An improved LB method for predicting dynamic characteristics of Vertical Axis Wind Turbine airfoils

One of the important challenges for Vertical Axis Wind Turbine (VAWT) is to fully understand its dynamic characteristics in different operating conditions. Meanwhile, it is necessary to seek a fast and accurate method to evaluate the dynamic characteristic of VAWT. In this study, we improve the LB model by considering the operating principle of VAWT to study the dynamic characteristics of the dedicated and commonly used VAWT airfoils in different operating conditions. The results show that the improved LB model is suitable for simulating the dynamic characteristic of VAWT with a thick airfoil. Although the asymmetric airfoil shows the higher lift coefficient, their dynamic characteristic appears huge fluctuation as the increases of tip speed ratio. Moreover, at a low tip speed ratio, the advantages of the asymmetric airfoil are not obvious. While the dynamic characteristic of the symmetric airfoil is relatively stable with the variation of tip speed ratio.

Aerodynamic Design and Wind Tunnel Tests of Small-scale Horizontal-axis Wind Turbines for Low Tip Speed Ratio Applications

In recent times, the application of small-scale horizontal axis wind turbines (SHAWTs) has drawn interest in certain areas where the energy demand is minimal. These turbines, operating mostly at low Reynolds number (Re) and low tip speed ratio (λ) applications, can be used as stand-alone systems. The present study aims at the design, development, and testing of a series of SHAWT models. On the basis of aerodynamic characteristics, four SHAWT models viz., M1, M2, M3, and M4 composed of E216, SG6043, NACA63415, and NACA0012 airfoils, respectively have been developed. Initially, the rotors are designed through blade element momentum theory (BEMT), and their power coefficient have been evaluated. Thence, the developed rotors are tested in a low-speed wind tunnel to find their rotational frequency, power and power coefficient at design and off-design conditions. From BEMT analysis, M1 shows a maximum power coefficient (Cpmax) of 0.37 at λ = 2.5. The subsequent wind tunnel tests on M1, M2, M3, and M4 at 9 m/s show the Cpmax values to be 0.34, 0.30, 0.28, and 0.156, respectively. Thus, from the experiments, the M1 rotor is found to be favourable than the other three rotors, and its Cpmax value is found to be about 92% of BEMT prediction. Further, the effect of pitch angle (θp) on Cp of the model rotors is also examined, where M1 is found to produce a satisfactory performance within ±5° from the design pitch angle (θp, design).

Experimental Investigation of Performance Characteristics for Savonius-Style VAWTs: A Comparative Study

Savonius-style wind turbines are mainly gauged by two types of coefficients namely: (i) coefficient of power (CP) and (ii) coefficient of torques (CT). Coefficient of power is defined as the ratio of power generated by the turbine to the total power available to the turbine from the free-flowing wind. This is synonymous to the operational efficiency of the wind turbine. Coefficient of torque reflects the torque generating ability of the turbine. In this manuscript, experiments have been performed using three different types of rotor profiles for Savonius-style wind turbines (SSWTs) namely, classical SSWT, Benesh type SSWT and elliptical shaped SSWT using oriented jets. Using deflector plates the orientation of jets have been varied from 20° to 70°. Addition of deflector plates to the wind turbines, assists in maximizing the utilization of wind energy. Experiments have been performed in the laminar air flow. Mechanical loads have been used to study Coefficient of performance (CP) and coefficient of torque (CT) as a function of tip speed ratio (TSRs). The velocity of the wind is adjusted by varying the rheostat that controls the AC motor for the wind tunnel systems. Experimental results indicated that optimum performance could be achieved from all three types of SSWT variants at TSR ∼ 0.70. Out of the three designs studied in this manuscript, elliptic shaped SWT yielded best coefficient of performance equal to 0.39 at TSR = 0.70.

Wake Expansion and the Finite Blade Functions for Horizontal-Axis Wind Turbines

This paper considers the effect of wake expansion on the finite blade functions in blade element/momentum theory for horizontal-axis wind turbines. For any velocity component, the function is the ratio of the streamtube average to that at the blade elements. In most cases, the functions are set by the trailing vorticity only and Prandtl’s tip loss factor can be a reasonable approximation to the axial and circumferential functions at sufficiently high tip speed ratio. Nevertheless, important cases like coned or swept rotors or shrouded turbines involve more complex blade functions than provided by the tip loss factor or its recent modifications. Even in the presence of significant wake expansion, the functions derived from the exact solution for the flow due to constant pitch and radius helical vortices provide accurate estimates for the axial and circumferential blade functions. Modifying the vortex pitch in response to the expansion improves the accuracy of the latter. The modified functions are more accurate than the tip loss factor for the test cases at high tip speed ratio that are studied here. The radial velocity is important for expanding flow as it has the magnitude of the induced axial velocity near the edge of the rotor. It is shown that the resulting angle of the flow to the axial direction is small even with significant expansion, as long is the tip speed ratio is high. This means that blade element theory does not have account for the effective blade sweep due to the radial velocity. Further, the circumferential variation of the radial velocity is lower than of the other components.

Some effects of flow expansion on the aerodynamics of horizontal-axis wind turbines

The flow upwind of an energy-extracting horizontal-axis wind turbine expands as it approaches the rotor, and the expansion continues in the vorticity-bearing wake behind the rotor. The upwind expansion has long been known to influence the axial momentum equation through the axial component of the pressure, although the extent of the influence has not been quantified. Starting with the impulse analysis of Limacher and Wood (2020), but making no further use of impulse techniques, we derive its exact expression when the rotor is a circumferentially uniform disc. This expression, which depends on the radial velocity and the axial induction factor, is added to the thrust equation containing the pressure on the back of the disc. Removing the pressure to obtain a practically useful equation shows the axial induction in the far wake is twice the value at the rotor only at high tip speed ratio and only if the relationship between vortex pitch and axial induction in non-expanding flow carries over to the expanding case. At high tip speed ratio, we assume that the expanding wake approaches the Joukowsky model of a hub vortex on the axis of rotation and tip vortices originating from each blade. The additional assumption that the helical tip vortices have constant pitch allows a semi-analytic treatment of their effect on the rotor flow. Expansion modifies the relation between the pitch and induced axial velocity so that the far-wake area and induction are significantly less than twice the values at the rotor. There is a moderate decrease – about 6 % – in the power production, and a similar size error occurs in the familiar axial momentum equation involving the axial velocity.

Attempt to quantify the impact of seasonal air density variation on operating tip-speed ratio of small wind turbines

This study presents the impact of seasonal variation in air density on the operating tip-speed ratio of small wind turbines. The air density, which varies depending on the temperature, atmospheric pressure, and relative humidity, has an annual amplitude of about 5% in Tokyo, Japan. This study quantified this impact using the rotational speed equation of motion in a small wind turbine informed by previous work. This governing equation has been simplified by expanding the aerodynamic torque coefficient profile for a wind turbine rotor to the tip-speed ratio. Furthermore, this governing equation is simplified by using nondimensional forms of the air density, inflow wind velocity, and rotational speed with their characteristic values. In this study, the generator’s load is set to be constant based on a previous analysis of a small wind turbine. By considering the equilibrium between the aerodynamic torque and the load torque of the governing equation at the optimum tip-speed ratio, the impact of the variation in the air density on the operating tip-speed ratio was expressed using a simple mathematical form. As shown in this derived form, the operating tip-speed ratio was found to be less sensitive to a variation in air density than that in inflow wind velocity.

Effect of solidity on the dynamic behaviour of the Darrieus turbine with leading-edge protuberance

The dynamic behaviour of the straight–type Darrieus turbine with leading-edge protuberance (LEP) was analysed under various solidity ratios at several tip speed ratios through experiments. The Darrieus turbine is a type of Vertical Axis Wind Turbine (VAWT) which uses wind energy to generate electricity. This type of turbine was subjected to vortex-induced and buffeting types of vibrations. These vibrations were more sensitive to the number of blades and tip speed ratios. Based on the experimental measurements, the results revealed that, at a low tip speed ratio, the four-bladed turbine exhibits lesser vortex-induced vibrations than those of the three and five-bladed turbines. However, at a high tip speed ratio, the three-bladed configuration operates well against the vortex-induced vibrations. In the case of buffeting, a three-bladed turbine diminishes the dynamic oscillations at both low and high tip speed ratios, whereas the four and five-bladed turbines induce dynamic oscillations at slightly higher amplitudes. However, the amplitude of buffeting is smaller than those of vortex-induced vibrations.

Numerical Analysis and Parameter Optimization of J-Shaped Blade on Offshore Vertical Axis Wind Turbine

In this study, the performance of offshore wind turbines at low tip speed ratio (TSR) is studied using computational fluid dynamics (CFD), and the performance of offshore wind turbines at low tip speed ratio (TSR) is improved by revising the blade structure. First, the parameters of vertical axis offshore wind turbine are designed based on the compactness iteration, a CFD simulation model is established, and the turbulence model is selected through simulation analysis to verify the independence of grid and time step. Compared with previous experimental results, it is shown that the two-dimensional simulation only considers the plane turbulence effect, and the simulation turbulence effect performs more obviously at a high tip ratio, while the three-dimensional simulation turbulence effect has well-fitting performance at high tip ratio. Second, a J-shaped blade with optimized lower surface is proposed. The study showed that the optimized J-shaped blade significantly improved its upwind torque and wind energy capture rate. Finally, the performance of the optimized J-blade offshore wind turbine is analyzed.

THE EFFECT OF NUMBER OF BLADES EXPERIMENTALLY ON THE HORIZONTAL WIND TURBINE SPEED

The aim of this research is to find the effect of the number of blades on the wind turbine speed and to find which number of blades is suitable for low wind areas and high wind areas. In wind turbine design; the number of blades, tip speed ratio, and the rotational speed of the rotor are the most important factors. At first, the tip speed ratio and the number of blades must be selected. The power of a wind turbine generator depends on the rotational speed of the rotor. The increase in wind velocity leads to an increase in the rotor speed. At wind velocity 2.36m/s, the rotational speed of 6 blades, 4 blades and 3 blades was 288, 54, and 34 rpm respectively. And, at wind velocity 13.85m/s, the rotational speed of 6 blades, 4 blades, and 3 blades are 1856, 2220, and 2103 rpm respectively. So, when the number of blades decreases, the rotational speed will increase at high wind velocity. But, at low wind velocity, the rotational speed is more effective when the number of blades increases. So, 6 rotor blades were found as suitable for low wind velocity areas as in Iraq.