The Effect of Wind on Multiple, Short, Natural-draft Dry Cooling Towers
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The deployment of concentrating solar thermal (CST) power plants in arid areas necessitates the use of dry cooling systems to reject heat from the condenser. Previous research has shown that the capacity of short natural draft dry cooling towers (NDDCTs), used as condensers for CST plants, can be significantly influenced by the prevailing wind condition. From the literature, it is apparent that there is a lack of work relating to how the interactions between multiple cooling towers during windy and no-wind conditions impact the cooling capacity of multiple cooling towers on a common site, and short NDDCTs in particular. This is a particular problem because as the capacity of CSP power plants is increased, additional cooling is required which necessitates the addition of more NDDCTs. When adding these cooling towers, there is a need to be able to position them correctly so that their performance as a group is maximised. To do this, an understanding of the effect they have on one another is needed. Hence, this work aims to characterise the interaction between multiple short NDDCTs on the cooling capacity of the multi-tower system on a common site over a range of typical operating conditions including wind speed, tower spacing, wind incidence angle, and the number of cooling towers. This study first investigated the effect of different tower spacings on the cooling performance of multiple short NDDCTs using computational fluid dynamics (CFD) under a no-wind condition. The simulated tower in all multi-tower simulations is identical and is representative of an actual steel-membrane cooling tower in a campus of the University of Queensland. The geometry of the used cooling tower in this study is a cylindrical shape with a horizontally arranged air-cooled heat exchanger and is 20 m high with a diameter of 12.525 m. This study has shown that, at a tower spacing of less than two tower diameters (2D) where D is the diameter of the tower, a reduction in the scavenging area ii between the towers limits the air supply to the towers and this interaction decreases the cooling performance of the towers. Secondly, the study investigated the effect of three major parameters: wind speeds (0-8 m/s), wind incidence angles (0°, 45°, and 90°), and tower spacings (1.8D, 2.6D, and 4.2D) on the thermo-flow performance of the cooling towers. It was shown that the interaction between the towers at a 90° wind incidence resulted in a performance improvement of the towers only at a tower spacing of 1.8D, while for the other tower spacings there was no interaction between the towers. At a wind incidence of 45°, the interference between the towers contributes to a decrease in the performance of the towers at tower spacings of 1.8D and 2.6D. It was found that the thermal performance of the NDDCTs in wind incidence of 0° is superior to other layouts at lower tower spacings. This study found that it helps to add the second tower in line with the prevailing wind direction. Thirdly, the performance of the three short NDDCTs is also investigated in an inline layout, labelled as windward, middle, and leeward towers. At all tower spacings, the windward tower shields the middle and leeward towers by deflecting the upcoming wind. Finally, the effect of tower dimensions on the interaction of three NDDCTs with similar sizes was examined and the results revealed that larger towers are less vulnerable to crosswinds due to their higher capacity of the drawing air into the cooling tower compared to smaller cooling towers. Overall, the most significant outcome of this investigation was to show that when adding additional cooling towers to an existing CSP site, they should be placed along the line of the prevailing wind direction, with a spacing determined by the average wind speed.