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Both scientific and industrial communities have a growing interest in marine renewable energies. There is a wide variety of technologies in this domain, with different degrees of maturity. This study focuses on two models of a mast-free vertical axis Darrieus tidal turbine with the objective of characterizing the effect of vertical confinement, rotor configuration, and fluid–structure interactions on their performances in free-surface flows. The first model comprised four straight rigid blades maintained by circular flanges on both ends of the rotor and the second model is equipped with free-ended interchangeable blades attached to a single upper flange. Two configurations of the second model mounted with either rigid or flexible blades were used, first for comparison with the dual-flange turbine, then in order to address the effect of fluid–structure interactions on the turbine performances. While the single-flange models exhibit a significantly lower efficiency at all operating points, it is observed that the use of flexible blades tends to enhance turbine performances at low Reynolds numbers. The flow topology obtained from PIV measurement at selected operating points is discussed with respect to the performance of each turbine model in order to highlight the role of the dynamic stall and blade–vortex interactions. [ABSTRACT FROM AUTHOR]

With the aim of finding efficient solutions for cross flow turbine (CFT) bi-directional diffusers able to harvest non perfectly rectilinear tidal currents, a 2D CFD analysis of ducted CFTs was carried out with focus on the effects of diffuser shape and yaw angle. The HARVEST hydrofoil shaped diffuser, equipped with a pair of counter-rotating turbines, and a bi-directional symmetrical diffuser were compared in terms of coefficient of power (CP), torque ripple, overall thrust on diffuser and wake characteristics. Slightly better CP were predicted for the symmetrical diffuser, due to the convergent walls that address the flow towards the blade with a greater attack angle during early and late upwind and to the viscous interactions between the turbine wakes and strong vortices shed by the diffuser. A CP's extraordinary improving resulted when yaw increased up to 22.5° for the hydrofoil shaped and up to 30° for the symmetrical diffuser. Similar behaviour in yawed flows also occurred in case of a ducted single rotor, demonstrating that it is a characteristic of CFTs. The insertion of a straight throat in the diffuser design proved to be an effective way to mitigate torque ripple, but a CP loss is expected. [ABSTRACT FROM AUTHOR]

This paper investigates the potential of Micro Gas Turbines (MGTs) in the global shift towards low-carbon energy systems, particularly focusing on their integration within microgrids and distributed energy generation systems. MGTs, recognized for their fuel flexibility and efficiency, have yet to achieve the commercialization success of rival technologies such as Internal Combustion Engines (ICEs), wind turbines, and solar power (PV) installations. Through a comprehensive review of recent techno-economic assessment (TEA) studies, we highlight the challenges and opportunities for MGTs, emphasizing the critical role of TEA in driving market penetration and technological advancement. Comparative analysis with ICE and RES technologies reveals significant gaps in TEA activities for MGTs, which have hindered their broader adoption. This paper also explores the learning and experience effects associated with TEA, demonstrating how increased research activities have propelled the success of ICE and RES technologies. The analysis reveals a broad range of learning and experience effects, with learning rates (α) varying from 0.1 to 0.25 and experience rates (β) from 0.05 to 0.15, highlighting the significant role these effects play in reducing the levelized cost of energy (LCOE) and improving the net present value (NPV) of MGT systems. Hybrid systems integrating MGTs with renewable energy sources (RESs) and ICE technologies demonstrate the most substantial cost reductions and efficiency improvements, with systems like the hybrid renewable energy CCHP with ICE achieving a learning rate of α = 0.25 and significant LCOE reductions from USD 0.02/kWh to USD 0.017/kWh. These findings emphasize the need for targeted TEA studies and strategic investments to unlock the full potential of MGTs in a decarbonized energy landscape. By leveraging learning and experience effects, stakeholders can predict cost trajectories more accurately and make informed investment decisions, positioning MGTs as a competitive and sustainable energy solution in the global energy transition. [ABSTRACT FROM AUTHOR]

In the context of energy shortages and the development of new energy sources, tidal current energy has emerged as a promising alternative. It is typically harnessed by deploying arrays of multiple water turbines offshore. Vertical axis water turbines (VAWTs), as key units in these arrays, have wake effects that influence array spacing and energy efficiency. However, existing studies on wake velocity distribution models for VAWTs are limited in number, accuracy, and consideration of influencing factors. A precise theoretical model (Lam's formula) for wake lateral velocity can better predict wake decay, aiding in the optimization of tidal current energy array designs. Turbulence in the ocean, serving as a medium for energy exchange between high-energy and low-energy water flows, significantly impacts the wake recovery of water turbines. To simplify the problem, this study uses software ANSYS Fluent 2020 R2 for two-dimensional simulations of VAWT wake decay under different turbulence intensities, confirming the critical role of turbulence intensity in wake velocity decay. Based on the obtained data, a new mathematical approach was employed to incorporate turbulence intensity into Lam's wake formula for VAWTs, improving its predictive accuracy with a minimum error of 1%, and refining some parameter calculations. The results show that this model effectively reflects the impact of turbulence on VAWT wake recovery and can be used to predict wake decay under various turbulence conditions, providing a theoretical basis for VAWT design, optimization, and array layout. [ABSTRACT FROM AUTHOR]

Oscillating hydrofoils were installed in a water tunnel as a surrogate model for a hydrokinetic cross-flow tidal turbine, enabling the study of the effect of flexible blades on the performance of those devices with high ecological potential. The study focuses on a single tip-speed ratio (equal to 2), the key non-dimensional parameter describing the operating point, and solidity (equal to 1.5), quantifying the robustness of the turbine shape. Both parameters are standard values for cross-flow tidal turbines. Those lead to highly dynamic characteristics in the flow field dominated by dynamic stall. The flow field is investigated at the blade level using high-speed particle image velocimetry measurements. Strong fluid–structure interactions lead to significant structural deformations and highly modified flow fields. The flexibility of the blades is shown to significantly reduce the duration of the periodic stall regime; this observation is achieved through systematic comparison of the flow field, with a quantitative evaluation of the degree of chaotic changes in the wake. In this manner, the study provides insights into the mechanisms of the passive flow control achieved through blade flexibility in cross-flow turbines. [ABSTRACT FROM AUTHOR]

A cross-flow air turbine, which is a self-rectifying, air-driven turbine, was designed and proposed for the power take-off (PTO) system of an oscillating water column (OWC) wave energy converter (WEC). To predict the complicated non-linear behavior of the air turbine in the OWC, numerical and experimental investigations were accomplished. The geometries of the nozzle and the rotor of the turbine were optimized under steady-flow conditions, and the performance analysis of the model in bi-directional flows was conducted by commercial computational fluid dynamics (CFD) code ANSYS CFX. Experimentation on the full-scale turbine was then undertaken in a cylindrical-type wave simulator that generated reciprocating air flows, to validate the numerical model. The optimized model had a peak cycle-averaged efficiency of 0.611, which is 1.7% larger than that of the reference model, and a significantly improved band width with an increase in flow coefficients. Under reciprocating-flow conditions, the optimized model had a more improved operating range with high efficiency compared to the performance derived from the steady-flow analysis, but the peak cycle-averaged efficiency was decreased by 4.3%. The numerical model was well matched to the experimental results with an averaged difference of 3.5%. The proposed optimal design having structural simplicity with high performance can be a good option to efficiently generate electricity. [ABSTRACT FROM AUTHOR]

Hydraulic turbines have become indispensable for harnessing renewable energy sources, particularly in-pipe hydraulic turbine technology, which leverages excess energy within pipeline systems like drinking water distribution pipes to produce electrical power. Among these turbines, the propeller-type axial turbine with circular blades stands out for its efficiency. However, there is a notable lack of literature on fluid dynamics and structural behavior under various operational conditions. This study introduces a comprehensive methodology to numerically investigate the hydraulic and structural responses of turbines designed for in-pipe installation. The methodology encompasses the design of circular blades, followed by parametric studies on fluid dynamics and structural analysis. The circular blade's performance was evaluated across different materials, incorporating static, modal, and harmonic response analyses. Results showed that the circular blade achieved a peak hydraulic efficiency of 75.5% at a flow rate of 10 l/s, generating 1.86 m of head pressure drop and 138 W of mechanical power. Structurally, it demonstrated a safety factor exceeding 1 across the entire hydraulic range without encountering resonance or fatigue issues. This research and its methodology significantly contribute to advancing the understanding of designing and assessing the fluid dynamic behavior and structural integrity of circular blades in axial propeller-type turbines for in-pipe installations, serving as a valuable resource for future studies in similar domains. [ABSTRACT FROM AUTHOR]

In this paper, we propose a new 3D model for vertical-axis tidal turbines (VATTs) embedded in the shallow-water code SHYFEM. The turbine model is based on the Blade-Element∖Momentum (BEM) theory and, therefore, is able to predict turbine performance based on the local flow conditions and the geometric characteristics of the turbine. It is particularly suitable for studying turbine arrays, as it can capture the interactions between the turbines. For this reason, the model is used to test a tidal farm of 21 devices with fluid dynamic simulations. In particular, we deploy the farm at Portland Bill, which is a marine site characterised by a wide spread in the direction of the tidal currents during a flood-ebb tide cycle. We optimised the lateral and longitudinal spacing of the turbines in a fence using computational fluid dynamics simulations and then performed a sensitivity analysis by changing the distance between the fences. The results show that the greater the distance between the fences, the higher the power output. The increase in power generation is around 16%, but this implies a huge increase in the horizontal extent of the farm. Further assessments should be carried out, as the expansion of a marine area dedicated to energy exploitation may conflict with other stakeholder interests. [ABSTRACT FROM AUTHOR]

A reduction in wake effects in large wind farms through wake-aware control has considerable potential to improve farm efficiency. This work examines the success of several emerging, empirically derived control methods that modify wind turbine wakes (i.e., the pulse method, helix method, and related methods) based on Strouhal numbers on the O (0.3) . Drawing on previous work in the literature for jet and bluff-body flows, the analyses leverage the normal-mode representation of wake instabilities to characterize the large-scale wake meandering observed in actuated wakes. Idealized large-eddy simulations (LES) using an actuator-line representation of the turbine blades indicate that the n = 0 and ± 1 modes, which correspond to the pulse and helix forcing strategies, respectively, have faster initial growth rates than higher-order modes, suggesting these lower-order modes are more appropriate for wake control. Exciting these lower-order modes with periodic pitching of the blades produces increased modal growth, higher entrainment into the wake, and faster wake recovery. Modal energy gain and the entrainment rate both increase with streamwise distance from the rotor until the intermediate wake. This suggests that the wake meandering dynamics, which share close ties with the relatively well-characterized meandering dynamics in jet and bluff-body flows, are an essential component of the success of wind turbine wake control methods. A spatial linear stability analysis is also performed on the wake flows and yields insights on the modal evolution. In the context of the normal-mode representation of wake instabilities, these findings represent the first literature examining the characteristics of the wake meandering stemming from intentional Strouhal-timed wake actuation, and they help guide the ongoing work to understand the fluid-dynamic origins of the success of the pulse, helix, and related methods. [ABSTRACT FROM AUTHOR]

Vertical-axis wind turbines (VAWTs) are an innovative solution for energy harvesting, as they harness the power of the wind by enabling rotational motion around a vertical shaft situated on the ground. This paper deals with the design optimisation of VAWT systems for highway energy harvesting. The four design parameters, blade number, blade curvature angle, blade thickness and blade diameter ratio, have been investigated to find their respective optimalities for the enhanced energy efficiency of VAWT systems. Computational fluid dynamics (CFD) simulations are conducted in Ansys Fluent using a Banki turbine model created in Solidworks®, with a constant velocity inlet of 4 m/s and rotational speeds ranging from 0.5 to 3 rad/s. The simulations consider the placement of the turbine in the central reservation of a highway with a windshield for enhanced performance. From the results, it was observed that increasing blade thickness and blade number improve turbine performance, with maximum power coefficients achieved at specific tip speed ratios (TSRs). The optimal blade diameter ratio has been found to be approximately 0.75 for TSR values between 0.1 and 0.5, whilst a ratio of 0.83 gave the best performance at higher TSR values. Also, a blade curvature angle of 60 degrees has been found optimal for slow rotations, while 100 degrees yielded the highest power coefficient for faster rotations. The study could also highlight the significance of blade curvature angle variation, resulting in a notable 14% performance increase compared to the baseline. The geometric changes proposed in the study allow for greater power extraction from the same turbine footprint, leading to increased energy efficiency in VAWT systems. [ABSTRACT FROM AUTHOR]

The purpose of this research work was to examine the hydroelectric potential of wastewater treatment plants by harnessing the kinetic and/or potential energy of treated wastewater for electricity generation. Such a concept encapsulates the essence of renewable energy and resonates with international sustainable development mandates and climate change adaptation strategies. The primary objective was to analyze the performance parameters of the Francis turbine, a key component of this energy generation system. An experimental analysis encompassed model tests on the Francis turbine, simulating varied flow conditions using the GUNT turbine. Additionally, historical data from the Toruń Wastewater Treatment Plant (WWTP) 2018 annual wastewater discharge were employed to validate the findings and shed light on real-world applications. The tested efficiency of the Francis turbine peaked at 64.76%, notably below the literature-reported 80%. The turbine system's overall efficiency was approximately 53%, juxtaposed against the theoretical value of 66.35%. With respect to the Toruń WWTP data, the turbine's power output was highest at 24.82 kW during maximum wastewater flow, resulting in a power production of 150.29 MWh per year. The observed turbine efficiencies were consistent with the previously documented range of 30% to 96%. The turbine displayed optimal outputs during heightened flow rates and maximized production at more frequent, lower flow rates throughout the year. Implementing such turbines in wastewater treatment plants not only aligns with global renewable energy goals but also boasts lower construction costs and environmental impacts, primarily due to the utilization of existing infrastructure. Furthermore, wastewater flow consistency counters the seasonal variability seen in conventional water treatment plants. These findings pave the way for more energy-efficient design recommendations for turbines within wastewater treatment and hydropower plants. [ABSTRACT FROM AUTHOR]

Standard technical solutions are not cost-effective for many small hydropower sites. This study aims to demonstrate the workflow for the tailor-made variable-speed axial propeller turbine and provide proof of this concept. The turbine is designed to meet the site's specific space limitations and operating range needs. The runner shape is adjusted to the variable-speed operation and defined hydraulic profile using a parametric geometry model and CFD-based optimization. The variable-speed propeller turbine shows excellent flow control while keeping the mechanical design simple. The tailor-made approach minimizes construction costs using existing structures and is highly suitable for mini-hydropower applications. The prototype—an atypical turbine designed for highly restricted space and installed on-site—serves as proof of the concept. The findings on the design of axial variable-speed turbines are presented. [ABSTRACT FROM AUTHOR]

While hydro turbines generate over 40% of the world's total renewable energy, these traditional turbines present a great environmental concern due to the potential of their sharp blades to damage aquatic lives and ecology, along with their harmful noise and vibration during operation. One effective solution to these environmental issues is to substantially increase the skew of these blades, which would result in a much safer blade operation for aquatic animals (such as fish, etc.) and a substantial reduction in noise and vibration. Adding skew to turbine rotors is known to reduce cavitation and noise, and hence, to mitigate the environmental impact on underwater fauna and flora. However, adding skew will compromise the power performance of the turbines. This study aimed to identify the effect of rotor skew on the hydrodynamic power performance of a series of horizontal-axis turbine rotors that were manufactured using 3D printing technology and tested in a towing tank. The diameter of the turbine rotor model was 0.3 m and the skewed angle contained positive and negative angles of 45, 60 and 90 degrees along with a non-skewed rotor. This study was conducted to analyze the hydrodynamics of a turbine rotor with different skew angles and a 0-degree skewed rotor. Various tip speed ratios, ranging from 2.3 to 4.3, were set in accordance with the RPM and the carriage speed. Gain and filter were applied to boost the signal, and post-calibration was conducted. The results show that (1) the non-skewed rotor had the highest power coefficient; (2) the rotor with a skew angle of 45 degrees had the lowest power loss, at 6.97%, compared with the zero-skew rotor blades; (3) while the larger the skew, the more loss in power production efficiency, the rotor with a negative 90-degree skewed angle had the largest power loss, 31.42%. It was then concluded that, based on the results and analysis, (1) to achieve the greatest reduction in noise and vibration, the rotor with a skew angle of 90 degrees would be the best choice, and (2) to mitigate noise/vibration and efficiency due to skew, the rotor with a skewed angle of 45 degrees would be the best design choice. [ABSTRACT FROM AUTHOR]

Renewable energy sources are rapidly increasing in demand and importance as governments and countries around the globe begin to understand their vital role in reducing climate change. This project aimed to design and create an optimised micro-hydro turbine system for downpipes to harness the currently untapped potential energy from rainwater. Experimental methods were used to determine the magnitude of voltage output available at different rainfall intensities by simulating such flow rates on a hydraulic bench. The viability of this energy to power household appliances was then evaluated, and methods of increasing the voltage output were assessed, such as layering the turbines in a single downpipe or placing multiple downpipes around the building. The study determined that, during average rainfall in the UK, a single turbine could produce a maximum of 7.21 V of DC voltage, or 50.49 V during heavy rainfall—enough energy to power a mobile device charger or a vacuum cleaner, respectively. Therefore, this proves a high potential in rainwater energy harvesting as a renewable energy source. It was also concluded that a positive correlation occurred for both the number of turbines in a downpipe and the number of pipes around the building with the voltage output of the whole system. [ABSTRACT FROM AUTHOR]

The following study investigates the effectiveness of a deep learning-based method for predicting the flow field and flow-driven rotation of a vertical-axis hydrokinetic turbine operating in previously unseen free-stream velocities. A Convolutional Neural Network (CNN) is trained and tested using the solutions of five two-dimensional (2-D), foil-resolved Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations, with free-stream velocities of 1.0, 1.5, 2.0, 2.5, and 3.0 m/s. Based on the boundary conditions of free-stream velocity and rotor position, the flow fields of x-velocity, y-velocity, pressure, and turbulent viscosity are inferred, in addition to the angular velocity of the rotor. Three trained CNN models are developed to evaluate the effects of (1) the dimensions of the training data, and (2) the number of simulations used as training cases. Reducing data dimensions was found to diminish mean relative error in predictions of velocity and turbulent viscosity, while increasing it in predictions of pressure and angular velocity. Increasing the number of training cases from two to three was found to reduce relative error for all predicted unknowns. With the best achieved CNN model, the variables of x-velocity, y-velocity, pressure, turbulent viscosity, and angular velocity were inferred with mean relative errors of 6.93%, 9.82%, 10.7%, 7.48%, and 0.817%, respectively. [ABSTRACT FROM AUTHOR]

The combination of an offshore wind turbine and a wave energy converter on an integrated platform is an economical solution for the electrical power demand in coastal countries. Due to the expensive installation cost, a prediction should be used to investigate whether the location is suitable for these sites. For this purpose, this research presents the feasibility of installing a combined hybrid site in the desired coastal location by predicting the net produced power due to the environmental parameters. For combining these two systems, an optimized array includes ten turbines and ten wave energy converters. The mathematical equations of the net force on the two introduced systems and the produced power of the wind turbines are proposed. The turbines' maximum forces are 4 kN, and for the wave energy converters are 6 kN, respectively. Furthermore, the comparison is conducted in order to find the optimum system. The comparison shows that the most effective system of desired environmental condition is introduced. A number of machine learning and deep learning methods are used to predict key parameters after collecting the dataset. Moreover, a comparative analysis is conducted to find a suitable model. The models' performance has been well studied through generating the confusion matrix and the receiver operating characteristic (ROC) curve of the hybrid site. The deep learning model outperformed other models, with an approximate accuracy of 0.96. [ABSTRACT FROM AUTHOR]

Energy accessibility, reliability and availability are key components of improved quality of life and human development in all spheres. As the United Nations' SDG 7 calls for access to electricity for all by 2030, Africa still has a wide gap to fill as the statistics show that 85% of the population that will not have access to electricity is in Africa. As the world tries to wean itself off non-renewable energy and transition to green through use of renewable energy sources, hydropower energy remains at the heart of Africa for this venture. With majority of the rural population in Africa lacking electricity, there is need for a low-tech system that utilizes river flow to generate just enough energy for normal operation in these regions. Micro-hydrokinetic river turbine technology (µ-HRT), which offers less intermittency, can potentially contribute to sustainably electrifying Africa rural areas. The technology has been adopted by few countries worldwide, with limited comprehensive study in Africa even though the technology seems viable for use in African rivers. This paper reviewed the status of the µ-HRT applications in Africa and some of the barriers to its development. The study found out that the technology has not been vastly developed in Africa. Despite numerous barriers, the technology is simply a low-tech technology that requires the use of local resources and capacity building for its sustainability in terms of construction, operation and maintenance requirements. It is therefore recommended that R&D and field trials be conducted for its possible adoption. [ABSTRACT FROM AUTHOR]

More than ever, the generation of energy from renewable sources has become one of the most critical and challenging areas of human activity. This is the result of a number of factors acting simultaneously, including the highly publicized greenhouse effect, the need to reduce CO2 emissions, the dwindling of natural resources and thus the need to protect them, including the preservation of wealth (among other natural resources) for future generations. This paper presents a numerical investigation of the operation of a vertical axis wind turbine with guide vanes. The work analyzes the obtained power on the turbine rotor for different numbers of rotor blades and different numbers of guide vanes, as well as for different proportions of the width of the rotor ring and the guide ring. The analyses carried out point to the potential feasibility of using this type of design in practice. The obtained power coefficient is higher than that of classical vertical axis wind turbines. The analyses also indicate that the proposed design solution can be optimized to increase the efficiency of wind energy extraction. [ABSTRACT FROM AUTHOR]

We investigated the effective use of cross-flow wind turbines for small-scale wind power generation to increase the output power by using a casing, which is a kind of wind-collecting device, composed of three flow deflector plates having the shape of a circular-arc airfoil. Drag-type vertical-axis wind turbines have an undesirable part of about half of the swept area where the inflow of wind results in low output performance. To solve this problem, we devised a casing consisting of three flow deflector plates, two of which were to block the unwanted inflow of wind and the remaining flow deflector plate having an angle of attack with respect to the wind direction to increase the flow toward the rotor. In this study, output performance experiments using a wind tunnel and numerical fluid analysis were conducted on a cross-flow wind turbine with three flow deflector plates to evaluate the effectiveness of the casing on output performance improvement. As a result, it was confirmed that the casing could improve the output performance of the cross-flow wind turbine by approximately 60% at the maximum performance point and could also maintain the output performance about 50% higher compared to the bare cross-flow wind turbine without the casing within a deviation angle of ±10 degrees, even when the casing direction was inclined against the wind direction due to changes in wind direction. [ABSTRACT FROM AUTHOR]

The article presents a study of the flow through a tunable check valve used as a hydraulic lock in a system with an actuator. Special attention is given to energy losses of the liquid stream in the poppet gap. In the first stage of the research, CFD methods were used to determine the speed and pressure distributions for the fixed values of the input flow rate and the poppet position. The values of the jet angle and pressures determined based on the CFD results were used to build a simulation model of the entire hydraulic system in Matlab/Simulink environment. The influence of the spring parameters pressing the poppet against the valve seat on the pressure drop and thus on the amount of energy lost was investigated. In particular, the spring stiffness and initial tension were studied. The obtained results were used to develop guidelines for constructing a valve prototype. Finally, the results of simulation tests were verified based on the actual valve characteristic obtained on a test bench. [ABSTRACT FROM AUTHOR]

In hydropower, the exploitation of small power sources requires the use of small turbines that combine efficiency and economy. Banki-Michell turbines represent a possible choice for their simplicity and for their good efficiency under variable load conditions. Several experimental and numerical tests have already been designed for examining the best geometry and optimal design of cross-flow type machines, but a theoretical framework for a sequential design of the turbine parameters, taking full advantage of recently expanded computational capabilities, is still missing. To this aim, after a review of the available criteria for Banki-Michell parameter design, a novel two-step procedure is described. In the first step, the initial and final blade angles, the outer impeller diameter and the shape of the nozzle are selected using a simple hydrodynamic analysis, based on a very strong simplification of reality. In the second step, the inner diameter, as well as the number of blades and their shape, are selected by testing single options using computational fluid dynamics (CFD) simulations, starting from the suggested literature values. Good efficiency is attained not only for the design discharge, but also for a large range of variability around the design value. [ABSTRACT FROM AUTHOR]

We develop a reduced order model to represent the complex flow behavior around vertical axis wind turbines. First, we simulate vertical axis turbines using an accurate high order discontinuous Galerkin--Fourier Navier--Stokes Large Eddy Simulation solver with sliding meshes and extract flow snapshots in time. Subsequently, we construct a reduced order model based on a high order dynamic mode decomposition approach that selects modes based on flow frequency. We show that only a few modes are necessary to reconstruct the flow behavior of the original simulation, even for blades rotating in turbulent regimes. Furthermore, we prove that an accurate reduced order model can be constructed using snapshots that do not sample one entire turbine rotation (but only a fraction of it), which reduces the cost of generating the reduced order model. Additionally, we compare the reduced order model based on the high order Navier--Stokes solver to fast 2D simulations (using a Reynolds Averaged Navier--Stokes turbulent model) to illustrate the good performance of the proposed methodology. [ABSTRACT FROM AUTHOR]

We explore the stability of wakes arising from 2D flow actuators based on linear momentum actuator disc theory. We use stability and sensitivity analysis (using adjoints) to show that the wake stability is controlled by the Reynolds number and the thrust force (or flow resistance) applied through the turbine. First, we report that decreasing the thrust force has a comparable stabilising effect to a decrease in Reynolds numbers (based on the turbine diameter). Second, a discrete sensitivity analysis identifies two regions for suitable placement of flow control forcing, one close to the turbines and one far downstream. Third, we show that adding a localised control force, in the regions identified by the sensitivity analysis, stabilises the wake. Particularly, locating the control forcing close to the turbines results in an enhanced stabilisation such that the wake remains steady for significantly higher Reynolds numbers or turbine thrusts. The analysis of the controlled flow fields confirms that modifying the velocity gradient close to the turbine is more efficient to stabilise the wake than controlling the wake far downstream. The analysis is performed for the first flow bifurcation (at low Reynolds numbers) which serves as a foundation of the stabilization technique but the control strategy is tested at higher Reynolds numbers in the final section of the paper, showing enhanced stability for a turbulent flow case. [ABSTRACT FROM AUTHOR]

Supplying power to remote areas may be a challenge, even for those communities already connected to the main grid. Power is often transmitted from long distances, under adverse weather conditions, and with aged equipment. As a rule, modernizing grid infrastructure in such areas to make it more resilient faces certain financial limitations. Local distribution may face stability issues and disruptions through the year and--equally important--it cannot absorb significant amounts of locally-produced power. The European policy has underlined the importance of energy production in local level towards meeting energy security and climate targets. However, the current status of these areas makes the utilization of the local potential prohibitive. This study builds on the observation that in the vicinity of such mountainous areas, irrigation dams often cover different non energy-related needs (e.g., irrigation, drinking water). Transforming these dams to small-scale hydropower (SHP) facilities can have a twofold effect: it can enhance the local energy portfolio with a renewable energy source that can be regulated and managed. Moreover, hydropower can provide additional flexibility to the local system and through reservoir operation to allow the connection of additional solar photovoltaic capacities. The developed methodological approach was tested in remote communities of mountainous Greece, where an earth-fill dam provides irrigation water. The results show a significant increase of renewables' penetration and enhanced communities' electricity autarky. [ABSTRACT FROM AUTHOR]

Complex systems of water distribution networks (WDS) are used to supply water to users. WDSs are systems where a lot of distributed energy is available. Historically, this energy is artificially dissipated by pressure reduction valves (PRVs), thanks to which water utilities manage the pressure level in selected nodes of the network. The present study explores the use of economic hydraulic machines, pumps as turbines (PATs) to produce energy in a small network located in a town close to Palermo (Italy). The main idea is to avoid dissipation in favor of renewable energy production. The proposed study is applied to a WDN typical of the Mediterranean countries, where the users, to collect water during the period of water scarcity conditions, install private tanks. The presence of private tanks deeply modifies the network from its designed condition. In the proposed analysis, the economic benefit of PATs application in water distribution networks has been investigated, accounting for the presence of users' private tanks. The analysis, carried out by mean of a mathematical model able to dynamically simulate the water distribution network with PATs, shows the advantage of their installation in terms of renewable energy recovery, even though the energy production of PATs is strictly conditioned by their installation position. [ABSTRACT FROM AUTHOR]

Cavitation is a common flow phenomena in most hydraulic turbines and has the potential to cause vibration, blade surface damage and performance loss. Despite the fact that crossflow turbines have been used in small-scale hydropower systems for a long time, cavitation has not been studied in these turbines. In this paper, we present the findings of a computational study on cavitation inception in crossflow turbines. Cavitation inception was assessed using three-dimensional (3D) Reynolds-Averaged Navier-Stokes (RANS) computations. A homogeneous, free-surface two-phase flow model was used. Pressure distributions on the blades were examined for different flow rates, heads and impeller speeds to assess cavitation inception. The results showed that cavitation occurs in the second stage of the turbine and was observed on the suction side near the inner edge of the blades. For the particular turbine studied, cavitation always occurred at shaft speeds greater than that, giving the maximum efficiency for each combination of flow rate and head. The implication is that the useful operating range of crossflow turbines is up to and including the maximum efficiency point. [ABSTRACT FROM AUTHOR]

In order to get rid of the impact of the global financial crisis and actively respond to global climate change, it has become a common choice for global economic development to develop clean energy such as wind energy, improve energy efficiency and reduce greenhouse gas emissions. With the advantages of simple structure, unnecessary facing the wind direction, and unique appearance, the vertical axis wind turbine (VAWT) attracts extensive attention in the field of small and medium wind turbines. The lift-type VAWT exhibits outstanding aerodynamic characteristics at a high tip speed ratio, while the starting characteristics are generally undesirable at a low wind speed; thus, how to improve the starting characteristics of the lift-type VAWT has always been an important issue. In this paper, a lift-drag combined starter (LDCS) suitable for lift-type VAWT was proposed to optimize the starting characteristics of lift-type VAWT. With semi-elliptical drag blades and lift blades equipped on the middle and rear part outside the starter, the structure is characterized by lift-drag combination, weakening the adverse effect of the starter with semi-elliptical drag blades alone on the output performance of the original lift-type VAWT and improving the characteristics of the lift-drag combined VAWT. The static characteristic is one of the important starting characteristics of the wind turbine. The rapid development of computational fluid dynamics has laid a solid material foundation for VAWT. Thus the static characteristics of the LDCS with different numbers of blades were investigated by conducting numerical simulation and wind tunnel tests. The results demonstrated that the static torque coefficient of LDCS increased significantly with the increased incoming wind speed. The average value of the static torque coefficient also increased significantly. This study can provide guidelines for the research of lift-drag combined wind turbines. [ABSTRACT FROM AUTHOR]

Lift force is an important parameter for the performance evaluation of an H-Darrieus wind turbine. The rotational direction of the streamlined force is effective on the performance of the wind turbine. In order to analyze the flow characteristics around the turbine blades in real-time, a numerical analysis using three-dimensional unsteady Reynold-averaged Navier–Stokes equations has been introduced. Experimental data were obtained from a field test facility constructed on an island in South Korea and was introduced to compare the numerical simulation results with measured data. The optimum tip speed ratio (TSR) was investigated via a multi-variable optimization approach and was determined to be 3.5 for the NACA 0015 blade profile. The turbine displays better performance with the maximum power coefficient at the optimum TSR. It is due to the delay in the flow separation from the blade surface and the relatively lower strength of the tip vortices. Furthermore, the ratio between lift and drag forces is also the highest at the optimum TSR, as most of the aerodynamic force is directly converted into lift force. For one rotation of the turbine blade at the optimum TSR, the first quarter of motion produces the highest lift as the static pressure difference is maximum at the leading edge, which helps to generate maximum lift. At a TSR less than the optimum TSR, small-lift generation is dominant, whereas at a higher TSR, large drag production is observed. Both of these lead to lower performance of the turbine. Apart from the TSR, the optimum wind angle of attack is also investigated, and the results are prepared against each TSR. [ABSTRACT FROM AUTHOR]

Energy is the prerequisite for social and economic development of a community and country. In Japan, national government is promoting small hydropower (SHP) through a renewable energy policy by providing a high FIT price of 34 yen (≒0.32 US$/kWh) on energy generated from an SHP of less than 200 kW. Until now, the energy generation was controlled by national government agencies, but now independent power generation businesses are growing at the local community level in rural Japan. For the future growth of SHP, it is necessary to make electricity generation at the local community level. Therefore, these local communities will install and manage their renewable electricity by themselves. It will help to make the community self-sustainable and independent from the national government, and at the same time, it will also lead them to achieve the Sustainable Developments Goals (SDGs) target from community-based action. This paper aimed to discuss an SHP development business model in which local community will become the business owner of the SHP. It means "of the community, by the community and for the community". The community identifies their renewable energy potential and needs, they borrow money from the financial organization or banks, install the power plant and do necessary maintenance and management by themselves. The revenue earned by selling electricity is used to repay the loan, and the rest is used for community development directly (such as local roads construction, agriculture land improvements, community hall maintenance, waterways maintenance, welfare, etc.). This paper also discussed a community-based 50 kW SHP installed in Miyazaki prefecture of Japan as a case study. This SHP is one of the best examples of a community ownership model (Community-based business model). A detailed explanation from planning to investment has been discussed. The local community is getting approximately 112,000 USD per year by selling the electricity, and 162-ton CO2 is estimated to decrease yearly, which will support the achievement of SDGs. Finally, installing this kind of SHP in remote areas will provide managerial skills to the local community directly, plant operation knowledge, and education to local students. Local communities learn the problem-solving skills, which lead them to solve the local problem on a community level by themselves. [ABSTRACT FROM AUTHOR]

This paper demonstrates the procedure of blade adjustment in a Kaplan-type water turbine, based on calculations of the flow system. The geometrical adjustment of a twisted blade with varying chord length is described in the study. Computational fluid dynamics (CFD) analysis was used to characterise aerofoil and turbine performance. Furthermore, two turbines, with a different number of blades, were designed, manufactured, and tested experimentally. The numerical model results were then compared with the experimental data. The studies were carried out with different rotational velocities and different stator blade incidence angles. The paper shows a comparison of the turbine efficiencies that were assessed, using numerical and experimental methods, of a flow system with four- and five-bladed rotors. The numerical model results matched up well with those of the experimental study. The efficiency of the proposed turbines reached up to 72% and 84% for four-bladed and five-bladed designs, respectively. These efficiencies, when considered with the turbine's simplicity, low production and maintenance costs, as well as their potential for harvesting energy from low energy flows, mean that Kaplan turbines provide a promising technology for processing renewable energy. [ABSTRACT FROM AUTHOR]

The article presents results of research on an adjustable check valve. In particular, the article deals with improvement of flow characteristics and reduction in pressure losses of an existing valve design. The subject of the research was the valve body in the form of a steel block intended for mounting a typical cartridge valve insert. Two variants of the valve body were analysed: a standard one, which is currently in production, and the proposed new solution, in which the geometry was modified based on the results of CFD simulations. The main research task was to properly shape and arrange holes and flow channels inside the body, between the cartridge valve and the connecting plate. Using CFD analyses, a solution for minimising the flow resistance was sought and then the method of modifying flow channels geometry was developed. The CFD simulation results showed a significant reduction in pressure loss, up to 40%. The obtained simulation results were verified on a test bench using a prototype of the proposed valve block. A high degree of consistency in the results of CFD simulations and laboratory experiments was achieved. The relative difference between simulation and experimental results in the entire considered range of the flow rate did not exceed 6.0 %. [ABSTRACT FROM AUTHOR]

Tidal energy is one of the major sources of renewable energy. To accelerate the development of tidal energy, improved designs of Tidal Current Turbine (TCT) are necessary. The effect of tower on performance and wake of TCT is investigated using Computational Fluid Dynamics (CFD) simulations. Transient analysis with transient rotor stator frame change model and shear stress transport turbulence model are utilized in ANSYS CFX. An experimentally validated numerical model with full scale tidal turbine with a blockage ratio of 14.27% and Tip Speed Ratio (TSR) 4.87 is used to simulate the effect of different tower diameters on performance and wake. The effect of different tower diameters is quantified in terms of coefficient of performance (CP). Coefficient of performance for a 3.5 m tower diameter is 0.472 which is followed by 3, 2.5 and 2 m with coefficients of performance of 0.476, 0.478 and 0.476 respectively. Similarly, the coefficient of thrust (CT) on the rotor for 3.5 m tower diameter is 0.902, for 3 m diameter 0.906 and for 2.5 and 2 m diameters are 0.908 and 0.906 respectively. [ABSTRACT FROM AUTHOR]

In the array design of the vertical axis wind turbines (VAWT), the wake effect of the upstream VAWT on the downstream VAWT needs to be considered. In order to simulate the velocity distribution of a VAWT wake rapidly, a new two-dimensional numerical method is proposed, which can make the array design easier and faster. In this new approach, the finite vortex method and vortex particle method are combined to simulate the generation and evolution of the vortex, respectively, the fast multipole method (FMM) is used to accelerate the calculation. Based on a characteristic of the VAWT wake, that is, the velocity distribution can be fitted into a power-law function, a new correction model is introduced to correct the three-dimensional effect of the VAWT wake. Finally, the simulation results can be approximated to the published experimental results in the first-order. As a new numerical method to simulate the complex VAWT wake, this paper proves the feasibility of the method and makes a preliminary validation. This method is not used to simulate the complex three-dimensional turbulent evolution but to simulate the velocity distribution quickly and relatively accurately, which meets the requirement for rapid simulation in the preliminary array design. [ABSTRACT FROM AUTHOR]

Aiming at the performance evaluation problem of tidal energy turbines in the application of periodic time-varying flow velocity, with the goal of maximizing the efficiency of energy harvesting in practical applications, an evaluation system combining the characteristics of flow velocity changes in practical applications is proposed. After long-term monitoring of tidal current flow velocity in the applied sea area, the actual measured tidal current periodic flow velocity is divided into several flow velocity segments by using statistical segmentation, and the evaluation flow velocity of each flow velocity segment and its time proportion in the tidal current cycle are obtained. A test device with constant torque regulation is built, and capture power tests of different torque loads are carried out under each evaluation flow rate. After comparison, the maximum captured power at each evaluation flow rate is determined. We calculate the weight based on the time proportion of each evaluation flow velocity and obtain the turbine average power of the tidal cycle, thereby evaluating the overall energy capture performance of the turbine under the periodic time-varying flow velocity. Finally, the application test of the turbine in the actual sea area shows that the thin-walled airfoil turbine is more suitable for the sea area, which is the same as the pool evaluation result. The result shows that the evaluation system is reliable and effective and has significance for guiding practical engineering. [ABSTRACT FROM AUTHOR]

Currently, vertical axis wind turbines (VAWT) are considered as an alternative technology to horizontal axis wind turbines in specific wind conditions, such as offshore farms. However, complex unsteady wake structures of VAWTs exert a significant influence on performance of wind turbines and wind farms. In the present study, instantaneous flow fields around and downstream of an innovative VAWT with inclined pitch axes are simulated by an actuator line model. Unsteady flow characteristics around the wind turbine with variations of azimuthal angles are discussed. Several fluid parameters are then evaluated on horizontal and vertical planes under conditions of various fold angles and incline angles. Results show that the total estimated wind energy in the shadow of the wind turbine with an incline angle of 30° and 150° is 4.6% higher than that with an incline angle of 90°. In this way, appropriate arrangements of wind turbines with various incline angles have the potential to obtain more power output in a wind farm. [ABSTRACT FROM AUTHOR]

The development of tidal energy in Australia is still a challenge with few studies performed on the characterisation of the resource, due to the difficulty to acquire data and uncertainties about the influence of this anthropogenic activity on the marine environment. Changes in flow could lead to alterations in sediment transport and have further influence on the marine habitat. A case study in a promising area, Banks Strait (Australia), was created using high resolution 2D and 3D models validated against in situ data to investigate changes to hydrodynamic conditions with two scenarios of tidal farms (100 and 300 turbines). Comparison between 2D and 3D is performed to find the best compromise between model accuracy and computational time for preliminary assessment. Changes to current speed and bed shear stress over a 35 day period were found to be localised around the tidal farms and did not extent more than 7 km from the farm (300 turbines) for both 2D and 3D. The results showed that for near field and far field, 2D models are sufficient to give a first approximation of the hydrodynamic influence of tidal farm deployment on its environment. [ABSTRACT FROM AUTHOR]

The mechanical power and wake flow field of a 1:40 scale model of the US Department of Energy's Reference Model 1 (RM1) dual rotor tidal energy converter are characterized in an open-channel flume to evaluate power performance and wake flow recovery. The NACA-63(4)-24 hydrofoil profile in the original RM1 design is replaced with a NACA-4415 profile to minimize the Reynolds dependency of lift and drag characteristics at the test chord Reynolds number. Precise blade angular position and torque measurements were synchronized with three acoustic Doppler velocimeters (ADV) aligned with each rotor centerline and the midpoint between the rotor axes. Flow conditions for each case were controlled to maintain a hub height velocity, u h u b = 1.04 ms − 1 , a flow Reynolds number, R e D = 4.4 × 10 5 , and a blade chord length Reynolds number, R e c = 3.1 × 10 5 . Performance was measured for a range of tip-speed ratios by varying rotor angular velocity. Peak power coefficients, C P = 0.48 (right rotor) and C P = 0.43 (left rotor), were observed at a tip speed ratio, λ = 5.1. Vertical velocity profiles collected in the wake of each rotor between 1 and 10 rotor diameters are used to estimate the turbulent flow recovery in the wake, as well as the interaction of the counter-rotating rotor wakes. The observed performance characteristics of the dual rotor configuration in the present study are found to be similar to those for single rotor investigations in other studies. Similarities between dual and single rotor far-wake characteristics are also observed. [ABSTRACT FROM AUTHOR]

This article presents the results of computational fluid dynamics (CFD) analysis of an innovative directional control valve consisting of four poppet seat valves and two electromagnets enclosed inside a single body. The valve has a unique design, allowing the use of any poppet valve configuration. Both normally opened (NO) and normally closed (NC) seat valves can be applied. The combination of four universal valve seats and two electromagnets gives a wide range of flow path configurations. This significantly increases the possibility of practical applications. However, due to the significant miniaturization of the valve body and the requirement to obtain necessary connections between flow paths, multiple geometrically complex channels had to be made inside the body. Hence, the main purpose of work was to shape the geometry of the flow channels in such a way as to minimize pressure losses. During the CFD analyses velocity distribution in flow channels and pressure distribution on the walls were determined. The results were used to obtain pressure loss as a function of flow rate, which was then verified by means of laboratory experiments conducted on a test bench. [ABSTRACT FROM AUTHOR]

Tidal stream turbines are subject to complex flow conditions, particularly when installed in staggered array configurations where the downstream turbines are affected by the wake and/or bypass flow of upstream turbines. This work presents, for the first time, methods for and results from the physical testing of three 1/15 scale instrumented turbines configured in a closely-spaced staggered array, and demonstrates experimentally that increased power extraction can be achieved through reduced array separation. A comprehensive set of flow measurements was taken during several weeks testing in the FloWave Ocean Energy Research Facility, with different configurations of turbines installed in the tank in a current of 0.8 m/s, to understand the effect that the front turbines have on flow through the array and on the inflow to the centrally placed rearmost turbine. Loads on the turbine structure, rotor, and blade roots were measured along with the rotational speed of the rotor to assess concurrently in real-time the effects of flow and array geometry on structural loading and performance. Operating in this closely-spaced array was found to improve the power delivered by the rear turbine by 5.7–10.4% with a corresponding increase in the thrust loading on the rotor of 4.8–7.3% around the peak power operating point. The experimental methods developed and results arising from this work will also be useful for further scale-testing elsewhere, validating numerical models, and for understanding the performance and loading of full-scale tidal stream turbines in arrays. [ABSTRACT FROM AUTHOR]

In this study, three-dimensional transient numerical simulations of the flow around a cross flow water turbine of the type H-Darrieus are performed. The hydrodynamic characteristics and performance of the turbine are investigated by means of a time-accurate unsteady Reynolds-averaged Navier–Stokes (URANS) commercial solver (ANSYS-Fluent v. 19) where the time dependent rotor-stator interaction is described by the sliding mesh approach. The transition shear stress transport turbulence model has been employed to represent the turbulent dynamics of the underlying flow. Computations are validated versus previous experimental work in terms of the turbine efficiency curve showing good agreement between numerical and experimental values. The behavior of the power and force coefficients as a function of turbine angular speed is analyzed. Moreover, visualizations and analyses of the instantaneous vorticity iso-surfaces developing at different blade rotational velocities are presented including a few movies as additional material. Finally, the fluid variables fields are averaged along a turbine revolution and are compared with the steady predictions of simplified steady approaches based on the blade element momentum theory and the double multiple streamtube method (BEM-DMS). [ABSTRACT FROM AUTHOR]

Vertical Axis Wind Turbines (VAWTs) are characterized by complex and unsteady flow patterns resulting in considerable challenges for both numerical simulations and measurements describing the phenomena involved. In this study, a 3D Actuator Line Model (ALM) is compared to a 2D and a 3D Vortex Model, and they are validated using the normal forces measurements on a blade of an operating 12 kW VAWT, which is located in an open site in the north of Uppsala, Sweden. First, the coefficient power ( C P ) curve of the device has been simulated and compared against the experimental one. Then, a wide range of operational conditions for different tip speed ratios (TSRs), with λ = 1.84, 2.55, 3.06, 3.44, 4.09 and 4.57 were investigated. The results showed descent agreement with the experimental data for both models in terms of the trend and magnitudes. On one side, a slight improvement for representing the normal forces was achieved by the ALM, while the vortex code performs better in the simulation of the C P curve. Similarities and discrepancies between numerical and experimental results are discussed. [ABSTRACT FROM AUTHOR]

An actuator surface model (ASM) to be employed to simulate the effect of a vertical axis turbine on the hydrodynamics in its vicinity, particularly its wake is introduced. The advantage of the newly developed ASM is that it can represent the complex flow inside the vertical axis turbine's perimeter reasonably well, and hence, is able to predict, with a satisfying degree of accuracy, the turbine's near-wake, with a low computational cost. The ASM appears to overcome the inadequacy of actuator line models to account for the flow blockage of the rotor blades when they are on the up-stream side of the revolution, because the ASM uses a surface instead of a line to represent the blade. The ASM was used on a series of test cases to prove its validity, demonstrating that first order flow statistics—in our study, profiles of the stream-wise velocity—in the turbine's vicinity, can be produced with reasonable accuracy. The prediction of second order statistics, here in the form of the turbulent kinetic energy (TKE), exhibited dependence on the chosen grid; the finer the grid, the better the match between measured and computed TKE profiles. [ABSTRACT FROM AUTHOR]

Preliminary design of a new installation concept of a drag-driven vertical axis hydrokinetic turbine is presented. The device consists of a three-bladed, wheel-shaped, turbine partially embedded in relatively shallow channel streambanks. It is envisioned to be installed along the outer banks of meandering rivers, where the flow velocity is increased, to maximize energy extraction. To test its applicability in natural streams, flume experiments were conducted to measure velocity around the turbine and power performance using Acoustic Doppler Velocimetry and a controlled motor drive coupled with a torque transducer. The experiment results comprise the power coefficient, the spatial evolution of the mean velocity deficit, and a description of the flow structures generated by the turbine and responsible for the unsteadiness of the wake flow. Applying a triple decomposition on the Reynolds stresses, we identify the dominant contribution to such unsteadiness to be strongly associated with the blade passing frequency. [ABSTRACT FROM AUTHOR]

The increasing demand for renewable and sustainable energy sources has encouraged the development of small run-of-river plants. Preliminary studies are required to assess the technical and economic feasibility of such plants. In this context, the identification of optimal potential run-of-river sites has become a key issue. In this paper, an approach that is based on GIS tools coupled with a hydrological model has been applied to detect potential locations for a run-of-river plant. A great number of locations has been analyzed to identify those that could assure the achievement of different thresholds of potential power. The environmental and economic feasibility for small hydropower projects in these locations has been assessed and a multi-objective analysis has been carried out to highlight the most profitable configurations. The Soil and Water Assessment Tool (SWAT) has been calibrated to simulate runoff in the Taw at Umberleigh catchment (South West England). The results showed that, in the area of study, different locations could be selected as suitable for run-of-river plants. [ABSTRACT FROM AUTHOR]

Experiments with a three-bladed, constant chord tidal turbine were undertaken to understand the influence of free surface proximity on blockage effects and near-wake flow field. The turbine was placed at various depths as rotational speeds were varied; thrust and torque data were acquired through a submerged sensor. Blockage effects were quantified in terms of changes in power coefficient and were found to be dependent on tip speed ratio and free surface to blade tip clearance. Flow acceleration near turbine rotation plane was attributed to blockage offered by the rotor, wake, and free surface deformation. In addition, particle image velocimetry was carried out in the turbine near-wake using time- and phase-averaged techniques to understand the mechanism responsible for the variation of power coefficient with rotational speed and free surface proximity. Slower wake propagation for higher rotational velocities and increased asymmetry in the wake with increasing free surface proximity was observed. Improved performance at high rotational speed was attributed to enhanced wake blockage, and performance enhancement with free surface proximity was due to the additional blockage effects caused by the free surface deformation. Proper orthogonal decomposition analysis revealed a downward moving wake for the turbine placed in near free surface proximity. [ABSTRACT FROM AUTHOR]

Water supply systems are one of the main hydraulic systems with significant potential for the installation of micro-hydropower devices. Although there are already some mini-hydropower applications in water supply systems, it is still a huge potential that continues to be under-exploited. The arrangement based on an axial turbine, such as an inline tubular propeller, with different diameters and rotational speeds suitable to exploit the existing potential in the water sector, is fully tested. The turbine with the nominal diameter of 85 mm was analysed through experimental campaigns and numerical models for a large range of heads and flow measurements to access its performance. A good correlation between the physical model and the numerical results were obtained, with mean values of deviation less than 5% regarding flow, mechanical power and efficiency. These results were extrapolated to other similar turbine with 170 mm, using affinity laws to investigate the power extraction performance. Both geometries were also numerically investigated using computational fluid dynamics (CFD) models and comparisons were made between the affinity model and experimental results. The results identified differences when compared with the classical affinity curves. Therefore, new formulations based on affinity equations were proposed for the analyzed axial turbine with different diameters without imposing a constant turbine efficiency since actually it does not keep constant as CFD calibrated model proved. Compared to experimental test efficiencies for different rotational speeds, the new proposed affinity laws provided a maximum error of 12% for both diameters. [ABSTRACT FROM AUTHOR]

Most vertical axis wind turbines (VAWTs) need arms connecting the blades with the rotational axis. The arms increase the power loss of VAWTs; however, the distribution between the pressure and friction influences and their degrees of influence have not yet been investigated in detail in past research. We applied computational fluid dynamics (CFD) targeting a small-sized straight-bladed VAWT to elucidate the effects of arms on turbine performance. In the analysis, three kinds of arms with different cross-sections (NACA 0018 airfoil, 18% rectangular, circular) with the same height were added to an armless rotor. The tangential forces and resistance torques caused by the added arms were recalculated by dividing the pressure and friction influences based on the surface pressure and friction distributions obtained by the CFD on an arm or a blade. The pressure-based tangential force of an arm, regardless of the cross-section, had a tendency to increase near the connection part between the arm and a blade. Though the value was small, the friction on the rectangular arm generated a driving force, whereas the friction on the other arms generated resistance forces. The pressure-based tangential force of a blade increased for a wide region around the connection part. The friction-based tangential force of a blade dropped around the connection part of every arm-equipped rotor. The arm resistance torque added to a VAWT by the existence of arms was larger than the added blade resistance torque in the cases of rectangular and circular arm rotors. Conversely, in the case of the airfoil arm rotor, the resistance torque added to blades became larger than that of arms. [ABSTRACT FROM AUTHOR]

The second part of this work describes a wind turbine Computational Fluid Dynamics (CFD) simulation capable of modeling wake effects. The work is intended to establish a computational framework from which to investigate wind farm layout. Following the first part of this work that described the near wake flow field, the physical domain of the validated model in the near wake was adapted and extended to include the far wake. Additionally, the numerical approach implemented allowed to efficiently model the effects of the wake interaction between rows in a wind farm with reduced computational costs. The influence of some wind farm design parameters on the wake development was assessed: Tip Speed Ratio (TSR), free-stream velocity, and pitch angle. The results showed that the velocity and turbulence intensity profiles in the far wake are dependent on the TSR. The wake profile did not present significant sensitivity to the pitch angle for values kept close to the designed condition. The capability of the proposed CFD model showed to be consistent when compared with field data and kinematical models results, presenting similar ranges of wake deficit. In conclusion, the computational models proposed in this work can be used to improve wind farm layout considering wake effects. [ABSTRACT FROM AUTHOR]

The flow developed on a tidal site can be characterized by combinations of turbulence, shear flows, and waves. Horizontal-axis tidal turbines are therefore subjected to dynamic loadings that may compromise the working life of the rotor and drive train components. To this end, a series of experiments were carried out using a 0.9 m horizontal-axis tidal turbine in a tow tank facility. The experiments included two types of regular waveforms, one of them simulating an extreme wave case, the other simulating a more moderate wave case. The second regular wave was designed to match the peak period and significant wave height of an irregular wave which was also tested. Measurements of torque, thrust, and blade-bending moments were taken during the testing campaign. Speed and torque control strategies were implemented for a range of operational points to investigate the influence that a control mode had in the performance of a tidal stream turbine. The results showed similar average power and thrust values were not affected by the control strategy, nor the influence of either the regular or irregular wave cases. However, it was observed that using torque control resulted in an increase of thrust and blade root bending moment fluctuations per wave period. The increase in fluctuations was in the order of 40% when compared to the speed control cases. [ABSTRACT FROM AUTHOR]

The crossflow turbines commonly used in small hydropower systems have a single nozzle. We are unaware of any studies of double-nozzle crossflow turbines which could have twice the power output of the single-nozzle design by doubling the flow through the same runner, with a high maximum efficiency. We present a computational analysis of a double-nozzle crossflow turbine, to determine the turbine efficiency and fundamental flow patterns. This work was based on a single-nozzle crossflow turbine with a maximum efficiency of 88%, one of the highest reported in the open literature through extensive experimental measurements. Previous numerical studies on this turbine have shown that the water flow in the runner was confined to less than half the runner periphery, implying that the other half could be used to double the runner power output by employing a second nozzle. We show that adding a second, identical nozzle without making any other changes to the design achieves a doubling of the power output. The dual-nozzle turbine, therefore, has the same efficiency as the original turbine. We also investigate the use of a slider to control the flow at part-load and show that part-load efficiency of the double-nozzle is very similar to that of the original turbine. This demonstrates the feasibility of using two nozzles for crossflow turbines. [ABSTRACT FROM AUTHOR]