Wind power generation

Transcription

1 6.2 Wind power generation Wind energy was the source of power for sailing ships and has been used for at least 3,000 years in windmills. However, this form of energy fell into disuse with the spread of electrification and when cheap fossil-fuelled engines became largely available. Although the sudden increase in oil prices in 1973 stimulated a large number of research programmes in several countries, this effort relaxed when oil prices declined in Nonetheless, the knowledge gathered during this period was sufficient to initiate the development of large wind turbines. Moreover, the recent attention given to climate change, the need to increase the share of renewable energies and the fear of a future decline in oil production has triggered a new interest in wind energy. Compared to other forms of renewable energies, wind power requires much less capital cost per unit of power, and the resource is accessible everywhere to some extent. It is available in large quantities in the temperate zones, where most of the industrially developed countries are located. During the last decades of the Twentieth century, many different wind turbine concepts were built and tested: with horizontal and vertical axis rotors; with various numbers of blades; with the rotor placed upwind or downwind of the tower, etc. The horizontal-axis turbine with a three-blade rotor facing the wind proved to be the most suitable type of machine and this resulted in its considerable development, marked by a rapid growth in size and power, as well as its widespread diffusion The wind resource Wind is a movement of air created by the action of the Sun s energy; the heating of surfaces located in various parts of the world produce different results and these differences create movements of the atmosphere. A general circulation is observed at various latitudes with the cyclic influence of the seasons. On a smaller scale, differential heating occurs between land and water bodies, resulting in the formation of daily breezes. The shape and the roughness of the land or water surface also exert a profound influence on wind pattern and the local resource (DNV, 2002). Wind blows at higher speeds over large flat surfaces like the sea, and this is the prime interest for coastal and offshore wind power. It is also stronger on the tops of hills or in valleys oriented parallel to the prevailing wind direction. Wind slows down over rough surfaces, like towns and forests, and wind speed variation with height or wind shear is affected by atmospheric stability conditions. In order to exploit wind energy, it is very important to take the strong variations of wind speeds which exist between different locations into account. Sites located a few kilometres apart can be subjected to a markedly different wind regime and be of different interest for the installation of wind turbines. The wind climate of a given site can be characterized statistically by a Weibull distribution. The probability density function (whose integral over any set of wind speeds gives the probability that the wind speed has values in this set) is given by the formula: f(v ) k (V k 1 C k ) e (V C) k where V is the wind speed, C is the scale parameter and k is the shape parameter. Fig. 1 compares an example of site data (Manwell et al., 2001) with a Weibull distribution curve calculated with a scale parameter C 7.9 m/s and a shape parameter k 2. It is common to characterize the local wind resource by the value VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 561

2 POWER GENERATION FROM RENEWABLE RESOURCES probability density site data 0.08 Weibull curve wind speed (m/s) Fig. 1. Comparison of wind speed measurements by Weibull distribution. of the average wind speed V ave. As a rule, one can consider that the mean wind speed is equal to the scale parameter of the Weibull distribution multiplied by 0.89 (Burton et al., 2001). Wind force changes within a few days or hours according to the weather conditions. Breezes are responsible for a diurnal component of the wind. Finally, it is a common experience that the wind force and direction fluctuate rapidly around a mean value. This is called turbulence, which is an important feature of the wind, because it creates force fluctuations on the wind turbine blades, thereby increasing fatigue and reducing the blades life expectancy. Turbulence intensity is determined statistically as the standard short-term deviation of the wind velocity. Over complex terrain, the turbulence level can be 15% to 20%, while over the sea (offshore), the value can be 10% to 14% (Manwell et al., 2001). On a wind farm, the wake of a turbine can affect other turbines (Barthelemie et al., 2004). A wind turbine must withstand the worst possible storm which may occur on the site during the whole project lifetime. If the turbine is installed for 20 years, the maximum gust taken into account is the 50 years return event. Standards give the indicative values to be considered. Table 1 reproduces the different classes considered by the International Electrotechnical Commission (IEC, 1999). The reference wind speed is defined as the 10 minutes average value of an extreme wind with a recurrence period of 50 years. Extreme gusts which may occur with a return period of 1 year or 50 years are also indicated. Longer term variations exist and the wind resource is not the same from one year to another. The potential effect of global warming on the future wind climate remains an open question. Variability is one of the main disadvantages of wind energy. As long as the amount of wind power is small compared to the local grid transport capacity, energy is delivered to the grid when the wind blows, and production is regarded as a negative demand for the conventional generators. In some countries, large scale wind farms are being considered, mainly as offshore turbine clusters. These farms will have a power of several hundred MW, equivalent to conventional power stations and will be required to predict a given energy production anticipated 24 hours in advance. This is a consequence of electricity market liberalization in western countries, where several power companies can compete on the same grid. The grid operator must be able to know in advance how the foreseeable demand will be balanced by offers from different producers (Makarov and Hawkins, 2003). The predictability of the production adds value to the energy (Nielsen et al., 2003). Failure to deliver is subject to penalties. Research is made to obtain a suitable prediction at least 24 hours in advance (Giebel et al., 2003). When a site on which to install a turbine is taken into consideration, a precise assessment of the true wind resource is essential. A meteorological mast is installed on the site for several months to monitor the wind speeds, directions and turbulence levels at several elevations above the ground. The data collected allow the evaluation of future energy production and the project s economic viability. Table 1. Wind speed parameters for wind turbine classes (IEC, 1999) Parameters Class I Class II Class III Class IV Reference wind speed V ref (m/s) Annual average wind speed V ave (m/s) year return - 10 minutes average gust speed (m/s) 50 year return - 10 minutes average gust speed (m/s) ENCYCLOPAEDIA OF HYDROCARBONS

3 WIND POWER GENERATION Theory of wind turbines The airflow pattern is illustrated schematically in Fig. 2. The rotor faces the incoming wind. The figure shows the shape of the stream tube tangent to the tips of the blades. As some kinetic energy is subtracted from the wind, the speed downstream of the rotor is lower than upstream. Therefore, the diameter of the tube is larger downstream than upstream. In the absence of any rotor, the air would cross the section S r with the velocity V 0. The power of the airflow would be: 1 E 23 rs r V where r is the air density. The upstream tube is in fact smaller than S r and the actual power P is only a fraction of the incoming power. We can then define a power coefficient C P so that: P C P E It can be demonstrated that the air velocity on the rotor plane is (Betz and Prandtl, 1919): 1 V r 23 (V 0 V w ) 2 where V w is the air velocity in the wake downstream. One can calculate the power coefficient as a function of the ratio between the velocity downstream in the wake to the upstream velocity. The optimum value of C P arises when this ratio is 1/3. In this case: C P max 16/ where the ratio 16/27 is derived from the theory of axial moment, by applying some approximate hypotheses. This is known as the Betz limit (Betz and Prandtl, 1919). No turbine can be designed with a higher power coefficient. Present day turbines have power coefficients of about 70 to 80% of the theoretical limit. The theory indicates that the power is proportional to the cube of the wind velocity. This justifies the interest in windy sites for the installation of wind turbines. The power is proportional to the air density and wind turbines must be de-rated when installed in hot climates or in the mountains. A blade is basically a wing. Fig. 3 shows the different forces acting on a blade segment. If the rotor angular velocity is W, the tangential velocity of a blade element at a distance r from the axis is Wr. The tangential air velocity V t has practically the same module. The resultant velocity vector makes an angle f with the rotor plane, determined as: V r V t V r tan f 1 21 Wr Angle b between the blade segment plane and the rotor plane is called the pitch angle and angle a between the inflow vector and the blade segment plane is the angle of attack. Therefore, we have: f a b The aerodynamic force on the blade segment of surface area A can be separated into a lift force F L perpendicular to the apparent wind direction W on the blade element and a drag force F D perpendicular to this direction: 1 F L 23 C L ArW 2 2 where C L is the lift force coefficient and Fig. 2. Schematic flow pattern around a horizontal-axis wind turbine. VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 563

4 POWER GENERATION FROM RENEWABLE RESOURCES V 0 a / V t 1 F D 23 C D ArW 2 2 Wr F M where C D is the drag force coefficient. As shown in Fig. 3, these forces combine to yield a propulsive force in the rotor plane F M and an axial force F T perpendicular to the first one. A given blade profile is characterized by the relationships between a, C L and C D. Fig. 4 shows, for example, the values of the lift and drag coefficients for the DU-91-W2-250 aerofoil. It can be seen that the lift is almost proportional to the angle of attack for a values lower than 10. For larger angles of attack, the lift drops and the drag increases dramatically. For small angles, in attached flow conditions, the air flows smoothly around the aerofoil. For larger angles, the flow is stalled and a turbulent wake is formed. The ratio between the tangential velocity at the blade tip to the wind velocity is noted l: WR l 123 V 0 where R is the rotor radius. The coefficient performance C p can be estimated as a function of l and of the solidity, defined as the total blade area divided by the rotor swept area. The solidity can be altered by the number of blades and the value of the chords (blade width). When the number of blades (or the solidity) is increased, the optimum tip speed l is lowered. V r Fig. 3. Forces acting on a blade segment. b W F L F T F D Wind turbine regulation systems Passive stall regulation with fixed rotation speed Let us consider a wind turbine rotating at a constant velocity. When the wind speed increases, the angle of attack on the blades becomes wider. Above a given speed, the airflow begins to detach from the blades outer surface. This stall phenomenon is first encountered close to the hub and progresses towards the tips for higher wind speeds. The progressive stall provides an automatic power regulation process. Passive stall regulation was largely used in the early commercial turbines with a nominal power of a few hundred kw, equipped with asynchronous generators (the so-called Danish wind turbine). Stall regulation suffers from problems associated with stall behaviour itself: vibration, instability, difficulty in predicting stall onset as well as the return to attached flow conditions. Vibration results in additional fatigue damage to the blades over time. If the rotor is able to rotate at different speeds, the beginning of the stall can somehow be adjusted. Many wind turbines of the 1990s were fitted with two-speed asynchronous generators. Depending on wind conditions, the rotor turned at the lower or the higher speed. Fig. 5 represents the power curve of a typical passive stall-regulated 400 kw turbine as well as the power curve of a 660 kw wind turbine fitted with these active regulation systems. As regards the 400 kw turbine, there is a maximum for the nominal wind speed, followed by a loss of power for higher wind speeds. When the wind exceeds the maximum acceptable value during storm conditions, the turbine is stopped. This is done using air brakes located at the tips of the blades. lift and drag coefficients C L (lift coefficient) C D 10 (drag coefficient) angle of attack ( ) Fig. 4. Shape and characteristics of the DU-91-W2-250 aerofoil (Jeppe, 1999). 564 ENCYCLOPAEDIA OF HYDROCARBONS

5 WIND POWER GENERATION power (kw) pitch and variable speed regulation stall regulation wind speed (m/s) Fig. 5. Typical power curves of turbines with different regulation systems. Pitch regulation It can be understood from Fig. 3 that if the pitch angle b is increased, the angle of attack a is reduced, and the lift force is decreased and the blade is said to feather. All large modern wind turbines are fitted with adjustable blade-pitch mechanisms. When the wind speed reaches excessive values, the rotor is stopped. This is obtained by rotating the blades into an idle position, with the leading edge facing the wind. The aerodynamic loads on the blades are then reduced to a minimum. When the wind speed increases, instead of increasing the pitch angle of the blades to feather, it is also possible to reduce the pitch angle, with the aim of deliberately provoking the stalling effect. With this method, the blade rotation amplitude necessary to control the power is less than for feathering, so that regulation is theoretically faster. Variable speed In large wind turbines, the rotor velocity can vary around the nominal speed (typically 30%). This is made possible by a specific arrangement of the generator with the incorporation of some power electronics, coupled with the blade-pitch regulation system, thus making it possible to achieve constant power output in spite of short-term fluctuations of the wind. When the wind force suddenly increases, the rotor is allowed to accelerate for a few seconds and the additional spinning velocity provides some storage of kinetic energy in the rotor. If the wind stays high, the blades are pitched to reduce the power capture and to maintain the rotor velocity below the acceptable maximum. During a subsequent wind drop, the energy stored in the rotor is released as the rotor slows down. If necessary, the blades are pitched again to restore rotor velocity. Fig. 5 shows the power curve of a 660 kw wind turbine fitted with these active regulation systems. Energy production starts at a minimum wind cut-in velocity (3-4 m/s). The power curve follows, more or less, the cubic theoretical one, as long as the wind speed is less than the nominal value (14-16 m/s). Above this velocity, the power remains practically constant. For safety reasons, the turbine is stopped for winds faster than the cut-out velocity (about 25 m/s). Energy production Depending on the local wind climate, the power curve parameters (cut-in, nominal and cut-out wind velocities) can be selected to design the most suitable wind turbine. Fig. 6 shows an example of the energy produced during the year by a 2,000 kw turbine erected at a site, characterized by Weibull distribution with a scale parameter of 9 m/s and a shape parameter of 2. Results are given as the cumulative number of hours per year with a power exceeding a given value. The yearly energy output is represented by the area below this curve. It can be seen that the turbine operates 7,500 hours over the year, only about 700 hours at the nominal power, and that most of the energy is produced at intermediate wind velocities. The efficiency of the turbine s utilization on the site is frequently assessed by the ratio of the yearly energy production (kwh) to the nominal power of the turbine (kw). The result is called the equivalent number of hours per year; this is 3,150 h in the example in Fig. 6. Dividing this number by the hours in a year, one obtains the equivalent utilization ratio, in this instance 36%. Wind farm projects are considered economic if the equivalent number of hours exceeds 2,000 h/yr. Windy locations are characterized by values of 2,500 h/yr and offshore sites may exceed 3,000 h/yr. The number of hours can be adjusted by judicious choice of the wind turbine. Turbine manufacturers offer different versions of a given turbine type power (kw) 2,500 2,000 1,500 1, ,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 cumulative number of hours/year Fig. 6. Energy production during the year by a 2,000 kw turbine. VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 565

6 POWER GENERATION FROM RENEWABLE RESOURCES depending on local resources; larger rotors, for instance, for less windy areas Turbine equipment Blades The blades are the components which interact with the wind. Their shape is designed in order to obtain good aerodynamic efficiency. Fig. 7 shows a typical wind turbine blade outline, together with several cross-sections at different locations along the length. Near the hub, the blade has a circular section (Rooij, 2004). A bearing fitted in the hub allows the blade pitching movement. A transition piece links the root with the aerodynamic part of the blade. As the distance from the hub axis (radius) increases, the thickness of the wing decreases, as well as the chord. The tangential velocity of a blade element increases with the radius. According to Fig. 3, the local pitch angle must be decreased in order to maintain a proper angle of attack. The blade is twisted, the total twist being about 25 between the beginning of the aerodynamic section and the tip. The aerodynamic forces vary with the square of the local relative air velocity and increase rapidly with the radius. It is therefore important to design the part of the blade near the tip with good lift and low drag coefficients. The blades are flexible and therefore can be deflected by the wind. In order to avoid the blades hitting the tower, the rotor axis is frequently tilted at a small angle. The cross-section of a wind turbine blade is quite thick in order to obtain the high rigidity necessary to resist the different mechanical loads acting during operation. The centrifugal force due to rotation is typically six to seven times larger than the blade weight at the root section. The weight of the blade itself creates a bending moment on the root which is alternated at each revolution. The wind exerts a force which is not constant; fluctuations result from turbulence, but also from the fact that wind velocity increases with altitude. A blade in a high position is subjected to a stronger wind than in a low position and the corresponding load fluctuation is also repeated at each revolution. All these alternating loads are responsible for fatigue, which is the biggest technical challenge in blade design. A thorough analysis is required to eliminate the risk of resonance between the different mechanical oscillators (blades, tower, drive train, etc.). The blades are made of lightweight materials, like fibre-reinforced plastics, which exhibit good fatigue properties. The fibres are mainly glass woven fabrics but for the largest blades, carbon fibres are utilized in the blade parts where loads are most critical. Some blades are entirely made of carbon fibres whereas wood laminates are also utilized by some manufacturers. The fibres are incorporated in a matrix of polyester, vinylester or epoxy resin and the blades are made up of two shells which are bonded together. Internal webbing reinforces the structure. The external blade surface is covered with a smooth coat of coloured gel intended to prevent ultraviolet ageing of the composite material. Lightning strikes are an important cause of failure; therefore lightning protection is provided by electrical conductors located on the surface and inside the blade. Depending on each manufacturer s technology and experience, the blades may be fitted with additional features like vortex generators to increase the lift, stall strips to stabilize the airflow, or tip winglets to reduce tip lift loss and noise. Drive train The blades are linked to the hub which contains the pitch adjustment mechanisms. The hub is generally a foundry piece cast out of steel or spheroidal graphite Fig. 7. Blade shape and cross-sections (enlarged). 566 ENCYCLOPAEDIA OF HYDROCARBONS

7 WIND POWER GENERATION iron. It is externally protected by the spinner, a shell with an oval shape. The rotor shaft is supported by bearings. It rotates at a relatively low speed (e.g rpm, revolutions per minute). The size and weight of electrical generators are almost proportional to the spinning velocity. It is therefore of interest to design the generators with a high rotation speed (e.g. 1,000 or 1,500 rpm) and to utilize an intermediate gearbox to transform the slow rotation of the shaft into the high spinning speed of the generator. Fig. 8 shows the schematic arrangement of a typical wind turbine. Gearbox The gearbox is designed to step up the speed of the rotor to a value suitable for conventional generators. In some turbines, the gear ratio may exceed 1:100. This is achieved in three separate stages. The first stage is generally a planetary gear, while the others are parallel or helical gears. However, the gearbox is a source of noise and manufacturers make efforts to control it, for instance through the use of helical gears instead of parallel spur gears. The gearbox is force-lubricated and the oil is continuously filtered and cooled. With preventive maintenance being standard practice, it is common to monitor the temperature of the gearbox, as well as the vibrations. Generator The generator is the unit that transforms the mechanical energy into electric power. There are two basic types of generators: asynchronous generators and synchronous generators. Asynchronous generators These machines are basically three-phase induction motors. They are characterized by a synchronous speed, determined by the number of poles of the rotor and the grid frequency. With a 50 Hz grid and a generator manufactured with two pairs of poles on the rotor, the synchronous speed is 1,500 rpm. If the mechanical torque on the shaft forces the machine to rotate faster, electrical energy is delivered to the grid by the generator. The difference between the actual rotating speed and the synchronous speed is called the slip. In conventional asynchronous generators equipped with a squirrel cage induction rotor, the slip is about 1%, so these generators are considered constant speed machines. The magnetizing current for the stator is provided by the grid itself. During start up, the stator is connected to the grid by a soft starter which limits the initial current. The generator consumes some reactive power which must be compensated for by on-board capacitor banks. When a wind gust hits the turbine, the energy output fluctuates and if the short-circuit power of the local grid is low this may result in rapid changes of the power of electrical appliances connected in the vicinity, for instance electric light bulbs. This light fluctuation, or flicker, is particularly unpleasant and blade 2 blade bearing 3 blade pitch actuator 4 hub 5 spinner 6 main bearing 7 main shaft 8 aviation light 9 gearbox 10 high-speed shaft and brake 11 hydraulic unit and cooler 12 generator 13 electrical equipment and controller 14 wind sensors 15 transformer 16 nacelle frame 17 tower 18 yaw drive Fig. 8. Typical arrangement of a wind turbine. VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 567

8 POWER GENERATION FROM RENEWABLE RESOURCES has pushed research towards variable speed systems. One solution is to utilize a coiled rotor supplied with a separate alternating current elaborated by an electronic frequency converter. The synchronous speed is therefore a function of the difference between the grid frequency and the rotor current frequency. A speed variation of 30% can be achieved and it is worth mentioning that the electrical energy required by the rotor is only a fraction (about 10%) of the useful energy output collected on the stator terminals. Synchronous generators The rotor is made up of a discrete number of electromagnets, or permanent magnets. The frequency of the current produced by this kind of generator is directly proportional to the rotation speed. Directly connected to the grid, such a generator rotates at a fixed speed, with no slip at all. In order to allow a variable speed mode of operation, the solution is to convert the generator s variable frequency current into direct current via an electronic rectifier, and to transform the direct current back into alternating current suitable for the grid. All direct-drive generators operate according to this principle. This type of generator is more expensive than the asynchronous one, but the absence of a gearbox eliminates a source of maintenance problems and reduces the overall amount of turbine noise. In order to develop the electrical power required, these generators are built with a large diameter. Fig. 9 shows a sketch of a direct-drive turbine. The nacelle is much wider than for the turbine equipped with a gearbox and the fast rotating generator, as appears in Fig. 8. Some turbine manufacturers propose a hybrid solution, with a generator rotating at an intermediate velocity and a gearbox with a limited speed multiplication ratio. Transformer and cabling The voltage level of the generator s output is relatively low (e.g. 690 V) and must be elevated to a medium voltage level (e.g. 36 kv) by means of a transformer to reduce transmission losses. The transformer is installed in the nacelle, or at the base of the tower. Flexible electric cables, which link the nacelle to the base of the tower, form a loop below the nacelle, to allow for its yawing movement. This movement is monitored; if the rotation exceeds two turns, the nacelle is yawed in the opposite direction during the next no-wind period to untwist the cables. Yaw system The whole nacelle is rotated on top of the tower by a yawing system to make the rotor face the wind. Wind blade 2 stator windings 3 rotor windings or permanent magnets 4 blade bearing 5 blade pitch actuator 6 hub 7 spindle 8 rotor bearing 9 rotor 10 stator 11 aviation light 12 wind sensors 13 electrical equipment and controller 14 nacelle frame 15 nacelle housing 16 tower 17 yaw drive Fig. 9. Direct-drive turbine with a synchronous generator. 568 ENCYCLOPAEDIA OF HYDROCARBONS

9 WIND POWER GENERATION Table 2. Typical characteristics of a large wind turbine Nominal power 4.5 MW Number of blades 3 Rotor diameter Control Blade length Blade maximum chord Mass of one blade Mass of the nacelle with rotor and blades Tower mass (steel tubular type) Tower height (depends on local wind conditions) Tower diameter at base Rotor rotation velocity 120 m blade pitch and variable speed 58 m 5 m 18 t 220 t 220 t m 5.5 m 9-15 rpm Gearbox ratio 100:1 Cut-in wind speed Nominal wind speed Cut-out wind speed 4 m s 12 m s 25 m s velocity and direction are continuously monitored by wind sensors installed on the roof of the nacelle. Generally, the rotor is positioned in the average direction of the wind calculated from the measurements of the last 10 minutes by the turbine computer. Tower Tower height depends on the local wind climate. Onshore, the nacelle is generally located at an altitude of 1 or 1.2 times the rotor diameter. On low wind areas, the nacelle is placed high in order to be exposed to stronger winds. Offshore, the nacelle can be lower, typically 0.8 times the rotor diameter. Tubular towers are made out of rolled steel, although some are built in concrete. They have a conical shape, with the base diameter larger than the diameter at nacelle level. The different sections are connected by bolted flanges. Tubular towers have the advantage of protecting the equipment located inside and maintenance access to the nacelle is much safer and more comfortable than for a lattice tower. The nacelle can be accessed by a ladder installed inside the tower, whereas in the largest turbines, a lift is provided. Towers, which have a cylindrical section for symmetry reasons (as the wind may blow from any direction), create a strong downwind wake, and this is the prime reason why most turbines have their rotors placed upwind. They are highly visible structures which have to retain a corrosion-free appearance for many years. Therefore, a coating system is chosen to take this into account. Early wind turbines were installed on lattice towers. Such towers can also be used to erect large turbines, and they are still preferred where local manufacturing capabilities make it the logical choice. The towers are fixed on the ground by strong foundations which are generally slabs of concrete, embedded at some depth below the surface. Ancillary equipment The main ancillary equipment items located in the nacelle include: a mechanical brake installed on the high-speed rotation shaft to block rotation during a storm or maintenance operations; a hydraulic unit to lubricate the gearbox and other mechanical equipment; heat exchangers to cool the oil and the generator. On the roof of the nacelle, the following components are located: anemometers and wind vanes for turbine control; aviation beacons; a helihoist platform (for access to offshore turbines). The equipment is continuously enhanced in order to improve reliability and the economic performance of the turbines. Many sensors are now utilized to monitor the state of the equipment and facilitate their VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 569

10 POWER GENERATION FROM RENEWABLE RESOURCES maintenance. This is particularly crucial for offshore turbines which cannot be accessed easily. These turbines are equipped with an on-board crane to simplify handling operations at sea Typical characteristics of modern large turbines Table 2 summarizes the main data of a large turbine. Precise values depend on the technology mastered by the different manufacturers, but the parameters indicated in this table can be considered as typical for a 4.5 MW wind turbine. Fig. 10 shows the nacelle of a 5 MW turbine. The rotor diameter is 126 m and the nacelle is 100 m above the ground. Fig. 11 shows a 2 MW direct-drive turbine, with the large diameter nacelle typical of this type of machine with a large generator and no gearbox Other types of wind turbines There are many different kinds of wind turbines, and even if the upwind three-bladed horizontal-axis type described in the above is the most frequent to date, some others have their place. The main existing systems are briefly described below. Horizontal-axis two-bladed turbines. Fig. 12 shows two-bladed turbines installed in the Netherlands. Rotors equipped with two blades must rotate faster than those with three blades and as a consequence, the aerodynamic noise level is higher. A two-bladed rotor is subject to severe imbalance due to wind speed variation with height and to gyroscopic effects when Fig. 10. Exercise of access by helicopter on the top of a 5 MW turbine (courtesy of Repower). Fig. 11. A 2 MW direct-drive turbine (courtesy of the Auhor). the nacelle is yawed. A method to reduce the corresponding loads is to use teetering hubs with the rotor hinged to the main shaft. Wind turbines with a single blade were mainly installed in Italy, but went into disuse some years ago. Multi-bladed turbines. The rotation speed decreases when the number of blades is increased, but the torque is raised. In low wind areas, these turbines are frequently utilized in agriculture to drive water pumps. Wind turbines for areas subject to cyclones. The turbines are installed on a tower which can be tilted. It is laid horizontal and secured to the ground when a cyclone is announced. Vertical-axis turbines. The main advantage of this kind of turbine is the absence of a yaw system. This kind of turbine is less efficient than the horizontal-axis type but its simplification is of interest for small units used in harsh zones like high mountains or the Arctic. The rotor can have high solidity and therefore strong mechanical resistance. Fig. 13 shows a 6 kw machine used for water heating at a ski resort in Valle d Aosta. 570 ENCYCLOPAEDIA OF HYDROCARBONS

11 WIND POWER GENERATION The capital cost of onshore wind power depends on several factors: a) the turbine rating; b) the number of turbines in the farm; c) the distance and the characteristics of the grid connection; d) the difficulties in erecting the turbines. An overall cost of about 900 to 1,100 /kw is generally considered (in 2006). The cost of the wind turbine itself is about 800 /kw. This cost decreased by 50% over 15 years (Morthorst and Chandler, 2004) and the energy cost is already lower than gas-based electricity under some circumstances (Milborrow, 2005) Development of offshore wind energy Fig. 12. Two-bladed turbines in Netherlands (courtesy of the Author) Development of onshore wind energy Wind energy has developed remarkably since the 1990s. Table 3 summarizes the capacity installed in some countries by the end of The global capacity amounted to c. 53 GW, most of it in Europe (Tishler and Milborrow, 2005). In Germany and Denmark, as it has become difficult to equip new sites, the existing sites have been repowered with more modern and larger turbines, replacing the old ones. The power installed differs largely between countries, and this reflects the political support and financial incentives made available locally. Some environmental issues are associated with the installation of a wind turbine project: the inherent visual obstruction must be acceptable for the nearby population; disturbance and loss of habitat must be investigated for birds, bats and other living creatures; the noise, although remarkably reduced for the latest turbines, must be evaluated around the turbine. Experience indicates turbines are inaudible at a distance of 500 m from the tower. The wind over the marine open surface is stronger and more stable than over the land, therefore offshore wind is a very attractive source of energy. In fact, the resource can be 30% to 40% larger than on the coast. The technology presently utilized to harness the wind offshore is very similar to the one existing onshore, at least as far as the aerial part of the turbine is concerned. These turbines are three-bladed horizontal-axis machines, which are firmly anchored to the seabed. Offshore wind turbines must face specific problems: The moment at the seabed of the loads on the rotor is aggravated by the additional length of the tower under the water surface. The waves induce supplementary loads and fatigue on the structure, which can be significantly higher than the wind loads. The mechanical soil characteristics of the seabed are frequently poor, so that the foundations must be increased in size. Table 3. Wind capacity (GW) in selected countries at the end of 2005 Germany 18.1 Netherlands 1.2 Spain 9.8 Portugal 1.0 USA 8.9 Japan 0.9 India 4.2 France 0.8 Denmark 3.1 Austria 0.7 Italy 1.7 China 0.7 UK 1.3 Sweden 0.5 VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 571

12 POWER GENERATION FROM RENEWABLE RESOURCES Fig. 13. Small vertical-axis turbines in Valle d Aosta (courtesy of Ropatec). The environmental impact assessment must consider a great variety of marine life, and the whole ecosystem is not yet sufficiently understood. The sea provides a workplace for many professions and subsea electric cables are often a threat to them. The support structures for offshore turbines may be of different types. These are schematized in Fig. 14. In shallow waters, the turbine can be supported by a concrete slab, or on a gravity base, laid on the seabed. If the water depth is less than approximately 20 m, the structure is a steel tube, or monopile which is driven into the seabed by a hydraulic hammer to over a depth sufficient to transfer the loads to the soil. However, if the soil is too hard to allow monopile penetration, a hole is bored into the seabed previously and the monopile is grouted into position. Although this is the cheapest type of foundation, its use is limited by the risk of having the structure resonant mode within the frequency range excited by the waves, the rotor spinning or the blade passage frequency. The resonance frequency decreases with length and increases with the diameter of the structure. In deep waters, the diameter of a monopile is unacceptable. Therefore, tripod structures are made of members welded together and are fixed to the seabed with piles at each corner, with a gravity base or bucket suction anchors according to the soil characteristics. However, this more complicated construction makes the foundations more expensive. The installation of offshore turbines requires special vessels, equipped with large cranes, and legs Fig. 14. Schematic view of support structures for offshore wind turbines. 572 ENCYCLOPAEDIA OF HYDROCARBONS

13 WIND POWER GENERATION bearing on the seabed to immobilize the platform during lifting operations. An example is shown in Fig. 15. The turbines must be very reliable as maintenance necessitates access to them and bad weather conditions may make access by boat impossible, for safety reasons. Redundancy of some equipment parts is accounted for and remote monitoring of sensors placed on the most critical components is standard practice. Offshore turbines are designed to withstand the marine environment: submarine structures are protected against corrosion by cathodic protection; the aerial parts are painted appropriately; insulation of the electrical equipment is reinforced; and the atmosphere inside the nacelle and the tower is conditioned to avoid condensation. The capacity installed by the end of 2004 was 600 MW and many large projects were under development. Offshore wind is of particular interest where shallow water seas are located close to heavily populated areas, e.g. around the North Sea, along the US East Coast, in China, etc. The capital cost of offshore wind power depends largely on: the local conditions; water depth; wave climate; soil characteristics; distance to shore and to the grid connection point. Values range from 1,500 /kw to 2,500 /kw or more (in 2006), but will decline in the future as capacity increases. References Barthelemie R. et al. (2004) ENDOW (Efficient Development of Offshore Wind Farms): modelling wake and boundary layer interactions, «Wind Energy», 7, Betz A., Prandtl L. (1919) Schraubenpropeller mit geringstem Energieverlust, Göttingen, Nachrichten von der Wissenchaften zu Göttingen, Mathematisch-Physikalische Klasse, Burton T. et al. (2001) Wind energy. Handbook, Chichester, John Wiley. DNV (Det Norske Veritas) (2002) Guidelines for design of wind turbines, Copenhagen, DNV/Risø National Laboratory. Giebel G. et al. (2003) The state of the art in short term prediction of wind power from a Danish perspective, in: Proceedings of the 4 th International workshop on largescale integration of wind power and transmission networks for offshore wind farms, Billund (Denmark), October. IEC (International Electrotechnical Commission) (1999) Wind turbine generator systems. International Standard Part 1: Safety requirements, Genève, IEC. Jeppe J. (1999) Unsteady airfoil flows with application to aeroelastic stability, Roskilde (Denmark), Report Risø R-1116 (EN), Risø National Laboratory. Makarov Y., Hawkins D.L. (2003) Scheduling of wind generation resources and their impact on power grid supplemental energy and regulation reserves, in: Proceedings of the 4 th International workshop on largescale integration of wind power and transmission networks for offshore wind farms, Billund (Denmark), October. Fig. 15. Installation of offshore turbines in Denmark (courtesy of A2SEA). VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 573

14 POWER GENERATION FROM RENEWABLE RESOURCES Manwell J.F. et al. (2001) Assessment of the Massachusetts offshore wind energy resource, in: 2001 European Wind Energy Conference, Copenhagen 2-6 July, München, WIP- Renewable Energies, Milborrow D. (2005) Goodbye gas and squaring up to coal, «Windpower Monthly News Magazine», 21, Morthorst P.E., Chandler H. (2004) The cost of wind power: the facts within the fiction, «Renewable Energy World», 7, Nielsen C.S. et al. (2003) Two wind power prognosis criteria and regulating power costs, Stockholm, in: Proceedings of the 4 th International workshop on large-scale integration of wind power and transmission networks for offshore wind farms, Billund (Denmark), October. Rooij R. van (2004) Design of airfoils for wind turbine blades, in: Proceedings of the energy workshop of the global climate and energy project, Palo Alto (CA), April. Tishler C., Milborrow D. (2005) The windicator, «Windpower Monthly News Magazine», 21, 50. Jacques Ruer Saipem/Bouygues St. Quentin en Yvelines, France 574 ENCYCLOPAEDIA OF HYDROCARBONS