new solution that encapsulates the functionality of the UniSky simulator for computing the sky luminance distribution under the arbitrary cloud coverage and the HOLIGILM tool for obtaining the light field below a hollow tubular light guide. In synergy, these tools represent a unique concept for a fast and accurate prediction of daylight illuminance conditions in building interiors. This new solution allows not only for random cloud sizes and distributions in the sky, but also for modeling cloud arrays drifting from or to a specific direction as observed in many situations. This way the availability of luminous energy can be computed in any site, under any exterior conditions and for any modeling domain. The demonstration is made for a hollow light pipe in order to avoid unwanted (parasitic) effects from pipe bends. However, the computational concept proposed in this paper is not limited to only straight pipes, but is applicable to arbitrary geometries. The UniSky simulator is able to generate the light field that is the input to any light-guide integrator. UniSky Simulator also allows to separate the contribution from the clouds, haze, or air pollution by performing the computations for clean atmosphere first, then for a turbid atmosphere, and then for a specific cloud arrangement, while holding all other parameters constant. Since the configurations differ only in the properties we have added, the relative contribution of the clouds can be obtained from a comparison of the illuminance distributions for the respective cloud model and cloud-free model. This way we can distinguish the effects from the air pollution, haze, or the purely cloudy day. We have shown that the models of homogeneous skies are successful in the prediction of the light guide efficiency even under partly cloudy conditions, but fail in reproducing the angular distribution of luminous energy in the room interiors. This is because the luminous efficiency is derived from the luminous fluxes at the top and bottom interfaces of a light guide. However, the peak luminance values that appear due to isolated bright clouds or any other imperfections are efficiently suppressed in the process of integration, so there is no reason to suppose that the light-guide efficiency derived from the homogeneous models will deviate much from that computed from an exact sky-luminance model. On the other hand, the cloud arrangement can have a large impact on the directionality of light beams below a light-pipe, and thus on the illuminance patterns as well. Although determined within an acceptable error tolerance the light-pipe efficiency derived from an idealized model is not a guarantee of correct assessment of daylighting quality in the interiors.