Monitoring and Damage Detection in Structural Parts of Wind Turbines 23 limitation lies in the fact that future research will be strongly connected to full scale tests and that such tests cannot be carried out by research organisations alone. Considered as a whole, these results suggest that there are monitoring methods that can be applied to wind turbines and thereby contribute to making the process of harvesting renewable energy more reliable. This research will serve as a base for future studies on the topic of combining the different classes of methods to develop a monitoring system that is able to predict the residual life-time of structures. Such systems are of great use for off-shore wind turbines, because they are the basis of a maintenance on demand. 6. Acknowledgements Parts of the present and still ongoing research in this field as well as the publication of this chapter are funded by the German federal state of Hesse (project „LOEWE-Zentrum AdRIA: Adaptronik Research, Innovation, Application“, grant number IIIL4-518/14.004 (2008)) as well as in the framework of the programme „Hessen ModellProjekte“ (HA-Projekt-Nr.: 214/09-44). This financial support is gratefully acknowledged. 7. References Andersen, P. (1997). Identification of Civil Engineering Structures using Vector ARMA models, PhD-Thesis, University of Aalborg, Aalborg. Andersen, P. & Brincker, R. (2000). The Stochastic Subspace Identification Techniques, www.svibs.com. Andersen, P., Brincker, R., Goursat, M. & Mevel, L. (2007). Automated Modal Parameter Estimation For Operational Modal Analysis of Large Systems, Proceedings of the 2nd International Operational Modal Analysis Conference, ed. Brincker R. & Møller N., Copenhagen, pp. 299–308. Asmussen, J. C. (1998). 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The wind power has been harnessed by mankind for a long time and the associated technology is more advanced than other clean energies. Nowadays wind power increasingly attracts interests and its utilization has entered a rapid development stage. There are two types of wind turbines, namely horizontal-axis wind turbine (HAWT) and vertical-axis wind turbine (VAWT). The latter has many advantages, such as low cost, simple-structured blades, convenient installation and maintenance, and the ability to utilize wind from all directions without the need of a steering mechanism. A special VAWT implemented with magnetic suspension and self-pitch blade design will be introduced in this chapter. It not only has the advantages of traditional VAWT, but also the advantages of low threshold starting wind-speed, high wind power efficiency, etc. Discussion in this chapter includes the benefits of applying magnetic bearing technology to the wind turbine. Specifically, the entire wind turbine rotor weight can be supported by magnetic bearings. The friction of the bearings is essentially non-existence. There is no need for bearing lubrication, and the maintenance cost can be reduced. Furthermore, the magnetic suspension technology can eliminate mechanical vibration and reduce noise. Since low friction also reduces starting torque, the magnetic bearings enable producing power at lower wind speed than conventional bearings. The discussion also includes a blade-pitch adjusting technique. Conventional VAWT applies special blade adjusting mechanism which is complicated in structure, costly to fabricate and wastes power. Herein, the blades in a magnetic suspended VAWT are designed to adjust the pitch automatically and do not require any dedicated special devices. The blade pitch is adjusted naturally during rotation for the best windward angle. As a result the blades always produce the maximum thrust wind force improving the wind turbine efficiency. Thus, the magnetically suspended and self-pitched vertical-axis wind turbine will be designed with uncomplicated structure, high efficiency and low cost. Fundamental and Advanced Topics in Wind Power 234 2. Vertical-axis wind turbine 2.1 Basic principle of vertical axis-machine Wind machine is a kind of energy conversion device, which converts wind energy into mechanical, electric or heat energy. According to their rotor layout, wind turbines can be categorized into horizontal-axis wind turbines (HAWT) and vertical-axis wind turbines. The rotor of a HAWT is horizontal and must point to the wind. The rotating plane of HAWT blades is perpendicular to the wind direction during operation. The turbine blades are radial and their number is commonly 2~3. The shape of a turbine blade is always similar to a wing. It is perpendicular to the rotating shaft and there is an angle between the blade and rotating plane. Since the HAWT works on the principle of airfoil lift, its torque tends to be large and efficiency high in utilizing wind energy. Yaw device is necessary to turn the blades rotating plane in line with the wind direction. The HAWTs have been investigated thoroughly in theory and have the advantages of high wind energy utilization efficiency. They are the mainstream commercial products of the current wind turbine technology. The shaft of a VAWT is perpendicular to the ground and the wind direction. VAWT accepts the wind from all horizontal directions; there is no need of a yaw control devices, making the structure design simple and reducing precession force on blades. Comparing to HAWT, a VAWT has the advantages of low noise and less adverse effect on environment. 1 nose clip, 2 blade nose, 3 blade support pole, 4 wheel arm, 5 upper cover, 6 baffle, 7 position disk, 8 bearing cover, 9 upper ratchet, 10 lower ratchet, 11 generator support, 12 turbine frame, 13 ventral shield, 14 support shelf. Fig. 1. Components of a vertical-axis wind turbine Magnetic Suspension and Self-pitch for Vertical-axis Wind Turbines 235 There are two kinds of VAWTs. The first kind works on the principle of wind drag. Its typical structure is the S-type (Savonius) wind turbine, such as the cup-shaped wind wheel blades for wind-speed measurement. The S-type wind turbine consists of two axis - staggered half cylinders, and the starting torque is large. Unsymmetrical airflows exist around the rotor producing the lateral thrust to the turbine blades. The second kind of VAWT works on the principle of airfoil lift. Its typical application is Darrieus wind turbines. Darrieus wind turbines have various forms, including H- type and ф-type. An H-type wind turbine has a simple structure, but centrifugal force may generate serious bending stress on its turbine wheel connections. The flexural blades in ф-type wind turbine only bear the tension without centrifugal load; the bending stress in blades of ф-type wind turbine is therefore mitigated. The VAWT wheel rotates in a horizontal plane and there is no vertical motion of the blades. Researchers used to believe that the tip-speed ratio (the ratio of blade tip rotational speed to wind speed) of VAWT cannot be greater than 1, and therefore the associated wind energy efficiency is lower than that of a HAWT. The VAWT blades were designed by using blade element momentum method. But the airflow through a VAWT, typically separated unstable flows, is more complicated than those of HAWT. The blades elements moment theory is not suitable for its analysis and design, and this is one reason for less development in VAWT. But as technology progressing, researchers have realized that only S-type VAWTs are limited by the tip-speed ratio less than 1. For the lift-type wind wheel (Darrieus-type) the tip-speed ratio can reach as high as 6. Therefore, its wind energy utilization efficiency is not lower than HAWT. Recognizing the advantage, many institutions have started investigation on VAWTs and achieved considerable advancement in recent years. 2.2 Different vertical axis wind turbines As the wind power technology develops, the unique advantages of VAWTs have been unveiled and appreciated, especially for those small wind turbine applications. The recent progress in VAWT research has enabled many commercializations of small VAWTs as follows. 2.2.1 Sail S-type roof VAWT Figure 2 shows the VAWT manufactured by Enviro-Energies Holdings, a Canadian company. The main line of its production is a 10KW wind turbine, with advantages of low- speed, high power output, quiet operation, and maintenance free. It can be installed on roof for domestic power need. 2.2.2 Light type VAWT As shown in Figure 3, Urban Green Energy (http://www.urbangreenenergy.com/ products/uge-4k) has developed and patented a revolutionary new dual axis design that eliminates the main concern of other vertical axis wind turbines that is premature bearing failure. Its vertical axis machines include those rated for 600W, 1KW and 4KW. They can be installed on top of a tower, roof and other suitable places. These wind turbines can be connected to utility grid. They are made of glass or carbon fibers. The 4KW model weighs about 461Kg, cuts in wind speed at 3.5m/s and is rated at wind speed 12m/s. Fundamental and Advanced Topics in Wind Power 236 Fig. 2. VAWT made by Enviro-Energies and installed on the company’s roof Fig. 3. VAWT made by Urban Green Energy Magnetic Suspension and Self-pitch for Vertical-axis Wind Turbines 237 2.2.3 H type VAWT Figure 4 shows a small VAWT developed by Ropatec of Italy. The Ropatec products are sold to more than thirty countries worldwide, supplying reliable electrical power for families, farms, remote pastoral areas, communications companies and enterprise groups. Its VAWTs have four ratings, i.e., 1KW, 3KW, 6KW and 20KW. The 20KW wind turbine has 5 blades; the others have 3 blades. Fig. 4. H-type VAWT made by Ropatec of Italy Fig. 5. Aesthetic VAWT made by Oy Windside in Finland Fundamental and Advanced Topics in Wind Power 238 2.2.4 Aesthetic VAWT A new type of VAWT as shown in Figure 5 has been developed by Oy Windside in Finland. The wind turbine as a derivative of marine engineering, can be used for charging batteries and provide an environment-friendly image. One of the applications of Oy Windside wind turbines is about “wind art”. The concept is to integrate the wind turbine into art and provide lighting. The aesthetics function and ecological concept are both considered in the turbine design. 2.2.5 Drag-type and lift-type combined VAWT Figure 6 shows the VAWT produced by Green Giant Tech, Taiwan. Green Giant Tech has combined the benefits of the Savonius-type blade and the Darrieus-type blade, applying both the drag and lift forces of wind power. Its VAWT includes three models rated at 400W, 3.6KW and 5KW. It also manufactures street lighting devices for wind and solar power. Fig. 6. VAWT made by Green Giant Tech's in Taiwan 2.3 Magnetic suspension and self-pitch for vertical axis wind turbine Maglev Engineering Research Center, Shandong University, China has committed to the magnetic bearing research and related product development. Recently, magnetic suspension technology has been applied to the vertical axis wind turbine, in which the entire rotor weight of a VAWT was suspended by magnetic bearing. The turbine friction was greatly reduced, and start-up wind speed decreased. Figure 7 shows the magnetically suspended and self-pitched VAWT for street lighting. Since the self-pitch technique and magnetic suspension were applied to the Shandong University VAWT, the wind power efficiency and system performance of the wind turbine have been greatly improved. The manufacturing cost and operational cost were also effectively reduced. This new magnetically suspended vertical-axis wind generator has irreplaceable advantages compared with other VAWTs in the market. Due to its low cost and suitability for high-power single devices, this new VAWT design will have broad commercial potential. [...]... the wind speed increased, the output power of turbine A increases rapidly and reaches the rated value, 300W at 9m/s At 11m/s wind speed, output power of wind turbine B is only a quarter of turbine A, and output power of wind turbine C reaches a half of turbine A Magnetic Suspension and Self-pitch for Vertical-axis Wind Turbines 247 Wind Turbine Power Curve Power (W) 350 300 250 Wind Turbine A Wind. .. parameters of wind turbine A, B and C The wind turbine rotational speed and output power as functions of wind speed were recorded from the experiments, and they are plotted in Figure 15 and 16, respectively Turbine Speed (r/min) Wind -Turbine Speed Curve 120 100 80 Wind Turbine A Wind Turbine B Wind Turbine C 60 40 20 0 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9. 00 10.00 11.00 12.00 Wind Speed (m/s)... (m/s) Fig 15 Turbine speed vs wind speed It can be seen from these two figures that wind turbines A and with self pitch had good start-up properties and starting wind speed was about 1.8m/s However, starting wind speed of wind turbine B with fixed pitch was above 4m/s At the same wind speed, the turbine rotational speed of turbine A was greater than turbine C, and the turbine speed of C is in turn, greater... been tested Their start-up properties and power outputs were measured and compared The parameters of these tested wind turbines are listed in table 1 246 Fundamental and Advanced Topics in Wind Power Parameters wind turbine A wind turbine B wind turbine C Pitch type Fixed-pitch Self-pitch Number of 5 blades 5 5 Blade shape Streamline symmetrical blades Streamline symmetrical blades Arc blades Sizes... pressure was mitigated, and the blade manufacturing cost reduced  Blades can be made with different sizes as spare parts, which can be used for the same wind turbine but according to local wind power resource needs This is a unique feature not available for other wind turbines  In order to lower wind power investment, the power capacity of any single wind turbine should be increased as much as possible... system of wind turbine with five degrees of freedom using radial and axial magnetic bearings 242 Fundamental and Advanced Topics in Wind Power 1 lower backup thrust ring, 2 axial magnetic bearing, 3 radial magnetic bearing, 4 generator, 5 upper backup bearing, 6 lower backup bearing, 7 rotor shaft, 8 radial magnetic bearing stator, 9 shell, 10 upper backup thrust ring, 11 axial magnetic bearing rotor... suspension to wind turbines has achieved the following advantages: 1 Starting wind speed is reduced by magnetic suspension due to reduced bearing friction and power output of wind turbine is increased for the same wind speed 2 Magnetic suspension has largely changed the traditional wind turbine rotor system design using special rolling-element or oil-film bearings These traditional bearings depend on... self-excitation in induction machine and the role of excitation-capacitors in its initiation Simulation results of the self-excited induction generator driven by the variable speed wind turbine are presented in the last section of this chapter The process of voltage build up and the effect of saturation characteristics are also explained in the same section 250 Fundamental and Advanced Topics in Wind Power 2 Induction... angle according to the wind speed in order to control the generator output power Moreover, it is necessary to control blade pitch to lower starting torque during wind turbine start-ups Comparing with the fixed-pitch wind turbines, the controlled-pitch designs can produce more wind power, because it can vary blade pitch according to variable wind speed For stable loading control, security and high efficiency,... University of Delft, Holland 198 9 H Camblong Digital robust control of a variable speed pitch regulated wind turbine for above rated wind speeds In: Control Engineering Practice, Vol.16, No 8, pp 94 695 8, ISSN 096 7-0661 Ahmet Serdar Yilmaz a, Zafer Ozer Pitch angle control in wind turbines above the rated windspeed by multi-layer perception and radial basis function neural networks In : Expert Systems with . parameters of these tested wind turbines are listed in table 1. Fundamental and Advanced Topics in Wind Power 246 Parameters wind turbine A wind turbine B wind turbine C Pitch type Self-pitch. review, Renewable and Sustainable Energy Reviews, Vol. 13, pp. 1– 39. 230 Fundamental and Advanced Topics in Wind Power Monitoring and Damage Detection in Structural Parts of Wind Turbines 25 Have,. large Fundamental and Advanced Topics in Wind Power 244 aerodynamic torque; improve the wind energy utilization and the self-starting capability of the wind turbine. The self-adjusting blade