| 1. |
- Abedi, Hamidreza, 1979, et al.
(author)
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Development of blade element momentum (BEM) method for hydropower
- 2022
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In: IOP Conference Series: Earth and Environmental Science. - : IOP Publishing. - 1755-1307 .- 1755-1315. ; 1079:1
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Conference paper (peer-reviewed)abstract
- The BEM method is extensively used for analyzing the aerodynamic performance of wind turbines and marine propellers. It is computationally fast and is easily implemented while it can give fairly accurate results. Application of the BEM method to predict the forces acting on rotor blades for a model scale axial shaft-driven Counter-Rotating Pump-Turbine (CRPT) is investigated. Some modifications have been proposed to adopt the classical BEM method for CRPT machine and the results are validated against results from Computational Fluid Dynamics (CFD). The results display that the proposed modifications can improve the loading predicted by BEM. However, the improvements are more pronounced in pump mode rather than turbine mode.
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| 2. |
- Fahlbeck, Jonathan, 1992, et al.
(author)
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A Head Loss Pressure Boundary Condition for Hydraulic Systems
- 2022
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In: OpenFOAM Journal. - : OpenCFD Ltd. - 2753-8168. ; 2, s. 1-12
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Journal article (peer-reviewed)abstract
- Despite the increase in computational power of HPC clusters, it is in most cases not possible to include the entire hydraulic system when doing detailed numerical studies of the flow in one of the components in the system. The numerical models are still most often constrained to a small part of the system and the boundary conditions may in many cases be difficult to specify. The headLossPressure boundary condition is developed in the present work for the OpenFOAM open-source CFD code to include the main effects caused by a large hydraulic system onto a component in the system. The main motivation is to provide a boundary condition for incompressible hydraulic systems where known properties are specified by the user and unknown properties are calculated. This paper is a guide to the developed headLossPressure boundary condition. It is based on the extended Bernoulli equation to calculate the kinematic pressure on the patch. An arbitrary number of minor and friction losses are considered to describe the system in terms of head losses. The boundary condition also provides the opportunity to specify the head (difference in height) in relation to a reference elevation. System changes during operations are modelled through Function1 variables, which enables time-varying inputs. The developments are validated against experimental test data, where the varying head between two free surfaces and a valve closing and opening sequence are modelled with the boundary condition. The main effects of the system are well captured by the headLossPressure boundary condition. It is thus a useful and trustworthy boundary condition for incompressible flow simulations of components in a hydraulic system.
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| 3. |
- Fahlbeck, Jonathan, 1992, et al.
(author)
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A Low-Head Counter-Rotating Pump-Turbine at Unsteady Conditions
- 2022
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In: OpenFOAM Workshop (OFW17) Book of abstracts.
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Conference paper (other academic/artistic)abstract
- With the increased amount of energy produced from variable renewable energy sources, such as wind and solar power, the need to store energy increases. The reason is that it is necessary to cope with the variation in energy being produced by the renewables to stabilise the electrical grids. The most widely used technology for energy storage on a large scale is today pumped hydro storage (PHS). For PHS to be economically feasible, a high head is typically required, which puts topographical constraints on where it can be built. However, the EU project ALPHEUS aims to develop PHS for low-head applications, hence allowing PHS at yet unexplored sites. In the project, new reversible counter-rotating pump-turbine (CRPT) concepts are explored as an alternative runner design for low-head situations. The CRPT consists of two runners rotating in opposite direction from one another and it is suggested that it can reach higher efficiencies and be more compact compared to a single runner arrangement. In the present work a model counter-rotating pump-turbine for the ALPHEUS project is numerically analysed with computational fluid dynamics (CFD) simulations. The simulations are carried out using unsteady CFD in OpenFOAM-v2012. In the simulations, the two runners rotate individually via prescribing a solid body rotation to the runner domains. The individually rotating runners causes a intricate rotor-rotor interaction which is resolved by the numerical model. An example of this is shown in Figure, where a complex vortical structure is developing by the runners and support-structures. Furthermore, the CRPT is in reality part of large hydraulic system which effects the performance of the machine. The system includes bends, valves, long pipes, and two large water reservoirs. To restrict the size of the computational domain, the novel \verb|headLossPressure| boundary condition, developed by Fahlbeck et al., is used to include the main effects of the hydraulic system. To summarise, this study will show the potentials with a CPRT in a PHS application through CFD simulations, explain the used numerical framework, and demonstrate a use case for the new headLossPressure boundary condition.
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| 4. |
- Fahlbeck, Jonathan, 1992, et al.
(author)
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Cavitation Simulations of a Low Head Contra-rotating Pump-turbine
- 2023
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In: OpenFOAM Workshop (OFW18) Book of abstracts. ; , s. 348-349
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Conference paper (other academic/artistic)abstract
- To meet the demands of a larger share of the electrical energy produced by intermittent renewable energy sources, an increasing amount of plannable energy sources is needed. One solution to handle this is to increase the amount of energy storage in the electrical grids. The most widespread energy storage technology today is by far pumped hydro storage (PHS). In an attempt to enable PHS at low-head sites, the ALPHEUS (augmenting grid stability through low head pumped hydro energy utilization and storage) EU Horizon 2020 research project was formed. In ALPHEUS, new axial flow, low-head, contra-rotating pump-turbine (CRPT) designs are investigated. A CRPT has two individual runners rotating in opposite directions. CRPTs developed within the ALPHEUS project have already been thoroughly analysed at stationary and transient operating conditions by the authors. However, the effects on the CPRT's performance due to potential cavitation on the runner blade surfaces have previously not been investigated. For that reason, the current study focuses on running cavitation simulations on a model scale CRPT using the OpenFOAM computational fluid dynamics (CFD) software. In the CFD simulations, cavitation is modelled as a two-phase liquid-vapour mixture using the interPhaseChangeDyMFoam solver. The two runner domains have a prescribed solid body rotation. Condensation and evaporation processes are handled with the Schnerr-Sauer model. Turbulence is managed with the k-omega shear stress transport-scale adaptive simulation (kOmegaSSTSAS) model. Flow-driving pressure differences over the computational domain are achieved with the headLossPressure boundary condition to emulate a larger experimental test facility of which the CRPT is part. Figure 1 shows a snapshot in time of an iso-surface (light blue) of cavitating cloud with alpha_liquid=0.9 in turbine mode. At this operating point, a small amount of cavitating flow is found by the suction side of the leading edges of the left runner, which is facing a lower reservoir. In Figure 2, the same type of iso-surface is shown, however now in pump mode. It is seen that the pump mode operating condition is much worse than the turbine mode. The cavitating cloud covers most of the suction side of the left runner, additionally, the tip-clearance region is also exposed to cavitation. Furthermore, traces of cavitation are found on the leading edges of the right runner as well as on the left small-support struts. It is thus important to, at least, analyse the pump mode to determine if and how much cavitation affects the CRPS's operating performance.
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| 5. |
- Fahlbeck, Jonathan, 1992, et al.
(author)
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Evaluation of startup time for a model contra-rotating pump-turbine in pump-mode
- 2022
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In: IOP Conference Series: Earth and Environmental Science. - : IOP Publishing. - 1755-1307 .- 1755-1315. ; 1079:1
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Conference paper (peer-reviewed)abstract
- A larger part of the electricity is today from intermittent renewable sources of energy. However, the energy production from such sources varies in time. Energy storage is one solution to compensate for this variation. Today pumped hydro storage (PHS) is the most common form of energy storage. Usually, it requires a large head, which limits where it can be built. In the EU project ALPHEUS, PHS technologies for low- to ultra-low heads are explored. One of the concepts is a contra-rotating pump-turbine (CRPT). The behaviour of this design at time-varying load conditions is today scarce. In the present work, the impact of the startup time for a CRPT is analysed through computational fluid dynamics (CFD) simulations. The analysis includes a comparison between a coarse and a fine CFD model. The coarse model produces acceptable results and is 50 times cheaper, this model is thus used to assess the startup time. It is found that longer startup times generate lesser loads and peak values. A startup time of 10 s may be a sufficient alternative as the peak loads are heavily reduced compared to faster startups. Furthermore, there is not much difference between a startup time of 20–30 s.
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| 6. |
- Fahlbeck, Jonathan, 1992, et al.
(author)
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Flow Characteristics of Preliminary Shutdown and Startup Sequences for a Model Counter-Rotating Pump-Turbine
- 2021
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In: Energies. - : MDPI AG. - 1996-1073 .- 1996-1073. ; 14:12
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Journal article (peer-reviewed)abstract
- Pumped Hydropower Storage (PHS) is the maturest and most economically viable technology for storing energy and regulating the electrical grid on a large scale. Due to the growing amount of intermittent renewable energy sources, the necessity of maintaining grid stability increases. Most PHS facilities today require a geographical topology with large differences in elevation. The ALPHEUS H2020 EU project has the aim to develop PHS for flat geographical topologies. The present study was concerned with the initial design of a low-head model counter-rotating pump-turbine. The machine was numerically analysed during the shutdown and startup sequences using computational fluid dynamics. The rotational speed of the individual runners was decreased from the design point to stand-still and increased back to the design point, in both pump and turbine modes. As the rotational speeds were close to zero, the flow field was chaotic, and a large flow separation occurred by the blades of the runners. Rapid load variations on the runner blades and reverse flow were encountered in pump mode as the machine lost the ability to produce head. The loads were less severe in the turbine mode sequence. Frequency analyses revealed that the blade passing frequencies and their linear combinations yielded the strongest pulsations in the system.
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| 7. |
- Fahlbeck, Jonathan, 1992
(author)
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Flow in contra-rotating pump-turbines at stationary, transient, and cavitating conditions
- 2024
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Doctoral thesis (other academic/artistic)abstract
- This thesis investigates contra-rotating pump-turbines (CRPT) through computational fluid dynamics (CFD) simulations. The research was carried out within the ALPHEUS EU research project, which examined low-head pumped hydro storage using CRPTs. The aim is to analyse and suggest operations for the CRPT at stationary, transient, and cavitating flow conditions. Stationary conditions are analysed using steady-state and unsteady CFD. It is found that the CRPT can produce a hydraulic efficiency of about 90% in both pump and turbine modes for a wide range of operating conditions. Transient startup and shutdown sequences are extensively analysed with the objective of finding load gradient limiting sequences. It is uncovered that the transient sequences in pump mode are more severe than those in turbine mode. This is partly because reversed flow is encountered when the CRPT is not able to overcome the elevation difference between the reservoirs. Therefore, it is suggested that a valve needs to be part of the sequences to avoid reversed flow and control the change in flow rate. For an optimal pump mode startup, the runners need to initially speed up so that the CRPT precisely balance the reservoirs' elevation difference. In the remaining part of the sequence, the valve should open during about three-quarters of the sequence. The runner facing the lower reservoir should use most of the sequence to speed up, while the runner facing the upper reservoir should speed up in the final third of the sequence. For the pump mode shutdown, the valve should close before speeding down the runners, or the runners can speed down as the valve is almost closed. Corresponding sequences in turbine mode are also examined. The suggested startup sequence in turbine mode consists of an initial valve opening, shortly followed by the simultaneous speedup of the runners. The turbine mode shutdown, on the other hand, utilises a multi-stage valve closure as the runners are brought to a standstill. Cavitating flow simulations are carried out at stationary operating conditions in both pump and turbine modes to determine how cavitation impacts the performance of the CRPT. It is found that the pump mode is more sensitive to cavitation than the turbine mode. Nonetheless, irrespective of the mode, the presence of cavitation invariably leads to a degradation in the CRPT performance. This is because the cavitating region causes flow separation on the runner blades, which disturbs the efficient flow guidance in the blade passages.
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| 8. |
- Fahlbeck, Jonathan, 1992
(author)
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Low head pumped hydro storage with contra-rotating pump-turbines
- 2022
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Licentiate thesis (other academic/artistic)abstract
- The increasing share of electrical energy production by intermittent sources pushes the demand for energy storage. Pumped hydro storage (PHS) has a long history of providing a cost-efficient energy storage solution. However, for PHS to be a viable option, a large head is typically required. This makes energy storage via PHS difficult in countries that lack high mountain regions. To address this problem, and thus allow for PHS in flat countries, the EU project ALPHEUS was formed. In ALPHEUS, new pump-turbine technologies intended for low head PHS are evaluated. One of the investigated designs is a shaft-driven contra-rotating pump-turbine (CRPT). In this thesis, CRPTs are numerically simulated with computational fluid dynamics at stationary and transient operating conditions. The stationary operation is studied through both steady-state and unsteady simulations. The steady-state computations are made to get an understanding of the operating range of the CRPT. The unsteady simulations are carried out on selected operating points with the aim to identify the complex flow behaviour. The transient operations cover startup and shutdown procedures in both pump and turbine modes. The pump mode startup procedure is the major focus since it was found when evaluating preliminary startup and shutdown sequences that the pump mode startup was exposed to the largest loads. Hence, three parallel studies are presented in this thesis to determine how to startup the CRPT in pump mode and limit high-amplitude load variations. This contributes to the machine's lifetime and flexible operation. The outcome of this research shows the potential of using CRPTs in a low head PHS situation and may also help with solving the inherent problems correlated to energy production from intermittent sources.
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| 9. |
- Fahlbeck, Jonathan, 1992, et al.
(author)
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Numerical analysis of an initial design of a counter-rotating pump-turbine
- 2021
-
In: IOP Conference Series: Earth and Environmental Science. - : IOP Publishing. - 1755-1307 .- 1755-1315. ; 774:1
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Conference paper (peer-reviewed)abstract
- Renewable sources of energy are on the rise and will continue to increase the coming decades. A common problem with the renewable energy sources is that they rely on effects which cannot be controlled, for instance the strength of the wind or the intensity of the sunlight. The ALPHEUS Horizon 2020 EU project has the aim to develop a low-head hydraulic pump-turbine which can work as a grid stabilising unit. This work presents numerical results of an initial hub-driven counter-rotating pump-turbine design within ALPHEUS. Computational fluid dynamics simulations are carried out in both prototype and model scale, for pump and turbine modes, and under steady-state and unsteady conditions. The results indicate that the initial design have a hydraulic efficiency of roughly 90 % in both modes and for a wide range of operating conditions. The unsteady simulations reveal a complex flow pattern downstream the two runners and frequency analysis show that the dominating pressure pulsations originates from the rotor dynamics. Given the promising high efficiency, this initial design makes an ideal platform to continue the work to optimise efficiency and transient operations further.
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| 10. |
- Fahlbeck, Jonathan, 1992, et al.
(author)
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NUMERICAL SIMULATIONS OF COUNTER-ROTATING PUMP-TURBINE WITH A NEW HEAD-LOSS PRESSURE BOUNDARY CONDITION
- 2021
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In: OpenFOAM Workshop (OFW16) Book of abstracts.
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Conference paper (other academic/artistic)abstract
- With an increasing amount of energy from renewable sources, such as wind and solar, the need of complementary controllable energy sources increases. Hydropower plays a key role to provide a stable and flexible electrical grid. By storing large amount of water when there is excess power in the grid, and later utilise the stored water when there is a lack of power, hydropower is a stabilising unit for the electrical grid [1]. The ALPHEUS EU project has the aim to develop a low-head to ultra low-head seawater based Pumped Hydropower Storage (PHS) solution with a pump-turbine unit [2, 3]. The main goal of the ALPHEUS project is that the pump-turbine unit should have a round-trip efficiency of 70 - 80 % and a switching time of about 120 s. PHS use the potential energy by pumping water to a reservoir. The potential energy is later extracted by reversing the pump to a turbine. Three pump-turbine concepts are to be investigated, a counter-rotating shaft-driven, a counter-rotating rim-driven, and a positive-displacement alternative. A rigorous optimisation process will be applied to maximize the round-trip efficiency for a wide range of operating conditions. In this work an initial design of a counter-rotating shaft-driven alternative is considered. In the ALPHEUS project an optimised counter-rotating shaft-driven pump-turbine will be experimentally evaluated in model scale, in the hydraulic laboratory at TU Braunschweig. The experiments are partly made in order to generate experimental test data. The numerical models are later going to be evaluated with the experimental test data. The experimental test facility consists of a two open reservoir surfaces, upper and lower. In turbine-mode, water flows from the upper to the lower reservoir, and it is pumped from the lower to the upper in pump-mode. The reservoirs are connected with a series of pipes, including bends and other obstacles in the flow path. The machine is going to be tested at different operating conditions and it is thus hard to estimate the flow rate for any given case. This is because head, or pressure, losses scale quadratic to a change in flow rate. An option to overcome this problem in a numerical framework is to include head losses at the boundaries of the computational domain. The flow rate in the simulation is calculated as a balance between the available pressure and the losses in the system. In OpenFOAM there is not any available boundary condition that can include up-/down-stream losses at a patch. This present work demonstrates a new pressure boundary condition, headLossPressure, developed by Fahlbeck [4]. The boundary condition is an extension of the available totalPressure boundary condition. It uses the volumetric flux to adjust the static pressure on the patch according the Bernoulli equation [5]. The headLossPressure is an incompressible pressure boundary condition for in-/outflow patches. If the patch has inflow the losses are subtracted, and for an outflow patch the losses are added. The basic functionality of the headLossPressure boundary condition is evaluated on a simple test case by Fahlbeck [4]. In this work the boundary condition is used together with the initial design of a model scale counter-rotating shaft-driven pump-turbine in the ALPHEUS project. The blade geometries shown in Figure 1a were designed by the Advanced Design Technology Ltd (ADT) company. The diameter of the runners is 27 cm, runner 1 (red) has eight blades, and runner 2 (blue) has seven blades. Runner 1 has a rotational speed of 1453 RPM in pump mode and 832 RPM in turbine mode, runner 2 rotates at 90 % of the speed of the first runner in each mode. The numerical simulations are made on the computational domain shown in Figure 1b. The numerical simulations are made with unsteady CFD at one operating condition in both pump and turbine modes. The numerical framework includes the two rotating runners, hub, support-struts, and contraction/extraction parts. The simulations utilise the unsteady incompressible pimpleFoam solver and the k-ω SST model is used to account for turbulence. The convective terms of the momentum equations are discretised using the LUST scheme, and temporal discretisation with the backward scheme. The pressureInletOutletVelocity and headLossPressure are used as boundary conditions for velocity and pressure, respectively, at both the inlet and the outlet. The pressure boundary condition is set to operate with a total height difference of 8 m, the full pipe length is roughly 16 m, one 90° bend, and some additional flow obstacles are included. The results from the unsteady simulations, shown in Figure 2, resolves the unsteady wakes of the runners and the support- struts. The complex flow pattern produced by the runners is caused by the downstream runner cutting the wakes of the upstream runner. The machine is operating at a high efficiency in both modes as the flow is rather axial after the runners. This is seen by that the vortex shedding of the support-strut is rather axial. A frequency analysis, not shown here, uncover that the pressure pulsations in the system are strongly connected to the blade passing frequencies and linear combinations of it. The headLossPressure boundary condition can be used to produce a plausible flow field as the solver calculates a flow rate that is not totally unphysical. The question still remains if the flow rate is correct and if the boundary condition can be used even for transient simulations. The numerical model and this new boundary condition will later be compared against experimental test data of an optimised counter-rotating pump-turbine. Acknowledgments The authors thank all those involved in the organisation of OFW16 and to all the contributors that will enrich this event. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 883553. The computations were enabled by resources provided by the Swedish National Infrastructure for Computing (SNIC) at NSC and C3SE partially funded by the Swedish Research Council through grant agreement No. 2018-05973. References [1] IEA, Will pumped storage hydropower expand more quickly than stationary battery storage? IEA, Paris, 2019. [Online]. Available: https://www.iea.org/articles/ will-pumped-storage-hydropower-expand-more-quickly-than-stationary-battery-storage [2] ALPHEUS H2020. Accessed: 2021-02-12. [Online]. Available: https://alpheus-h2020.eu/ [3] M. Qudaih and et al., “The contribution of low-head pumped hydro storage to a successful energy transition,” in Proceedings of the Virtual 19th Wind Integration Workshop, 2020. [4] J. Fahlbeck, “Implementation of an incompressible headlosspressure boundary condition,” in Proceedings of CFD with OpenSource Software, 2020, Edited by Nilsson. H., http://dx.doi.org/10.17196/OS CFD#YEAR 2020. [5] F. M. White, Fluid mechanics, ser. McGraw-Hill series in mechanical engineering. McGraw-Hill, 2011.
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