loads investigations of HAWT with wind tunnel tests and site measurements Shigeto HIRAI, Senior Researcher, Nagasaki R&D Center, Technical Headquarters, MITSUBISHI HEAVY INDSUTRIES, LTD, Fukahori, Nagasaki, 8510392 JAPAN Phone: +81-95-834-2820/Fax: +81-95-834-2385/E-mail: shigeto_hirai@mhi.co.jp Akihiro HONDA, Research Manager, Nagasaki R&D Center, Technical Headquarters, MITSUBISHI HEAVY INDSUTRIES, LTD, Fukahori, Nagasaki, 8510392 JAPAN Kai KARIKOMI, Researcher, Nagasaki R&D Center, Technical Headquarters, MITSUBISHI HEAVY INDSUTRIES, LTD, Fukahori, Nagasaki, 8510392 JAPAN Abstract loads acting on HAWT (Horizontal Axis Turbine) are quite different from those of other structures. At first, wind loads largely depend on the natural wind on the site, which is accompanied with random fluctuations and probabilistic phenomena. Secondly, even in the same wind condition, the wind loads also depend on the configuration of HAWT against wind and on its operational status. Especially in Japan, the former factor may play bigger role because of the increase of fluctuation by more complicated surrounding terrain and higher extreme wind speed caused by typhoons, which may be somewhat common in East Asia. Therefore, it is important to understand how wind loads are action on HAWT with various conditions for more sophisticated load design. In this study, wind tunnel experiments have been conducted using large scale model of HAWT with 2 meter diameter of model rotor. The wind loads were measured with strain gauges attached on the model. The basic tendency of wind loads are shown according with basic parameters, like azimuth angle, wind direction, blade configuration and so on. The results are also compared with site measurements which have been obtained in prototype. In the data obtained at the site, large scattering may be observed which is affected by the fluctuations of natural wind. However, common characteristics can be found between wind tunnel tests and site measurements. The obtained data are being used to publish new guideline for introducing HAWT in Japan and it would reduce the risk of mismatch between the site condition and the introduced turbines. Key Words energy, tunnel experiment, load, HAWT 1. Introduction loads are the most important factor for the design of HAWT (Horizontal Axis Turbine) as well as other big structures, like high buildings, long bridges and so on. However, those of HAWT have quite different characteristics when compared with those of other structures. At first, wind loads largely depend on the natural wind on the site, which is accompanied with random fluctuations and probabilistic phenomena. Since most of the wind loads are acting on the rotor and it is a part which keeps constantly rotating, fatigue loads play more important roles in the design loads of HAWT and they are sensitive to turbulence of natural wind. For other structures, it is common to focus mainly on extreme loads. Furthermore, HAWT is often located where mean wind speed is expected to be higher, like on top of hill or ridge. This makes the extreme wind speed for design also higher. Secondly, even in the same wind condition, the wind loads also depend on the configuration of HAWT against wind and on its operational status. When the wind speed goes up, HAWT stops power production and changes the configuration of its rotor to some resting position in order to reduce the increase of wind loads. Even in lower speed during HAWT is producing energy, the sudden change of wind may cause the increase of wind loads and HAWT starts to change the pitch angle of blades or the yaw angle of rotor to avoid the increase of wind loads. These are quite different from other structures with fixed shape, which means constant wind force coefficient. Especially in Japan, the former factor may play bigger role because of the increase of fluctuation by more complicated surrounding terrain and higher extreme wind speed caused by typhoons, which may
be somewhat common in East Asia. Therefore, it is important to understand how wind loads are acting on HAWT with various conditions for more sophisticated load design. In this study, wind tunnel experiments have been conducted using large scale model of HAWT and wind loads were measured. Many researches have been reported with various ideas for modeling wind turbines in wind tunnel, sometimes in a very big scale 1). Simulating the wind load in prototype is more focused in this study. The basic tendency of wind loads are shown according with basic parameters. The results are also compared with site measurements which have been obtained in prototype. 2. Tunnel Experiments tunnel experiments have been conducted to understand the basic aerodynamic characteristics of HAWT. In this study, two factors which may govern wind loads have been focused. One is unsteadiness of the flow and the other is the various configuration of HAWT. The size of the model was intended to be set as comparatively large with 2 meter diameter of model rotor, though Reynolds Number between the model and the prototype is still different for more than one order. The prototype was set to 1 MW class HAWT and the model scale was around 1/30. The model rotor was designed to have similar shape as the prototype. The nacelle and the tower was also modeled with some deformation to house sensors and electric motor to control the rotor. The model properties are shown in Table 1 and Figure 1. The rotational speed was adjusted with electric motor. The pitch angle of blades and the yaw angle of the rotor was changed manually. The rotor torque and horizontal force was measured. The bending moment at blade root was also measured. Table 1 Model Properties Item Prototype Model Scale 1 1 / 30.7 Rotor Daimeter 61.4 m 2.0 m Hub Height 68 m 2.2 m Rotational (Normal Operation) 19.8 rpm Adjustable Rotor Hub Blade Blade root bending moment sensor Rotor torque sensor Horizontal force sensor Tower Fig. 1 Model Setup The above model was placed in wind tunnel with cross section area of 30 square meters. This resulted in blockage ratio of around 10% when rotor swept area is used as index. To simulate the unsteadiness of natural wind, two techniques were used. One is boundary layer flow similar to the natural wind, which includes the vertical profile of mean wind speed and the turbulence. The other is artificial change of mean wind speed or wind direction generated by the movable cascades. These
changing flow conditions were intended to qualitatively simulate the gust described in IEC 61400-1 standard 2). To simulate the different operating conditions and configuration, the experimental conditions were changed as in Table 2. Table 2 Typical Experimental Conditions Simulated Operating Rotational Condition *1 Tip *1 Tip Reynolds *1 Ratio Number *2 Power Production Rated 600rpm 62.8m/s 9m/s 7.0 2.4*10 5 Power Production 220rpm 23.0m/s 9m/s 2.6 0.9*10 5 Idling (Fine Pitch) Extreme Slow (0m/s) 9m/s - 0.3*10 5 Idling (Feather Pitch) Extreme Slow (0m/s) 9m/s - 0.3*10 5 *1:Standard value for experiment *2:Based on average chord length of blade as typical length and relative wind speed at blade tip 3. Experiment Results 3.1. Model Rotor Properties To check the aerodynamic properties of model rotor, the power coefficient and thrust force coefficient was measured in smooth flow. The maximum power coefficient was around 0.3 which is little lower than the prototype. The main difference is expected to be from the low value of Reynolds Number and the lift and drag properties are worsened. The effect of yaw angle of rotor and tip speed ratio is shown in Figure 2. The tip speed ratio has remarkable effect especially for power coefficient and gives maximum value near that of the prototype. The effect of yaw angle is also clearer for power coefficient. C P [-] 0.45 0.40 0.35 0.30 0.25 0.15 0.10 0.05 0 2 4 6 8 10 12 14 16 Tip 周速比 [-] Ratio (a) Power Coefficient Cp 0deg -15deg +15deg Yaw Angle C Fx [-] 1.20 1.00 0.80 0.60 0deg 0.40-15deg +15deg Yaw Angle 0 2 4 6 8 10 12 14 16 Tip 周速比 [-] Ratio (b) Thrust Force Coefficient CFx Fig. 2 Model Rotor Properties (Smooth Flow) 3.2. in Turbulent Flow Figure 3 shows the time series of wind loads measured in boundary layer turbulent flow. The turbine is supposed to be in power production mode at rated wind speed. Each component of wind load shows random fluctuation, which is mainly due to the turbulence of wind. In natural wind in the atmosphere, the variation of wind load acting on HAWT is expected to be quite dependent on the turbulence. Therefore it becomes important to estimate the wind properties where the HAWT is located. 3.3. Effect of Rotor Configuration When the wind speed increases, HAWT stops power production and changes the configuration of its rotor to reduce the high wind loads. There are mainly two types in the view of pitch angle of blades.
Pitch-regulated turbine moves the blades into feather pitch, where the blade plane is almost perpendicular to the rotor plane. The rotor plane is kept perpendicular to the wind direction, though in some cases the rotor is changed to face downwind. In stall-regulated turbines, the blade pitch angle is fixed to fine pitch, where the blade plane is almost parallel to the rotor plane. The rotor plane is changed to be parallel to the wind direction. Figure 4 shows the wind loads in various rotor configurations when the turbine is idling at high wind speed. The wind loads show big change depending on both pitch angle and wind direction offset. In the view of favorable configuration to reduce the wind loads, it is clear that each type of turbine is to operate in order to minimize the wind load by above-mentioned configuration change. C P(-) 0.60 0.50 0.40 0.30 0.10 (a) Power Coefficient C Mx(-) 0.02 0.01-0.01-0.02-0.03-0.04-0.05 (b) Blade Root Moment (Edge Direction) C My(-) 0.25 0.15 0.10 0.05 (c) Blade Root Moment (Flap Direction) C Fx(-) 1.2 1 0.8 0.6 0.4 0.2 0 (d) Thrust Force Fig. 3 Time Series of Load (Power Production Mode at Rated, Boundary Layer Turbulent Flow) 0.08 Thrust Force C Fx 0.06 ファイン Fine 0.04 フェザー Feather 0.02-0.02-0.04-0.06-0.08 0 60 120 180 240 300 360 Direction 風向偏差 Offset (deg) (deg.) Direction Offset θ Side Force Fy 0.06 0.04 Thrust Force Fx Side Force C Fy 0.02-0.02-0.04 ファイン Fine フェザー Feather Top View -0.06 0 60 120 180 240 300 360 Direction 風向偏差 Offset (deg) (deg.) Fig. 4 Load Depending on Rotor Configuration (Idling at High, Boundary Layer Turbulent Flow)
From these data, the wind loads can be summarized as in Table 3. Table 3 Depending on Turbine Configuration and Type Type During Power Production Idling in High Fine pitch Feather Pitch Rotor plane to face upwind Rotor plane to face upwind (otherwise Force coefficient to vary and to decrease downwind) above rated wind speed loads to increase by the power of loads to have maximu around two rated wins speed Possibility of loss of yaw control Pitch- Regulated Turbine Rated Stall- Regulated Turbine Fine Pitch Rotor plane to face upwind Larger force coefficient above rated wind speed loads to show some increease abovbe rated wind speed and to become maximum at cut-out wind speed Rated Fine Pitch Rotor plane to be parallel to wind axis loads to increase by the power of two Possibility of loss of yaw control Note: The above description shows general properties and may differ for each make. 4. Comparison with Site Measurements In several HAWT site in Japan, site measurements were carried out to investigate the wind load 3). The obtained data includes tower base moment. To make comparison this data with the data from wind tunnel test, the tower base moment was calculated from the thrust force. Figure 5 shows the comparison between them. The target turbine in site measurements is pitchregulated type. The data from site measurements shows typical dependency on wind speed shown in Table 3. Large scattering may be observed which is affected by the fluctuations of natural wind for fluctuating components. However, common characteristics can be found between wind tunnel tests and site measurements, especially for time-averaged values. The data from wind tunnel test agrees well with them. As for wind loads along the wind axis governed by rotor thrust force, the characteristics can be simulated by wind tunnel test, though careful attention may be necessary for the similarity of the model and experimental conditions.
Normalized Tower Base Moment タワー基部の合成モーメント (Along wind) / 定格風速の平均モーメント 2.5 2.0 1.5 1.0 0.5 0.0 実測平均 Site 実験平均 Tunnel 0 10 20 30 40 Mean ナセル平均風速 (m/s) (m/s) Fig.5 Comparison of wind loads between site measurements and wind tunnel test (Pitch-regulated, Both power production and idling) 5. Conclusion loads acting on HAWT were investigated by wind tunnel test. The data can explain the various changes of wind loads depending on wind speed, turbine type, operational mode and so on. These are more complicated than other big structures. Comparison between wind tunnel test and site measurements shows good agreement at least for thrust component. tunnel test can be useful to understand the phenomenon concerning with such wind loads, though special attention should be necessary for the model and experimental conditions. The obtained data are being used to publish new guideline 4) for introducing HAWT in Japan and it would reduce the risk of mismatch between the site condition and the introduced turbines. This study was conducted in part of Japanese project funded by The New Energy and Industrial Technology Development Organization (NEDO) and the authors would like to thank the committee members, NEDO, Toyo Sekkei and The Central Research Institute of Electric Power Industry (CRIEPI). 6. References 1) http://wind.nrel.gov/amestest/ 2) IEC 61400-1, turbines Part 1: Design requirements, Edition 3.0, 2005. 3) A Honda et al., Site Measurement, Tunnel Test and Numerical Simulation of the aerodynamic load acting on wind turbine generator (in Japanese), Journal of Japan Association for Engineering, Vol.33, No.2, 2008. 4) The New Energy and Industrial Technology Development Organization (NEDO), Guideline for Turbines in Japan (Part for Typhoons and Turbulence) (in Japanese), 2008 (to be published).