A wind turbine design method for a three-bladed rotor

Abstract

The application discloses a wind turbine design method for three wind wheels, comprising the following steps: building a wind turbine model test table in a wind tunnel test section and placing a grid at the entrance of the wind tunnel test section; setting output power collection parameters, opening the wind tunnel, collecting output power, and closing the wind tunnel; repeating the above experiment to collect power of the three wind wheel wind turbine; processing data to estimate the degree of electricity cost with the test data; estimating the cost and calculating the annual power generation of the single wind wheel wind turbine unit and the three wind wheel wind turbine unit, and comparing and evaluating the design effect of the designed three wind wheel wind turbine. The application optimizes the existing single wind wheel wind turbine, solves the problems of low power generation and large occupation area of small wind turbines by designing the three wind wheel wind turbine, solves the problems of transportation, installation and maintenance of large wind turbines, and has important engineering significance for the research on the aerodynamic characteristics of the wind turbine.

Description

Technical Field

[0001] This invention relates to the field of wind energy optimization and utilization, and in particular to a design method for a three-rotor wind turbine. Background Technology

[0002] Compared to large wind turbines with higher power output, smaller and medium-sized wind turbines have become more technologically mature thanks to the continuous efforts of scholars worldwide. Smaller wind turbines offer unparalleled advantages over larger ones, including longer lifespan, lower size and weight, and easier installation, making them suitable for a wider range of applications. However, smaller wind turbines also have their drawbacks: they require a larger footprint for the same power output, and their wind energy utilization coefficient is lower than that of larger turbines. Therefore, addressing the shortcomings of smaller and medium-sized wind turbines is a major research direction in the future of wind power generation and deserves close attention. Considering the various problems currently facing smaller wind turbines, many scholars have focused on multi-rotor wind turbines. Multi-rotor wind turbines can not only solve the problems of low power generation and large footprint of smaller wind turbines, but also address the difficulties of transportation, installation, and maintenance of larger wind turbines. Furthermore, multi-rotor wind turbines offer better dynamic stability compared to single-rotor wind turbines. The advantages of multi-rotor wind turbines are obvious, but they also have specific disadvantages. The tower structure of multi-rotor wind turbines is more complex, and due to severe aerodynamic interference, the requirements for blade strength and stiffness are also higher. Secondly, the yaw system of multi-rotor wind power generation systems is also more complex. Further research is needed in these areas.

[0003] Domestic scholars have conducted limited research on multi-rotor wind turbines. For example, Tianjin University used a 2kW horizontal axis wind turbine as the unit model for a multi-rotor horizontal axis wind power generation system. Based on the wind alignment accuracy at the cut-in wind speed, the tail rudder was designed, and the impact of wind shear on power generation was analyzed. The results show that the power generation of a multi-rotor wind power generation system using tail rudder yaw is higher than that of a single-rotor wind turbine using active yaw. Furthermore, a tower for a 6kW three-rotor wind power generation system was designed, and its mechanical performance was studied through numerical simulation. The results show that the longer the rotor radius, the greater the wind shear coefficient, and that a multi-rotor wind power system can effectively reduce the impact of wind shear. Existing research on multi-rotor wind turbines mainly focuses on uniform inflow, with almost no research on multi-rotor wind turbines with turbulent inflow. Previous studies have primarily involved numerical simulations, lacking sufficient wind tunnel test data for verification. Summary of the Invention

[0004] The purpose of this invention is to overcome the defects of the prior art and provide a design method for a three-rotor wind turbine. This method optimizes the aerodynamic characteristics of the multi-rotor wind turbine from multiple perspectives, thereby selecting a wind turbine with appropriate power suitable for local wind resource conditions, maximizing the utilization of wind resources, optimizing costs, and obtaining the greatest economic benefits.

[0005] The objective of this invention is achieved as follows: a design method for a three-rotor wind turbine, comprising the following steps:

[0006] (1) A three-rotor wind turbine test bench was built in the wind tunnel test section. The single-rotor wind turbine used in the test was fixed and placed in the wind tunnel test section. A grid was placed at the entrance of the wind tunnel test section to modulate the grid turbulence field to simulate the real wind conditions.

[0007] (2) Set the sampling frequency and sampling time of the data acquisition device; start the wind tunnel, adjust the control cabinet frequency to obtain the set wind speed, adjust the wind turbine speed, obtain the optimal tip speed ratio, and collect data;

[0008] (3) Close the wind tunnel, fix the three-rotor wind turbine used in the test and place it in the wind tunnel test section. Repeat step (2) to test the three-rotor wind turbines with different spacing. After completion, close the wind tunnel.

[0009] (4) Data processing: Estimating the total cost of single-rotor and three-rotor wind turbines based on experimental data;

[0010] (5) Calculate the annual power generation of single-rotor wind turbines and three-rotor wind turbines;

[0011] (6) The design effect of the three-wheel wind turbine is evaluated by comparing and calculating the cost per kilowatt-hour.

[0012] As a further limitation of the present invention, the three-rotor wind turbine in step (1) adopts the DTU-LN221 airfoil as the wind turbine design airfoil, and the three-rotor wind turbine tower is arranged in a triangular arrangement structure; the distance between each wind turbine in the three-rotor wind turbine unit is s, and the total swept area of ​​the three wind turbines is consistent with the swept area of ​​a single wind turbine.

[0013] As a further limitation of the present invention, the data acquisition in step (2) specifically includes: wind tunnel measurement of the axial thrust of the wind turbine rotor, the wake velocity field and the output shaft power of the single wind turbine rotor and the three wind turbine rotor. The axial thrust is obtained by measuring a six-component balance placed under the tower. The balance is fixed to the bottom of the wind tunnel by bolts and tape. In the wake measurement, for the single wind turbine rotor, the measuring points are arranged in the horizontal direction of the hub height. For the three wind turbine rotor, the wake of the upper single wind turbine rotor and the two lower wind turbine rotors are measured in the horizontal direction. Wake monitoring points are arranged at the positions of 1, 2, 3, 5 and 8 times the diameter of the single wind turbine rotor at the axial distance of the wind tunnel.

[0014] To minimize tower costs, tower mass must be minimized. Therefore, the weight borne by each section of the tower in the three-rotor wind turbine is calculated separately. The tower is divided into three sections according to the different weights borne. The weight of the wind turbine borne by each tower is equivalent to the radius in the empirical formula based on the swept area. The total cost calculation of the tower of the three-rotor wind turbine can be simplified to three sections of tower. The total cost mentioned in step (4) includes blade cost and total tower cost.

[0015] Blade cost C blade The empirical formula is as follows:

[0016]

[0017] Tower cost C tower The empirical formula is as follows:

[0018] C tower =1.50(0.3973πR) 2 H-1414) (2)

[0019] Where R is the radius of the wind turbine and H is the height of the hub;

[0020] The total cost of the tower for a single-rotor wind turbine is obtained directly using empirical formula (2); the weight of the wind turbine borne by the three-rotor tower is equivalent to the radius R in empirical formula (2) based on the swept area, and the equivalent wind turbine radius R corresponding to each of the three sections a, b, and c of the tower is respectively. a R b R c for:

[0021]

[0022] The total cost of a three-rotor wind turbine tower is the sum of the costs of the three tower sections.

[0023] As a further limitation of the present invention, the annual power generation calculation for single-rotor wind turbines and three-rotor wind turbines in step (5) is calculated using the following formula:

[0024]

[0025] Among them, v cut-in To cut off the wind speed, v cut-out To determine the wind speed, p(v) represents the probability distribution of wind speed v. m (v) represents the output power at wind speed v;

[0026] The wind speed distribution probability p(v) is described by the Weibull distribution function, and its expression is:

[0027]

[0028] Where k is the shape factor and c is the size distribution factor; c is determined by the statistical average of the wind speed distribution.

[0029] Output power P m (v) The expression is:

[0030]

[0031] Where ρ is the air density, A is the swept area of ​​the wind turbine, and C P denoted as the wind turbine power coefficient, and v as the wind speed.

[0032] Wind energy utilization coefficient C P The relationship between the tip speed ratio λ and the blade pitch angle β is as follows:

[0033]

[0034]

[0035] Among them, c1-c6 are parameters characterizing the properties of the wind turbine.

[0036] As a further limitation of the present invention, the cost of electricity (COE) in step (6) is defined as the ratio of the total cost of the wind turbine to the annual power generation:

[0037]

[0038] Where C is the total cost of the wind turbine, and P AE This represents the annual power generation of the wind turbine.

[0039] Compared with existing technologies, the present invention employs the above technical solutions, and its beneficial effects are as follows: The present invention uses wind tunnel experiments to compare and improve the power characteristics and wake velocity field of single-rotor and three-rotor wind turbines. Wind tunnel experiments are used to verify the accuracy of data, and to evaluate and predict the dynamic stall of wind turbine airfoils. Compared with data obtained through numerical calculations and theoretical analysis, the experimental method is more direct and accurate. Furthermore, the constants obtained through experiments can more accurately help determine subsequent empirical constants. The use of wind tunnel experiments to study multi-rotor wind turbines and single-rotor wind turbines of equal area, measuring the turbine thrust, output power, and wake velocity field, makes the experiments more intuitive and accurate.

[0040] By adding a grid, the test results are identical to those of megawatt-class wind turbines; moreover, the manufacturing process for wind turbines of the same diameter is the same, and the cost difference is not significant. Based on this assumption and referring to the cost of wind turbines of the same diameter, the costs of multi-rotor wind turbines and equivalent-area single-rotor wind turbines are estimated.

[0041] This invention optimizes existing single-rotor wind turbines by designing a three-rotor wind turbine to address the problems of low power generation and large footprint of small wind turbines, as well as the difficulties in transportation, installation, and maintenance of large wind turbines. This is because as blades become larger, higher demands are placed on blade design theory and manufacturing processes, energy loss becomes more severe, and high-speed transmission components are more prone to failure. The multi-rotor design can replace large-capacity wind turbine units, which has significant engineering implications for the study of wind turbine aerodynamic characteristics. Attached Figure Description

[0042] Figure 1 A schematic diagram of the overall process of this invention.

[0043] Figure 2 A schematic diagram of the design of the three-rotor wind turbine of this invention.

[0044] Figure 3 A schematic diagram of the grid arrangement in this invention.

[0045] Figure 4 A schematic diagram of wake measurement in this invention.

[0046] Figure 5 The wake velocity profile in this invention.

[0047] Figure 6 A schematic diagram of the load-bearing capacity of each component of the three-wind turbine tower in this invention.

[0048] Figure 7 A schematic diagram of the coordinates of the three wind turbines in the cost model calculation of this invention. Detailed Implementation

[0049] like Figure 1 The method for designing a three-rotor wind turbine, as shown, includes the following steps:

[0050] Step 1) Construct a three-rotor wind turbine test bench in the wind tunnel test section. First, fix the single-rotor wind turbine used in the test and place it in the wind tunnel test section. Place a grid at the entrance of the wind tunnel test section and modulate the grid turbulence field to simulate real wind conditions. Both the single-rotor and three-rotor wind turbines use the DTU-LN221 airfoil as the airfoil design for the wind turbine. The three-rotor wind turbine tower is arranged in a triangular structure. The distance between each rotor of the three-rotor wind turbine unit is s, and the total swept area of ​​the three rotors is the same as that of the single rotor.

[0051] In this embodiment of the invention, the test section dimensions for the wind tunnel experiment are 3m × 1.5m × 3m, and the low-speed test section dimensions are 3m × 3m × 7m. The maximum wind speed designed for the wind tunnel is 50m / s for the high-speed section. The single-rotor wind turbine used has a diameter D = 0.4m, while the three-rotor wind turbine used has a diameter d = 0.231m, ensuring consistent swept area of ​​the rotors. Specific design schematics of the single-rotor and three-rotor wind turbines are shown below. Figure 2 As shown, the distance between each rotor of the three-rotor wind turbine is s, and the value of s is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9.

[0052] The turbulence generator used in the experiment, such as Figure 3 As shown in the figure, the black dots represent calibration points for measuring the turbulence intensity of the grid. Experiments show that when the model wind turbine is placed 1.6m behind the grid, the turbulence intensity is 10.5%. This grid turbulence device is constructed using aluminum alloy grid strips and bolts; the aluminum alloy grid strips are 3cm wide and thick. For connection, rubber pads are installed between the top of the grid frame and the wind tunnel wall, and six adjustable anchor bolts are installed at the bottom. Tightening the anchor bolts secures the grid turbulence device, providing high flexibility and facilitating installation and disassembly.

[0053] Step 2) Set the sampling frequency and sampling time of the data acquisition unit; start the wind tunnel, adjust the control cabinet frequency to obtain the set wind speed of 10m / s, adjust the wind turbine speed to obtain the optimal tip speed ratio, and collect data.

[0054] Wind tunnel measurements were performed on single-rotor and three-rotor wind turbines to measure the axial thrust of the rotors, the wake velocity field, and the output shaft power. Axial thrust was measured using a six-component balance placed beneath the tower, secured to the bottom of the wind tunnel with bolts and tape. In this implementation, the data acquisition unit was an NI-USB6210, and the six-component balance used was an ATI Gamma six-axis force / torque sensor with a sampling frequency of 1kHz and a sampling time of 10s. For wake measurements, for single-rotor turbines, measurement points were arranged horizontally along the hub height. For three-rotor wind turbines, two sets of wakes were measured horizontally from the upper single rotor and the two lower rotors, with wake monitoring points placed at axial distances of 1, 2, 3, 5, and 8 times the diameter of a single rotor. Figure 4 As shown; wake measurements were collected using a hot-wire anemometer at a sampling frequency of 5 kHz and a sampling time of 20 s; for power measurement, an indirect shaft power measurement method was used, specifically:

[0055] The first step involves using two selected motors, one of which is connected to a 12V DC power supply to simulate wind turbine rotation and provide stable power. The other motor acts as a generator, outputting voltage and current. Both motors are connected to the two ends of a selected dynamic torque sensor, and an electronic load is connected to the generator's lead wire. Next, the power source motor voltage is sequentially set from 3V to 12V, and the resistance of the electronic load is adjusted at each given voltage (3.5Ω, 5Ω, 10Ω, 15Ω, 50Ω, 100Ω). The output current, torque, and speed of the generator under 10 given power conditions and different loads are measured.

[0056] The second step involves interpolating the 10 sets of measured data and plotting the curves relating the output current, torque, and speed. Then, the 10 curves are interpolated again to plot the three-dimensional surface relationship between the generator's output current, torque, and speed. The corresponding torque expression is shown in formula (A).

[0057] T = a0 + a1I + a2N + a3I 2 +a4IN+a5N 2 (A)

[0058] Where a0 = 0.00443, a1 = 0.01418, and a2 = 3.968 × 10 -6 a3 = 6.253 × 10 -5 a4 = -4.881 × 10 -8 a5 = -63289 × 10 -10 .

[0059] The third step involves connecting the wind turbine to a DC motor via a coupling. Two leads from the motor are connected to an NI data acquisition unit to collect the output voltage generated by the motor. A 0.05Ω sampling resistor and an adjustable potentiometer (0Ω-100Ω) are connected in series in the circuit. The potentiometer adjusts the load resistance to obtain different wind turbine rotation speeds under the same wind speed conditions, thus yielding the wind turbine power at different tip speed ratios (λ). The tip speed ratio λ is defined as follows:

[0060]

[0061] Where R is the radius of the wind turbine and ω is the angular velocity of the wind turbine.

[0062] Step 3) Close the wind tunnel, fix the three-rotor wind turbine used in the test and place it in the wind tunnel test section. Repeat step 2) to test the three-rotor wind turbines with different spacing. After completion, close the wind tunnel.

[0063] In the embodiments of the present invention, some experimental results are as follows: Figure 5 As shown, Figure 5 The figure compares the wake velocity deficit of a three-rotor wind turbine and a single-rotor wind turbine with the same area under uniform flow and turbulence intensity of 10.5%. It shows that the velocity deficit of the three-rotor wind turbine wake under turbulent conditions is significantly lower than that under uniform flow, and the difference is even greater than that of the single-rotor turbine. This indicates that turbulence has a greater effect on the wake velocity recovery of the three-rotor wind turbine than on the single-rotor wind turbine. This phenomenon may be explained by the more intense collision of wake vortices under turbulent conditions in the three-rotor wind turbine compared to the single-rotor wind turbine.

[0064] Step 4) Data processing: Estimate the total cost of single-rotor and three-rotor wind turbines using experimental data. The three-rotor wind turbine is designed to consist of three scaled-down versions of the rotors of a real wind turbine. Assuming that the real wind turbine and the experimental wind turbine have similar aerodynamic trends, and that the increased output power remains effective for megawatt-class wind turbines, the three-rotor wind turbine is based on the same design conditions as the single-rotor wind turbine, resulting in the following design conditions:

[0065] I) Power coefficient (0.468);

[0066] II) Mechanical efficiency (100%);

[0067] III) Electrical efficiency (94.4%);

[0068] IV) Rated wind speed (11.4 m / s).

[0069] Total cost includes blade cost and tower cost;

[0070] Blade cost C blade The empirical formula is as follows:

[0071]

[0072] Tower cost C tower The empirical formula is as follows:

[0073] C tower =1.50(0.3973πR) 2 H-1414) (2)

[0074] Where R is the radius of the wind turbine and H is the height of the hub;

[0075] Meanwhile, the main costs of wind farm investment include: infrastructure construction, transportation, civil engineering, installation design, and the costs of transformers and lines. In this regard, the initial investment for single-rotor and multi-rotor wind turbines can be considered the same and will not be compared. In this invention, the three rotors of the three-rotor wind turbine are arranged in an equilateral triangle, with s / d = 0.4 as the calculation benchmark and a side length of 80.025 meters. Figure 6 Schematic diagram of the load-bearing capacity of each component of the three-windmill tower. Figure 7 The center coordinates of each wind turbine are shown. The weight of the wind turbine borne by the three wind turbine towers is equivalent to the radius R in the empirical formula (2) based on the swept area. The equivalent wind turbine radii R of the three segments a, b, and c of the tower are respectively. a R b R c for:

[0076]

[0077] The total cost of the tower for a three-rotor wind turbine is the sum of the costs of the three tower sections; the total cost of the tower for a single-rotor wind turbine is obtained directly using the empirical formula (2).

[0078] The costs of each component of the single-rotor wind turbine and the three-rotor wind turbine can be seen in Table 1.

[0079] Table 1

[0080]

[0081] Step 5) Calculate the annual power generation of single-rotor and three-rotor wind turbines;

[0082] The formula for calculating annual power generation is:

[0083]

[0084] Among them, v cut-in To cut off the wind speed, v cut-out To determine the wind speed, p(v) represents the probability distribution of wind speed v. m (v) represents the output power at wind speed v;

[0085] The wind speed distribution probability p(v) is described by the Weibull distribution function, and its expression is:

[0086]

[0087] Where k is the shape factor and c is the size distribution factor; usually the shape factor k is 2, and c is determined by the statistical average value of the wind speed distribution.

[0088] Wind speed varies with altitude in a power-law manner, and the formula for the wind shear function is:

[0089]

[0090] in, α represents the wind speed at heights h1 and h2, respectively, and α is the wind shear coefficient.

[0091] The mechanical power that a wind turbine extracts from the wind, i.e., its output power P. m (v) The expression is:

[0092]

[0093] Where ρ is the air density, A is the swept area of ​​the wind turbine, and C P denoted as the wind turbine power coefficient, and v as the wind speed.

[0094] Wind energy utilization coefficient C P The relationship between the tip speed ratio λ and the blade pitch angle β is as follows:

[0095]

[0096]

[0097] Among them, c1-c6 are parameters characterizing the properties of the wind turbine.

[0098] Step 6) Compare and evaluate the design effectiveness of the three-rotor wind turbine by calculating the cost per kilowatt-hour;

[0099] The cost of electricity (COE) is defined as the ratio of the total cost of a wind turbine to its annual power generation.

[0100]

[0101] Where C is the total cost of the wind turbine, and P AE This represents the annual power generation of the wind turbine.

[0102] To calculate the annual power generation of a single-rotor wind turbine, we assume that the wind speed follows a Rayleigh distribution, the wind shear coefficient is 1 / 7, and the air density is 1.225 kg / m³. 3 The levelized cost of electricity (LCOE) for a single-rotor wind turbine generator set is shown in Table 2.

[0103] Table 2. Cost per kilowatt-hour of a single-rotor wind turbine generator set

[0104]

[0105] Based on the previous assumptions, the increased output power of the three-rotor wind turbine remains effective for megawatt-class wind turbines. The output power of the three-rotor wind turbine is 4.56% higher than that of the single-rotor wind turbine, while its total cost is 95% of that of the single-rotor wind turbine. Therefore, the levelized cost of electricity (LCOE) of the three-rotor wind turbine is 91% of that of the single-rotor wind turbine, resulting in an LCOE of 0.2793 yuan / kWh.

[0106] This invention provides a design method for a three-rotor wind turbine, optimizing the aerodynamic characteristics of the multi-rotor wind turbine from multiple perspectives. Compared to a single-rotor wind turbine, the multi-rotor wind turbine exhibits increased output power and reduced total load. The advantages of the multi-rotor wind turbine extend beyond output power; it also offers advantages in wake characteristics. Compared to an equivalent single-rotor wind turbine, the multi-rotor wind turbine, with its rotor circumference to swept area ratio being twice that of a single-rotor wind turbine, exhibits faster wake recovery and lower wake turbulent kinetic energy. Compared to an equivalent single-rotor wind turbine with the same total swept area, power, and thrust, the multi-rotor structure demonstrates faster wake recovery.

[0107] This invention is not limited to the above embodiments. Based on the technical solutions disclosed in this invention, those skilled in the art can make some substitutions and modifications to some of the technical features without creative effort, and all such substitutions and modifications are within the protection scope of this invention.

Claims

1. A design method for a wind turbine with three rotors, characterized in that, Includes the following steps: (1) A three-rotor wind turbine test bench was built in the wind tunnel test section. The single-rotor wind turbine used in the test was fixed and placed in the wind tunnel test section. A grid was placed at the entrance of the wind tunnel test section to modulate the grid turbulence field to simulate the real wind conditions. (2) Set the sampling frequency and sampling time of the data acquisition device; start the wind tunnel, adjust the control cabinet frequency to obtain the set wind speed, adjust the wind turbine speed to obtain the optimal tip speed ratio, and collect data; (3) Close the wind tunnel, fix the three-rotor wind turbine used in the test and place it in the wind tunnel test section. Repeat step (2) to test the three-rotor wind turbines with different spacing. After completion, close the wind tunnel. (4) Data processing: using experimental data to estimate the total cost of single-rotor and three-rotor wind turbines; The total cost mentioned in step (4) includes the cost of the blades and the total cost of the tower; Blade cost The empirical formula is as follows: ； Tower cost The empirical formula is as follows: ； Where R is the radius of the wind turbine and H is the height of the hub; The total cost of the tower for a single-rotor wind turbine is obtained directly using empirical formula (2); the weight of the wind turbine borne by the three-rotor tower is equivalent to the radius R in empirical formula (2) based on the swept area, and the equivalent wind turbine radii corresponding to the three segments a, b, and c of the tower are respectively. , , for: ； The total cost of a three-rotor wind turbine tower is the sum of the costs of the three tower sections. (5) Calculate the annual power generation of single-rotor wind turbines and three-rotor wind turbines; The annual power generation calculation for single-rotor and three-rotor wind turbines described in step (5) is as follows: ； in, To cut into wind speed, To determine the wind speed, p(v) represents the probability distribution of wind speed v. m (v) represents the output power at wind speed v; The wind speed distribution probability p(v) is described by the Weibull distribution function, and its expression is: ； Where k is the shape factor and c is the size distribution factor; c is determined by the statistical average of the wind speed distribution. Output power P m (v) The expression is: ； in, Where A is the air density and A is the swept area of ​​the wind turbine. Where v is the wind turbine power coefficient and v is the wind speed; Wind energy utilization coefficient Speed ​​ratio with leaf tip and pitch angle The relationship is: ； ； in, - Parameters characterizing the properties of a wind turbine; (6) The design effect of the three-wheel wind turbine is evaluated by comparing and calculating the cost per kilowatt-hour.

2. The wind turbine design method for a three-rotor system according to claim 1, characterized in that, The three-rotor wind turbine in step (1) adopts the DTU-LN221 airfoil as the wind turbine design airfoil, and the three-rotor wind turbine tower is arranged in a triangular arrangement structure; the distance between each wind turbine in the three-rotor wind turbine unit is s, and the total swept area of ​​the three wind turbines is consistent with the swept area of ​​a single wind turbine.

3. The wind turbine design method for a three-rotor system according to claim 1, characterized in that, The data acquisition in step (2) specifically includes: wind tunnel measurement of the axial thrust of the wind turbine rotor, the wake velocity field and the output shaft power of the single-rotor and three-rotor wind turbines. The axial thrust is obtained by measuring the six-component balance placed under the tower. The balance is fixed to the bottom of the wind tunnel by bolts and tape. In the wake measurement, for the single-rotor wind turbine, the measuring points are arranged in the horizontal direction of the hub height. For the three-rotor wind turbine, the wake of the upper single rotor and the two lower rotors is measured in the horizontal direction. Wake monitoring points are arranged at the axial distance of 1, 2, 3, 5 and 8 times the diameter of the single rotor in the wind tunnel.

4. The wind turbine design method for a three-rotor system according to claim 1, characterized in that, The cost of electricity (COE) mentioned in step (6) is defined as the ratio of the total cost of the wind turbine to its annual power generation: ； Where C represents the total cost of the wind turbine unit. This represents the annual power generation of the wind turbine.

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