Design of steam generator systems for concentrating solar power plants

Concentrating solar power plants employ reflecting mirrors to concentrate the incident solar irradiation, firstly converting it into high temperature heat and secondly into electric energy. The stochastic and fluctuating nature of the solar irradiation requires more operating flexibility as compared to traditional systems. The rate of response to frequent load changes and start-up procedures is however constrained by the use of conventional components, designed to operate under steady-state conditions. In particular, steam generator systems were designed for baseload applications and therefore their responsiveness to load changes is limited by thermo-mechanical stress, which might a˙ect the components lifetime or cause failure.
This thesis work has two main objectives. The first one is to quantify the impact of the heating rate constraints on the economy of parabolic trough power plants to assess the prospects for improvement using more responsive steam generators. This is carried out by modelling the transient performance of such power plants and implement control strategies, which account for these constraints. The second objective is to define a method to design the steam generator system considering maximum allowable heating rates and their impact on the performance of the power plant. This is achieved by evaluating the significance of implementing such constraints in the sizing procedure and assessing the transient performance of the new designs. The models are validated against either data available in literature or provided by power plant operators.
The results from this thesis indicate that it is of significant importance to account for the steam generator thermo-mechanical limitations. The use of suitable designs to reach heating rates in the order of 7-10 K/min could increase the electric energy production of the power plant, while at the same time allowing for operating decision flexibility throughout the year. Designing the steam generator systems for such constraints results in di˙erent optimal configurations as compared to traditional sizing methods. However, the derived increase in the steam generator cost can be compensated by the gain in electric energy production and associated revenues.