An analytical expression for the power coefficient of an ideal horizontal-axis wind turbine

The paper shows that an analytical evaluation of the power coefficient C_p for an ideal horizontal-axis wind turbine can be made via an expression giving the direct relationship of C_p and the axial induction factor a. The results obtained agree closely with those obtained numerically from the usual integral expression involving several variables.     

Research on wind turbine rotor models using NACA profiles

In this study, rotation rates and power coefficients of miniature wind turbine rotor models manufactured using NACA profiles were investigated. For this purpose, miniature rotor models with 310 mm diameter were made from “Balsa” wood. When all properties of rotor models were taken into account, a total of 180 various combinations were obtained. Each combination was coded with rotor form code. These model rotors were tested in a wind tunnel measurement system. Rotation rates for each rotor form were determined based on wind speed. Power coefficient values were calculated using power and tip speed rates of wind. Rotor models produced a rotation rate up to 3077 rpm, with a power coefficient rate up to 0.425. Rotor models manufactured by using NACA 4412 profiles with 0 grade twisting angle, 5 grade blade angle, double blades had the highest rotation rate, while those manufactured by using NACA 4415 profiles with 0 grade twisting angle, 18 grade blade angle, 4 blades had the highest power coefficient.     

Investigating the influence of dimensional scaling on aerodynamic characteristics of wind turbine using CFD simulation

Wind turbines are used in a variety of applications with different performance requirements. Investigating the influence of scaling on wind turbine characteristics can pave the way to utilize the experience gained from a smaller turbine for a larger one. In this paper, the effects of wind turbine size on aerodynamic characteristics of a rotor blade are examined using CFD simulation. NREL phase VI wind turbine rotor was simulated in order to validate the results and ensure the accuracy of the CFD model. A 2 MW wind turbine was then chosen as a large turbine and a scaled down model of its rotor was simulated numerically. The results of the simulation were introduced to Similarity Theory relations in order to predict the aerodynamic characteristics of the 2 MW wind turbine. The 2 MW turbine was also simulated and the results of the simulation were compared to predictions of Similarity Theory. It was observed that the results of the simulation completely follow the values predicted by Similarity Theory. Both Similarity Theory predictions and simulation results demonstrated that the torque increases with the cube of change in rotor diameter whereas the thrust value and aerodynamic forces grow with the square of change in diameter.     

Testing basic performance of a very small wind turbine designed for multi-purposes

A very small wind turbine system for multi-purposes was developed and its performance was reported in this paper. The rotor diameter of the turbine is 500 mm. The tests of the energy output, turbine speed, power coefficient, and torque of turbine were carried out for a wide rage of free stream velocity. The flow around the wind turbine and the influence of the turbulence were investigated with a particle image velocimetry. Experimentally obtained power coefficient was 0.4 in maximum and 0.36 in the rated running condition, respectively. The tip speed ratio corresponding to the optimum driving condition was 2.7. Comparing with the other commercial turbines, the performance was excellent at a slow turbine speed. By the flow visualization and PIV measurement around the wind turbine, the approaching flow velocity and the accelerated flow field passing the blade tip was obtained. It was confirmed that the actual flow passed through the blades was about 20% slower than the ideal flow. Tip vortex shed from the blade tip was also visualized clearly.     

Open loop wind turbine emulator

A new wind turbine emulator (WTE) is presented, which is able to simulate the turbine power curves without using a closed loop control system. The state of the art emulators use DC or AC motors, closed loop controlled by processors with the turbine power curves recorded. The presented emulator has a DC voltage source, a power resistor and a DC motor with independent excitation. The motor power curve has a shape similar to a wind turbine power curve for a given wind speed; the wind speed variations can be emulated by the variations of the DC voltage source. The open loop emulator is completely different and new, because it works in open loop and does not require the presence of a processor.The following elements are included: the theoretical foundations of the emulator, the emulator power curves adjustment procedure to simulate a commercial wind turbine and the experimental tests.     

Power curve control in micro wind turbine design

In this work, a micro wind turbine will be designed and built for a series of wind tunnel tests (rotor dynamics and Wind Turbine (WT) start-up velocity). Its design stems from an original numerical code, developed by the authors, based on the Blade Element Momentum (BEM) Theory.     

Innovative improvement of a drag wind turbine performance

Drag type wind turbines have strong potential in small and medium power applications due to their simple design. However, a major disadvantage of this design is the noticeable low conversion efficiency. Therefore, more research is required to improve the efficiency of this design. The present work introduces a novel design of a three-rotor Savonius turbine with rotors arranged in a triangular pattern. The performance of the new design is assessed by computational modeling of the flow around the three rotors. The 2D computational model is firstly applied to investigate the performance of a single rotor design to validate the model by comparison with experimental measurements. The model introduced an acceptable accuracy compared to the experimental measurements. The performance of the new design is then investigated using the same model. The results indicated that the new design performance has higher power coefficient compared with single rotor design. The peak power coefficient of the three rotor turbine is 44% higher than that of the single rotor design (relative increase). The improved performance is attributed to the favorable interaction between the rotors which accelerates the flow approaching the downstream rotors and generates higher turning moment in the direction of rotation of each rotor.     

Assessment of wind characteristics and wind turbine characteristics in Taiwan

Wind characteristics and wind turbine characteristics in Taiwan have been thoughtfully analyzed based on a long-term measured data source (1961–1999) of hourly mean wind speed at 25 meteorological stations across Taiwan. A two-stage procedure for estimating wind resource is proposed. The yearly wind speed distribution and wind power density for the entire Taiwan is firstly evaluated to provide annually spatial mean information of wind energy potential. A mathematical formulation using a two-parameter Weibull wind speed distribution is further established to estimate the wind energy generated by an ideal turbine and the monthly actual wind energy generated by a wind turbine operated at cubic relation of power between cut-in and rated wind speed and constant power between rated and cut-out wind speed. Three types of wind turbine characteristics (the availability factor, the capacity factor and the wind turbine efficiency) are emphasized. The monthly wind characteristics and monthly wind turbine characteristics for four meteorological stations with high winds are investigated and compared with each other as well. The results show the general availability of wind energy potential across Taiwan.     

Load sharing behavior analysis method of wind turbine gearbox in consideration of multiple-errors

Conducting a further analysis on loading sharing among compound planetary gear system in wind turbine gearbox, and making a meshing error analysis on the eccentricity error, gear thickness error, base pitch error, assembly error, and bearing error of wind turbine gearbox respectively. In view of the floating meshing error resulting from meshing clearance variation caused by the simultaneous floating of all gears, this paper establishes a refined mathematical model of two-stage power split loading sharing coefficient calculation in consideration of multiple errors. Also obtains the regular curves of the load sharing coefficient and floating orbits of center gears, and conducts a load sharing coefficient test experiment of compound planetary gear system to verify the research results, which can provide scientific theory evidence for proper tolerance distribution and control in design and process.     

Wind tunnel study on wind and turbulence intensity profiles in wind turbine wake

In recent years, there has been a rapid development of the wind farms in Japan. It becomes very important to investigate the wind turbine arrangement in wind farm, in order that the wake of one wind turbine does not to interfere with the flow in other wind turbines. In such a case, in order to achieve the highest possible efficiency from the wind, and to install as many as possible wind turbines within a limited area, it becomes a necessity to study the mutual interference of the wake developed by wind turbines. However, there is no report related to the effect of the turbulence intensity of the external flow on the wake behind a wind turbine generated in the wind tunnel. In this paper, the measurement results of the averaged wind profile and turbulence intensity profile in the wake in the wind tunnel are shown when the turbulence intensity of the external wind was changed. The wind tunnel experiment is performed with 500mm-diameter two-bladed horizontal axis wind turbine and the wind velocity in wake is measured by an I-type hot wire probe. As a result, it is clarified that high turbulence intensities enable to the entrainment of the main flow and the wake and to recover quickly the velocity in the wake.     

Calculation of hoisting forces of the wind turbine rotor based on wind conditions

The assembly and hoisting process of the wind turbine rotor in an open wind environment are regarded to improve the hoisting safety, efficiency and quality. The wind turbine rotor model of a 1.5 MW wind turbine are given, and the hoisting forces of the wind turbine rotor in different poses with various azimuth angles, yaw angles and pitch angles in 3D coordinate system are calculated based on the defined wind conditions model. The maximum and minimum hoisting forces of the wind turbine rotor are acquired and the corresponding azimuth angle, yaw angle and pitch angle of the wind turbine rotor are obtained with respect to the wind conditions in the hoisting process. For four specific poses with particular azimuth angles, yaw angles and pitch angles of the wind turbine rotor, the hoisting forces of the wind turbine rotor are calculated along its hoisting height increment. The change processes of the hoisting forces of the wind turbine rotor in the hoisting process are analyzed and the conclusions are drawn.     

Fault analysis of wind turbine generators in India

Wind energy has become a techno-economically viable source of energy and is considered as a preferable renewable energy resources option in the power sector in India. If the current pace of development is maintained for at least a few decades, India would soon possess the highest windfarm installation in the world and a significant portion of the country's energy needs could be met through wind power. The available wind energy resource can be utilised to the greater extent by optimally siting the windfarms, by appropriate machine selection and by proper maintenance. An attempt has been made to evaluate the performance of wind turbine generators for the largest demonstration windfarm (10 MW) in Asia. This windfarm is situated at Lamba, Gujarat State with 50 wind turbine machines of 200 kW capacity. The technical availability, real availability, capacity factor and maximum down time of the wind turbine generators have been calculated and plotted over the year. About 30 fault conditions have been identified and analysed by pareto diagram.     

LCA sensitivity analysis of a multi-megawatt wind turbine

During recent years renewables have been acquiring gradually a significant importance in the world market (especially in the Spanish energetic market) and in society; this fact makes clear the need to increase and improve knowledge of these power sources. Starting from the results of a Life Cycle Assessment (LCA) of a multi-megawatt wind turbine, this work is aimed to assess the relevance of different choices that have been made during its development. Looking always to cover the largest possible spectrum of options, four scenarios have been analysed, focused on four main phases of lifecycle: maintenance, manufacturing, dismantling, and recycling. These scenarios facilitate to assess the degree of uncertainty of the developed LCA due to choices made, excluding from the assessment the uncertainty due to the inaccuracy and the simplification of the environmental models used or spatial and temporal variability in different parameters. The work has been developed at all times using the of Eco-indicator99 LCA method.     

Investigation of wind shear coefficients and their effect on electrical energy generation

Wind measurements are generally performed below wind turbine hub heights due to higher measurement and tower costs. In order to obtain the wind speed at the hub height of the turbine, the measurements are extrapolated, assuming that the wind shear is constant. This assumption may result in some critical errors between the estimated and actual energy outputs. In this paper wind data collected in Bal?kesir from October 2008 to September 2009, has been used to show the effects of wind shear coefficient on energy production. Results of the study showed that, the difference between wind energy production using extrapolated wind data and energy production using measured wind data at hub height may be up to 49.6%.     

The concept of a smart wind turbine system

A smart wind turbine concept with variable length blades and an innovative hybrid mechanical-electrical power conversion system was analyzed. The variable length blade concept uses the idea of extending the turbine blades when wind speeds fall below rated level, hence increasing the swept area, and thus maintaining a relatively high power output. It is shown for a typical site, that the annual energy output of such a wind turbine that could double its blade length, could be twice that of a corresponding turbine with fixed length blades. From a cost analysis, it is shown that the concept would be feasible if the cost of the rotor could be kept less than 4.3 times the cost of a standard rotor with fixed length blades. Given the variable length blade turbine system exhibits a more-or-less linear maximum power curve, as opposed to a non-linear curve for the standard turbine, an innovative hybrid mechanical-electrical power conversion system was proposed and tested proving the feasibility of the concept.     

Wind tunnel and numerical study of a small vertical axis wind turbine

This paper presents a combined experimental and computational study into the aerodynamics and performance of a small scale vertical axis wind turbine (VAWT). Wind tunnel tests were carried out to ascertain overall performance of the turbine and two- and three-dimensional unsteady computational fluid dynamics (CFD) models were generated to help understand the aerodynamics of this performance.Wind tunnel performance results are presented for cases of different wind velocity, tip-speed ratio and solidity as well as rotor blade surface finish. It is shown experimentally that the surface roughness on the turbine rotor blades has a significant effect on performance. Below a critical wind speed (Reynolds number of 30,000) the performance of the turbine is degraded by a smooth rotor surface finish but above it, the turbine performance is enhanced by a smooth surface finish. Both two bladed and three bladed rotors were tested and a significant increase in performance coefficient is observed for the higher solidity rotors (three bladed rotors) over most of the operating range. Dynamic stalling behaviour and the resulting large and rapid changes in force coefficients and the rotor torque are shown to be the likely cause of changes to rotor pitch angle that occurred during early testing. This small change in pitch angle caused significant decreases in performance.The performance coefficient predicted by the two dimensional computational model is significantly higher than that of the experimental and the three-dimensional CFD model. The predictions show that the presence of the over tip vortices in the 3D simulations is responsible for producing the large difference in efficiency compared to the 2D predictions. The dynamic behaviour of the over tip vortex as a rotor blade rotates through each revolution is also explored in the paper.     

Maximum thermodynamic power coefficient of a wind turbine

According to the centenary Betz‐Joukowsky law, the power extracted from a wind turbine in open flow cannot exceed 16/27 of the wind transported kinetic energy rate. This limit is usually interpreted as an absolute theoretical upper bound for the power coefficient of all wind turbines, but it was derived in the special case of incompressible fluids. Following the same steps of Betz classical derivation, we model the turbine as an actuator disk in a one dimensional fluid flow but consider the general case of a compressible reversible fluid, such as air. In doing so, we are obliged to use not only the laws of mechanics but also and explicitly the laws of thermodynamics. We show that the power coefficient depends on the inlet wind Mach number , and that its maximum value exceeds the Betz‐Joukowsky limit. We have developed a series expansion for the maximum power coefficient in powers of the Mach number that unifies all the cases (compressible and incompressible) in the same simple expression: .     

A momentum explanation for the unsatisfactory Betz model prediction in highly loaded wind turbines

The present study attempts to provide a direct explanation for the strong deviation between the predictions of the well‐known Betz model and the actual results for the performance of highly loaded wind turbines. The new model accounts for the inner and outer stream interaction (momentum and energy transfers) by reformulating the relevant one‐dimensional flow equations. This results in a very good prediction of the experimentally measured C_T = f(a) relationship in ‘windmilling propeller’ tests. It is shown that although the maximum power coefficient is near that predicted by Betz, for rotor loadings beyond the ‘optimum’ point, this drops sharply (instead of remaining steady, near the C_P = 0.5 value). Further analysis shows that the main contributor for this ‘efficiency’ degradation is the reduction in the mass flux through the turbine rotor, apparently because of the wake vortices that block its passage in the inner section. Copyright © 2010 John Wiley & Sons, Ltd.     

Challenges and options for a large wind power uptake by the European electricity system

The contribution of renewable energies (in particular of wind power) to the electrical power generation has been continuously increasing in the recent decades. This article focuses on the necessary options that manage the variability of wind turbine output and enable the large scale integration of wind power with the current electricity system, such as additional power reserves, distributed storage technologies, in particular electric vehicles, and cross-boarder power transmission. The influence of geographical distribution of wind turbines on the produced power variability is described as well. The article highlights that even though state-of-art technologies for higher wind integration are present, there is a necessity for the proper management and integration of mentioned options.     

Investigation on wind power potential on Hong Kong islands—an analysis of wind power and wind turbine characteristics

This paper discusses the potential for electricity generation on Hong Kong islands through an analysis of the local weather data and typical wind turbine characteristics. An optimum wind speed, u_op, is proposed to choose an optimal type of wind turbine for different weather conditions. A simulation model has been established to describe the characteristics of a particular wind turbine. A case study investigation allows wind speed and wind power density to be obtained using different hub heights, and the annual power generated by the wind turbine to be simulated. The wind turbine's capacity factor, being the ratio of actual annual power generation to the rated annual power generation, is shown to be 0.353, with the capacity factor in October as high as 0.50. The simulation shows the potential for wind power generation on the islands surrounding Hong Kong.