Principles of operational optimization of CSP plants based on carbon dioxide mixtures

This research, developed in the framework of the SCARABEUS project, studies the off-design performance of transcritical power cycles running on C₂-S₂ mixtures in Concentrated Solar Power applications. The objective of this work is to identify optimum operational strategies that maximize net energy production when exposed to variable ambient temperature, with special focus on the operation of the Heat Rejection Unit (Air-Cooled Condenser).

Four different strategies are identified, depending on ambient temperature: variable or constant condensation pressure for ambient temperatures lower than the design value, and constant turbine inlet temperature or constant return temperature of the heat transfer fluid for ambient temperature higher than design value. The results show that a combination of variable and constant minimum cycle pressure stands as the most promising alternative for low ambient temperatures, enabling net system efficiencies higher than 41%. As a drawback, this strategy poses potential issues on the control of the compressor inlet, which must be further investigated in future works. On the other hand, constant turbine inlet temperature enables higher net performance than constant return temperature of the heat transfer fluid, even if at the expense of a reduction in energy storage capacity for the same inventory of molten salts and the creation of detrimental thermal transients. Finally, it is found that the Air-cooled condenser configuration with all fans equipped with Variable Frequency Driver stands as the best alternative to maximize the techno-economic performance of the system.