Key Takeaways & Executive Findings
- •• • Ambient wind speed is the dominant factor: at wind speeds >8 m/s, the back pressure difference between towers exceeds 1 kPa, and the upstream tower (Tower 2) exhibits significantly larger fluctuations and reductions in air intake and heat dissipation than the downstream tower (Tower 1), directly impacting unit load and efficiency. • • Ambient temperature primarily affects system back pressure: as temperature rises from 5°C to 34.9°C, back pressure increases from approximately 6.1 kPa to 28.2 kPa, a 4.6-fold rise that can severely constrain summer peak output. • • Closing windward rolling shutters or reducing louver openings improves circumferential air redistribution but reduces total tower air flow and heat dissipation; the loss from the windward zone is not fully compensated by leeward gains, so such measures are not recommended under high-wind summer conditions. • • Bottom bypass windows outperform top bypass windows: at 12 m/s wind, each 20% increase in bottom bypass opening raises heat dissipation by about 0.8%, and increasing from 70% to 100% opening yields an additional ~3% improvement; top bypass windows at 50% opening actually reduce heat dissipation by ~2.2%, so top bypass installation is discouraged. • • NDC systems demonstrate superior economics: compared to design values, the 660 MW NDC unit reduces auxiliary power consumption rate by 1.1 percentage points, lowers net coal consumption by 4 g/(kW·h), and achieves a minimum load of 20% THA (better than the design 30% THA); annual net coal consumption is 3.37 g/(kW·h) lower than ISC and 2.61 g/(kW·h) lower than ACC systems.
Abstract
To investigate the flow and heat transfer characteristics of natural draft direct air-cooling towers (NDC) for large coal-fired power generating units, a three-dimensional CFD numerical model covering major plant buildings, air-cooled radiators, and ambient wind fields was established based on the NDC systems of a 2×660 MW unit of a power plant. The influences of meteorological factors, including ambient wind speed, ambient temperature, and ambient wind direction, as well as regulation measures such as rolling shutters, louvers, and bypass windows on the heat dissipation performance of NDC towers were systematically analyzed. The results demonstrate that ambient wind speed acts as the dominant factor governing the performance of the NDC system. As wind speed rose, the uneven distribution of air intake volume and heat dissipation among each cooling delta increased remarkably, which elevated the unit back pressure, and the upstream tower suffered more severe impacts than the downstream one. Ambient temperature exerted a slight effect on circumferential flow distribution, yet substantially changed the overall back pressure of the system. In terms of regulation strategies, closing rolling shutters in the windward zone and reducing the opening of partial louvers can improve air flow redistribution to a certain extent, but will reduce the total air flow rate and total heat dissipation of the entire tower. By contrast, bottom bypass windows can effectively optimize the air intake on the leeward side and boost heat dissipation under high-wind operating conditions, whereas top bypass windows deliver only limited improvement effects. This research can provide fundamental data and technical references for the optimal design, operational regulation, and energy-saving retrofitting of large NDC units.
1. Introduction
Coal-fired power generation remains a cornerstone of China's energy infrastructure, particularly for baseload supply. Under the dual-carbon strategy, improving unit efficiency and reducing coal and water consumption are critical. Traditional wet cooling systems, despite high heat transfer efficiency, consume enormous amounts of water, limiting their application in water-scarce regions and under tightening environmental regulations. Direct air-cooling technology has been widely adopted in the coal-rich but water-poor northeastern, northwestern, and northern China due to its significant water-saving advantages. Among these, natural draft direct air-cooling towers (NDC) utilize the density difference between internal and external air to drive natural convection, eliminating the need for large fans, thereby reducing auxiliary power consumption and noise, and offering superior economic and environmental benefits.
However, NDC performance is highly sensitive to ambient wind speed, direction, and temperature, as well as tower geometry. Adverse meteorological conditions can induce hot air recirculation, internal and external vortices, and uneven heat dissipation, leading to elevated turbine back pressure during summer, which curtails unit output and economic operation, and in severe cases, triggers unit trips. Existing commercial approaches often rely on forced draft systems or lack effective regulation strategies to mitigate these effects. This study addresses this bottleneck by establishing a comprehensive CFD model that includes plant buildings, radiators, and ambient wind fields, systematically analyzing the impact of meteorological factors and regulation measures such as rolling shutters, louvers, and bypass windows. The findings provide quantitative guidance for optimizing NDC design and operation, particularly highlighting the effectiveness of bottom bypass windows in high-wind conditions, thereby offering a practical solution to enhance cooling performance and operational reliability.
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YANG Qian, SUN Zhenguo, SUN Xinfeng, NING Wengang, ZHANG Xiaobing, WANG Haijun, GU Hongfang, XU Kuan, DENG Shuanghui, WANG Xuebin (2026). Numerical Simulation and Application of Natural Draft Direct Air-Cooling Tower for Large Coal-Fired Power Units. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202608009
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Frequently Asked Questions
What is the quantitative impact of ambient wind speed on the back pressure difference between upstream and downstream towers, and how does this affect unit load?
Under the prevailing SSE wind direction, when wind speed exceeds 4 m/s, the back pressures of the two towers begin to diverge noticeably. At wind speeds greater than 8 m/s, the back pressure difference between the towers exceeds 1 kPa. The upstream tower (Tower 2) experiences larger fluctuations and a greater reduction in air intake and heat dissipation compared to the downstream tower (Tower 1). This differential can lead to uneven condenser performance, potentially limiting the overall unit output and requiring operational adjustments to maintain safe back pressure limits.
How does ambient temperature affect the system's back pressure and cooling performance, and what are the implications for summer operation?
Ambient temperature primarily influences system back pressure rather than flow distribution. As temperature increases from 5°C to 34.9°C, the back pressure rises from approximately 6.1 kPa to 28.2 kPa, a significant increase that reduces the cooling capacity and can constrain the unit's maximum output during hot summer days. This necessitates careful monitoring and possibly load curtailment to avoid exceeding back pressure limits.
What is the effectiveness of rolling shutters and louver adjustments in improving cooling performance under high wind conditions?
Closing rolling shutters in the windward zone or reducing louver openings can improve circumferential air redistribution by suppressing excessive windward intake and enhancing leeward ventilation. However, this comes at the cost of reduced total air flow and heat dissipation, as the loss from the windward side is not fully compensated by leeward gains. The greater the adjustment, the more significant the reduction in cooling capacity. Therefore, such measures are not recommended under high-wind summer conditions, as they may exacerbate back pressure issues.
How do bottom bypass windows compare to top bypass windows in enhancing heat dissipation, and what are the recommended operational settings?
Bottom bypass windows are significantly more effective than top bypass windows. At a wind speed of 12 m/s, each 20% increase in bottom bypass opening raises heat dissipation by approximately 0.8%. Increasing the opening from 70% to 100% yields an additional ~3% improvement due to reduced cold air mixing. In contrast, top bypass windows provide limited benefits, and at 50% opening, they actually reduce heat dissipation by about 2.2%. Therefore, it is recommended to prioritize the installation and operation of bottom bypass windows, while avoiding top bypass windows altogether.
What are the economic and environmental advantages of NDC systems compared to other cooling technologies, and how do they translate into operational benefits?
NDC systems demonstrate superior economics and environmental benefits. Compared to design values, the 660 MW NDC unit reduces auxiliary power consumption rate by 1.1 percentage points, lowers net coal consumption by 4 g/(kW·h), and achieves a minimum load of 20% THA, which is better than the design value of 30% THA. When compared to similar-scale ISC and ACC systems, NDC systems have annual net coal consumption that is 3.37 g/(kW·h) and 2.61 g/(kW·h) lower, respectively. These advantages translate into lower operating costs and reduced carbon emissions, making NDC a favorable choice for coal-fired power plants in water-scarce regions.
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