An experimental and numerical study of a three-lobe pump for pumped hydro storage applications

Pumped hydro storage (PHS) plays an important role as a matured technology that accounts for the vast majority of global energy storage capacity, and its expansion is therefore desirable. The expansion of PHS in mid- and high-water heads is limited to topographic features, but there is an untapped potential in low-head applications. For most of the PHS applications, a Francis reversible pump-turbine (RPT) is regarded as the most common and cost-effective machine, but it is not a suitable option for water heads of less than 30m. In its place, positive displacement machines like lobe pumps could potentially work as RPT machines and unleash new possibilities for low-head pumped hydroelectric storage. In addition, unlike bladed pumpturbines, lobe pumps-turbines present a fish friendliness design, an important attribute to preserve the aquatic wildlife. This work will therefore present a three-lobe pump that could potentially be used in low-head PHS. An experimental model for a lobe machine will be presented, and its results will be used to validate the computational fluid dynamic simulations. Numerical investigations will address the characteristic curves regarding water-head, rotation speed and flow rate. © 2023 Institute of Physics Publishing. All rights reserved.An experimental and numerical study of a three-lobe pump for pumped hydro storage applicationspublishedVersio

Simulations of the dynamic load in a francis runner based on measurements of grid frequency variations

In the Nordic grid, a trend observed the recent years is the increase in grid frequency variations, which means the frequency is outside the normal range (49.9-50.1 Hz) more often. Variations in the grid frequency leads to changes in the speed of rotation of all the turbines connected to the grid, since the speed of rotation is closely related to the grid frequency for synchronous generators. When the speed of rotation changes, this implies that the net torque acting on the rotating masses are changed, and the material of the turbine runners must withstand these changes in torque. Frequency variations thus leads to torque oscillations in the turbine, which become dynamical loads that the runner must be able to withstand. Several new Francis runners have recently experienced cracks in the runner blades due to fatigue, obviously due to the runner design not taking into account the actual loads on the runner. In this paper, the torque oscillations and dynamic loads due to the variations in grid frequency are simulated in a 1D MATLAB program, and measured grid frequency is used as input to the simulation program. The maximum increase and decrease in the grid frequency over a 440 seconds interval have been investigated, in addition to an extreme event where the frequency decreased far below the normal range within a few seconds. The dynamic loading originating from grid frequency variations is qualitatively found by a constructed variable Tstress, and for the simulations presented here the variations in Tstress are found to be around 3 % of the mean value, which is a relatively small dynamic load. The important thing to remember is that these dynamic loads come in addition to all other dynamic loads, like rotor-stator interaction and draft tube surges, and should be included in the design process, if not found to be negligible

Simulation and Discussion of Models for Hydraulic Francis Turbine Simulations

The paper presents simulations using three different models for a hydraulic turbine. The models are too simple and unrealistic to be used for actual simulations, but the intention is to discuss behaviour intrinsic to the models that provide insight into the physics involved. The presented results point out several behaviours that are contradictory to actual turbine behaviour, and suggest what can be done to avoid this. The models can be used as a basis for later modifications that make the models applicable for simulations, modifications the authors are currently working on

A test of the v 2-f k-epsilon turbulence model for the prediction of vortex shedding in the Francis-99 hydrofoil test case

A test of the v 2 − f k − epsilon turbulence model for the flow around the Francis-99 hydrofoil geometry is conducted in order to assess it's accuracy of trailing edge vortex shedding prediction. The model is based on the k − epsilon turbulence model, but needs no wall damping function, and also allows near-wall turbulence anisotropy. For reference, the model results are compared with the the SST k − w, in addition to preliminary experimental results previously published. It is indicated that the v 2 − f k − epsilon model gives at least as good, or better results than the more commonly used SST k − w model for the present case, though further measurements are needed in order to make a proper conclusion

Dynamic load on a Francis turbine runner from simulations based on measurements

The frequency of the Nordic power grid has become more volatile during recent years. This gives rise to two effects in synchronous machines. Firstly, as grid frequency changes the rotational speed of synchronous machines must change likewise. Secondly, the Nordic grid uses speed droop operation extensively as the primary governing; hence the power produced is a function of the grid frequency. These two coupled effects will lead to the runner and axle on synchronous machines having to cope with a varying level of torque. Even if the unit is supposedly operating at steady state via a fixed set point for the production, the influence of the varying grid frequency is that the torque in not steady at all. Recent years' new high head Francis runners in Norway have shown a tendency towards experiencing fatigue to a greater extent than what seem to be the case for new runners decades ago. Leading to this paper, measurements have been made of the rotational speed; generator power; main servo motor position and grid frequency at a Francis turbine unit. Based on these measurements simulations that include the hydraulic domain have then been performed. From these simulation results a property is constructed which is intended as a qualitative measure of the material stresses induced in the rotating masses of the unit, and is representative of the dynamical loads on the material of the rotating masses. The work is a part of a longer term goal, namely identifying the stress oscillations in a Francis turbine runner operating at speed of rotation oscillating because of grid frequency variations

Pump-Turbines in Conventional Hydropower Plants

This paper proposes an innovative approach to retrofit existing hydropower plants with suitable upper and lower reservoirs into pumped storage. The conventional methods are both comprehensive and expensive, mainly due to the increased risk of cavitation during pumping operation. In an existing facility, the installed runner is already sufficiently submerged to avoid unfortunate cavitation, but replacing the runner with a reversible pump-turbine demands an even further increase of the static pressure at the low-pressure side of the turbine. An alternative to the traditional method is discussed in this research by introducing the concept of a booster pump installed in the draft tube. Increasing the pressure at the inlet of the pump-turbine during pumping could eliminate the risk of cavitation and enable operation of a pumped storage plant without submerging the runner further. This conceptual paper discusses the motive for developing pumped storage and presents two uses for the proposed booster pump; eliminating cavitation and correcting for some of the necessary lifting head of the pump-turbine. The project is a part of the Norwegian Research Centre for Hydropower Technology, HydroCen

A New Technical Concept for Water Management and Possible Uses in Future Water Systems

A new degree of freedom in water management is presented here. This is obtained by displacing water, and in this paper is conceptually explained by two methods: using an excavated cavern as a container for compressed air to displace water, and using inflatable balloons. The concepts might have a large impact on a variety of water management applications, ranging from mitigating discharge fluctuation in rivers to flood control, energy storage applications and disease-reduction measures. Currently at a low technological readiness level, the concepts require further research and development, but the authors see no technical challenges related to these concepts. The reader is encouraged to use the ideas within this paper to find new applications and to continue the out-of-the-box thinking initiated by the ideas presented in this paper

Measurements and simulations of turbines on common grid

Speed droop control is of basic importance for the primary governing in the Nordic grid. The speed droop control. a mandatory and build-in regulatory loop on all larger units. is automatically changing the produced power on synchronous units as the grid frequency changes. This part of the governor allows a certain deviance from the nominal 50 Hz grid frequency. If the grid frequency is decreasing this means that the load on the grid is greater than the power delivered into the grid. and the local speed droop regulatory loop on each unit then autonomously increases the production to obtain a new balance between load and production. which will be at a lower frequency than 50 Hz. If the power delivered into the grid is greater than the load. the rotating masses will be accelerated (thus increasing the grid frequency) and the speed droop operation will act to reduce the power produced to obtain a new balance. this time at a higher frequency than 50 Hz. The frequency in the Nordic power grid has in recent years for increasing duration been outside the allowed steady state frequency band of 50 ± 0.1 Hz. In order to study the behaviour of a turbine operating on a common grid, measurements have been done at site. The measurements performed are the generator power, main servo motor position, the rotational speed of the unit and the grid frequency. The purpose of the measurements was to see if it is possible to observe the behaviour of the machine as it is linked together with all the other machines on a synchronous grid. It is interesting to observe the response to deviations in the frequency due to the speed droop operation. In order to better understand the behaviour, a simulation model of two power plants, complete with individual conduit system, turbine and generator, connected to the same grid was used

Optimization procedure for variable speed turbine design

This article outlines a design procedure for variable speed Francis turbines using optimization software. A fully parameterized turbine design procedure is implemented in MATLAB. ANSYS CFX is used to create hill diagrams for each turbine design. An operation mode of no incidence losses is chosen, and the mean efficiency in the range of the best efficiency point is used as optimization criterion. This characteristic is extracted for each design, and optiSLang is used for system coupling and optimization. In the global optimization loop, the downhill simplex method is used to maximize the turbine performance. For this article, the bounding geometry of the runner is kept as in the original configuration. This way, the performance of the different variable speed turbines can be compared directly. Two optimization parameters describing the blade leading-edge geometry have been used in the optimization procedure. The resulting design was an almost circular leading edge, and shows an increase in mean efficiency of 0.25% compared to the reference case. There was a significant change in the turbine performance, with close to no change at the best efficiency point, and an increase in efficiency of almost 1% at low rotational speed. The outlined procedure is parallelizable and can be performed within an industrial timeframe.publishedVersion© 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited

Two-way coupled simulation of the Francis-99 hydrofoil using model order reduction

The Francis-99 hydrofoil is simulated using a quasi two-way Fluid-Structure Interaction procedure. The structural domain is reduced by the use of modal decomposition, and solved for inside the commercial fluid solver ANSYS CFX. Both the first order Backward Euler and second order Crank-Nicolson time discretization scheme is used in the structural equations, with significantly different results. Several coupled fluid-structure phenomena is observed that would be unobtainable in a normal one-way approach. The most interesting is an "added stiffness" effect, where the eigenfrequency of the foil increases when the flow velocity is increased. This trend corresponds well with available experimental results. The same phenomenon is observed in the hydrodynamic damping on the foil. Self-induced vibration due to vortex shedding is also simulated with good results. The implemented two-way approach allows the different forcing terms to be tracked individually, due to the discretization of the second order structural system. This provides insight into the underlying physics behind the different FSI phenomena seen, and helps us explain why the damping and eigenfrequency characteristics change as the flow velocity passes the lock-in region