Performance of a Natural Draft Direct Dry Cooling System for a Supercritical CO2 Power Cycle Using a Coupled Simulation Approach

This work develops a coupled simulation framework between a 1-D thermofluid network process model and a 3-D CFD air-side model of a natural draft direct dry cooling system (NDDDCS) for a supercritical carbon dioxide (sCO2) power cycle. The method leverages the computational efficiency of the network model to capture sCO₂-side property distributions within the heat exchanger tubes, while harnessing the high-fidelity capabilities of CFD to resolve intricate airflow phenomena on the air-side. An NDDDCS sized for a 50 MWe sCO2 application is simulated under no-wind conditions. Heat exchangers, serving both the precooler (PC) and intercooler (IC) heat loads, are included in the same cooling tower, featuring vertically arranged tube banks around the circumference. Results show that the model captures airflow effects such as recirculation at the heat exchangers and flow separation from the internal tower walls, which reduce the overall air mass flow rate by 6.25 % and heat transfer rate by 13.85 %, in comparison to a previous 1-D model, raising the sCO2-side outlet temperatures. Furthermore, the model investigates the variation in system performance of a lumped configuration, where the PC and IC tube banks are grouped, and an alternating configuration, where the tube banks are interspersed. The alternating configuration yields a relative performance increase of 1.88 % in the air mass flow rate and 1.02 % in heat transfer, as chaotic mixing and swirling flows are minimised, producing more uniform airflow through the tower. To assess whether implementing this alternating arrangement is justified, performance under crosswind conditions should be investigated.