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The efficiency gains observed in a pair of closely spaced Darrieus turbines suggest the deployment of multiple turbines as an appealing solution for wind and, particularly, hydrokinetic applications, where the inflow direction is constant. The present study develops some design guidelines for closely spaced hydrokinetic Darrieus turbines by analyzing the trends of both power augmentation and wake development within arrays of multiple rotors, including not only an even number of rotors, which is the usual case in literature, but also an odd one. The analysis is carried out by means of two-dimensional computational fluid dynamics simulations and includes not only the assessment of instantaneous blade forces but also locally sampled flow fields past each blade that allowed the reconstruction of dynamic polar data, contributing to a more comprehensive understanding of the physical mechanisms at play in such compact setups. The study demonstrates a consistent power augmentation mechanism across different layouts, even in the case of an odd number of rotors. This enhancement originates from flow blockage in the mutual interaction areas, favorably altering the inflow angle and subsequently increasing the angle of attack and lift generation. While this mechanism aligns with previous observations on arrays of counter-rotating pairs, its application to multiple turbines introduces complexities due to potential asymmetries in inflow, leading to an uneven power enhancement across turbines within the array. The identified efficiency improvement pattern suggests that maximizing leeward mutual interactions within an array of multiple Darrieus turbines would enhance the overall efficiency of the setup. [ABSTRACT FROM AUTHOR]

The global climate and energy crisis have underscored the importance of sustainability in energy systems and their efficiency. In the case of vertical axis turbines (VATs) for hydrokinetic applications, the increment in efficiency is a topic of interest. Using winglets as passive flow control devices has the potential to improve the power coefficient of straight-bladed (SB) Darrieus turbines highly due to their impact in the dynamics of the flow close to the tip blade and the general impact in the hydrodynamic performance of each blade. The aim of the present work is to study the influence of the geometric parameters of a symmetric winglet in the performance of an SB-VAT for hydrokinetic applications via numerical simulations based on Computational Fluid Dynamics (CFD). Several simulations were performed in Star CCM+ v2206 varying the cant and sweep angles of the designed winglet. Numerical results show that a cant angle of 45° in combination with a sweep angle of 60° achieved the highest power coefficient with an increment around 20% with respect to the model without winglets. Furthermore, the vortical flow structures that form around straight and winglet blades are examined. This involves assessing the distribution of pressure and skin friction coefficients at different blade azimuthal positions during a turbine revolution. In general, the predicted increment in performance is related to the influence of the winglets in the strength of the tip vortices and in the delay in the flow separation. [ABSTRACT FROM AUTHOR]

Hydrokinetic turbines interact with the dynamics of the sedimentary bottom at small and large scale. Despite the interest that the study of these interactions deserves, little research has been published in the field. In this paper, we investigate by numerical simulation the interaction between modeled hydrokinetic turbines and bed load under clear-water scour conditions. The mixture of water and sediment is accounted for by an Eulerian multiphase model developed in the open source platform OpenFOAM. The turbine blades are parameterized by two models: the Blade Element Method (BEM) and the Actuator Disc Theory (AD). A good agreement with measurements is obtained in the near wake. The effects of the two different turbine models on the bedload sediment transport and on the wake characteristics are then examined. The local impact of the turbine modeled by BEM is more pronounced on the bed than the one modeled by the AD. So, the BEM modeling is preferable to the AD one for local studies of the impact of a turbine on the bed morphology. However, combining BEM with the two-phase fluid-sediment is time consuming and make difficult the use of this method to study a farm of several stream turbines. In such a case the use of AD is still remains recommended for now. Moreover, more studies must be done at real scale as the turbine rotational speed is relatively low and could induced less impact of the swirl on the bottom shear stress. [ABSTRACT FROM AUTHOR]

Tidal turbines are one source of marine renewable energy but development of tidal power is hampered by uncertainties in fish-turbine interaction impacts. Current knowledge gaps exist in efforts to quantify risks, as empirical data and modeling studies have characterized components of fish approach and interaction with turbines, but a comprehensive model that quantifies conditional occurrence probabilities of fish approaching and then interacting with a turbine in sequential steps is lacking. We combined empirical acoustic density measurements of Pacific herring (Clupea pallasii) and when data limited, published probabilities in an impact probability model that includes approach, entrainment, interactions, and avoidance of fish with axial or cross-flow tidal turbines. Interaction impacts include fish collisions with stationary turbine components, blade strikes by rotating blades, and/or a collision followed by a blade strike. Impact probabilities for collision followed by a blade strike were lowest with estimates ranging from 0.0000242 to 0.0678, and highest for blade strike ranging from 0.000261 to 0.40. Maximum probabilities occurred for a cross-flow turbine at night with no active or passive avoidance. Estimates were lowest when probabilities were conditional on sequential events, and when active and passive avoidance was included for an axial-flow turbine during the day. As expected, conditional probabilities were typically lower than analogous independent events and literature values. Estimating impact probabilities for Pacific herring in Admiralty Inlet, Washington, United States for two device types illustrates utilization of existing data and simultaneously identifies data gaps needed to fully calculate empirical-based probabilities for any site-species combination.

Shrouded wind and hydrokinetic turbines are being widely studied nowadays. Although actuator disk studies have concluded that the addition of a diffuser around a rotor can improve the Power Coefficient, not many designs have realized those benefits when compared to standard high performance bare turbines, as shown by Nunes et al. (2020) [1]. One reason for low power coefficient on several designs is massive flow separation on the hub surface due to high adverse pressure gradient inside the diffuser, resulting on low mass flow capture and, hence, poor performance. This work presents a novel design methodology for shrouded rotors, which takes into consideration the influence of the entire linear cascade on each annular section. Also, the blade root is left unloaded to guarantee that no boundary layer separation occurs on the hub surface by allowing a layer of energized fluid to bypass the rotor. A turbine modeled by this method has been numerically studied and is shown herein to deliver a peak power coefficient of 0.415 normalized by the diffuser's largest cross sectional area. [Display omitted] [ABSTRACT FROM AUTHOR]

The global climate and energy crisis have underscored the importance of sustainability in energy systems and their efficiency. In the case of vertical axis turbines (VATs) for hydrokinetic applications, the increment in efficiency is a topic of interest. Using winglets as passive flow control devices has the potential to improve the power coefficient of straight-bladed (SB) Darrieus turbines highly due to their impact in the dynamics of the flow close to the tip blade and the general impact in the hydrodynamic performance of each blade. The aim of the present work is to study the influence of the geometric parameters of a symmetric winglet in the performance of an SB-VAT for hydrokinetic applications via numerical simulations based on Computational Fluid Dynamics (CFD). Several simulations were performed in Star CCM+ v2206 varying the cant and sweep angles of the designed winglet. Numerical results show that a cant angle of 45° in combination with a sweep angle of 60° achieved the highest power coefficient with an increment around 20% with respect to the model without winglets. Furthermore, the vortical flow structures that form around straight and winglet blades are examined. This involves assessing the distribution of pressure and skin friction coefficients at different blade azimuthal positions during a turbine revolution. In general, the predicted increment in performance is related to the influence of the winglets in the strength of the tip vortices and in the delay in the flow separation.

The implementation of renewable energies represents a crucial step in meeting the sustainable development goal of the United Nations for affordable and clean energy. The Terminos Lagoon region in Campeche, the largest coastal lagoon in Mexico, offers potential for renewable energy sources such as wind, photovoltaic, and current energy. This study presents a renewable energy potential assessment for the main city at Terminos Lagoon, Ciudad del Carmen, which has the largest oil activity in Mexico and high electricity consumption. The outputs of high-resolution numerical models were analyzed to evaluate wind and photovoltaic resources and currents. A hybrid system consisting of 24 wind turbines, 5516 photovoltaic panels, and 32 hydrokinetic turbines could generate 521.33 GWh, which is 39.63% of the state's energy demand and exceeds the energy consumption of Ciudad del Carmen by 10.24%. Wind and photovoltaic energy are the most significant contributors (517.15 GWh and 3.77 GWh, respectively), while hydrokinetic energy contribution is marginal (0.407 GWh) and requires further research and development. The results suggest that the region has the potential for clean and renewable energy technologies to reduce greenhouse gas emissions and contribute to the energy transition. [ABSTRACT FROM AUTHOR]

The design of hydrokinetic plants in hydropower canals involves the choice of the array layout, rotor geometry, turbine spacing, and array spacing, and necessitates the control of the resultant backwater to avoid upstream flooding hazards. Several works in the literature have shown that array power optimization is feasible with small spacings between the arrays, disregarding the limitation in the power output induced by backwater upstream. In this study, a 1-D channel model with a Double Multiple Streamtube code and wake sub-models are integrated to predict an array layout that will maximize the array power. The outputs of the conducted sensitivity analysis confirm that this design enabled improved power conversion with closely spaced turbines and largely spaced arrays, thus allowing for a partial recovery of the total head variation for a new array deployed upstream. In addition to the quantitative assessment of the mechanical power converted, the tool enables depth control from the downstream undisturbed flow station to the backwater exhaustion far upstream, thereby increasing its flexibility. Furthermore, it overcomes the limitations of actuator disc models by considering rotor's fluid dynamic losses. The results show that power output linearly scales for a limited number of arrays (≤5), whilst the variation in water depth variation follows a power law from the most downstream array towards upstream, regardless of the plant size. Finally, the maximum upstream inflow depth is demonstrated to become asymptotic for large multi-array plants under ideal conditions. [ABSTRACT FROM AUTHOR]

The hydrokinetic turbine is used worldwide for electrical generation purposes, as such a technology may strongly reduce environmental impact. Turbines designed using backward swept blades can significantly reduce the axial load, being relevant for hydro turbines. However, few works have been conducted in the literature in this regard. For the case of hydrokinetic rotors, backward swept blades are still a challenge, as the authors are unaware of any optimization procedures available, making this paper relevant for the current state of the art. Thus, the present work develops a new optimization procedure applied to hydrokinetic turbine swept blades, with the main objective being the design of blades with reduced axial load on the rotor and possibly a reduction in the cavitation. The proposed method consists of an extension of the blade element momentum theory (BEMT) to the case of backward swept blades through a radial transformation function. The method has low computational cost and easy implementation. Once it is based on the BEMT, it presents good agreement when compared to experimental data. As a result, the sweep heavily affects the chord and twist angle distributions along the blade, increasing the turbine torque and power coefficient. In the case of the torque, it can be increased by about 18%. Additionally, even though the bound circulation demonstrates a strong change for swept rotors, Prandtl's tip loss seems to be not sensitive to the sweep effect, and alternative models are needed. [ABSTRACT FROM AUTHOR]

This paper presents a novel theoretical and computational methodology for the generation of an onset turbulent field with prescribed properties in the numerical simulation of an arbitrary viscous flow. The methodology is based on the definition of a suitable distribution of volume force terms in the right-hand side of the Navier–Stokes equations. The distribution is represented by harmonic functions that are randomly variable in time and space. The intensity of the distribution is controlled by a simple PID strategy in order to obtain that the generated turbulent flow matches a prescribed turbulence intensity. A further condition is that a homogeneous isotropic flow is established downstream of the region where volume force terms are imposed. Although it is general, the proposed methodology is primarily intended for the computational modelling of hydrokinetic turbines in turbulent flows representative of tidal or riverine installations. A first numerical application is presented by considering the injection of homogeneous and isotropic turbulence with 16% intensity into a uniform unbounded flow. The analysis of statistical properties as auto-correlation, power spectral density, probability density functions, demonstrates that the generated flow tends to achieve satisfactory levels of stationarity and isotropy, whereas the simple control strategy used determines underestimated turbulent intensity levels. [ABSTRACT FROM AUTHOR]

Design methods and concepts that can increase the energy conversion efficiency of marine and river current turbines are of great importance for the development and wider utilization of this emerging renewable energy technology. This work is aimed to improve hydrodynamic performances of the hydrokinetic turbines to get as close as possible to the theoretical energy conversion efficiency limitation of 59.3%, known as the Lanchester–Betz–Joukowsky limit. The winglets are integrated at the turbine blade tips to reduce the effect of the tip vortex. The winglet design concept is adopted from the jet aircraft and adjusted to the hydrokinetic turbine application. The numerical investigation of the winglets impact on the hydrodynamic performances was performed using computational fluid dynamic (CFD). The results indicate that winglets subdue the strength of tip vortices, which is reflected through the increase in the power extracted. It has been confirmed that winglet height has a great influence on the power coefficient increase, which for higher winglets reaches the value above 50%. The analysis of the wake regions behind turbines pointed out that the turbines with winglets form stronger vortices in the far wake zone which may influence the back-flow turbine installed in row array. • Increase of maximum power coefficient up to 13% at design tip speed ratio. • Energy extraction improvements are achieved with higher winglets. • Blade tip losses reduction are obtained by diffusing tip vortices. • The detailed analysis of the flow around hydrokinetic turbines with winglets is performed. [ABSTRACT FROM AUTHOR]

This paper focuses on a model for increasing the power generation of a hydroelectric power station using the combined application of hydrokinetic turbines installed at the tailrace of the existing dam. Hydrokinetic turbines will capture additional power from the energy remaining in water currents exiting draft-tube outlets and tailrace of the dam. Two commercially available hydrokinetic turbines of different orientation were used for the analysis to theoretically estimate the increased power generation capability of Shiroro hydropower station using an array of 75, 150, 225 and 300 units. Preliminary results showed that at 50% reliability, combined-cycle hydropower can increase the generation capacity of Shiroro hydropower station by 2.5% and that there is a considerable power generation potentials that can be harnessed to augment existing output from the power station and alleviate the existing power problems in the country. [ABSTRACT FROM AUTHOR]

This paper focuses on a model for increasing the power generation of a hydroelectric power station using the combined application of hydrokinetic turbines installed at the tailrace of the existing dam. Hydrokinetic turbines will capture additional power from the energy remaining in water currents exiting draft-tube outlets and tailrace of the dam. Two commercially available hydrokinetic turbines of different orientation were used for the analysis to theoretically estimate the increased power generation capability of Shiroro hydropower station using an array of 75, 150, 225 and 300 units. Preliminary results showed that at 50% reliability, combined-cycle hydropower can increase the generation capacity of Shiroro hydropower station by 2.5% and that there is a considerable power generation potentials that can be harnessed to augment existing output from the power station and alleviate the existing power problems in the country.

At a global level, hydrokinetic power has been considered as a renewable energy source, and it has become an attractive alternative for the rural electrification of non-interconnected areas with the presence of water resources. Aspects such as the low rural electrification rate, the increase in energy demand, the decrease in fossil reserves and the climate change, are some of the factors that have driven the use of this technology for the electricity production. The aim of this work is to give a review of the hydrokinetic energy potential of water resources, the requirements and impacts of the implementation of hydrokinetic technology in different countries, and the current development in the Colombian case. At present, it can be observed that the implementation of this technology in different regions of the world, especially in Colombia, has several challenges and barriers, including gaps in knowledge, information and data, such as well as limitations of water resources and infrastructure, finally, impacting on a low adoption of this technology. On the other hand, publications on studies of implementation and potential of hydrokinetic technology have been increasing over time, indicating that this topic has been gaining interest despite the challenges.

This paper presents a novel theoretical and computational methodology for the generation of an onset turbulent field with prescribed properties in the numerical simulation of an arbitrary viscous flow. The methodology is based on the definition of a suitable distribution of volume force terms in the right-hand side of the Navier–Stokes equations. The distribution is represented by harmonic functions that are randomly variable in time and space. The intensity of the distribution is controlled by a simple PID strategy in order to obtain that the generated turbulent flow matches a prescribed turbulence intensity. A further condition is that a homogeneous isotropic flow is established downstream of the region where volume force terms are imposed. Although it is general, the proposed methodology is primarily intended for the computational modelling of hydrokinetic turbines in turbulent flows representative of tidal or riverine installations. A first numerical application is presented by considering the injection of homogeneous and isotropic turbulence with 16% intensity into a uniform unbounded flow. The analysis of statistical properties as auto-correlation, power spectral density, probability density functions, demonstrates that the generated flow tends to achieve satisfactory levels of stationarity and isotropy, whereas the simple control strategy used determines underestimated turbulent intensity levels.

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Copyright of DYNA - Ingeniería e Industria is the property of Publicaciones Dyna SL and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Due to the increase in electricity demand worldwide, the unstable oil prices and the vulnerability of the environment for the greenhouse gases issued, numerous ways to generate electricity through alternative sources of energy are currently being investigated. One of these renewable sources of energy is the hydrokinetic turbines. In this investigation the hydrokinetic turbines are analyzed, making an analysis of different aspects by the elaboration of a software, wish provides the necessary information to make the study in any water resource of the country, calculating the energy estimate for Cuba conditions, having into account the decide location, hydrokinetic turbine type and its characteristics. The study case for possible installation of hydrokinetic turbines is Banes bay. [ABSTRACT FROM AUTHOR]

This work reports a study on a tidal current system named GEMSTAR, composed of two hydrokinetic turbines supported by a floating submerged structure. A flexible mooring cable links the floating support to the seabed allowing the free rotation of the system and its alignment to changing current direction. The work is mainly focused on the design of the turbine blade with two objectives: keeping a constant power output above the rated power condition and limiting the thrust increase between rated and maximum operating speed. A fixed pitch blade was considered, in order to reduce the costs associated to a complex pitch control mechanism. An analytical procedure for blade design is proposed and validated by comparison to other numerical methods. Above the rated current speed, the power is held constant while the rotational speed of the turbine and of the generator increases, assuming an over‐speeding control strategy. The adopted design configuration relies on the possibility of the generator to operate in de‐fluxing working conditions. As reported in the last part of this study, a numerical model of the generator has also been developed and applied to a test case in order to analyse the electric behaviour of the system. [ABSTRACT FROM AUTHOR]

The research presented in this paper involves the design of a power control system for a hydrokinetic turbine previously tested in real operating conditions. A maximum power point tracking (MPPT) algorithm was designed and simulated using the required parameters for a specific electric generator. The proposed system consists of a generator connected to the hydrokinetic turbine, a three-phase uncontrolled rectifier, a direct current (DC) boost converter with MPPT control to extract maximum available power, and a buck converter to control the amount of power delivered to the load. In order to test the MPPT algorithm, we built the individual blocks on the basis of the corresponding equations of each component. The algorithm considered the specific parameters of the previously tested turbine as input data and simulated the same water velocities for which the turbine had been tested. Thus, the simulation predicted a power output of 105 W for a water velocity of 1.33 m/s, 60 W for 1 m/s, and 30 W for 0.83 m/s. The efficiency of the control system was demonstrated when the instantaneous power value was maintained at a maximum point, regardless of the rotational speed according to the experimental power curves of the driving rotor obtained for certain water velocities. [ABSTRACT FROM AUTHOR]

The aim of the present paper is to discuss the monitoring strategies for a modular hydrokinetic turbine system. A floating triangular-frame arrangement of three propeller hydrokinetic turbines is installed in the outflow channel of a hydropower plant, in the interest of remaining water stream energy conversion. The monitoring techniques based on Wireless Sensor Networks (WSN) and the Internet of Things (IoT) are reported, taking into account the specific framework of the instrumentation and data management in field experiments. [ABSTRACT FROM AUTHOR]

A numerical formulation for the modeling of turbulent cavitating flows is presented. The flow field is governed by the 3D, time-dependent Navier–Stokes equations for a compressible isothermal mixture. The Arbitrary Lagrangian–Eulerian Variational Multiscale (ALE-VMS) formulation is adopted to model the turbulent flow on moving domains with no-slip boundary conditions imposed weakly. The formulation is first tested on the cavitating flow over a 2D NACA0012 airfoil and compared to published numerical results. Next, the framework is applied to the benchmark problem for the flow over a hemispherical fore-body. The numerical results are compared to the reported experimental data, showing a good agreement over the range of cavitation numbers. Finally, the simulation of a hydrokinetic turbine in cavitating flow at a low cavitation number is presented in order to test the stability of the formulation and the capability to handle real engineering problems involving turbulent cavitating flows on moving domains. [ABSTRACT FROM AUTHOR]

Copyright of Tecnología y Ciencias del Agua is the property of Instituto Mexicano de Tecnologia del Agua (IMTA) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Brazilian hydrological information network does not provide data series of daily velocities. The river velocities are important for the study of hydrokinetic potential. Therefore, the work proposes a model called flow-velocity that estimates the average daily velocity and the distribution of the velocity profile of the cross section of rivers. The model was applied to the Amazon basin, using the highest and lowest flow rates of the historical series. The highest and lowest average velocities found in the Amazon River were 2.27 m s-1 and 0.735 m s-1, respectively. The main contributors to the Amazon River presented average daily velocities close to 2.0 ms-1 for the flood period, but in the dry season these velocities did not exceed 0.5 m s-1. Thus, it was verified that the Amazon River has hydrokinetic potential throughout the year and its tributaries during the flood period. [ABSTRACT FROM AUTHOR]

Summary: Increased demand of energy leads to exploration of new sources of energy. In the last few years, hydrokinetic energy technology emerged as an important area in the field of renewable energy generation. Various hydrokinetic turbines have been studied and used to harness the hydrokinetic potential. Among all hydrokinetic turbine technology, cross‐flow hydrokinetic turbine is considered as the most suitable approach for the riverine system. There are various configurations of cross‐flow hydrokinetic turbine exist for hydrokinetic energy extraction. A number of numerical and experimental studies were carried out on the performance enhancement and design optimization of different configurations under variable operating conditions. Under the present paper, review of different rotor for the configurations of the cross‐flow hydrokinetic turbines are discussed. The paper will be useful to understand the cross‐flow hydrokinetic technology in order to explore the methods for selection, performance enhancement, and design optimization of cross‐flow hydrokinetic turbines. [ABSTRACT FROM AUTHOR]

• An assessment of powertrain resistance of shrouded turbines is developed. • Diffuser technologies strongly increase the axial load on turbines. • Dissipative torques in turbine powertrains need to include Stribeck effect. • Bearings must be properly dimensioned, considering the hydrodynamic load. • Diffuser can significantly reduce starting flow velocities by about 27%. A growing interest has been devoted to turbines with diffusers, which induce a pressure drop downstream of the turbine to accelerate the fluid in the axial direction. This effect increases the axial aero- or hydrodynamic load on the turbine powertrain, such that frictional torque presents a challenge to turbine starting. Hence, this work is concentrated on how a diffuser can affect the dissipative torque of a wind or hydrokinetic turbine, in which the increased axial load and starting torque need to be considered. The approach uses a quasi-steady Blade Element Momentum model, adapted to consider the action of a diffuser, to predict instantaneous aerodynamic torque and power. The mass-moment of inertia of the entire system is taken into account, as well as the frictional losses in the bearings. As the diffuser affects the axial load on the rotor blades, the dissipative torque can undergo relevant variations. The Stribeck effect on the starting behavior of a small wind turbine is analyzed in order to evaluate the influence of the diffuser on the minimal flow velocity necessary to start rotating the turbine. The rotational speed and power output of a shrouded hydrokinetic turbine with a 10 m diameter are computed and compared to those of a bare turbine; at steady-state, a rotational speed 20% higher than that of the bare turbine is achieved, generating about 40% more net energy despite increased powertrain losses. [ABSTRACT FROM AUTHOR]

In the present study, the effect of the turbine solidity on the performance of helical cross-flow hydrokinetic turbines is numerically investigated under different flow conditions. Four turbines, with different solidities of 0.15, 0.2, 0.25 and 0.3, were investigated in five dissimilar flow conditions with Froude numbers of 0.0714, 0.143, 0.214, 0.286 and 0.357. The devices employed in the analyses were three-bladed with the diameter of 1 m and length of 1.5 m. They were positioned horizontally in a rectangular-shaped channel with a perpendicular alignment to the flow direction. Symmetrical NACA0018 hydrofoils, which are twisted with an angle of 120°, were used in the blade design. The investigation results have shown that the turbines with the solidity values 0.15 and 0.2 are more efficient than the other two cases. Considering all five flow conditions, the models have displayed the worst performance at a tip speed ratio of 4.0. [ABSTRACT FROM AUTHOR]

Wind and hydrokinetic turbines are designed for steady operation, and hence their efficiency suffers in unsteady environments. In this study, the response of a low-inertia rotor model to axial gusts is investigated experimentally. Gust profiles are modelled as ramp functions with differing slopes, and are simulated by accelerating the rotor model in a towing-tank facility. Rotor speed and torque are recorded during the system's response to the variation in freestream velocity. Furthermore, time-resolved particle image velocimetry (PIV) is carried out in the rotor's wake to gain insights on the change of bound circulation. The power produced is observed to be higher during the gust than during steady operation. For instance, the power output of the rotor during the fastest gust is, at maximum, 27% higher than during steady operation. Using the PIV data, the circulation of the tip and trailing vortices is estimated as a proxy for the bound circulation, and is found to have higher absolute magnitudes during the gust. As a result, the increase of the circulatory forces is concluded to be the source of the increased power performance. The current findings are relevant to rotor systems with directly coupled generators and no speed control. • The unsteady response of a generic low-inertia rotor is investigated experimentally. • The vortex dynamics in the rotor's wake is examined using time-resolved 2D-PIV. • Insights on the bound circulation were gained using a vortex model of the wake. • Compared to steady operation, the rotor model produces higher power during the gust. • The circulatory forces are the source of the increased power output during the gust. [ABSTRACT FROM AUTHOR]

The Brazilian hydrological information network does not provide data series of daily velocities. The river velocities are important for the study of hydrokinetic potential. Therefore, the work proposes a model called flow-velocity that estimates the average daily velocity and the distribution of the velocity profile of the cross section of rivers. The model was applied to the Amazon basin, using the highest and lowest flow rates of the historical series. The highest and lowest average velocities found in the Amazon River were 2.27 m s-1 and 0.735 m s-1, respectively. The main contributors to the Amazon River presented average daily velocities close to 2.0 ms-1 for the flood period, but in the dry season these velocities did not exceed 0.5 m s-1. Thus, it was verified that the Amazon River has hydrokinetic potential throughout the year and its tributaries during the flood period. [ABSTRACT FROM AUTHOR]

Abstract Blade element momentum (BEM) theory is a commonly used tool to predict the performance of horizontal axis conversion systems, such as wind and water turbines. Moreover, BEM theory can be easily integrated into many optimization techniques to improve the turbine structure and performance reliability. BEM theory though conceptually simple has different sources of convergence issues. The main focus of this work was to introduce a computational intelligence technique, namely, multilayer perceptron (MLP) neural networks (NNs) to overcome the convergence issues regardless of their sources. To improve the BEM accuracy, NNs were also employed as a multivariate interpolation tool to calculate the lift and drag coefficients over an operational range of local Reynolds numbers. This technique was found to be easy to integrate into any modified BEM model such those account for blockage in channels. The BEM-NNs model was able to operate at a higher tip speed ratio, with no convergence problems, compared to other models. Integration of NNs as multivariate interpolation tool for hydrodynamic coefficient calculation further improved the power prediction compared to that when using a constant representative Reynolds number. Highlights • Artificial neural networks (NNs) overcomes the convergence issue that occurs during blade element momentum (BEM) iteration • NNs were employed as multivariate interpolation tool to calculate the lift/drag coefficients which improved BEM accuracy • Integrating the suggested BEM-NNs model into a modified BEM model to account for blockage in channels. • The BEM-NNs model successfully overcame the convergence problem at high tip speed ratios compared to other modified models [ABSTRACT FROM AUTHOR]

The research presented in this paper involves the design of a power control system for a hydrokinetic turbine previously tested in real operating conditions. A maximum power point tracking (MPPT) algorithm was designed and simulated using the required parameters for a specific electric generator. The proposed system consists of a generator connected to the hydrokinetic turbine, a three-phase uncontrolled rectifier, a direct current (DC) boost converter with MPPT control to extract maximum available power, and a buck converter to control the amount of power delivered to the load. In order to test the MPPT algorithm, we built the individual blocks on the basis of the corresponding equations of each component. The algorithm considered the specific parameters of the previously tested turbine as input data and simulated the same water velocities for which the turbine had been tested. Thus, the simulation predicted a power output of 105 W for a water velocity of 1.33 m/s, 60 W for 1 m/s, and 30 W for 0.83 m/s. The efficiency of the control system was demonstrated when the instantaneous power value was maintained at a maximum point, regardless of the rotational speed according to the experimental power curves of the driving rotor obtained for certain water velocities.

The increased use of renewable energy resources for rural electrification has encouraged research and experimental projects decreasing designs uncertainties. This paper presents a deep and critical review of the design of micro hydro-generation technologies for river applications. This article shows three principal aspects: design, analysis and computational tools to study vertical micro-turbines, including the ones that are not vertical but that are relevant for the actual research. An exhaustive review is presented and analyzed hereafter. An in-depth review analysis focused on design is carried out. As a result of this work, it is clear that despite the need for interest in using renewable resources technologies, there is still a lack of research about design focused on computational analysis, as evidenced by the limited number of publications so far. [ABSTRACT FROM AUTHOR]

Due to the Venturi effect caused by a diffuser, which speed-up the velocity through the rotor, shrouded turbines are able to exceed the Betz-Joukowsky limit if the power coefficient is based on the rotor diameter. However, on hydrokinetic turbines this increased velocity may also promote cavitation on the blade. As this subject is still not clear on the current literature, this work presents a novel approach for optimizing hydrokinetic turbines free of cavitation under diffuser effect. The model uses the minimum pressure coefficient as the criterion to keep the pressure near blade tip above water vapor pressure. It includes an extension of Vaz & Wood's optimization in order to take into account the influence of the diffuser speed-up ratio regarding cavitation effect. A changing on the thrust coefficient is assumed to optimize chord and twist angle distributions along the blade. As a result, the proposed model shows that cavitation is indeed sensitive to the diffuser speed-up ratio, demonstrating that such a phenomenon needs to be considered in the design of diffuser-augmented hydrokinetic turbines. Also, the optimization method corrects the chord without relevant changing in the turbine power coefficient, where the increased power output is about 42 % higher than the bare turbine for a water velocity of 2.5 m/s. In this case, the model is assessed through comparisons using a 3-bladed hydrokinetic turbine with 10 m diameter, in which the diffuser speed-up ratio is varied. Furthermore, an evaluation is made with models available in the literature, suggesting good performance concerning the cavitation analysis on shrouded rotor design with the proposed optimization procedure. [ABSTRACT FROM AUTHOR]

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This paper presents a review of the stream current power sector, with a distinction made between the marine (MCP) and the river/estuary current power (RECP). Although scientific literature about MCP is actually well defined, that about RECP seems small, though this domain has some research interest. This paper has thus a special emphasis on this latter, with comparative studies done between these domains. The assessment of the academic and industrial interests for the RECP is first addressed, based on two main scientific resources and a qualitative highlight of its potential. Then, a review of actual constraints restricting its development is introduced, followed by a non-exhaustive presentation of industrial projects. Finally, some development prospects allowing constraints to be mitigated are proposed. Globally, MCP and RECP are treated unconcernedly, with a primary interest on the mechanical converter study and the location energy potential estimation. It has been highlighted that countries with RECP potential are more plentiful, and that undertaken projects can be classified mainly into two categories following the nominal power of the production unit. Furthermore, the river current power growth has been confirmed in recent years, with a majority part of patented hydrokinetic technologies, although commercial deployments are still scarce.

This paper deals with an aeroelastic problem that consists into self-sustained, pitch-heave oscillations of an elastically-mounted airfoil. Such oscillations of an airfoil could be used in order to develop a novel, fully-passive hydrokinetic energy flow harvester that is relatively simple from a mechanical point of view. Indeed, the motion of such an airfoil emerges as a result of the fluid-structure interaction between the flow, the airfoil and its elastic supports, and is sustained through a net transfer of energy from the flow to the structure. In this numerical study, the OpenFOAM-2.1.x CFD toolbox is used for solving the aeroelastic problem. Through unsteady two-dimensional viscous simulations at a Reynolds number of 500,000, the fully-passive turbine is optimized and investigated to develop a better understanding of the physics at play. Following a gradient-like optimization of the turbine, two-dimensional efficiencies as high as 34% have been obtained, and two fundamental mechanisms have been found to be very beneficial for enhancing the performances of the turbine: the adequate synchronization between both degrees-of-freedom, and the nonsinusoidal shape of the pitching motion. [ABSTRACT FROM AUTHOR]

Maximizing the performance of hydropower plants by taking advantage of the remaining energy downstream of dams via the installation of hydrokinetic turbines is feasible. In these cases, the design of the rotor diameter and velocity are fundamental and depend on the depth and velocity of the river, respectively. In this study, the Saint-Venant model was applied and calibrated by linear regression of measured and simulated flow rates, which resulted in a correlation of 0.99. The validation was performed using measured velocities that are comparable to the simulated velocities. A power curve was generated for the measured flow rates and simulated velocities and obtained a correlation of 0.96. The curve was used to estimate the velocity, calculate the energy density, and define a design velocity of 2.35 m/s. Ten points in the study area were selected for the turbine locations, and the velocities were determined using the power curve for the measured flow rates and simulated velocities. The rotor design was performed using the blade element method (BEM), and a rotor diameter of 10 m was defined. The 10 turbines can generate 2.04 GWh/year of electricity. The results demonstrate the potential for utilising the remaining energy of hydroelectric plants. [ABSTRACT FROM AUTHOR]

Hydrokinetic turbines are a promising technology for renewable energy production from river, tidal and marine currents. This paper proposes an innovative approach applied to optimization of horizontal axis hydrokinetic turbines (HAHTs) considering the possibility of cavitation. The minimum pressure coefficient is the criterion used for identifying cavitation on blades. Blade Element Momentum (BEM) theory is employed for the rotor design. During the optimization procedure, chord length at each blade section is corrected by a modification on the local thrust coefficient in order to prevent cavitation. The hydraulic parameters as lift, drag and minimum pressure coefficients are calculated by XFoil. Additionally, Computational Fluid Dynamics (CFD) techniques are used to validate the proposed methodology. Cavitation volume in the water flow through the rotor, with and without geometrical modifications, is evaluated using a Reynolds Averaged Navier–Stokes (RANS) approach coupled to the Rayleigh-Plesset model to estimate the vapor production rate. The methodology is applied to the design of a 10 m diameter Hydrokinetic Turbine (HT) rated to 250 kW output power, for a flow velocity of 2.5 m/s. The flow around the optimized rotor presents a reduction of the vapor volume without a major variation upon the turbine output power. A comparison with the Horizontal Axis Rotor Performance Optimization (HARP_opt) code was carried out, demonstrating good behavior. CFD simulations revealed that the proposed design method minimizes cavitation inception, yielding a useful tool for efficient HT design at rated conditions. [ABSTRACT FROM AUTHOR]

Utilization of electrical energy is the key to economic growth and improvement in people׳s living standards, especially in developing countries. The growing demand for electrical energy and the environmental effect of fossil fuel usage are the main topics driving us towards renewable technology. Hydropower, mainly hydrokinetic energy technology, is a well-known source of renewable energy. This study was carried out to present the potential of hydrokinetic energy in the world and Malaysia. Relevant research literature, based on developments, applications, design, operation, efficiency as well as different hydrokinetic technologies have been reviewed. This work critically considers the main characteristics of Malaysian current and water depth and the challenges associated with enhancing the efficiency of hydrokinetic turbines and providing electricity to remote areas with access to running water but little electricity. This paper will aid researchers to identify areas that need to be improved, as well as encourage public bodies to implement proper energy policies regarding hydrokinetic energy technology usage in rural areas with low-speed currents. [ABSTRACT FROM AUTHOR]

A stand-alone hydrokinetic energy conversion system (HECS), comprising a permanent magnet synchronous generator (PMSG), coupled to a horizontal axis hydrokinetic turbine through a mechanical transmission. The rotor performance is given by a characteristic curve of power as function of the rotation, given in non-dimensional quantities. The transmission is assumed to be of single stage with known mechanical efficiency. Park’s transform is applied to obtain the PMSG model, which is connected to resistive and inductive loads. A new method for the rotor angular speed control underwater speed variations, consisting on changing a resistive load connected to the generator, is presented. We present an analytic expression for the value of the resistive load, which keeps the HECS in the optimal operational condition. In addition, the numerical model in used to perform an investigation on the influence of the rotor power curves on the generation system stability and conversion efficiency. The generation system is submitted to variations of terminal load and water speed in order to assess its response in several situations of practical interest. It is shown that rotors which sharp characteristic curves are most likely subjected to severe stopping in the case of stream speed variation or demand variation oscillation, when compared to rotors with more smooth power curves. [ABSTRACT FROM AUTHOR]