A model based control is used to control a complex system which has a large settling time. The system that is shown in this paper is a drum boiler system integrated with a solar water heating system to provide pre-heated feed water as input to it. Thus, making a hybrid energy system where energy conservation is obtained by the use of solar energy in conventional thermal power plants. Matlab/Simulink is used to model the plant dynamics and use the same model to estimate current state and validate the real time results by the use of processor in loop simulation.