Off-design optimization for solar power plant coupling with a recompression supercritical CO₂ Brayton cycle and a turbine-driven main compressor

The recompression supercritical carbon dioxide Brayton cycle is one of the most promising candidate power blocks for the solar power tower plant. As an important component, the main compressor can be driven by an electric motor or a turbine. The layout with the turbine-driven main compressor has the advantages of low cost, high efficiency, and safety during loss-of-load conditions. For such a system, the main compressor and the turbines are connected with the synchronous generator and assumed to be operated with a constant shaft speed, which has a big challenge for the operation under off-design conditions with various ambient temperatures, and the corresponding optimal operation parameters and control schemes are lacking. Especially in the condition of low ambient temperature, the traditional control scheme, which usually maintains the main compressor inlet temperature at the design-point value, obviously underestimates the efficiency of the solar power tower system. In the present study, an integrated model of solar power tower coupling with the Brayton power cycle is developed, and the particle swarm optimization algorithm is utilized to search for the optimal operation parameters and control schemes under off-design conditions with various ambient temperatures. Finally, annual performance analyses are conducted to investigate the actual effects of the proposed control schemes. The results indicate that: in the case with low ambient temperature, the proposed control scheme improves the power cycle efficiency by 0.78%; the total power output of 390 MW.h (accounting for 1.02% of the yearly power output) can be saved. In the case with high ambient temperature, the power cycle efficiency experiences an inevitable decrease, which causes the total wasted power output of 253 MW.h (accounting for 0.66% of the yearly power output); the losses caused by high ambient temperature are restrained in a small range by the proposed control schemes.