Design guidelines for H-Darrieus wind turbines: Optimization of the annual energy yield

H-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

Influence of the Displacement Profile on the Performance and Mechanical Stresses of an Axial Piston Compressor for Refrigeration Applications

Abstract Axial piston compressors are commonly equipped with rotating disk plates that make the pistons following a sinusoidal displacement. The variation of the plate angle leads to stroke increments without changing the displacement profile. The axial piston architecture allows one to make piston displacement profiles that are different from a sinusoidal one by using rotating disk with a shaped circumferential profile. In this work, a detailed analysis on the thermodynamic cycle of compressors with different disk geometries was carried out.A lumped parameter numerical model of a compressor for refrigeration application was developed. The compressor performance (i.e. indicated power, compressed mass of gas and specific power) was estimated by imposing piston displacement profiles that are different from the sinusoidal one. The influence on the cycle COP in which the compressor runs was evaluated for each analysis. For each profile, the study of the forces acting on the rotating plate was also investigated. A sensitivity analysis allowed the definition of a profile design that guarantees the optimization of both the thermodynamic cycle and the mechanical stresses

Development of an engine variable geometry intake system for a Formula SAE application

The Formula SAE is an international competition for vehicle fully designed and built by students from worldwide Universities. The engine and vehicle design in the Formula SAE competition has to comply with a strict regulation. Regarding the engine intake line an air restrictor of circular cross-section no greater than 20 mm must be fitted between the throttle valve and the engine inlet. The aim of the throat is to limit the engine air flow rate as it strongly influences the volumetric efficiency and then the maximum power. The present paper is focused on the design of the engine intake system of the Firenze Race Team vehicle in order to optimize its performance in terms of both the maximum power and the drivability of the vehicle. One of the typical solutions for limiting the air restrictor influence consists of a plenum chamber placed along the intake line downstream of the restrictor. However the plenum involves also a delay in the engine response during the transient phases. The greater is the plenum, the lower are the power losses but the greater is the engine response delay. Taking advantage of a calibrated 1D model of the engine and a simplified vehicle model, the authors numerically analyzed an innovative solution that is constituted by a variable length duct inside the plenum. When the duct is at the maximum extension, the plenum is excluded from the intake line improving the engine response time. The optimization of the plenum volume and the definition of a preliminary control logic of the innovative system were done in order to obtain the maximum advantages in terms of both performance and engine drivability

A hybrid time-frequency domain approach for numerical modeling of reciprocating compressors

In the reciprocating compressor field, strong attention is paid to the study of pressure wave propagation in the discharge and suction pipelines. Oscillating pressure waves may lead to mechanical vibrations and failures and affect the machine performance. For this reason, an accurate analysis of the acoustic response of suction and discharge pipelines in a reciprocating compressor plant is of great interest. By solving a linear system of equations, the acoustic domain of a piping system can be easily determined. Usually, the acoustic pulsation analysis of the pipelines is carried out without considering the interaction between the machinery and the pipelines. Consequently, the reciprocal interaction between the compressor and the pipelines can not be considered. The aim of this work is to perform a fluid-dynamic analysis of the full compressor-pipelines system. For this purpose, a hybrid time-frequency domain approach is adopted. The reciprocating compressor thermodynamic cycle is simulated with a 0D timedomain model, while the pressure wave propagation in the pipelines is modelled by mean of a transfer matrix approach in the frequency domain. This analysis allows one to take into account the mutual interaction between the compressor and its pipelines by using the FFT and the Inverse FFT alternatively. The methodology was assessed by comparing the results of the simulation of a test case performed with both the hybrid approach and a commercial 1D code

Virtual incidence effect on rotating airfoils in Darrieus wind turbines

Small Darrieus wind turbines are one of the most interesting emerging technologies in the renewable energies scenario, even if they still are characterized by lower efficiencies than those of conventional horizontal-axis wind turbines due to the more complex aerodynamics involved in their functioning. In case of small rotors, in which the chord-to-radius ratios are generally high not to limit the blade Reynolds number, the performance of turbine blades has been suggested to be moreover influenced by the so-called "flow curvature effects". Recent works have indeed shown that the curved flowpath encountered by the blades makes them work like virtually cambered airfoils in a rectilinear flow. In the present study, focus is instead given to a further effect that is generated in reason of the curved streamline incoming on the blades, i.e. an extra-incidence seen by the airfoil, generally referred to as "virtual incidence". In detail, a novel computational method to define the incidence angle has been applied to unsteady CFD simulations of three airfoils in a Darrieus-like motion and their effective angles of attack have been compared to theoretical expectations. The analysis confirmed the presence of an additional virtual incidence on the airfoils and quantified it for different airfoils, chord-to-radius ratios and tip-speed ratios. A comparative discussion on BEM prediction capabilities is finally reported in the study

Critical Analysis of Dynamic Stall Models in Low-Order Simulation Models For Vertical-Axis Wind Turbines

Abstract The efficiency of vertical-axis wind turbines (VAWTs) still lacks from those of horizontal-axis rotors (HAWTs). To improve on efficiency, more accurate and robust aerodynamic simulation tools are needed for VAWTs, for which low-order methods have not reached yet a maturity comparable to that of HAWTs' applications. In the present study, the VARDAR research code, based on the BEM theory, is used to critically compare the predictiveness of some dynamic stall models for Darrieus wind turbines. Dynamic stall, connected to the continuous variation of the angle of attack on the airfoils, has indeed a major impact on the performance of Darrieus rotors. Predicted lift and drag coefficients of the airfoils in motion are reconstructed with the different dynamic stall models and compared to unsteady CFD simulations, previously validated by means of experimental data. The results show that low-order models are unfortunately not able to capture all the complex phenomena taking place during a VAWT functioning. It is however shown that the selection of the adequate dynamic stall model can definitely lead to a much better modelling of the real airfoils' behavior and then notably enhance the predictiveness of low-order simulation methods

Experimental investigation on industrial drying process of cotton yarn bobbins: energy consumption and drying time

Abstract In the textile industry, the drying process is a time consuming and energy expensive operation that influences strongly the cost of the textile finishing operations. For this reason, the study of innovative techniques plays a key role to decrease the energy consumption, the costs and the environmental impact. After a first mechanical process, the moisture is removed from yarn fibers by a thermal convection dryer that delivers hot air through the material. In this study, the drying process of cotton yarn bobbins is experimentally analyzed. With this aim, an experimental test rig was developed based on the geometry of industrial dryers. The influence of the drying air path and the air working conditions was assessed by performing tests with different configurations, temperatures and pressures. The results were analyzed in terms of drying time and energy consumption as the optimum drying condition is a trade-off between these parameters

Entrepreneurial Behaviour and New Venture Creation: the Psychoanalytic Perspective

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potential of the virtual blade model in the analysis of wind turbine wakes using wind tunnel blind tests

Abstract The present research frontier on wind turbine wake analysis is leading to a massive use of large-eddy simulations to completely solve the flow field surrounding the rotors; on the other hand, there is still room for lower-fidelity models with a more affordable computational cost to be used in extended optimization analyses, e.g. for a park layout definition. In this study, a customized version of the Virtual Blade Model (VBM) for ANSYS ® FLUENT ® is presented. The model allows a hybrid solution of the flow, in which the surrounding environment is simulated through a conventional RANS approach, while blades are replaced by a body force, calculated by a simplified version of the Blade Element Theory. The potential of the newly-customized VBM was evaluated by applying it to the famous NOWITECH-NORCOWE blind tests for horizontal axis wind turbines. Several test cases were analyzed and discussed including: 1) a single turbine; 2) an array of two turbines with one rotor working in the wake of the other one; 3) an array of two staggered rotors; 4) several configurations of rotors working in yawed-flow. The study proves that the VBM model can represent a valuable tool for the analysis of wind turbines wakes and of their interaction with near rotors

Critical issues in the CFD simulation of Darrieus wind turbines

Computational 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