Aerodynamic design method of small wind turbine for variable wind speed conditions

Abstract

The present invention discloses an aerodynamic design method for a small wind turbine under variable wind speed conditions, specifically: selecting a wind farm area where the wind turbine is arranged and determining the wind resource conditions in the area, selecting and calculating the initial parameters of the small wind turbine to be designed and the basic design parameters of the wind rotor; dividing the wind rotor blade profile and correcting and optimizing the installation angle and chord length of each profile, improving the wind energy capture capability of the wind turbine to be designed under low wind speed conditions, and then improving the power conversion output capability under variable wind speed conditions. After the aerodynamic design of the small wind turbine wind rotor is performed using this method, the utilization degree of low-grade wind energy under variable wind speed conditions can be improved, and the power generation under low wind speed conditions can be increased by 8% to 15%; and the rated design point conditions meet the design requirements, and the design deviation is about 1%, which generally increases the power generation by 5% to 10%. By carrying out wind turbine design by this method, the power generation of small wind turbines in variable wind speed areas can be greatly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of aerodynamic design methods for wind turbines, and in particular relates to an aerodynamic design method for a small wind turbine used under variable wind speed conditions. Background Art

[0002] At present, with the rapid development of economy and science and technology, problems such as energy shortage and environmental pollution have become more and more prominent. People need to explore a new energy source with huge reserves, friendly ecological environment, wide distribution of resources, and convenient development and utilization. Although the large-scale wind power industry has developed rapidly, it has also encountered many problems, especially in inland low wind speed areas, where its application in response to variable wind speed conditions is greatly restricted, and this problem has not been effectively solved. This requires giving full play to the advantages of small wind power. Compared with high-power wind turbines, small wind turbines have outstanding performance advantages in terms of construction area, available wind speed range, power transmission configuration, and energy storage technology application. In addition, small wind turbines have a high degree of wind energy resource utilization and a large number of full-time hours when operating under variable wind speed conditions. Therefore, the promotion and application of small wind turbines has a high economic investment and return value, and the market prospects are good.

[0003] Small wind turbines designed based on the current mainstream wind turbine aerodynamic design methods have weak wind energy capture capabilities when operating under variable wind speed conditions, which leads to low power generation and poor power output stability, large fluctuations, and overall weak ability to match variable wind speeds. In addition, since the design and application of small wind turbines have not received enough attention, the development of theoretical research on aerodynamic design of small wind turbines specifically for variable wind speed conditions has been slow. It is urgent to explore new wind turbine rotor aerodynamic design methods to address this issue, increase total power generation under variable wind speed conditions, and improve the level of low-grade wind energy utilization. Summary of the invention

[0004] The purpose of the present invention is to provide an aerodynamic design method for a small wind turbine under variable wind speed conditions, which solves the problem that wind turbines designed by existing methods cannot adapt to variable wind speed conditions and it is difficult to improve the power generation of wind turbines under low wind speed conditions.

[0005] The technical solution adopted by the present invention is an aerodynamic design method for a small wind turbine under variable wind speed conditions, which is specifically implemented according to the following steps:

[0006] Step 1: Select the wind farm area where the wind turbine is to be arranged and determine the wind resource conditions in the area, calculate and select the initial parameters of the proposed small wind turbine and the basic design parameters of the wind rotor;

[0007] Step 2: Divide the profile of the wind turbine rotor blade to be designed, and obtain the installation angle of each profile at the span length r of the final blade through correction optimization. The chord length C improves the wind energy capture capability of the proposed wind turbine under low wind speed conditions, thereby improving the power conversion output capability under variable wind speed conditions.

[0008] The present invention is also characterized in that:

[0009] In step 1, specifically:

[0010] Step 1.1, establish the Weibull curve f(v) of the wind resource conditions of the wind farm, and its expression is shown in formula (1);

[0011]

[0012] In formula (1), K is the shape parameter, Z is the size parameter, and v is the wind speed in front of the wind wheel;

[0013] Step 1.2, draw the distribution curve W(v) of wind power density changing with wind speed from the Weibull curve f(v), and its expression is shown in formula (2);

[0014]

[0015] In formula (2), ρ is the air density, S is the swept area of ​​the wind wheel;

[0016] Step 1.3, select the wind speed corresponding to the peak point of the wind power density distribution curve W(v) as the rated wind speed V;

[0017] Step 1.4, setting the speed N of the wind turbine and the power P of the wind turbine to be between 0.3kW and 50kW;

[0018] Step 1.5, determine the wind farm air density ρ, wind turbine transmission efficiency η1 and generator efficiency η2 and wind energy utilization coefficient C P , and then calculate the wind rotor diameter D and wind rotor radius R, as shown in equations (5) and (6);

[0019]

[0020]

[0021] Step 1.6, according to the rotor radius R, the rated speed Ω and the rated wind speed V, the blade tip speed ratio λ0 is determined. The blade tip speed ratio λ0 is calculated as shown in formula (7);

[0022]

[0023] Step 1.7, set the number of wind turbine blades B to 3, and select the NACA series airfoil.

[0024] In step 1.3, the rated wind speed V can also be calculated using formula (3);

[0025]

[0026] In step 2, specifically:

[0027] Step 2.1, divide the wind turbine rotor blade into 10 sections along the span length r, i.e., 10 blade element sections, with each section interval being 0.1R, and calculate the blade speed ratio λ at each section at the span length r, as shown in formula (8);

[0028]

[0029] In formula (8), i is the count of the wind rotor blade section, i = 1, 2, 3...9;

[0030] Step 2.2: Let the wind energy utilization coefficient C of each section of the wind turbine be P The maximum value is used as the objective function to solve the problem, as shown in formula (9);

[0031]

[0032] In formula (9), a and b are the axial and circumferential induction factors, respectively, and F is the tip loss coefficient;

[0033] Step 2.3, the calculation formula of blade tip loss coefficient F is shown in formula (10);

[0034]

[0035] f t is the loss factor of the blade tip loss coefficient F, as shown in formula (11);

[0036]

[0037] In formula (11), is the initial installation angle of the blade;

[0038] Step 2.4, after introducing the blade tip loss, the corrected energy equation is obtained, as shown in equation (12);

[0039] a(1-aF)=b(1+b)λ 2 (12);

[0040] Step 2.5, calculate the initial installation angle of each section of the blade according to the induction factors a, b and the blade speed ratio λ As shown in formula (13);

[0041]

[0042] Step 2.6, to solve for the wind energy utilization coefficient C PThe maximum is the objective function, the constraint condition is the energy equation, and then the induction factors a and b are iteratively calculated so that a and b can satisfy the energy equation at the same time. p reach the maximum;

[0043] Step 2.7, calculate the Reynolds number Re at each section according to the divided wind rotor blade section, and based on this, select the optimal angle of attack at the corresponding position as the initial angle of attack α0 of the airfoil at each section; the Reynolds number Re is calculated as shown in formula (16);

[0044]

[0045] In formula (15), μ is the dynamic viscosity, l is the characteristic length;

[0046] Step 2.8, calculate the twist angle θ of each blade section, as shown in formula (17);

[0047]

[0048] Step 2.9, calculate the solidity σ, as shown in formula (18);

[0049]

[0050] In formula (18), S Cross is the sum of the projected areas of the blades on the rotor rotation plane, and S is the rotor swept area;

[0051] Step 2.10, introduce two angle of attack corrections Δα1 and Δα2 to correct the angle of attack of the wind turbine, and integrate the two angle of attack corrections Δα1 and Δα2 into a total angle of attack correction Δα, as shown in equations (19), (20) and (21);

[0052]

[0053]

[0054] Δα=Δα1+Δα2 (21);

[0055] In formula (20), t is a constant related to the blade shape;

[0056] Step 2.11, introduce the total angle of attack correction Δα to perform superimposed correction on the initial angle of attack α0 of each airfoil section, as shown in formula (22);

[0057] α=α0+Δα (22);

[0058] Step 2.12, keep the twist angle θ of each section of the blade unchanged, and correct the angle of attack α to obtain the corrected installation angle As shown in formula (23);

[0059]

[0060] Step 2.13, introduce parameters e, drag-lift ratio ε and β as correction factors for chord length C, as shown in equations (24), (25) and (26);

[0061]

[0062]

[0063] cosβ-sinθ+cos 2 β-cosβsinθ=sinβ (26);

[0064] In formula (25), C l , C d Select the lift coefficient and drag coefficient of the airfoil corresponding to the optimal angle of attack α for each section, and β is the installation angle of each blade section The complementary angle of

[0065] Step 2.14, optimize the chord length C of each airfoil section according to the correction factor, as shown in formula (27);

[0066]

[0067] Step 2.15, select a cubic polynomial curve to perform nonlinear fitting on the torsion angle and chord length;

[0068] Step 2.16, use Profili software to export the airfoil data and establish the three-dimensional geometric model of the wind rotor according to the design parameters.

[0069] In step 2.15, specifically:

[0070] The position r value of each section of the blade along the span direction is taken as the independent variable of the fitting function, and the ordinates of the sample points are the chord length C and the installation angle The fitting curve equations are shown in equations (28) and (29);

[0071] C=m0+m1r+m2r 2 +m3r 3 (28);

[0072]

[0073] In formula (28) and formula (29), m and n represent the fitting coefficients of the third-order polynomial, respectively.

[0074] The beneficial effects of the present invention are: after adopting this method to carry out aerodynamic design of the small wind turbine rotor, the utilization degree of low-grade wind energy arranged under variable wind speed conditions can be improved, and the power generation under low wind speed conditions can be increased by 8% to 15%; and the rated design point conditions can meet the design requirements, and the design deviation is about 1%, which can increase the power generation by 5% to 10% overall. By carrying out wind turbine design by this method, the power generation of small wind turbines in variable wind speed areas can be greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0075] Figure 1 is a flow chart of the aerodynamic design method of a small wind turbine under variable wind speed conditions of the present invention;

[0076] Figure 2 It is a force diagram of a certain section of a wind turbine rotor blade in the method of the present invention;

[0077] Figure 3 It is a three-dimensional geometric model diagram of a wind turbine designed by the method of the present invention;

[0078] Figure 4 It is a pressure distribution cloud diagram of the whole machine after adopting the design method of the present invention;

[0079] Figure 5 It is a velocity distribution cloud diagram of the whole machine after adopting the design method of the present invention;

[0080] Figure 6 It is the velocity, pressure and turbulence distribution cloud diagram at the blade root, blade middle and blade tip sections after the design method of the present invention is adopted;

[0081] Figure 7 It is a comparison diagram of the distribution curves of the installation angles of each section along the blade span before and after optimization by the design method of the present invention;

[0082] Figure 8 It is a comparison diagram of the distribution curves of the chord length of each section along the blade span before and after optimization by the design method of the present invention;

[0083] Fig. 9 The wind energy utilization coefficient C of each operating condition before and after optimization by the design method of the present invention is p Distribution curve comparison chart;

[0084] Fig.10 The torque coefficient C of each operating condition before and after optimization by the design method of the present invention is m Distribution curve comparison chart;

[0085] Fig.11 The thrust coefficient C of each operating condition before and after optimization by the design method of the present invention is t Distribution curve comparison chart. DETAILED DESCRIPTION

[0086] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0087] The present invention is used for the aerodynamic design method of a small wind turbine under variable wind speed conditions, such as Figure 1 As shown, the specific implementation steps are as follows:

[0088] Step 1: Select the wind farm area where the wind turbine is to be arranged and determine the wind resource conditions in the area, select and calculate the initial parameters of the small wind turbine to be designed and the basic design parameters of the wind rotor; specifically:

[0089] Step 1.1, complete the site selection of the wind farm from the macro and micro levels, and establish the Weibull curve f(v) of the wind resource conditions of the wind farm, that is, the wind frequency and wind speed distribution curve, whose expression is shown in formula (1);

[0090]

[0091] In formula (1), K is the shape parameter, Z is the size parameter, and v is the wind speed in front of the wind wheel;

[0092] The parameters K and Z in this function are determined by the regional wind resource conditions and are affected by the region, period, etc. Generally, the parameters of a certain region can be calculated by continuously detecting 30 years of wind data;

[0093] Step 1.2, draw the distribution curve W(v) of wind power density changing with wind speed from the Weibull curve f(v), and its expression is shown in formula (2);

[0094]

[0095] In formula (2), ρ is the air density, S is the swept area of ​​the wind wheel;

[0096] Step 1.3, by analyzing the wind power density distribution curve W(v), the wind speed corresponding to the peak point of the curve is selected as the rated wind speed V, which can also be obtained by the calculation formula of the rated wind speed V, as shown in formula (3);

[0097]

[0098] Step 1.4, the speed N is divided into low speed, medium speed and high speed. The power P of the small wind turbine is between 0.3kW and 50kW. The speed and power parameters can be flexibly selected according to actual needs. At this point, the initial parameters of the small wind turbine are determined. The speed unit can be converted into radians, as shown in formula (4);

[0099]

[0100] Step 1.5, determine the wind farm air density ρ, wind turbine transmission efficiency η1 and generator efficiency η2 and wind energy utilization coefficient CP , C P The value of is generally not higher than 0.45 due to process structure limitations, and then the wind wheel diameter D is calculated and the wind wheel radius R is determined, as shown in formula (5) and formula (6);

[0101]

[0102]

[0103] Step 1.6, according to the rotor radius R, rated speed Ω and rated wind speed V, determine the blade tip speed ratio λ0. Generally, the blade tip speed of a small wind turbine is about 4. The blade tip speed ratio λ0 is calculated as shown in formula (7);

[0104]

[0105] Step 1.7, most existing wind turbines are three-blade wind turbines. Because the three-blade structure can eliminate the system's periodic load and help balance the force, the hub is easy to design, manufacture and install, so the number of blades B is generally 3;

[0106] Step 1.8, airfoil selection is a key part of wind turbine design. For example, a large airfoil lift-to-drag ratio may lead to increased output fluctuations, while a small lift-to-drag ratio may cause the output power to be too low to meet the requirements. In addition, problems that may arise from high Reynolds number turbulence, manufacturing process, installation cost, etc. must also be considered. Therefore, it is appropriate to select the NACA series airfoil;

[0107] Step 2: Obtain the installation angle of each section at the span length r of the final blade through correction optimization The chord length C is used to improve the wind energy capture capability of the proposed wind turbine under low wind speed conditions, thereby improving the power conversion output capability under variable wind speed conditions. Specifically:

[0108] Step 2.1, divide the wind turbine blade section, divide the wind turbine blade into 10 sections along the span length r, that is, 10 blade element sections. The force condition of the blade section is as follows: Figure 2 As shown, the interval between each section is 0.1R, and the blade speed ratio λ at the span length r of each section is calculated as shown in formula (8);

[0109]

[0110] In formula (8), λ0 is the tip speed ratio, that is, the blade speed ratio corresponding to the blade tip section, i is the count of the wind rotor blade section, i = 1, 2, 3...9;

[0111] Step 2.2: In order to make the entire wind turbine rotor output the maximum power generation, consider the wind energy utilization rate of the blades and set the wind energy utilization coefficient C of each section of the wind turbine to PThe maximum value is used as the objective function to solve, as shown in formula (9);

[0112]

[0113] In formula (9), a and b are the axial and circumferential induction factors, respectively, and F is the tip loss coefficient;

[0114] Step 2.3, this design method considers the influence of blade tip loss on wind turbine power, and derives the calculation formula of blade tip loss coefficient F as shown in formula (10);

[0115]

[0116] f t is the loss factor of the blade tip loss coefficient F, as shown in formula (11);

[0117]

[0118] In formula (11), it is defined here is the initial installation angle of the blade, which needs to be corrected later to get the final installation angle

[0119] Step 2.4, after introducing the blade tip loss, the energy equation is corrected, as shown in equation (12);

[0120] a(1-aF)=b(1+b)λ 2 (12);

[0121] Step 2.5, calculate the initial installation angle of each blade section based on the induction factors a, b and blade speed ratio λ As shown in formula (13);

[0122]

[0123] Step 2.6, to solve for the wind energy utilization coefficient C P The maximum is the objective function, the constraint condition is the energy equation, and then the induction factors a and b are iteratively calculated so that a and b can satisfy the energy equation at the same time. p To reach the maximum; in order to speed up the iterative calculation speed of the iteration coefficients a and b, an initial value can be provided, and the calculation formula is shown in formula (14) and formula (15);

[0124]

[0125]

[0126] Step 2.7, calculate the Reynolds number Re at each section according to the divided wind rotor blade section, and select the optimal angle of attack at the corresponding position according to the Reynolds number Re as the initial angle of attack α0 of the airfoil at each section. The calculation of the Reynolds number Re is shown in formula (16);

[0127]

[0128] In formula (15), μ is the dynamic viscosity, which is generally taken as μ = 1.822x10 -5 ; l is the characteristic length, here substitute the spanwise length r of each section;

[0129] The best angle of attack at the corresponding position of each section can be selected as the initial angle of attack α0 by using special airfoil selection software such as Profili and referring to the Reynolds number Re;

[0130] Step 2.8, calculate the twist angle θ of each blade section, as shown in formula (17);

[0131]

[0132] Step 2.9, the small wind turbine with low tip speed ratio has shorter and wider rotor blades, so the solidity σ is larger, and the calculation formula is shown in formula (18);

[0133]

[0134] In formula (18), the solidity σ is defined as the sum of the projected areas of the blades on the rotor rotation plane S Cross The ratio of the swept area S of the wind wheel;

[0135] Step 2.10, introduce two attack angle correction values ​​Δα1 and Δα2 to correct the attack angle of the large solid small wind turbine, and integrate the two attack angle correction values ​​Δα1 and Δα2 into a total attack angle correction value Δα, as shown in equations (19), (20) and (21);

[0136]

[0137]

[0138] Δα=Δα1+Δα2 (21);

[0139] In formula (20), t is a constant related to the blade shape, usually 0.2;

[0140] Step 2.11, introduce the total angle of attack correction Δα to perform superimposed correction on the initial angle of attack α0 of each airfoil section, as shown in formula (22);

[0141] α=α0+Δα (22);

[0142] Step 2.12, keep the twist angle θ of each section of the blade unchanged, and correct the angle of attack α to obtain the corrected installation angle As shown in formula (23);

[0143]

[0144] Step 2.13, consider wind resource conditions, airfoil performance and installation angles of each blade section The influence of the parameters e, drag-lift ratio ε and β (installation angle The complementary angle of the chord length C is used as the correction factor of the chord length C, as shown in equations (24), (25) and (26);

[0145]

[0146]

[0147] cosβ-sinθ+cos 2 β-cosβsinθ=sinβ (26);

[0148] In formula (25), C l , C d Select the lift coefficient and drag coefficient of the airfoil corresponding to the optimal angle of attack α for each section respectively;

[0149] Step 2.14, optimize the chord length C of each airfoil section according to the correction factor, as shown in formula (27);

[0150]

[0151] In formula (27), the meanings of some parameters are summarized in Table 1;

[0152] Table 1 Meaning of parameters in formula (27)

[0153]

[0154]

[0155] Step 2.15, because the above calculation results are the theoretical optimal solutions after dividing the sections, the obtained chord length and twist angle are only the solution values ​​at each section, which are generally different from the real smooth physical geometry. Therefore, considering that the actual wind turbine blade is an infinite number of blade element sections with smooth transition and processing issues, it is necessary to fit the curves of these two parameters. Select the cubic polynomial curve for nonlinear fitting of the twist angle and chord length, and use the position r value of each section of the blade along the span as the independent variable of the fitting function. The ordinates of the sample points are the chord length C and the installation angle. The fitting curve equations are shown in equations (28) and (29);

[0156] C=m0+m1r+m2r 2 +m3r 3 (28);

[0157]

[0158] In formula (28) and formula (29), m and n represent the fitting coefficients of the third-order polynomial respectively;

[0159] Step 2.16, use Profili software to export airfoil data and establish Figure 3 The three-dimensional geometric model of the wind wheel is shown.

[0160] The wind turbine designed by the original Wilson method and the wind turbine designed by this method are divided using the same grid topology. In the numerical simulation, the wind turbine flow field determines the rotation axis and gives the speed condition, the inlet is set as the speed inlet and gives the speed condition, and the outlet is set as the pressure outlet and gives the total pressure condition, so as to calculate and compare the wind energy utilization coefficient C of the two wind turbines under different wind speed conditions. p , torque coefficient C m and thrust coefficient C t , in order to verify the advanced nature of the design.

[0161] The advantages of the method of the present invention are mainly as follows:

[0162] (1) Compared with the traditional Wilson design method, this wind turbine aerodynamic design proposes a method of correcting the installation angle by the angle of attack (Formula 20-23), as well as a design method for the chord length (Formula 24-27). After the wind turbine is aerodynamically designed using this method, the installation angle of the blades in each section is reduced and the amount of reduction in each section is basically the same, and the chord length near the middle of the blade is significantly reduced. This makes the actual incoming flow angle of attack and the designed installation angle more consistent under low wind speed operating conditions, and the wind turbine performance is improved. In addition, the wind turbine performance remains basically unchanged under high wind speed operating conditions.

[0163] (2) The annual power generation of wind turbines designed using this method is significantly improved, which can be increased by 5-10%. This is mainly reflected in the enhanced wind energy capture capability under low wind speed conditions, the significant increase in power generation below rated operating conditions and the overall power generation, and the annual utilization hours index can be increased to far more than the current level of 3,500 hours. Therefore, the wind turbines designed by this method can be applied to the efficient utilization of wind energy under variable wind speed conditions, thereby improving the economic benefits of wind turbines.

[0164] Example

[0165] Aiming at the low wind speed area in the inland of northern Shaanxi, it is planned to arrange a three-blade horizontal axis small wind turbine with a rated power of 20kW in this area. The rated wind speed is given to be 7.8m / s and the rated speed is 25r / min. The aerodynamic design of the wind turbine is carried out based on these initial parameters. First, the parameters of each section of the wind turbine rotor blade are calculated based on the original Wilson design method. A three-dimensional geometric model is established based on the obtained parameters, and the numerical simulation of the wind turbine performance under various operating conditions, i.e. different wind speeds, is completed using Fluent software. The overall performance of the wind turbine of this design is good, but the cut-in wind speed is 4m / s, and the wind energy capture ability is poor when operating under low wind speed conditions, resulting in the unreasonable utilization of low-grade wind energy. Therefore, the performance is poor and the power generation is low when dealing with variable wind speed conditions.

[0166] The design is based on the aerodynamic design method of the small wind turbine under variable wind speed conditions of the present invention. The method calculates various profile parameters, including the angle of attack α, the torsion angle θ, the installation angle The chord length is C, etc. The design results are shown in Table 2.

[0167] Table 2 Blade data parameters of small wind turbine aerodynamic design method under variable wind speed conditions

[0168]

[0169] With these parameters, the modeling and numerical simulation work are completed. The proposed wind turbine rotor model is given in Figure 3, and the simulation results are given in Figure 4 , Figure 5 and Figure 6 Given. It can be analyzed that after this aerodynamic design method, the actual incoming flow angle of attack of each cross-section airfoil is basically consistent with the designed installation angle, and the aerodynamic performance is good. The pressure difference between the upper and lower surfaces of the airfoil at the tip and the middle of the blade increases significantly, so the cross-section airfoil is in a higher lift-to-drag ratio operating condition, and there is little or no turbulence. In addition, the rotation effect behind the wind rotor is weakened and the vortex is reduced. Therefore, the rated operating performance of the wind turbine designed by the aerodynamic design method of a small wind turbine under variable wind speed conditions is greatly improved, and the ability to capture and convert wind energy is further improved.

[0170] In addition, the comparison diagrams of the installation angle and chord length distribution of each section obtained by the two design methods are shown in Figures 7 and Figure 8 Given, and wind energy utilization coefficient C p , torque coefficient C m and moment coefficient C m The distribution curve comparison diagram is respectively Fig. 9 , Fig.10 and Fig.11In contrast, the aerodynamic design method for small wind turbines based on variable wind speed conditions has significantly improved and enhanced the performance of the wind turbine to be designed. The cut-in wind speed is 3.5m / s, and the wind energy capture capacity of low wind speed conditions is improved. It is more suitable for deployment in variable wind speed areas to efficiently convert wind energy, while the performance of higher wind speed conditions remains basically unchanged, and the power output of the rated design point conditions meets and is close to the design requirements. Therefore, the aerodynamic design method has a good effect and improves the comprehensive wind energy capture capacity. The design point power of the wind turbine to be designed based on the aerodynamic design method for small wind turbines with variable wind speed conditions, as well as the results of power generation and annual utilization hours are shown in Table 3.

[0171] Table 3 Statistical comparison of power and power generation level of design methods before and after optimization

[0172]

[0173] It is obvious that this aerodynamic design has been improved on the basis of the original Wilson design method and has achieved the optimization goal. The power generation and overall power generation under conditions below the rated wind speed have been greatly improved, and the annual utilization hours have reached 3753.17 hours, indicating that the wind turbine can efficiently utilize and convert wind energy. After optimization through this aerodynamic design method, the performance of the airfoils of each section of the blade has been greatly improved, with very little or even no turbulence, and the rotation effect behind the wind wheel has been weakened and the vortex has been reduced. Therefore, the performance of each operating condition of the wind turbine has been greatly improved, and the ability to capture and convert wind energy has been further improved. The power generation under conditions below the rated wind speed has increased by 11.47%, and the total power generation has increased by 6.10%.

[0174] The present invention, i.e., the aerodynamic design method of a small wind turbine under variable wind speed conditions, is applicable to the optimized design of all small wind turbine rotors. Before the optimization using this method, the cut-in wind speed is high and the wind energy capture ability is poor when operating under low wind speed conditions, resulting in low power generation efficiency, low overall power generation and large volatility. After the aerodynamic design of the small wind turbine rotor is performed using this method, the utilization of low-grade wind energy under variable wind speed conditions can be improved, and the power generation under low wind speed conditions can be increased by 8% to 15%; and the rated design point conditions meet the design requirements, and the design deviation is about 1%, which generally increases the power generation by 5% to 10%. By carrying out wind turbine design using this method, the power generation of small wind turbines in the variable wind speed area can be greatly improved.

Claims

1. An aerodynamic design method for a small wind turbine under variable wind speed conditions, characterized in that: Follow the steps below to implement it: Step 1: Select the wind farm area where the wind turbine is to be arranged and determine the wind resource conditions in the area, calculate and select the initial parameters of the proposed small wind turbine and the basic design parameters of the wind rotor; specifically: Step 1.1, establish the Weibull curve f(v) of the wind resource conditions of the wind farm, and its expression is shown in formula (1); In formula (1), K is the shape parameter, Z is the size parameter, and v is the wind speed in front of the wind wheel; Step 1.2, draw the distribution curve W(v) of wind power density changing with wind speed from the Weibull curve f(v), and its expression is shown in formula (2); In formula (2), ρ is the air density, S is the swept area of ​​the wind wheel; Step 1.3, select the wind speed corresponding to the peak point of the wind power density distribution curve W(v) as the rated wind speed V; The rated wind speed V can also be calculated by formula (3); Step 1.4, setting the speed N of the wind turbine and the power P of the wind turbine to be between 0.3 kW and 50 kW; Step 1.5, determine the wind farm air density ρ, wind turbine transmission efficiency η1 and generator efficiency η2 and wind energy utilization coefficient C P , and then calculate the wind rotor diameter D and wind rotor radius R, as shown in equations (5) and (6); Step 1.6, according to the rotor radius R, the rated speed Ω and the rated wind speed V, the blade tip speed ratio λ0 is determined. The blade tip speed ratio λ0 is calculated as shown in formula (7); Step 1.7, set the number of wind turbine blades B to 3, and select the NACA series airfoil; Step 2: Divide the profile of the wind turbine rotor blade to be designed, and obtain the installation angle of each profile at the span length r of the final blade through correction optimization. The chord length C improves the wind energy capture capability of the proposed wind turbine under low wind speed conditions, thereby improving the power conversion output capability under variable wind speed conditions; specifically: Step 2.1, divide the wind turbine rotor blade into 10 sections along the span length r, i.e., 10 blade element sections, with each section interval being 0.1R, and calculate the blade speed ratio λ at each section at the span length r, as shown in formula (8); In formula (8), i is the count of the wind rotor blade section, i = 1, 2, 3...9; Step 2.2: Let the wind energy utilization coefficient C of each section of the wind turbine be P The maximum value is used as the objective function to solve the problem, as shown in formula (9); In formula (9), a and b are the axial and circumferential induction factors, respectively, and F is the tip loss coefficient; Step 2.3, the calculation formula of blade tip loss coefficient F is shown in formula (10); f t is the loss factor of the blade tip loss coefficient F, as shown in formula (11); In formula (11), is the initial installation angle of the blade; Step 2.4, after introducing the blade tip loss, the corrected energy equation is obtained, as shown in equation (12); a(1-aF)=b(1+b)λ 2 (12); Step 2.5, calculate the initial installation angle of each blade section according to the induction factors a, b and blade speed ratio λ As shown in formula (13); Step 2.6, to solve for the wind energy utilization coefficient C P The maximum is the objective function, the constraint condition is the energy equation, and then the induction factors a and b are iteratively calculated so that a and b can satisfy the energy equation at the same time. p reach the maximum; Step 2.7, calculate the Reynolds number Re at each section according to the divided wind rotor blade section, and select the optimal angle of attack at the corresponding position according to the Reynolds number Re as the initial angle of attack α0 of the airfoil at each section; the Reynolds number Re is calculated as shown in formula (16); In formula (15), μ is the dynamic viscosity, l is the characteristic length; Step 2.8, calculate the twist angle θ of each blade section, as shown in formula (17); Step 2.9, calculate the solidity σ, as shown in formula (18); In formula (18), S Cross is the sum of the projected areas of the blades on the rotor rotation plane, and S is the rotor swept area; Step 2.10, introduce two angle of attack corrections Δα1 and Δα2 to correct the angle of attack of the wind turbine, and integrate the two angle of attack corrections Δα1 and Δα2 into a total angle of attack correction Δα, as shown in equations (19), (20) and (21); Δα=Δα1+Δα2 (21); In formula (20), t is a constant related to the blade shape; Step 2.11, introduce the total angle of attack correction Δα to perform superimposed correction on the initial angle of attack α0 of each airfoil section, as shown in formula (22); α=α0+Δα (22); Step 2.12, keep the twist angle θ of each section of the blade unchanged, and correct the angle of attack α to obtain the corrected installation angle As shown in formula (23); Step 2.13, introduce parameters e, drag-lift ratio ε and β as correction factors for chord length C, as shown in equations (24), (25) and (26); cosβ-sinθ+cos 2 β-cosβsinθ=sinβ (26); In formula (25), C l , C d Select the lift coefficient and drag coefficient of the airfoil corresponding to the optimal angle of attack α for each section respectively; β is the installation angle of each blade section The complementary angle of Step 2.14, optimize the chord length C of each airfoil section according to the correction factor, as shown in formula (27); In formula (27), r h b is the blade span length of the blade root section; h is the circumferential induction factor of the blade root section; F h is the tip loss coefficient of the blade root section; C is the blade installation angle of the blade root section; lh is the airfoil lift coefficient corresponding to the angle of attack of the blade root section; b t is the circumferential induction factor of the blade tip section; F t is the tip loss coefficient of the blade tip section; C is the blade installation angle of the blade tip section; lt is the airfoil lift coefficient corresponding to the angle of attack of the blade tip section; Step 2.15, select a cubic polynomial curve to perform nonlinear fitting on the torsion angle and chord length; Step 2.16, use Profili software to export the airfoil data and establish the three-dimensional geometric model of the wind rotor according to the design parameters.

2. The aerodynamic design method for a small wind turbine under variable wind speed conditions according to claim 1, characterized in that: In the step 2.15, specifically: The position r value of each section of the blade along the span direction is taken as the independent variable of the fitting function, and the ordinates of the sample points are the chord length C and the installation angle The fitting curve equations are shown in equations (28) and (29); C=m0+m1r+m2r 2 +m3r 3 (28); In formula (28) and formula (29), m and n represent the fitting coefficients of the third-order polynomial, respectively.

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