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No full textMicroeolic turbines in the built environment: influence of the installation site on the potential energy yield
No full textGeneric proposals for an effective integration of renewable energy sources in the urban environment are frequently carried out by project developers, local governments and media, although an in-depth knowledge of the technical and energetic limitations is often missing. In particular, the installation of small wind turbines on the rooftops of tall buildings is considered to represent an attractive solution thanks to the supposed possibility of exploiting local flow accelerations induced by the building façades. The real feasibility of this scenario has, however, yet to be proved, both in terms of real energy harvesting and of compatibility of the machines with a densely populated area.In this study, a critical examination of the flow conditions on the rooftop of a building in an urban environment has been carried out by means of CFD simulations. The main goal of the analysis was the assessment of some general criteria to evaluate the convenience of a microeolic turbine installation on the roof of a selected building as a function of both its geometrical features and those of its upwind building along the prevailing wind direction. In each configuration, the flow velocity and skew angle on the rooftop area of the installation building were calculated for different incoming wind profiles and compared to their levels in the undisturbed flow. Finally, an energy-based comparison between the flow potentials in the investigated environment is provided and some general tendencies are outlined
Design guidelines for H-Darrieus wind turbines: Optimization of the annual energy yield
Full text linkH-Darrieus wind turbines are gaining popularity in the wind energy market, particularly as they are thought to represent a suitable solution even in unconventional installation areas. To promote the diffusion of this technology, industrial manufacturers are continuously proposing new and appealing exterior solutions, coupled with tempting rated-power offers. The actual operating conditions of a rotor over a year can be, however, very different from the nominal one and strictly dependent on the features of the installation site. Based on these considerations, a turbine optimization oriented to maximize the annual energy yield, instead of the maximum power, is thought to represent a more interesting solution. With this goal in mind, 21,600 test cases of H-Darrieus rotors were compared on the basis of their energy-yield capabilities for different annual wind distributions in terms of average speed. The wind distributions were combined with the predicted performance maps of the rotors obtained with a specifically developed numerical code based on a Blade Element Momentum (BEM) approach. The influence on turbine performance of the cut-in speed was accounted for, as well as the limitations due to structural loads (i.e. maximum rotational speed and maximum wind velocity). The analysis, carried out in terms of dimensionless parameters, highlighted the aerodynamic configurations able to ensure the largest annual energy yield for each wind distribution and set of aerodynamic constraints
Pitch optimization in small-size darrieus wind turbines
No full textSmall Darrieus wind turbines are standing out as one of the most promising technologies in view of a widespread availability of delocalized energy production from wind. In view of a performance optimization ofDarrieus rotors, the application of a pitch angle to blades has been often suggested. In the present work, the pitch modeling in an advanced BEM code has been exploited to investigate different pitch control strategies. In particular, the prospects of a pitch preset based on both the maximum power and the maximum energy extraction have been evaluated and compared to those connected to a progressive pitch variation with the turbine revolution speed. The pitch effects have been discussed both in terms of aggregated power output and of variation of the airfoil functioning conditions throughout a revolution. The analysis confirmed that the application of a proper pitch angle could definitely provide an increase of the turbine performance, especially at medium-high solidities
An improved model for the performance estimation of an H-Darrieus wind turbine in skewed flow
No full textSmall turbines are considered one of the most promising technologies for an effective diffusion of renewable energy sources in new installation contexts with a high degree of integration with human activity (e.g. the urban environment). In these new installations, however, the real working conditions can be far from the nominal ones. In particular, the turbine functioning can be noticeably affected by misalignments between the oncoming flow and the axis of the rotor; differently from horizontal-axis wind turbines, whose performance is decreased by a skew angle, H-Darrieus turbines are thought to take advantage from this condition in some cases.
In this study, an improved model for the performance prediction of H-Darrieus under skewed flow was developed. In detail, a theoretical approach based on Momentum Models was properly modified to account for the variations induced by the new direction of the flow which invests the rotor. In particular, the modifications in the aerodynamic characteristics of the airfoils, the swept area and the streamtubes distribution were modeled. The performance predictions of the new model were compared both with experimental data available in the technical literature and with the results of wind tunnel tests purposefully carried out on a full scale model of an H-Darrieus turbine. Notable agreement has been constantly obtained between simulations and experiments
Critical issues in the CFD simulation of Darrieus wind turbines
Full text linkComputational Fluid Dynamics is thought to provide in the near future an essential contribution to the development of vertical-axis wind turbines, helping this technology to rise towards a more mature industrial diffusion. The unsteady flow past rotating blades is, however, one of the most challenging applications for a numerical simulation and some critical issues have not been settled yet.In this work, an extended analysis is presented which has been carried out with the final aim of identifying the most effective simulation settings to ensure a reliable fully-unsteady, two-dimensional simulation of an H-type Darrieus turbine.Moving from an extended literature survey, the main analysis parameters have been selected and their influence has been analyzed together with the mutual influences between them; the benefits and drawbacks of the proposed approach are also discussed.The selected settings were applied to simulate the geometry of a real rotor which was tested in the wind tunnel, obtaining notable agreement between numerical estimations and experimental data. Moreover, the proposed approach was further validated by means of two other sets of simulations, based on literature study-cases
Effectiveness of two-dimensional CFD simulations for Darrieus VAWTs: a combined numerical and experimental assessment
Full text linkThanks to the continuous improvement of calculation resources, computational fluid dynamics (CFD) is expected to provide in the next few years a cost-effective and accurate tool to improve the understanding of the unsteady aerodynamics of Darrieus wind turbines. This rotor type is in fact increasingly welcome by the wind energy community, especially in case of small size applications and/or non-conventional installation sites. In the present study, unique tow tank experimental data on the performance curve and the near-wake structure of a Darrieus rotor were used as a benchmark to validate the effectiveness of different CFD approaches. In particular, a dedicated analysis is provided to assess the suitability, the effectiveness and the future prospects of simplified two-dimensional (2D) simulations. The correct definition of the computational domain, the selection of the turbulence models and the correction of simulated data for the parasitic torque components are discussed in this study. Results clearly show that, (only) if properly set, two-dimensional CFD simulations are able to provide - with a reasonable computational cost - an accurate estimation of the turbine performance and also quite reliably describe the attended flow-field around the rotor and its wake
A computational procedure to define the incidence angle on airfoils rotating around an axis orthogonal to flow direction
No full textNumerical simulations provided in the last few years a significant contribution for a better understanding of many phenomena connected to the flow past rotating blades. In case of airfoils rotating around an axis orthogonal to flow direction, one of the most critical issues is represented by the definition of the incidence angle on the airfoil from the computed flow field. Incidence indeed changes continuously as a function of the azimuthal position of the blade and a distribution of peripheral speed is experienced along the airfoil's thickness due to radius variation. The possibility of reducing the flow to lumped parameters (relative speed modulus and direction), however, would be of capital relevance to transpose accurate CFD numerical results into effective inputs to low-order models that are often exploited for preliminary design analyses. If several techniques are available for this scope in the case of blades rotating around an axis parallel to flow direction (e.g., horizontal-axis wind turbines), the definition of a robust procedure in case the revolution axis is orthogonal to the flow is still missing. In the study, a novel technique has been developed using data from Darrieus-like rotating airfoils. The method makes use of the virtual camber theory to define a virtual airfoil whose pressure coefficient distributions in straight flow are used to match those of the real airfoil in curved flow. Even if developed originally for vertical-axis wind turbines, the method is of general validity and is thought to represent in the near future a valuable tool for researchers to get a new insight on many complex phenomena connected to flow past blades rotating around an axis orthogonal to flow direction, like for example dynamic stall
Optimization of the Performance of a Formula SAE Engine by Means of a Wastegate Valve Electronically Actuated
No full textThe engine and vehicle design in Formula SAE competition has to accomplish a strict regulation. In order to limit the maximum power, an air restrictor of 20mm of diameter is imposed in the intake line. To overcome the limitations caused by the restrictor, Firenze Race Team equipped its one-cylinder engine with a turbocharger, which is conventionally provided with a wastegate (WG) valve to limit the maximum boost pressure and avoid knocking phenomena. Typically, the WG valve is controlled by a pneumatic actuator, which opens the valve according to a defined and constant maximum boost pressure downstream the compressor in the whole engine operating range. Therefore, the boost pressure at high engine speed, in which knocking problems are less intense and the volumetric efficiency is lower, is limited by the threshold value defined at medium-low engine speeds, i.e. the pneumatic WG limits the maximum power that the engine can supply. In this study, the implementation of an electronic control system for the WG valve is described together with a dedicated control strategy aimed at providing the desired boost pressure at full load for each engine speed, in order to get the maximum power avoiding knocking phenomena. The electronic WG provided higher power values and a more extended torque curve in comparison to the conventional pneumatic one
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