Integrating solar power in large combined-cycle power plants

Large Solar Thermal Electric Generation Systems are in rapid development in many areas of the world: many of them are based on Concentrated Solar Power technology, using either a central receiver or rows of large parabolic trough solar collectors. The solar thermal thermodynamic conversion appears interesting for integration with high-efficiency combined cycle power plants, running on natural gas and using the combination of gas and steam cycles with advanced matching of the two by accurate design of the Heat Recovery Steam Generator (HRSG). In most of the proposed approaches, however, integration of the solar section for producing steam is performed using a typical "parallel-boiler" scheme. This approach produces an advantage in terms of reducing the consumption of the primary fossil fuel (natural gas); however, the original efficiency of the combined cycle is negatively affected, as the performance of the HRSG decreases. The idea here proposed is to follow design and off-design control guidelines which try to maintain a high efficiency of the HRSG (including operation with variable radiation conditions), and possibly to improve its performance, using the solar contribution to alleviate the pinch/heat capacity mismatching between the gas and the water/steam circuits. The fundamental idea is that of adding a parallel external solar evaporator to the specific heat transfer bundle located inside the HRSG. The application refers to a large HRSG with three pressure levels, using advanced parabolic trough solar collectors. Two innovative control strategies are proposed for correctly managing the off-design performance: control of the temperature increase across the collector in order to maximize its exergy efficiency (in converting the original energy from the sun) and implementing a dynamic reconfiguration for the solar field in the medium/high pressure sections, which use the same type of collector. The simulation over one representative year produced promising results.