One- and three-dimensional coupling flow simulations of pumped-storage power stations with complex long-distance water conveyance pipeline system
Introduction
The large-scale developments and technological progress in wind and solar power generation should significantly reduce its cost. Consequently, wind power and solar power are expected to replace fossil fuels as the world's major power sources. However, wind and solar power generation involves natural unstable and intermittent shortcomings (Menendez et al., 2020). For overcoming the challenge of low-quality clean energy generation in the power grid, the development of energy-storage technologies must be prioritized (Li et al., 2019). Currently, pumped-storage power technology is the only commercially proven grid-scale energy-storage technology (Zuo et al., 2015). Therefore, it is imperative to develop a pumped-storage power-generation technology.
To reduce construction costs, an important hydraulic system layout is extensively adopted in numerous diversion-type pumped-storage power stations. In this layout, several hydraulic turbine-generator sets generally share a common penstock and tailrace tunnel (Yin et al., 2019). However, the interaction among units significantly influences the safe and stable operation of pump-turbines in a water conveyance system. In particular, for transient processes, the water hammer waves in common water conveyance systems can be superimposed on each other and interact with pump-turbines. This can further threaten the safe and stable operation of pumped-storage power stations (Rezghi and Riasi, 2016). Simulations and experimental investigations suggest that a more extreme water hammer phenomenon occurs under transient conditions, in which one pump-turbine rejects loads after another pump-turbine rejects loads (Hu et al., 2018; Zeng et al., 2016). Consequently, it is important to study the transient flow in a pumped-storage power station, containing a complex long-distance water conveyance pipeline system, to eliminate the threat of extreme water and pressure fluctuations on its safe operation.
The transient flow in pumped-storage power station can be studied using three methods. The first is the theory calculation method, which only calculates the simple hydraulic system. This method cannot perform complex calculations of the transient flow in pumped-storage power stations containing a complex long-distance water conveyance pipeline system. The second is the experimental method. The transient flow tests require high-performance experimental equipment and instruments that are very expensive. Currently, only limited research institutes, including the Wuhan University of China, Norwegian University of Science and Technology, and Ecole Polytechnique Federale de Lausanne in Switzerland, possess model test rigs from across the world. Additionally, the severe water hammer and pressure fluctuations in transient processes often cause fatigue crack in the impeller and shorten the lifetime of the equipment (Trivedi and Dahlhaug, 2018). Thus, the experimental method is also not widely adopted. In contrast, with recent advances in computing performance, various simulation methods have been gradually developed and widely adopted by numerous researchers owing to their low cost and superior performances.
Among the various simulation methods of transient flow, the one-dimensional (1D) transient flow simulation method was first proposed to study the water hammer phenomena in pipeline system by solving the 1D momentum and mass conservation equation of fluid. The pure 1D method can simulate the transient flow in various complex pipeline systems with low computational cost. However, owing to the use of the simplified 1D solution of pump-turbines, 1D simulation results cannot reflect the nonlinear pressure fluctuations and detailed three-dimensional (3D) flow properties in pump-turbines (Ghidaoui et al., 2005; Kan et al., 2020).
To overcome these disadvantages of 1D transient flow simulation methods, various 3D transient flow simulation methods have gradually been developed. Some researchers simulated the 3D transient turbulence flow in a pump-turbine using a 3D computational fluid dynamics code. Time-varying boundary conditions were assigned depending on transient experimental data obtained at the entrance and exit of the pump-turbine (Li et al., 2017). To eliminate the dependence on transient experimental data, the full 3D simulation methods was proposed. In this method, complete water conveyance pipeline system and pump-turbine are both simulated using 3D model. Constant boundary conditions were assigned at the flow entrance and exit of a pumped-storage power station (Liu et al., 2019; Zhou et al., 2018). The full 3D flow simulation method can provide all transient performances and internal flow distributions in a complete flow passage. However, this method requires large amount of 3D computational mesh nodes for resolving 3D flow distribution accurately. Particularly, for the pumped-storage power station containing complex long-distance water conveyance pipeline system and surge tanks, the required computational mesh nodes for this method are too many to be afforded for existing computer performance. Moreover, because the full 3D two-phase flow simulation method uses high computational cost unnecessarily, it is not widely adopted in the field.
Therefore, to overcome these shortcomings, a 1D–3D coupling transient flow simulation method was proposed. In this coupling method, the transient flow in long-distance water conveyance pipeline systems of pumped-storage power stations was simulated using 1D flow momentum and mass conservation equation to consider the water hammer effect. The unsteady turbulence flow in the local pump-turbine was simulated using the 3D flow Navier–Stokes equation and continuity equation to consider the nonlinear fluctuations caused by turbulence. The transient flow simulation results of the 1D pipeline and 3D pump-turbine at the present time step were exchanged at the 1D–3D coupling boundaries to provide time-varying boundary conditions for the simulations of 3D and 1D at the next time step. The coupling method considers the interaction between the transient flow in 1D pipelines and unsteady turbulence flow in 3D pump-turbines. The simulation results of the proposed method can demonstrate the actual physical process of transient flow in pumped-storage power stations. Additionally, the fluctuation process of the free water surface in surge tanks were simulated using the 1D method. This solution avoided the simulation of the two-phase flow in complete water conveyance pipeline systems for considering the surge tanks. Therefore, this coupling method saved significant computational cost, compared to the full 3D method, owing to the simulation of only the local pump-turbine using the 3D method. However, all the information, including the water hammer in the pipeline system and nonlinear pressure fluctuations and internal flow distributions in the pump-turbine, can be obtained using the proposed coupling simulation method. The coupling method combines all the advantages of 1D and 3D transient flow simulation methods, as well as overcomes their shortcomings. Therefore, it is potential and has been gradually developed (Fan et al., 2012; Panov et al., 2014; Yang et al., 2015; Zhang et al., 2016).
By reviewing the existing studies on the transient flow simulation in pumped-storage power stations, we found the following research gaps to be filled. First, in the previous studies on transient flow in complex pipeline systems of pumped-storage power stations, pure 1D flow simulation method was primarily used. However, using this method, complex interactions between the transient flow in pipeline systems and turbulence flow in pump-turbines cannot be considered. Moreover, the pure 1D flow simulation method cannot be used to analyze the nonlinear fluctuations of pressures and hydraulic thrusts on the impellers, which are related to the turbulence fluctuations, and reveal their underlying internal flow mechanisms. Instead, the 3D simulation method should be used to study the hydraulic interactions in complex water conveyance pipeline systems, analyze the nonlinear fluctuations of pressures and hydraulic thrusts on the impellers, and reveal their underlying flow mechanisms. Second, in the previous studies on the 1D–3D coupling flow simulation in pumped-storage power station, only the sample long-distance water conveyance pipeline in pumped-storage power stations was studied, in addition to the transient processes of a single pump-turbine in the water conveyance pipeline system. However, in actual pumped-storage power stations, several parallel pump-turbines share a common water diversion penstock and tailrace tunnel with a surge tank. Comparing the existing transient flow simulations in pumped-storage power stations using the 1D–3D coupling method, the water conveyance pipeline system and transient operation models of pump-turbines in actual pumped-storage power stations are more complex. Therefore, more complex pipeline systems and combined operation modes must be considered to develop the 1D–3D coupling flow simulation method.
To fill the abovementioned research gaps, this study developed a 1D–3D coupling flow simulation method considering the complex long-distance water conveyance pipeline system in which two parallel pump-turbines share a common water diversion penstock and tailrace tunnel with a surge tank in an actual pumped-storage power station. Subsequently, three combined operation modes of two parallel pump-turbines were simulated to test the simulation performances of the developed 1D–3D coupling flow simulation method in this study. To summarize, this study contributed two novelties as follows:
First, considering that the existing simplified water conveyance pipeline systems cannot represent the actual situations in pumped-storage power stations, this study successfully developed a 1D–3D coupling flow simulation method considering the complex long-distance water conveyance pipeline system in which two parallel pump-turbines share a common water diversion penstock and tailrace tunnel with a surge tank in an actual pumped-storage power station. Second, the proposed 1D–3D coupling flow simulation method can simulate the nonlinear fluctuations of hydraulic thrusts on the impeller and reveal their underlying flow mechanisms, which are unachievable by the pure 1D flow simulation method, considering the existing studies on various complex transient processes of pumped-storage storage power stations comprising complex long-distance water conveyance pipeline systems.
The remainder of this study is structured as follows. Section 2 introduces the research objective and 1D–3D coupling transient flow simulation method. Additionally, the simulation accuracy of the 1D–3D coupling method is justified. Section 3 discusses three combined operation conditions of two parallel pump-turbines, which were simulated to test the performances of the 1D–3D coupling simulation method. Moreover, the nonlinear fluctuations of the pressures and thrusts on the impeller were analyzed and their flow mechanism was revealed. In Section 4, several valuable conclusions are drawn on the performance and superiority of the developed 1D–3D coupling flow simulation method.
Section snippets
Computational domain
In this study, an approximately entire water conveyance system in an actual pumped-storage power station was investigated, as shown in Fig. 1. In this pumped-storage power station, two pump-turbines shared a common water diversion penstock and tailrace tunnel with a surge tank. The two pump-turbines were installed in two water diversion branches, and two main spherical valves were installed at the high-pressure side entrances of the two pump-turbines. Additionally, two tailrace gates were
Comparison of transient performance and flow mechanism analysis
For calculating the regulation guarantee in conventional hydropower stations, the increase in the impeller speed and the extreme water hammer pressure values at the end of the spiral-casing and the draft-tube entrance are three primary factors requiring attention (Ye et al., 2020). Fluctuating hydraulic forces and their induced structural vibrations should also be considered in the transitions of pumped-storage power stations. In this section, the transient performance characteristics of the
Conclusions
In this study, a 1D–3D coupling flow simulation method was developed successfully for investigating the transient flow in the pumped-storage power station containing a complex long-distance water conveyance pipeline system. Subsequently, three transient combined operation modes of two parallel pump-turbines in a common penstock and tailrace tunnel system were simulated and compared using the proposed method to test the performance of the 1D–3D coupling simulation method. Finally, several
CRediT authorship contribution statement
Xiaolong Fu: Conceptualization, Methodology, Software, Validation. Deyou Li: Supervision, Writing – review & editing, Funding acquisition. Hongjie Wang: Supervision, Writing – review & editing, Resources. Zhenggui Li: Writing – review & editing. Qin Zhao: Writing – review & editing. Xianzhu Wei: Supervision, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors thank the National Natural Science Foundation of China (No. 52079034); the Natural Science Foundation of Heilongjiang Province, China (Grant No. ZD2020E002); the Sichuan Provincial Science and Technology Department Key R & D Plan Project, China (Grant No. 20SYSX0236); and the China Postdoctoral Science Foundation (Grant No.2019T120266) for their financial support.
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