Natural Draft Direct Dry Cooling System Performance at Various Application Scales Under Steady and Transient Conditions
Date
2024-12
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Stellenbosch : Stellenbosch University
Abstract
Natural draft direct dry cooling systems (NDDDCSs) provide an efficient alternative to traditional forced draft air-cooled condensers and indirect nat- ural draft dry cooling systems. NDDDCSs combine the advantages of these systems, including reduced system complexity, direct condensing, and lower auxiliary power consumption. This study developed 1-Dimensional (1-D) numerical, 3-Dimensional (3-D) computational fluid dynamics (CFD), and co-simulation models to character- ize the steady-state and transient performance of NDDDCSs with vertically arranged heat exchanger bundles. These models were applied to three appli- cations: a large coal-fired power plant (900 MWt), a concentrated solar power plant (100 MWt), and a water desalination plant (1 MWt). The steady-state 1-D and 3-D CFD models were validated against 3-D CFD results from lit- erature, which in turn matched large-scale experimental data under no-wind conditions. The transient models were validated against their steady-state counterparts. Steady-state 1-D simulation results show that increasing the ratio of to- tal tower height to inlet diameter (H₅/d₃) enhances NDDDCS performance. Larger outlet diameter to inlet diameter ratios (d₅/d₃) can also improve perfor- mance, although this reduces outlet air velocities, possibly resulting in cold in- flow effects. Lower inlet diameter to inlet height ratios (d₃/H₄) increase steam velocity through the condenser tubes, whereas higher ratios reduce steam-side pressure drops, leading to higher average saturated steam temperatures. Thus, optimal NDDDCS design parameters vary with scale. Steady-state 3-D CFD simulations reveal that recirculation reduces ND- DDCS performance. Air streams from the upper and lower tower regions converge to create low-velocity vortices in front of most heat exchanger deltas, causing localised reductions in air mass flow rates and increased inlet air tem- peratures, resulting in reduced outlet air temperatures. Under a 6 m/s cross- wind, NDDDCS performance decreases at all scales, with medium- and small- scale systems losing 40-50% in heat
transfer rate. Wind mitigation measures are recommended for all scales. Novel performance recovery points are identi- fied as crosswind speeds reach 18 m/s, 15 m/s, and 9 m/s for large-, medium-, and small-scale NDDDCSs, respectively.
Transient 1-D simulations indicate that NDDDCS start-up performance does not constrain overall power plant start-up. Step inputs in steam flow admission of 6.25% and 18% from cold start conditions can be managed by large- and medium-scale systems without exceeding statutory pressure limits. Industry-standard thermal load ramps are also effectively managed. Addition- ally, results highlight substantial full and partial load turbine islanding capa- bilities, with load increases of 150% and 135%, respectively, being managed without exceeding typical dry-cooled turbine pressure protection limits. Transient co-simulation results demonstrate substantial reductions in the permissible cold start-up
steam flow admission step input for large- (0.8%) and medium-scale (3.25%) NDDDCSs compared to 1-D results. A no-wind, cold start load ramp presents the worst case for NDDDCS start-up performance, exceeding the statutory pressure limit before plant start-up is achieved. How- ever, NDDDCSs achieve start-up effectively under crosswind conditions. Ad- justment of the selected tower-to-heat exchanger arrangement is recommended to improve start-up performance. Finally, co-simulation results confirm the considerable turbine islanding capacity of the system.
Description
Thesis (PhD)--Stellenbosch University, 2024.