Low-head Turbines Research Articles

Optimization of Elbow Draft Tubes for Variable Speed Propeller Turbine

The design of the elbow draft tubes is challenging due to the complexity of the flow. The whole turbine unit’s power output strongly depends on the draft tube function, especially for the low-head turbines. The article presents a novel approach to optimizing elbow draft tubes for a variable-speed propeller turbine designed for low-head applications. First, the study addresses the specifics of the propeller variable speed turbine by comparing the classical Kaplan turbine. Then, the grid scaling test is conducted to evaluate the uncertainty of the pressure regeneration. Further, a new approach to parameterising the elbow draft tube geometry is introduced. The study employs ANSYS CFX 2021 R1 software for numerical simulation to optimise the elbow draft tube geometry in the CAESES environment. After the sensitivity test and deselecting the non-sensitive parameters, we perform multi-objective genetic algorithm (MOGA) optimization. The optimization process results in a Pareto front of optimised elbow draft tube shapes with the best pressure regeneration for different draft tube construction heights, enabling the selection of suitable candidates for various locations. Minimal difference in the performance of the selected elbow draft tube shapes with the simple straight draft tube confirms a high-quality draft tube optimization achievement.

Numerical and experimental study of a very low head (VLH) turbine in an open channel at no-load conditions

Very low head (VLH) turbines are axial turbines with the capability of extracting power at a high efficiency of over 80% from very low heads (less than 4.5 m) and high flow rates of up to 30 m3/s per unit, with a significant reduction in construction costs, compared to conventional hydropower turbines. At no-load conditions, the turbine reaches its runaway speed, which is its theoretical maximum speed, causing damage to the equipment and the turbine structure, thereby imposing a considerable financial burden. As a result, understanding the performance of the turbine at the runaway speed is of utmost importance. The present paper centers its experimental and simulation efforts on predicting the runaway speed of the turbine rotor. An experimental study is conducted to determine the relationship between the runaway speed of the turbine and the flow rate, as well as compare its value with the rotational speed at the design point. Through the conducted experiments, it has been observed that at the mass flow rate corresponding to the design point, the runaway speed is 2.12 times higher than the rated value. Transient simulations using Computational Fluid Dynamics (CFD) are employed to investigate the performance under no-load conditions by estimating the generated torque as a key parameter. Simulations are carried out in an open channel configuration using the Ansys CFX software, and the results are compared to the experiments. In this study, a portion of the work focuses on simulating the free surface flow over the turbine, which allows for more realistic simulations of the turbine's operational behavior. The results of these simulations indicate that the homogeneous model is able to correctly predict free surface flow downstream and upstream. The numerical approach can also predict the runaway state of the VLH turbine with high accuracy, exceeding 95%; however, this variation is due to mechanical and hydraulic losses that have not been factored into the simulation. In this CFD modeling, setting up the boundary conditions accurately is vital, especially for the water levels at the inlet and outlet.

Exploration of transient flow perturbations in a bulb turbine diffuser using proper orthogonal decomposition

Hydraulic turbines with high efficiency over a wide range of operating conditions offer a much sought-after flexibility to electricity producers. However, some low-head turbines exhibit sharp losses of efficiency close to their peak efficiency discharge that are linked to draft tube flow separations whose causes remain misunderstood. This paper presents the latest results obtained in the scope of the BulbT project that focused on the flow dynamics related to the efficiency drop. The main objectives are to document the transient characteristics of the flow in the hub-wake region and investigate interactions between the core flow and wall separations to identify potential mechanisms explaining the efficiency drop. Using proper orthogonal decompositions of synchronized time-resolved velocity and pressure measurements, highly energetic modes representing stochastic perturbations across BulbT's draft tube are identified. These perturbations occur only for the discharges affected by the efficiency drop, past the best efficiency point. Despite the absence of near-wall velocity measurements, the modal decompositions provide evidence that the onset of the efficiency drop is the result of two independent types of flow separation that occur on opposite sides of the draft tube. Upstream separations are found to happen simultaneously with an asymmetric acceleration of the flow in a region surrounding the turbine's axis and the tip of the runner hub and become the most important contributor to the efficiency drop at the highest measured flow rate. Furthermore, the likeliness of observing one kind of flow separation increases after it has already occurred, pointing to a strong history effect.

Investigations of Spoilers to Mitigate Columnar Vortices in Propeller Turbines at Speed-No-Load Based On Steady and Unsteady Flow Simulations

Abstract With the introduction of an ever-larger share of renewable but intermittent energy sources on electrical grids, hydraulic turbines are more often used as network stabilizers. In such a role, they are generally operated in off-design operations like speed-no-load (SNL). No energy is extracted from the flow at SNL operation, but the runner rotates at the synchronous speed linked to the electrical grid. The flow inside the runner of low-head turbines operating at SNL is often dominated by a columnar vortex array that may induce damaging pressure fluctuations. This paper presents the study of a control device to mitigate those vortices. At SNL, the small guide vane opening leads to a high swirl in the runner generating secondary flows such as columnar vortices and backflows. The proposed concept is to move SNL operation toward a higher guide vane opening and hence lower swirl, preventing the formation of a columnar vortex array. Lowering the input swirl of SNL is accomplished by opening up the guide vanes while using a control device to limit the discharge. The control device, like a spoiler on an aircraft wing, is introduced on the guide vanes to generate added head losses, significantly decreasing the discharge. This paper compares the hydrodynamics of the flow in a propeller turbine with different spoiler geometries. The study is based on both RANS and URANS flow simulations. It highlights how such devices can successfully mitigate columnar vortices and their associated pressure fluctuations on runner blades.

Experimental Study of The Pressure-Time Method With Potential Application for Low-Head Hydropower

AbstractThe pressure-time method allows measuring the flowrate in hydraulic turbines, according to IEC 60041 standard. According to this standard, the applicability of the pressure-time method is limited to a straight pipe with a constant cross section with specific limits for length and velocity. However, low-head hydropower plants usually have an intake with a variable cross section and short length making this method difficult to be applied. In this paper, a test rig has been developed to extend the method's applicability. The test rig is designed for developing flows condition and small measurement lengths for pipes with constant and variable cross section, which could be similar to low-head turbine conditions. For pipe with constant cross section, three measuring lengths of 0.5, 1, and 1.5 m can be obtained on this test rig to study the pressure-time method. For pipe with variable cross section, the method is applied across a concentric reducer with a length of 255 mm, i.e., a 9.46 deg reducing angle. Different flow assumptions for estimation of the head loss and dynamic pressure variation are considered to compare the accuracy of discharge measurement. The results showed that the quasi-steady assumption for friction factor and kinetic energy, along with friction factor correction, increases the method accuracy up to 0.4% compared with the current standard recommendations. Moreover, the Monte Carlo method (MCM) is applied to estimate the transient measurement uncertainty and compared with the Taylor series method (TSM).

Numerical study on the effect of guide vane opening and draft tube outlet pressure on cavitation in Kaplan turbine

variations in electricity demand and site conditions like mass flow rate and net head, force the turbines to operate under off-design conditions. This increases cyclic loading in stationary and rotating components and severe cavitation in low head turbines like Kaplan turbine. The present analysis aims to examine the influence of various operating conditions on hydraulic efficiency and cavitation characteristics using ANSYS CFX. The stages of cavitation development in the runner are extensively studied for different operating conditions. The key point of present work is the critical analysis of various flow parameters for cavitating and non-cavitating conditions. The value of critical cavitation factor is found to increase from 0.34 to 0.61 with an increase in guide vane opening from 35º to 60º. The blade loading and flow parameters are found to be affected due to changes in guide vane opening and cavitation. The computed results are validated with the experimental model testing results and found to bear close comparison.

Low-Head Hydropower for Energy Recovery in Wastewater Systems

Hydraulic turbines for energy recovery in wastewater treatment plants, with relatively large discharges values and small head jumps, are usually screw Archimedes or Kaplan types. In the specific case of a small head jump (about 3 m) underlying a rectangular weir in the major Palermo (Italy) water treatment plant, a traditional Kaplan solution is compared with two other new proposals: a Hydrostatic Pressure Machine (HPM) located at the upstream channel and a cross-flow turbine (CFT) located in a specific underground room downstream of the same channel. The fluid mechanical formulations of the flow through these turbines are analyzed and the characteristic parameters are stated. Numerical analysis was carried out for the validation of the HPM design criteria. The efficiency at the design point of the CFT and HPM are estimated using the ANSYS CFX solver for resolution of 3D URANS analysis. The strong and weak points of the three devices are compared. Finally, a viability analysis is developed based on several economic indicators. This innovative study with a theoretical formulation of the most suitable turbomachine characterization, the potential energy estimation based on hydraulic energy recovery in a real case study of a wastewater treatment plant and the comparison of the three different low-head turbines, enhancing the main advantages, is of utmost importance towards the net-zero water sector decarbonization.

Interactions of a temperate North American fish community with a very low head hydropower facility in Ontario, Canada

AbstractEfforts are underway to re‐evaluate the use of existing instream infrastructure (e.g., weirs, water control dams) for the purposes of hydroelectric generation, with new very low head turbine technology that is purportedly “fish friendly” making retrofitting a viable option. This is the case at Wasdell Falls on the Severn River, ON, Canada, where the first very low head (VLH) turbines in Canada were put into operation at a long‐standing low‐head dam site. There is little information regarding fish usage of areas upstream from these structures and how this may relate to entrainment risk. Therefore, we assessed the risk of entrainment based on fish use of the forebay areas upstream from the infrastructure, including the forebay of three operating VLH turbines. Acoustic telemetry was used to determine movements and entrainment events of eight north temperate fish species. Entrainment through the VLH turbines did not occur over the course of one year, however, several fish did move downstream via other paths (e.g., over the water control structure). Forebay use was exclusive to rock bass, smallmouth bass, northern pike and largemouth bass, whereas channel catfish, walleye, white sucker and pumpkinseed avoided the forebay. When near the dam, fish tended to select deeper areas located away from the VLH forebay. Fish use of the VLH forebay was limited to brief forays indicating exploratory movements rather than prolonged residence. The findings suggest that entrainment risk at this VLH turbine site is low for the species and life stages studied.

Application of CFD to the Design of Manifolds Employed in the Thermodynamic Method to Obtain Efficiency in a Hydraulic Turbine

This study presents the design and implementation of different types of manifolds (sampling system) to measure water flow properties (velocity, pressure, and temperature) through the high- and low-pressure section of a Francis-type low head hydraulic turbine (LHT of 52 m) to calculate it is efficiency using the Thermodynamic Method (TM). The design of the proposed manifolds meets the criteria established in the “International Electrotechnical Commission—60041” Standard for the application of the TM in the turbine. The design of manifolds was coupled to the turbine and tested by the Computational Fluid Dynamics (CFD) application, under the same experimental conditions that were carried out in a power plant, without the need for on-site measurements. CFD analyses were performed at different operating conditions of volumetric flow (between values of 89.67 m3/s and 35.68 m3/s) at the inlet of turbine. The mechanical power obtained and the efficiency calculated from the numerical simulations were compared with the experimental measurements by employing the Gibson Method (GM) on the same LTH. The design and testing of manifolds for high- and low-pressure sections in a low head turbine allows for the constant calculation of efficiency, avoiding breaks in the generation of electrical energy, as opposed to other methods, for example, the GM. However, the simulated (TM) and experimental (GM) efficiency curves are similar; therefore, it is proposed that the design of the manifolds is applied in different geometries of low-head turbines.

Investigating barotrauma in juvenile salmon passing through a very low head turbine using response surface methodology

Although hydropower is a clean source of energy, in some cases, it can jeopardize the life of some species of riverine fish. Very Low Head (VLH) water turbine is an innovative design that aims at reducing the adverse effects of such hydroelectric facilities. In this research, two methodologies are integrated to investigate barotrauma in juvenile salmons passing through this particular turbine. First, to quantify barotrauma, we implement a method known as BioPA (Biological Performance Assessment) by combining the results of some laboratory experiments on juvenile Chinook salmon moving through a simulated turbine passage with the Computational Fluid Dynamics (CFD) simulation of the flow field in this environment. In the second part, we added surrogate-based modeling as a tool, which enabled us to study the effects of two geometrical parameters on the environmental performance of the VLH turbine with low computational costs. The results indicate a significant dependency between the installation angle of the VLH turbine and the severity of the barotrauma of this particular fish. In addition, further investigations suggest that the region near the middle of blades is the safest for fish in the case of decompression.

Editorial: Green or red: Challenges for fish and freshwater biodiversity conservation related to hydropower

Editorial: Green or red: Challenges for fish and freshwater biodiversity conservation related to hydropower

Modelling the Energy Extraction from Low-Velocity Stream Water by Small Scale Archimedes Screw Turbine

In recent years, there has been a growth of interest in the development of micro-hydropower power generation, especially in the low-head turbine. Low-head turbines gained popularity due to their high efficiency, relatively low cost, ability to operate at a low flow rate, and low environmental impact. In this aspect, the Archimedes Screw Turbine (AST) could be the primary key to electrifying the rural area in Sarawak, Malaysia, which is surrounded by rivers. This study starts with a conceptual design based on literature review findings. Eventually, the small-scale prototype is then being built and tested in the laboratory. The experiment is set up to simulate the actual Sarawak river velocity to determine the relationship between key performance variables such as the inclination angle of AST and water flow velocity. The findings revealed that the 45° angle of inclination was the optimum angle of AST within the water velocity of 1.0 m/s until 1.5 m/s. At this angle, the highest revolution per minute (RPM) generated by the AST shaft was 179.8, and the highest torque recorded was 0.9Nm. The results were validated through statistical means. It was found that both angles of inclination and river water velocity are significant to RPM and torque generation (p < 0.05). Two statistical models were generated based on linear regression to explain the contribution of water velocity and angle of inclination as inputs to torque and RPM as outputs, with a Pearson R2 value of more than 60%. The maximum mechanical power generated is about 1.54 kW, with a maximum efficiency of 94.6%. The outcome of this study would be useful for designing a small-scale AST power generation system by utilizing a low-flow river (velocity < 1.5 m/s) as a power source. This study would contribute to the existing knowledge stock of small-scale AST, primarily to operate in low-flow velocity rivers. For the future study, it is recommended for conducting a pilot study to test the actual performance of AST in the Sarawak River or rivers with similar flow characteristics.

Physical Modelling of the Effect on the Wave Field of the WaveCat Wave Energy Converter

The WaveCat is a moored Wave Energy Converter design which uses wave overtopping discharge into a variable v-shaped hull, to generate electricity through low head turbines. Physical model tests of WaveCat WEC were carried out to determine the device reflection, transmission, absorption and capture coefficients based on selected wave conditions. The model scale was 1:30, with hulls of 3 m in length, 0.4 m in height and a freeboard of 0.2 m. Wave gauges monitored the surface elevation at discrete points around the experimental area, and level sensors and flowmeters recorded the amount of water captured and released by the model. Random waves of significant wave height between 0.03 m and 0.12 m and peak wave periods of 0.91 s to 2.37 s at model scale were tested. The wedge angle of the device was set to 60°. A reflection analysis was carried out using a revised three probe method and spectral analysis of the surface elevation to determine the incident, reflected and transmitted energy. The results show that the reflection coefficient is highest (0.79) at low significant wave height and low peak wave period, the transmission coefficient is highest (0.98) at low significant wave height and high peak wave period, and absorption coefficient is highest (0.78) when significant wave height is high and peak wave period is low. The model also shows the highest Capture Width Ratio (0.015) at wavelengths on the order of model length. The results have particular implications for wave energy conversion prediction potential using this design of device.

Archimedean screw as fish-friendly turbines for harnessing hydropower potential

Hydraulic energy is one of the most important sources of renewable energy today. It is also a complementary source to other renewable energy sources, being the only one that offers an important nonpolluting storage capacity (through pump storage facilities). Another essential quality of hydraulic energy is its ability to regulate the energy system to allow the integration of other sources, in which the process of generating energy depends on factors that cannot be controlled (sun, wind, etc.). However, hydropower plants can damage ecosystems, especially by affecting fish within their turbines. Usually, fish cannot pass unharmed through most of the turbines. The main exceptions generally include some low head (below 8 m) turbines, such as the Very Low Head (VLH) turbine and the screw turbine, but lately strategies have also been developed for high head hydropower plants such as: fish passage facilities and fish friendly turbines such as Alden turbine, Minimal Gap Runner turbine (MGR). However, this article only deals with screw-type turbines, low-head fish-friendly turbines and how to implement them in order to obtain sustainable green energy.

Transient Analysis of self excited Induction generator with ELC

— Fast depletion of fossil fuels has drawn attention towards the use of nonconventional energy sources like wind, biomass, tidal/wave, and small hydro potential. In remote locations, harnessing of electrical energy from such local resources can be cheaper and easier compared to grid connection, which involves long transmission lines and associated losses. A micro-hydro system using natural hydro potential with minimal civil works is a strong candidate in this race. Uncontrolled low head turbines are prescribed for such applications. In this paper, a transient analysis of a self-excited induction generator (SEIG) with electronic load controller (ELC) utilized in stand-alone micro-hydro power generation employing uncontrolled turbines. In view of the need to fed single phase loads from such systems, the transient behavior due to switching in of such loads is of interest and is carried out here. A composite mathematical model of the total system has been implemented by combining the modeling of prime mover, SEIG, ELC, and load. Simulated results are matched with the experimental ones, obtained on a implemented prototype of an SEIG-ELC system for the starting of an IM and switching in a resistive load. To starting of an IM, a star/delta starter is utilized to avoid inrush current.

Extent of injury and mortality arising from entrainment of fish through a Very Low Head hydropower turbine in central Ontario, Canada

Because of growing interest in deploying newer very low head (VLH) turbine technology to generate electricity in rivers, there is a need to assess how fish fare in interactions with VLH turbines. We assessed injury and mortality rates from experimental VLH turbine entrainment of fish species local to the study site at Wasdell Falls on the Severn River, Ontario, which is one of the first such VLH installations in North America. Using balloon tags to recapture fish and before/after entrainment assessments, we found minimal injury and mortality differences between control (no entrainment) and treatment (entrainment) groups. One adult northern pike (Esox lucius Linnaeus, 1758; 1.16% of total entrained fish) was killed by turbine strike. Abrasion-related injuries (i.e., scale loss, torn fins) were the most common form of injury in both control and treatment fish, which was likely attributed to handling and not turbine passage per se. Telemetry monitoring of a subset of fish revealed that post-passage mortality was low. These results suggest that VLH turbine entrainment has negligible effects on the fish species studied here, and thus, VLH turbines may be suitable for increasing generating capacity at low head dam sites with minimal risk to fish.

Study of low head turbine propellers axial flow for use of micro-hydropower plant (MHP) in Aceh, Indonesia

Low head hydropower has the potential to produce green energy with a minimum on the environment and is one of the best choices for decentralized power plants. Aceh is one of many regions in Indonesia with hilly topography and many river streams. Of the 66 rivers in Aceh that have potential energy sources that can be developed 98.5 percent are low head. Generally, the technique for overcoming altitude problems in low river flows is the use of low head turbines. An important aspect of head power generation techniques is the selection of turbines. The study aimed to study low head propeller turbines for the use of micro hydropower plants. By calculating the main dimensions and main components of the turbine, field experiments were carried out to determine turbine rotation and energy, as well as turbine efficiency at a flow rate of 0.06 m3/sec and a 3.5-meter head. The calculation results obtained by a specific turbine speed of 1.53, runner diameter of 0.30 meters, hub diameter of 0.06 meters, shaft diameter 0.03, runner length 1.08, triangular speed of 30 degrees with the number of blades 4 pieces. From the results of field experiments, the maximum turbine rotation speed is 1828 rpm, the turbine power is 2,530 watts with a voltage between 220 to 240 volts while the maximum efficiency is 57 percent. It can be concluded that propeller-type turbines with the low head are suitable for micro power plants, especially in remote areas.

Effects of runner change on the Winter-Kennedy flow measurement method – A numerical study

The Winter-Kennedy (WK) method is a popular choice to estimate the relative flow rates, and thus the expected improvement in the efficiency of a low head turbine after its refurbishment. Runner refurbishment is a common way to improve the plant’s efficiency. However, a previous experiment on a model turbine reported deviations between the WK coefficients obtained from two different runners ‒ suggesting a deviation between the estimated and actual improvement in the efficiency. Without formal proof, the deviation was attributed to flow changes in the spiral casing. This paper presents a numerical investigation of the effects of a runner change on the WK method. For this purpose, unsteady Reynolds-averaged Navier-Stokes equations (URANS) simulations of a turbine model with two different runners were conducted. The runner’s impact on the average flow conditions upstream and its subsequent effect on the WK coefficients were studied. The study shows the dependence of the WK coefficients to the runner ‒ with a maximum deviation on the coefficient up to 0.7%. The larger deviations were observed in regions prone to strong secondary flow. Following a radial and circumferential sensitivity study, a suitable location to minimize the effects of runner change on the WK method is reported.

Numerical estimation of prototype hydraulic efficiency in a low head power station based on gross head conditions

Low-head hydraulic power units typically have a short intake pipe or no that structure. For this reason, the upstream and downstream flows in reservoirs could influence the internal flow characteristics in the turbine, which sometimes increase the flow non-uniformity and the local hydraulic losses. Meanwhile, the efficiency estimation methods for prototype turbines still have a limitation for determining the unit discharge, which is generally predicted by using a Winter-Kennedy method. This paper proposes a hydraulic performance estimating method focusing on low-head turbine units, which is conducted by using CFD analysis and based on site test results. A bulb type turbine unit was adopted as a research object, which is operating in a low-head run-of-the-river power station. The flow behaviors in turbine were simulated based on two water levels under single unit operation, with two reservoir modeling. The simulation results agreed well with test ones for the turbine power as well as the relative discharge curve. The prototype efficiency in operation was estimated at the same time. In addition, this paper analyzed the relationship between the gross head and net head, and introduced the concept of plant hydraulic efficiency, as regards the flow interaction between the turbine unit, intake and tailrace.

Design for localisation: A case study in the development and implementation of a low head propeller turbine in Nepal

This paper proposes a methodology for “design for localisation” that addresses the challenge of designing a product for local manufacture and use, whilst considering the production process availability and the context of the local geographic area. The methodology is derived from a case study of the development of a propeller turbine in Nepal. In the case study, the initial challenge was the absence of a low head turbine that could be manufactured, used and repaired in Nepal. A potential solution from previous academic work was identified however its intended operating environment differed considerably. Through identification of the specific local requirements, the design priorities for individual sub-systems in the new context were developed. Using three examples, design changes driven by improving the ease of manufacture and applicability to the local context are explained. Multiple phases of field and laboratory-based testing were used to validate, adapt and improve the design and its method for implementation. The experiences of the case study lead to three rules for design for localisation using an identified potential solution for a local problem: firstly, to derive local product requirements; secondly to develop solutions appropriate for local manufacturing; and finally, to conduct field-testing phases to ensure the product is suitable for its intended application.