Numerical and experimental study of small natural draft dry cooling tower for concentrating solar thermal power plant

Numerical and experimental study of small natural draft dry cooling tower for concentrating solar thermal power plant Thesis

School of Mechanical and Mining Engineering

  • Author(s): Li, Xiaoxiao
  • Published: 2018
  • Publisher: The University of Queensland
  • Volume: Doctor of Philosophy

Abstract: The typically arid climates in the likely locations for renewable thermal power plants such as Concentrating Solar Thermal (CST) power plants provide the motivation for dry cooling technology applications. Natural draft dry cooling tower (NDDCT) with low maintenance cost and no parasitic power consumption offers a feasible and cost effective option for such applications. Compared with the conventional coal fired and nuclear power plants, the size of the CST power plant proposed for Australian regional communities is much smaller. However, the existing cooling tower design is optimised for large steam power plants, and is not optimal for these renewable power plants. To help address this issue, Queensland Geothermal Energy Centre of Excellence (QGECE) at The University of Queensland was developing the small NDDCT technology for renewable power plants. A 20 m high NDDCT was built on the Gatton campus of the University of Queensland as testing base for developing dry cooling technologies for future small scale (<10 MWe) CST power plants. Using both numerical simulations and full-scale experimental tests, this thesis investigated the cooling performance of this small NDDCT in different ambient conditions, discussed the feasibility of the small NDDCT in CST power plant, identified two specific potential cooling issues for the small NDDCT and proposed the mitigation solutions. The main findings of this thesis are summarized as following: (1) This research developed the 1D analytical model and the 3D CFD model of the 20 m high Gatton NDDCT. The cooling performance of this cooling tower is investigated under different ambient temperatures, different hot water inlet temperatures and different crosswind speeds. The results show that both ambient temperature and the crosswind have significant influence on the performance of the cooling tower. (2) The performance of the experimental tower was tested at two constant heat loads of 600 kW and 840 kW, respectively, with various ambient temperatures. The 1D numerical model was refined and validated with the experimental data. The thermodynamic model of a 1 MW CST power plant running with sCO2 cycle was developed and integrated with the updated cooling tower model. Significant differences were observed between the steam Rankine and sCO2 cycles in terms of the effect of the ambient temperature on power generation. The differences between steam Rankine and sCO2 Brayton cycle in this context were analysed and discussed. (3) Two potential cooling issues with small NDDCT, the crosswind and the cold inflow, were identified and the detail experimental data were presented. The crosswind effect on small cooling tower is different from the conventional big cooling towers. With the increase of the crosswind speed, the overall cooling performance of the small NDDCT first decreases and then increases. The mixed convection theory and the Richardson Number were proposed to explain this phenomenon. On the other hand, repeated cold inflow events were observed during the tests, which cause a significant decrease of the air temperature inside the cooling tower. The water outlet temperature can be increased up to 3°C as a result of the cold inflow effect. Further analysis of the mechanism shows that the cold air incursion at the top of the cooling tower could decrease the driving force and also form an extra flow resistance for the airflow through the heat exchanger. (4) Based on the working mechanism of the air-cooled heat exchanger and the crosswind effect on NDDCT, this study proposed a new method to increase the cooling performance of NDDCT under crosswind conditions by optimizing the water mass flow rate in the air-cooled heat exchangers. By optimising the water distribution, the water outlet temperature of each heat exchanger is more uniform and the adversely influence of the crosswind can be effectively relieved. The findings in this paper can lay an important foundation for future small cooling tower design and operation.

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Suggested Citation
Li, Xiaoxiao, 2018, Numerical and experimental study of small natural draft dry cooling tower for concentrating solar thermal power plant, Volume:Doctor of Philosophy, Thesis, viewed 19 May 2019, https://www.nintione.com.au/?p=13632.

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