59 research outputs found
The Ability of Convergent–Divergent Diffusers for Wind Turbines to Exploit Yawed Flows on Moderate-to-High-Slope Hills
No full textSmall-to-medium-sized wind turbines operate with wind speeds that are often modest, and it is therefore essential to exploit all possible means to concentrate the wind and thus increase the power extracted. The advantage that can be achieved by positioning the turbine on hilly reliefs, which act as natural diffusers, is well known, and some recent studies can be found on the effects of the characteristics of hilly terrain on the turbine performance. The literature shows numerous investigations on the behavior of ducted wind turbines, i.e., equipped with a diffuser. But so far, there is a lack of studies on the flow acceleration effects achievable by combining natural relief and a diffuser together. In this study, we analyze the performance of a 50 kW ducted turbine positioned on the top of hills of various shapes and slopes, with the aim of identifying the geometric characteristics of the diffuser most suitable for maximizing power extraction. The results show that a symmetrical convergent–divergent diffuser is well suited to exploit winds skewed by the slope of the hill, and therefore characterized by significant vertical velocity components. Due to its important convergent section, the diffuser is able to convey and realign the flow in the direction of the turbine axis. However, the thrust on the diffuser and therefore on the entire system increases dramatically, as does the turbulence released downwind
Improved Lift for Thick Flatback Airfoils in the Inboard Blades of Large Wind Turbines
No full textThick airfoils are often used in the inboard sections of blades in commercial wind turbines. The main reason for this is to give the blade greater structural strength, but it is well known that thick airfoils degrade aerodynamic performance by stalling at relatively small angles of attack. The adoption of flatback airfoils instead of sharp trailing edges allows high lift coefficient to be maintained in thick airfoils. In this paper, we propose a novel airfoil design based on a passive flap to further improve the lift coefficient. This new design was tested by numerical simulation on several airfoils with different maximum thickness and different TE thickness. The improved design for flatback airfoils yields a higher lift coefficient, while the drag behaviour is strictly related to the baseline airfoil shape: some airfoils show a decrease in drag at certain angles of attack, while others exhibit a drag increase. In conclusion, the practical implications of the flap’s utilisation on a state-of-the-art blade designed for a 5 MW wind turbine are analysed. The findings demonstrate that, due to the enhanced lift coefficient, it is feasible to shorten the chord while maintaining the power output, thereby reducing material costs
Vertical-Axis Tidal Turbines: Model Development and Farm Layout Design
No full textIn 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
A DMST-based tool to establish the best aspect ratio, solidity and rotational speed for tidal turbines in real sea conditions
No full textA Double Multiple Stream Tube (DMST) routine to predict the performance of cross-flow hydrokinetic turbines in real environments is presented, along with a site assessment application to identify the most efficient turbine aspect ratio, solidity and configuration (single, or paired) for a selected area of the Northern Adriatic Sea. The peculiarity of this DMST tool is its 3D character, since it allows to reproduce the vertical distribution of the torque generated by the turbine. To this end, correlations for fluid dynamic phenomena, based on high-fidelity fully CFD simulations, were implemented. The marine circulation code SHYFEM is adopted to obtain velocity profiles for a half lunar cycle period. The sites with the highest mean kinetic power were identified. The DMST routine is equipped with an iterative process able to establish which rotational speed maximizes the power output. Indeed, a spatially non-uniform velocity profile requires to determine the flow velocity more suitable to obtain the rotational speed via Tip Speed Ratio (TSR) definition. To this end, the section of the blades working at optimal TSR varies from top to bottom, until the maximum power is reached. It works as a virtual Maximum Power Point Tracking system able to adapt the turbine operating conditions for the different turbine geometries, and for changes in flow conditions. The results show that for the case study, the performance curve shape influences the optimal TSR blade section: the latter is often located in the upper part of the turbine for the low solidity, whereas, for high solidity turbines, in the bottom half part
Embedding of a blade-element analytical model into the shyfem marine circulation code to predict the performance of cross-flow turbines /
No full textOur aim was to embed a 2D analytical model of a cross-flow tidal turbine inside the open-source SHYFEM marine circulation code. Other studies on the environmental impact of Tidal Energy Converters use marine circulation codes with simplified approaches: performance coefficients are fixed a priori regardless of the operating conditions and turbine geometrical parameters, and usually, the computational grid is so coarse that the device occupies one or few cells. In this work, a hybrid analytical computational fluid dynamic model based on Blade Element Momentum theory is implemented: since the turbine blades are not present in the grid, the flow is slowed down by means of bottom frictions applied to the seabed corresponding to forces equal and opposite to those that the blades would experience during their rotation. This simplified approach allowed reproducing the turbine behavior for both mechanical power generation and the turbine effect on the surrounding flow field. Moreover, the model was able to predict the interaction between the turbines belonging to a small cluster with hugely shorter calculation time compared to pure Computational Fluid Dynamics
A turbines-module adapted to the marine site for tidal farms layout optimization
No full textThe ocean energy exploitation is arousing growing interest in the renewable energy sector. In the short term, horizontal axis tidal turbines are the most promising technology due to the accumulated know-how in the field of wind energy. In order to maximize the performance of the devices in a cluster, it is essential to optimize the layout. The marine environment offers different conditions than
atmospheric situations, in terms of confinement and turbulence intensity. Moreover, tidal currents exhibit a highly predictable pattern in speed intensity and direction unlike the wind resource, which has a more random behaviour. Nonetheless, most of tidal sites are characterized by the inversion of flow where the two prevailing directions are not perfectly aligned and opposite, hence the angle
between those directions should be a design variable. In this work we will consider as a case study the site proposed in [1], where this angle is ±20°. For those sites with a flow inversion of almost 180°, the staggered configuration is preferable to avoid wakes interference as mentioned in [2]. Furthermore, many studies [3] had analysed positive interaction between neighbouring devices in a cluster, hence it is important to establish the optimal relative position accounting for fluid dynamic positive effects, and not only negative aspects such
as wake interactions. For this reason, in this work we present a novel approach to determine the best configuration of a cluster of few turbines, a ”module”, which will be the optimized ”building block” for the whole farm. The procedure to be followed consist of two phases in which both the characteristics of the site and those of the turbine are taken into consideration. To place the devices in an optimal configuration, we first consider the change of flow direction during the tidal cycle for the site of interest, allowing only those configurations which avoid wake interference for both prevailing flow directions; then, we assess the best layout by exploiting positive interactions between devices in the cluster. The mutual fluid dynamic influence is analysed by means of a 3D Blade Element Momentum model of the turbine [4] implemented in the Open Source SHYFEM code. A series of simulations is performed to outline the power production trend of the module, and consequently find the optimal distancing between the machines. CFD simulations are also used to
extract the module wake characteristics
A Double Multiple Stream Tube (DMST) routine to identify efficient geometries of cross-flow tidal turbines in site assessment analyses: Paper 594
No full textA routine to predict the performance of cross-flow hydrokinetic turbines, based on the Blade Element Momentum theory, for site assessment purposes is here presented. The routine uses as input the flow data obtained with the open-source marine circulation code SHYFEM. The routine consists in a Double Multiple
Stream Tube model making use of 1D flow simplifications for fast analyses. The dynamic stall sub-model and two original sub-models, implemented to include the effects of blade tip losses and the lateral deviation of streamlines approaching the turbine, have been validated versus results of 3D and 2D CFD simulations. As a
case study, the tool is applied to an area of the northern Adriatic Sea in order to quickly identify locations with the highest hydrokinetic potential and, at the same time, to find the most efficient turbine aspect ratio and configuration (single or paired turbines) taking into account the bathymetric constraints. The results show that turbines, with short aspect ratio, and paired turbines (with the same overall frontal area of a single rotor) can give the best power outputs thanks to higher flow speeds available at the top of the water column and more favorable Reynolds number and distribution of tip speed ratios along the blade
A Double Multiple Stream Tube (DMST) routine for site assessment to select efficient turbine aspect ratios and solidities in real marine environments
No full textA MATLAB routine, based on a Double Multiple Stream Tube model, developed to quickly predict the performance of cross-flow hydrokinetic turbine, here is presented. The routine evaluate flow data obtained with the open-source marine circulation code SHYFEM. The tool can establish the best locations to place tidal devices taking into account bathymetric constraints and the hydrokinetic potential. Hence, it can be used to decide the best set of geometrical parameters. The geometrical variables of our analysis are turbine frontal area, aspect ratio and solidity. Several sub-models, validated with 3D and 2D CFD simulations, reproduce phenomena such as dynamic stall, fluid dynamic tips losses and the lateral deviation of streamlines approaching the turbine. As a case study, the tool is applied to an area of the northern Adriatic Sea. After having identified some suitable sites to exploit the energy resource, we have compared behaviours of different turbines. The set of geometrical parameters that gives the best performance in terms of power coefficient can vary considering several locations. Conversely, the power production is always greater for turbine with low aspect ratio (for a fixed solidity and area). Indeed, shorter devices benefit from higher hydrokinetic potentials at the top of the water column
A BEM-Based Model of a Horizontal Axis Tidal Turbine in the 3D Shallow Water Code SHYFEM
No full textWe present a novel 3D implementation of a horizontal axis tidal turbine (HATT) in the shallow water hydrostatic code SHYFEM. The uniqueness of this work involves the blade element momentum (BEM) approach: the turbine is parameterized by applying momentum sink terms in the x and y momentum equations. In this way, the turbine performance is the result of both the flow conditions and the turbine’s geometric characteristics. For these reasons, the model is suitable for farm-layout studies, since it is able to predict the realistic behavior of every turbine in a farm, considering the surrounding flow field. Moreover, the use of a shallow water code, able to reproduce coastal morphology, bathymetry wind, and tide effects, allows for studying turbine farms in realistic environments
“But the skin of the earth is seamless”: Liberating Symbols of Nature in "Borderlands/The New Mestiza" by Gloria Anzaldúa
No full text“Build bridges, not walls!” Recently, this old slogan has boldly resounded in the streets of America. One of the greatest advocates of that belief was certainly Gloria Anzaldúa, author of Borderlands/La Frontera: The New Mestiza (1987) and co-author of This Bridge We Call Home (2002), among others. In Borderlands she offered a dramatic and revolutionary view on the life in the borderlands, nourishing with new life-blood the Latino, Feminist e Post-colonial studies. Being herself a Chicana grown up in the Texas-Mexico border area, Anzaldúa denounces the “terrorism” practiced by the U.S. government but also shows a possible path to follow in order to positively re-think the complex border identity: “I am participating in the creation of yet another culture, a new story to explain the world and our participation in it, a new value system with images and symbols that connect us to each other and to the planet” (81). Anzaldúa goes beyond the rationally cold, artificially created citizenship identity, towards a conception of the self more linked to the truth of being simply humans living on a planet whose surface is “seamless”. Nature is indeed central in her “new value system” and an endless source of symbols for her discourse. The aim of this article is to bring out the meanings attributed to Nature by Anzaldúa, proposing a close-reading of three poems included in Borderlands, where Nature (concreted in both natural space and animal life), becomes a symbol of the fight for liberation. “Horse”, “Wind tugging at my sleeve”, and “Dead”, are representative of the three main meanings we believe Anzaldúa ascribed to Nature: Nature as the strength we should – but frequently don’t - find in ourselves to fight oppression; Nature as the rebellion to the untruthful duality imposed by the border; Nature as the possibility of rebirth in a new identity
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