Modeling hybrid solar gas-turbine power plants: Thermodynamic projection of annual performance and emissions

The annual performance, fuel consumption and emissions of a hybrid thermosolar central tower Brayton plant is analyzed in yearly terms by means of a thermodynamic model. The model constitutes a step forward over a previously developed one, that was satisfactorily validated for fixed solar irradiance and ambient temperature. It is general and easily applicable to different plant configurations and power output ranges. The overall system is assumed as formed by three subsystems linked by heat exchangers: solar collector, combustion chamber, and recuperative Brayton gas-turbine. Subsystem models consider all the main irreversibility sources existing in real installations. This allows to compare the performance of a real plant with that it would have in ideal conditions, without losses. Furthermore, the improved version of the model is capable to consider fluctuating values of solar irradiance and ambient temperature. Numerical calculations are presented taking particular parameters from a real installation and actual meteorological data. Several cases are analyzed, including plant operation in hybrid or pure combustion modes, with or without recuperation. Previous studies concluded that this technology is interesting from the ecological viewpoint, but that to be compelling for commercialization, global thermal efficiency should be improved (currently yearly averaged thermal efficiency is about 30% for recuperative plants). We analyze the margin for improvement for each plant subsystem, and it is concluded that, the Brayton heat engine, by far, is the key element to improve overall thermal efficiency. Numerical estimations of achievable efficiencies are presented for a particular plant and real meteorological conditions.