In a context of sustainable development, enthusiasm for concentrated solar power technologies is developing. So, the CSPIMP (Concentrated Solar Power efficiency IMProvement) project started in 2013 in order to achieve a better competitiveness for the CSP plants. Its main target is to develop new procedures to improve steam turbine start up cycles, maintenance activities and plant control. One challenge in the project is to better forecast the solar resource in order to optimize plants operation. The development of a clear sky model is therefore an essential step to forecast Direct Normal Irradiance (DNI) because clear sky represents the nominal operating conditions of a concentrating solar power plant. This paper focuses on estimating DNI under clear-sky conditions using a knowledge model based on the coefficient of relative optical air mass, DNI and atmospheric turbidity fluctuations. The satisfactory results obtained validate the proposed methodology.