The crosswind effects on the start-up process of natural draft dry cooling towers in dispatchable power plants

Abstract

To investigate the suitability of natural draft dry cooling towers (NDDCTs) for PV/CST hybrid power plants in practice, the crosswind effects on transient start-up of NDDCTs are analyzed first using a simplified theoretical approximation and then numerical computations. The study covers crosswind speeds from 1 to 15 m/s. Simplified model identifies two distinct flow patterns that correspond to two ends of the wind speed range. The intersection of asymptotes technique is adopted to find when the flow switches from one pattern to the other. The switching wind speed also corresponds to the longest start-up time. Results of numerical analysis agree with these observations and provide more detailed insights. It is found that natural draft dominates the air flow at wind speeds below the cross-over and crosswind is dominant at higher wind speeds. The cold air incursion at the top plays an increasingly important role as the wind speed is increased and finally shuts off the tower top completely preventing any outflow from the top. The uniformity index is introduced as a single parameter to describe the flow distribution characteristics. The flow field is uniform if only a single mechanism becomes dominant: either natural draft or forced convection caused by crosswind. The start-up time is shorter when the flow is uniform and the longest when the flow field is least uniform. All results have been validated against experimental data collected on the University of Queensland natural draft dry cooling tower Gatton test rig. This research provides a practical perception for the crosswind effects on the transient start-up of NDDCT for dispatchable thermal power plants, guaranteeing a better cooperation between PV and CST part in PV/CST hybrid power generation system.