France is behind its objectives regarding the development of solar thermal energy. The company NEWHEAT has the objective to help reducing this gap by designing, financing, building and operating solar thermal plants which provide solar heat to large consumers: industrial processes and district heating networks. The present work focuses on the optimization of the operation of a solar thermal plant working at low temperatures (<100 degrees C) from a techno-economic point of view. The plant's architecture is defined and the models (solar field, thermal energy storage, plate heat exchanger) are validated with experimental data from the largest solar thermal plant in France (Condat-sur-Vezee). An optimization problem is formulated: the objective is to maximize, on a short time horizon, the profits from the heat sale to the consumer by taking into account the operating costs. An equation-oriented solution strategy is used to solve the NonLinear Programming (NLP) problem. Very often, operating costs are not taken into account but the results of this work show that they have an impact on the operation of the plant which differs from standard control strategies. A major part of this work was done using a tilted solar field but in a second part, as it was the case for the solar plant in Condat-sur-Vezee, the use of a solar tracking device was demonstrated and found to be an interesting alternative for the future of this field.