Planning over the intermediate and long terms for the operation of solar energy systems must be based on the use of measured data and performance models of system components. We present a dynamic model for predicting the thermal performance of a low-temperature solar power plant. We analyse the optimal control regime for energy management. Validation of the model was performed on a large solar plant located in the north of Tunisia. The results showed good agreement between measured and model values. During the cold season, the plant efficiency increases considerably when a greenhouse is connected to the power plant.