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.

Natural draft direct dry cooling systems (NDDDCSs) combine the benefits of two traditional dry cooling systems, air-cooled condensers (ACCs) and indirect natural draft dry cooling systems, to offer advantages such as reduced system complexity, high thermal efficiency, and low parasitic power losses and operational costs. These scalable systems can potentially operate in both large coal-fired power plants (900 MWt) and medium-scale concentrated solar power (CSP) plants (100 MWt). While prior research focused on NDDDCS steady-state performance, this study delves into the transient behaviour and start-up characteristics of this cooling system, crucial for power plants with rapid start-up requirements or load ramps during electricity demand peaks. The paper employs a transient 1-D mathematical model to characterize the NDDDCSs' response to start-up and load ramp conditions under no-wind, presenting results for both large- and medium scales while identifying limiting factors.