A.Belaid STUDY OF SHADING EFFECTS IN HELIOSTATS – TOWER SYSTEM USING A SIMULINK MODEL A.Belaid 1 , B.Bezza 1 , T. Arrif 1 , A. Gama 1 , R.Khelifi 1 1 Applied Research Unit on Renewable Energies, EPST Development center of renewable energies, Ghardaia- Algeria ABSTRACT In this work we present a design of a mathematical model used in the simulation of the behavior of a heliostats- tower optical system. The model permits the calculation of the orientation angles of a heliostat (two axes tracking system) in a solar tower system, it also allows to calculate the shading area of a heliostat on another. The reflected radiation to the target (the tower) is calculated from the incident radiation from the sun and calculated shaded area. For each heliostat, the cosine effect (angle of incidence) is included in the calculation of the reflected radiation, and then, the shadow effect is taken into account on reducing the radiation reflected by an amount which depends on the shadow area Keywords: Shading, Solar tower, Solar tracking, Simulink INTRODUCTION In central solar power, heliostats follow and reflect sunlight onto a receiver located at the top of a tower. The heliostat field plays a key role in contributing to the performance and the total cost of solar tower power plants [1]. The first development of the formula net-altitude tracking (spinning-elevation) to replace the formula Azimuth-elevation tracking which is typically used was made by Chen [2]. The performance of the heliostat field is defined in terms of optical performance which is the ratio of the net power absorbed by the receiver to the power incident normally on the field. The optical losses of heliostats field take account of losses of the field disposition such as cosine effect, shading and blocking and losses due to other factors such as imperfect mirrors reflectivity, atmospheric attenuation and spillage of the receptor [3]. Although several number of heliostat field arrangement is possible, most of the heliostat field is designed in radial stagger pattern [3],[6].This arrangement ensures that no heliostat is consigned in front of another heliostat in adjacent rings along a spoke to the tower. Therefore, the beam reflected from heliostats can pass between the neighbours to the receiver. Several studies have indicated that the radial offset disposition is most effective because it reduces the surface field and losses due to atmospheric attenuation [4]. Our work consists of designing a mathematical model that allows the calculation of flux reflected to a fixed target (tower) and calculating the effects of shading between heliostats.