Natural draft direct dry cooling system performance at various application scales under windless and windy conditions

Abstract

The natural draft direct dry cooling system (NDDDCS) presents a promising alternative to conventional forced draft air-cooled condensers (ACCs) and indirect natural draft dry cooling systems, overcoming their limitations by efficiently condensing process steam directly and passively. The NDDDCS achieves superior thermal efficiencies while reducing system complexities, operational costs, and auxiliary power consumption. Potential reductions in operational expenses for the system may offset initial capital investments, resulting in lower lifetime costs. While research to date has focused on large-scale NDDDCS performance under varying wind conditions, this work addresses a critical gap by investigating the system’s scalability for various applications, including a large-scale coal-fired power plant (900 MWt, 165 m total height), a medium-scale concentrated solar power plant (CSP) (100 MWt, 65 m total height), and a small-scale water desalination plant (1 MWt, 11.5 m total height) under both windless and windy conditions. To achieve this, a steady-state 3-D computational fluid dynamics (CFD) model is developed, validated and scaled, incorporating innovative features such as an atmospheric pressure gradient and the ability to capture reversed flows through the system’s heat exchangers. Additionally, the model assesses inlet air temperature data at a per-cell resolution, enabling the ability to predict non-uniform temperature distributions in the heat exchangers, including hot-spots caused by airflow recirculation. Analysis of velocity and temperature fields across all scales reveals that recirculation effects negatively impact NDDDCS performance, particularly at the middle circumferential heat exchanger sector. The hyperbolic shape of the tower shell induces internal recirculation zones and reduces the effective flow volume. Under windy conditions, performance losses of 44%, 40%, and 40% are observed at crosswind speeds of 3 m/s, 6 m/s, and 9 m/s for small, medium, and large-scale towers, respectively. Beyond these speeds, the relationship between system performance and draft driving potential becomes non-linear due to crosswind effects dominating. Furthermore, a novel effect is identified at wind speeds of 9 m/s, 15 m/s, and 18 m/s for small, medium, and large-scale systems, respectively, where an inflection point allows for system performance recovery. This study underscores the NDDDCS’s potential as a cooling solution for modern power cycles across various scales, while highlighting the implications of scaling the system for medium- and small-scale thermal applications.