Co-simulation of transient start-up performance in natural draft direct dry cooling system under varying wind conditions

The natural draft direct dry cooling system (NDDDCS) offers advantages over traditional forced draft air-cooled condensers and indirect natural draft systems, including reduced complexity, lower auxiliary power consumption, and improved thermal efficiency via direct condensation. This study investigates the transient cold start-up and operational performance of a large-scale (900 MWt) coal-fired power plant with vertically arranged heat exchangers under no-wind and crosswind conditions. A novel co-simulation approach couples a transient 3-D CFD air-side model (Ansys FLUENT) with a 1-D transient steam-side model (Python), integrated through PyFLUENT. Validated against previous steady-state models, the transient simulations capture complex airflow and thermal start-up behaviour. Four operational scenarios are analysed: windless start-up with step input, windless ramped load, ramped start-up under 9 m/s crosswind, and full-load turbine islanding. Results reveal significant flow and thermal inertia, causing a delayed response during start-up, though condenser pressure limits remain within safe bounds. Crosswind conditions enhance start-up performance, indicating beneficial airflow effects. The system demonstrates strong turbine islanding capability (100% to 265% load increase) without breaching low-pressure turbine protection limit. Design adjustments to the tower-to-heat exchanger layout are recommended to further improve start-up efficiency.