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Aerodynamics of Horizontal Axis Wind Turbines

To study the aerodynamics of wind turbines some knowledge of fluid dynamics in general is necessary and, in particular, aircraft aerodynamics. Excellent text books on aerodynamics are readily available, a bibliography is given at the end of this chapter, and any abbreviated account of the subject that could have been included in these pages would not have done it justice recourse to text books would have been necessary anyway. Some direction on which aerodynamics topics are necessary for the study of wind turbines would, however, be useful to the reader. A wind turbine is a device for extracting kinetic energy from the wind. By removing some of its kinetic energy the wind must slow down but only that mass of air which passes through the rotor disc is affected. Assuming that the affected mass of air remains separate from the air which does not pass through the rotor disc and does not slow down a boundary surface can be drawn containing the affected air mass and this boundary can be...

Wind Turbine Concept In Civil Engineering

Windmills have been used for at least 3000 years, mainly for grinding grain or pumping water, while in sailing ships the wind has been an essential source of power for even longer. From as early as the thirteenth century, horizontal-axis windmills were an integral part of the rural economy and only fell into disuse with the advent of cheap fossil-fuelled engines and then the spread of rural electrification. The use of windmills (or wind turbines) to generate electricity can be traced back to the late nineteenth century with the 12 kW DC windmill generator constructed by Brush in the USA and the research undertaken by LaCour in Denmark. However, for much of the twentieth century there was little interest in using wind energy other than for battery charging for remote dwellings and these low-power systems were quickly replaced once access to the electricity grid became available. One notable exception was the 1250 kW Smith-Putnam wind turbine constructed in the USA in 1941. This...

International Wind Energy Associations

British Wind Energy Association Lincoln's Inn House 42 Kingsway London WC2B 6EX UNITED KINGDOM Phone +44 171 404 3433 Fax +44 171 404 3432 Canadian Wind Energy Association 2415 Holly Lane, Suite 250 Ottawa, Ontario KIV 7P2 CANADA Danish Wind Power Association (Wind Turbine Owners' Organization) Phone +45 53 83 13 22 Fax +45 53 83 12 02 European Wind Energy Association 26 Spring street London W2 1JA UNITED KINGDOM Phone +44 (0) 171 402 7122 Fax +44 (0) 171 402 7125 E-mail ewea ewea.org Web http www.ewea.org Finnish Wind Power Association Phone +358 0 456 6560 Fax +358 0 456 6538 FME Groep Windenergic (Dutch wind energy trade association) Phone +31 79 353 1100 Fax +31 79 353 1365 French Wind Energy Association 63 Collet de Darbousson Valbonne 06560 FRANCE German Wind Energy Association Dorfangerweg 15 Unterfoehring 85774 GERMANY Phone +49 899506411 Hellenic Wind Energy Association (Greece) Norwegian Wind Energy Association Phone +47 66 84 63 69 Fax +47 66 98 11 80 Romanian Wind Energy...

Periodicals on Wind Energy

Wind Energy Weekly American Wind Energy Association 122 C Street. NW, Fourth Floor Washington, D.C. 20001 Phone (202) 383-2500 Published 50 times a year, Wind Energy Weekly provides up-to-date information on utility requests for proposals, federal & state regulatory activity, project development, international business opportunities, and energy and environmental policy. This bimonthly magazine provides information about residential wind systems as well as information on other renewable technologies and products for residential use. Published 10 times a year, this magazine provides information for the independent power industry and includes periodic updates on the wind industry. Wind Energy Technology Generating Power From the Wind National Technical Information Service This bimonthly publication provides current worldwide information on wind turbine design, performance, wind resource identification, legal institutional implications, and environmental aspects of wind power....

941 Modelling and prediction of EMI from wind turbines

There are two fundamental interference mechanisms for EMI from wind turbines, back-scattering and forward-scattering (Moglia, Trusszi and Orsenigo, 1996). These are shown in Figure 9.14. Forward-scattering occurs when the wind turbine is located between the transmitter and receiver. The interference mechanism is one of scatter or refraction of the signal by the wind turbine and, for TV signals, it causes fading of the picture at the rotational speed of the blades. Back-scattering occurs when the turbine is located behind the receiver. This results in a time delay between the wanted signal and the reflected interference and gives rise to ghost or double images on a TV screen.

Wind Energy Information Sources

As wind energy technology gains more widespread acceptance as an economically and technically viable resource, an increasing number of people representing electric utilities, the news media, the research community, political and regulatory leaders, and local communities are searching for up-to-date, accurate information on this clean, renewable energy resource. The following sources can provide a range of general and technical information about wind. American Wind Energy Association (AWEA) National Wind Technology Center 1617 Cole Boulevard Golden, CO 80401 Phone (303) 384-6900 Fax (303) 384-6901

University Wind Energy Programs in the USA

A number of universities offer courses in renewable energy technology through environmental science or engineering departments. In addition to classes, they work with industry partners to conduct valuable research. Institutions offering specific wind energy curriculum and research programs include the following

1053 Measurement and assessment of power quality characteristics of grid connected wind turbines

Determination of the power quality of wind turbines and prediction of their performance in service is not straightforward and IEC 614200-21 (IEC, 2000b) has been written to provide guidance. There are a number of difficulties when assessing the power quality of wind turbines as their performance will depend on the design of the entire wind turbine (including the aerodynamic rotor and control system), short-circuit level and interpolated for the X R ratio of the point of connection. A weighting factor, based on an assumed Raleigh distribution of wind speed, is also applied to provide flicker coefficients which may be used on sites with various average annual mean wind speeds. The Standard also defines methods to evaluate the impact of wind turbine startup at cut-in and rated wind speeds and during speed changing of two speed generators. Again currents are measured, combined with the 'fictitious grid' to provide a voltage time series and then passed through a flicker algorithm. For...

1561 Wind Farm Screening Chart

Figure 15-4 is a wind farm profitability screening chart based on cost of capital (discount rate) of 8 percent, project life of 15 years, initial cost of 1,100 European currency units per kW capacity, and the operating and maintenance cost of 3 percent of the initial project cost.3 The chart takes into account the inflation at a constant rate, i.e., the costs and revenues rising at the same rate. The taxes on sales, if any, must be deducted from the average selling price before it is entered in reading the chart. An example for using the chart follows At a site with 7.5 m s wind speed at hub height, the profitability index would be zero (a break-even point) if the energy can be sold at 0.069 ECU kWh. If the energy price is 0.085 ECU kWh, the profitability index would be 0.30, generally an attractive value for private investors. At a site with only 7 m s wind speed, the same profitability can be achieved if the energy can be sold at 0.097 ECU kWh.

Effects Of Wind Speed And Direction

Horizontal winds play a significant role in the transport and dilution of pollutants. As wind speed increases, the volume of air moving by a source in a period of time also increases. If the emission rate is relatively constant, a doubling of the wind speed halves the pollutant concentration, as the concentration is an inverse function of the wind speed.

102 Earthing Grounding of Wind Farms

Counterpoise Earthing Image

Wind farms, however, have rather unusual requirements for earthing. They are often very extensive stretching over several kilometres, subject to frequent lightning strikes because of the height of modern wind turbines, and are often on high-resistivity ground being located on the tops of hills. Thus, normal earthing practice tends not to be easily applicable and special consideration is required. The IEEE Recommended Practice (1991), which is no longer current, recommended 'that the entire wind farm installation have a continuous metallic ground system connecting all equipment. This should include, but not be limited to, the substation, transfor mers, towers, wind-turbine generators and electronic equipment.' This practice is generally followed with bare conductor being laid in the power-collection cable trenches to provide both bonding of all parts of the wind farm as well as a long horizontal electrode to reduce the impedance of the earthing system. A wind farm earthing system is...

Wind Power System Suppliers in the USA

The following is a partial list of wind turbine manufacturers and developers in the U.S.A. A complete list and company profile can be obtained from American Wind Energy Association Bergey Windpower Company 2001 Priestley Avenue Norman, OK 73069 Phone (405) 364-4212 Fax (405) 364-2078 E-mail mbergey bergey.com Southwest Windpower Wind Turbine Industries Corp. 16801 Industrial Circle, SE Prior Lake, MN 55372 Phone (612) 447-6064 Fax (612) 447-6050

Wind Energy Conversion Systems

As airplane technology evolved, research into airfoil design was readily adapted into propeller blade design for wind turbines. By the early part of the 20th century, thousands of windmills were scattered across the United States, serving farms and other rural areas for pumping and other mechanical applications, and later for powering small electric generators. Figures 14-14 and 14-15 show traditional type (Aermotor) multi-bladed windmills in a farm setting. Generally used for water pumping, the blades or sails are mounted at an oblique angle on the horizontal shaft. The fantail rudder steers the bladed wheel into the wind. This same basic design has endured for more than a century and is still applied today. In the 1920s and 1930s, there were some 300 reported companies producing small wind turbines throughout the world, with more than 100 of them located in North America. Two of the largest producers were Jacobs Manufacturing and the Windcharger Company, which reported to have...

24 Estimating Wind Speed from Localized Damages

One of the problems in dealing with wind damages is the estimation of wind speed when the subject building is located far from a weather reporting station, or is in an area that obviously experienced wind conditions different from that of the nearest weather station. In such cases, wind speed can actually be estimated from nearby collateral damage by the application of the Beaufort wind scale. The Beaufort wind scale is a recognized system introduced in 1806 by Admiral Beaufort to estimate wind speed from its effects. Originally it was used to estimate wind speeds at sea. The methodology, however, has been extended to estimating wind speeds over land as well. The Beaufort wind scale is divided into 12 levels, where each level corresponds to a range of wind speeds and their observable effects. A brief version of the currently accepted Beaufort wind scale is provided below.

1061 Wind farm and generator protection

Figure 10.20 shows a typical protection arrangement for a wind farm of fixed-speed wind turbines with generator voltages of 690 V and with a collection circuit voltage of 11 kV. The 11 kV circuit is fed from a 33 11 kV Delta Star wound transformer with the 11 kV neutral grounded either directly or through a resistor. The 11 0.69 kV transformers are also wound Delta Star and so the 690 V neutral points of each circuit may be directly grounded. The neutral point of the generators is not connected to ground. There are a number of zones of protection. At the base of the wind turbine tower a 690 V circuit breaker (usually a moulded case type as shown in Figure 10.1) will be fitted to protect the pendant cables and the generator. This is shown as Zone D. Figure 10.20 Protection of a Wind Farm with an 11 kV Connection Circuit (RMU - Ring Main Unit) fitted to the 690 V side of the turbine transformer to provide protection of the cables and also a point of isolation so that all the electrical...

963 Support mechanisms for wind energy

Historically, electrical energy from wind turbines was not competitive in commercial markets with other forms of generation, particularly the use of a combined cycle gas turbine (CCGT) plant burning natural gas. Hence, in order to take account of external costs, and to meet commitments to reduce CO2 emissions, various support mechanisms have been used by governments to encourage the development of wind power as well as other forms of renewable energy. These support mechanisms, together with the markets for electrical energy, are subject to very rapid change but the main principles are described. Perhaps the most obvious approach to support wind power is to require fixed premium tariffs to be paid for all power generated by renewable sources. This was the basis for the 'Public Utilities Requirement to Purchase Act (PURPA) introduced in the USA in 1978 but abandoned in the late 1980s and the German 'Electricity Feed-in-Law'. A similar approach was adopted in Spain and Denmark for a...

23 Variation of Wind Speed with Height

Wind blows slower near the surface of the ground than it does higher up. This is because the wind is slowed down by friction with the ground and other features attached to the ground, like trees, bushes, dunes, tall grass, and buildings. Because of this, wind speeds measured at, say 50 feet from the ground, are usually higher than wind speeds measured at only 20 feet from the ground. In fact, the wind speed measured at 50 feet will usually be 14 higher than the speed at 20 feet, assuming clear, level ground, and even wind flow. As a general rule, the wind speed over clear ground will vary with 1 7th the power of the height from the ground. This is called the 1 7th power rule. where v wind speed measurement, h height from ground, and k units conversion and proportionality constant. For this reason, when wind data from a local weather station is being compared to a specific site, it is well to note that most standard wind measurements are made at a height of 10 meters or 32.8 feet. If,...

Wind Speed and Energy Distributions

The wind turbine captures the wind's kinetic energy in a rotor consisting of two or more blades mechanically coupled to an electrical generator. The turbine is mounted on a tall tower to enhance the energy capture. Numerous wind turbines are installed at one site to build a wind farm of the desired power production capacity. Obviously, sites with steady high wind produce more energy over the year. Two distinctly different configurations are available for the turbine design, the horizontal axis configuration (Figure 4-1) and the vertical axis configuration (Figure 4-2). The vertical axis machine has the shape of an egg beater, and is often called the Darrieus rotor after its inventor. It has been used in the past because of specific structural advantage. However, most modern wind turbines use horizontal-axis design. Except for the rotor, all other components are the same in both designs, with some difference in their placement.

Wind Turbine Installations and Wind Farms

For any wind turbine installation, there are certain additional activities (e.g., construction of foundations and access roads, electrical connections, site erection, as well as project development and management) that must be undertaken. For flat onshore sites, which might be found typically in Denmark or North Germany, the total investment cost is approximately 1.3 times the ex-works turbine cost (EUREC Agency, 1996). In the UK, where sites are often located in more remote, upland areas the balance-of-plant costs (i.e., all costs other than the wind turbines) tend to be higher and a more typical breakdown is shown in Table 9.1. Commercial developers of wind farms will often prefer larger projects as, in that way, the fixed-costs, particularly electrical network connection and project development and management costs, may be spread over a bigger investment. A further encouragement for large projects is that the fixed costs of arranging project finance are high. However, there are...

Wind Power

The first use of wind power was to sail ships in the Nile some 5000 years ago. The Europeans used it to grind grains and pump water in the 1700s and 1800s. The first windmill to generate electricity in the rural U.S.A. was installed in 1890. Today, large wind-power plants are competing with electric utilities in supplying economical clean power in many parts of the world. The average turbine size of the wind installations has been 300 kW until the recent past. The newer machines of 500 to 1,000 kW capacity have been developed and are being installed. Prototypes of a few MW wind turbines are under test operations in several countries, including the U.S.A. Figure 2-1 is a conceptual layout of modern multimegawatt wind tower suitable for utility scale applications.1 Improved turbine designs and plant utilization have contributed to a decline in large-scale wind energy generation costs from 35 cents per kWh in 1980 to less than 5 cents per kWh in 1997 in favorable locations (Figure 2-2)....

The Wind Resource

Van Der Hoven Spektrum

The energy available in the wind varies as the cube of the wind speed, so an understanding of the characteristics of the wind resource is critical to all aspects of wind energy exploitation, from the identification of suitable sites and predictions of the economic viability of wind farm projects through to the design of wind turbines themselves, and understanding their effect on electricity distribution networks and consumers. From the point of view of wind energy, the most striking characteristic of the wind resource is its variability. The wind is highly variable, both geographically and temporally. Furthermore this variability persists over a very wide range of scales, both in space and time. The importance of this is amplified by the cubic relationship to available energy. At a given location, temporal variability on a large scale means that the amount of wind may vary from one year to the next, with even larger scale variations over periods of decades or more. These long-term...

163 Wind Energy

Wind energy to generate electricity is most feasible at sites where wind velocities are consistently high and reasonably steady. Ideally these sites should be remote from densely populated areas, since noise generation, safety, and disruption of TV images may be problems. On the other hand the generators must be close enough to a consumer that the energy produced can be utilized without lengthy transmission. An article in the EPRI journal (11) gives a good update on wind energy in the electric utility industry as of 1999. Another very useful source of information about wind energy is available from the American Wind Energy Association (12) and from its web site. This group publishes the AWEA Wind Energy Weekly and maintains an archive of back issues. According to Awea 3,600 megawatts of new wind energy capacity were installed in 1999 worldwide, bringing total installed capacity of 13,400 MW. In the United States 895 MW of new generating capacity was added between July 1998 and June...

33 Wind Power

Wind-driven electric generators can also be used to charge the batteries of stationary electric vehicles. Stationary wind generators, such as smaller versions of the one illustrated in Figure 10.6, are common methods of supplying power in areas without mains electricity. A stationary wind generator could be used in the same way as a stationary photovoltaic array. Alternatively it would be possible to mount a small generator on the roof of an electric vehicle, for charging when the vehicle was stationary. There would be no point in using it when the vehicle was in motion, as the power gained from the wind generator would be considerably less than the power lost by dragging the wind generator through the wind, the efficiency being less than 100 . Ideally, for aerodynamic reasons the wind generator would fold away when the vehicle was travelling. The concept of an onboard wind generator is illustrated in Figure 3.1. In windy places a wind generator 1.2m in diameter could produce up to...

Wind Power System

The wind power system is fully covered in this and the following two chapters. This chapter covers the overall system level performance, design considerations and trades. The electrical generator is covered in the next chapter and the speed control in Chapter 7. The wind power system is comprised of one or more units, operating electrically in parallel, having the following components the wind turbine with two or three blades. Because of the large moment of inertia of the rotor, the design challenges include the starting, the speed control during the power producing operation, and stopping the turbine when required. The eddy current or other type of brake is used to halt the turbine when needed for emergency or for routine maintenance. In the multiple tower wind farm, each turbine must

Wind Speed

The wind speed is the mean wind speed over the vertical distribution of a plume. However, usually the only wind speed available is that monitored at ground-level meteorological stations. These stations record ambient atmospheric characteristics, usually at the 10-m level, and typically with lower wind speeds than those affecting the plume. These lower speeds are due to the friction caused by the surface as shown in Figure 5.8.9. Therefore, the wind speed power law must be used to convert near-surface wind speed data into a wind speed representative of the conditions at the effective plume height. The wind speed power law equation is as follows where u1 and z1 correspond to the wind speed and vertical height of the wind station, while u2 and z2 pertain to the characteristics at the upper elevation. This formula is empirical, with the exponent derived from observed data. The exponent (p) varies with the type of ambient weather conditions, generally increasing with stability and surface...

A42 Wind Speed

In Gaussian equation the pollutant concentration is inversely proportional to wind speed. The effective wind speed (ue) is assumed to be the sum of the ambient wind component (u) and wind speed correction (uo). The 'uo' accounts for lateral dispersion caused due to traffic wake and also concentration divergence when wind speed approaches to zero (calm wind condition) or direction becomes parallel to the roadway 92 . Ideally, u is the mean wind speed at source height. However, some parameters 92 which are being used in the DFLS model formulation are based on wind speed measurements at 4.5 m height above the ground. In case of vehicles, it is difficult to measure wind speed such a small source height ( 0.3 m). Therefore, wind speeds at 4.5 m have been used. In general, wind speed and directions are measured at 10 m height. Hence, Power law relationship between wind speed and height has been used to get the wind speed values at 4.5 m height. The wind speed correction is dependent on the...

National Associations

Austrian Wind Energy Association IG Windkraft -Osterreich Mariahilferstrasse 89 22 1060 Vienna Austria British Wind Energy Association Danish Wind Turbine Manufacturers Assoc. Tel + 45 33 73 0330 Fax + 45 33 73 0333 E-mail danish windpower.dk Web www.windpower.dk Dutch Wind Energy Bureau Postbus 10 6800AA Arnhem The Netherlands Tel + 31 26 355 7400 Fax + 31 26 355 7404 Finnish Wind Power Association PO Box 846 FIN-00101 Helsinki Finland Hellenic Wind Energy Association c o PPC, DEME 10 Navarinou Str 10680 Athens Greece Irish Wind Energy Association Japanese Wind Energy Association c o Mechanical Engineering Lab. 1-2 Namiki, Tsukuba 305 Ibaraki-Ken Japan Netherlands Wind Energy Association Turkish Wind Energy Association EiE Idaresi Genel Mudurlugu Eskisehir Yolu 7km No. 166 06520 Ankara Turkey Romanian Wind Energy Association Power Research Institute Bd Energeticienilor 8 76619 Bucharest Romania

47 Windturbine Performance Measurement

In the final analysis, wind-turbine performance is concerned with the estimation of long-term energy production expected on a given site. The wind resource is described by the probability distribution (usually annual) of 1 h (sometimes, 10 min) mean wind speeds. To calculate the average energy production for a given probability distribution of wind speeds a relationship between wind speed and wind power is needed this is the power curve of the wind turbine. As dynamic effects are not of interest for long-term performance, averaging of the measured wind speed and wind turbine power is carried out which improves the correlation between them and attenuates the effects of wind turbulence. This is not to say that site turbulence is irrelevant a point which will be dealt with later. It is also important to note the power from the wind turbine which is of relevance here is the net power, defined as the power available from the wind turbine less power needed for control, monitoring, display...

1045Power flows slowvoltage variations and network losses

If the output from an embedded wind turbine generator is absorbed locally by an adjacent load then the impact on the distribution network voltage and losses is likely to be beneficial. However, if it is necessary to transport the power through the distribution network then increased losses may occur and slow-voltage variations may become excessive. If the wind generator operates at unity power factor (i.e., reactive power Q 0), then the voltage rise in a lightly-loaded radial circuit (Figure 10.9), is given approximately by

51 National and International Standards 511 Historical development

The preparation of national and international standards containing rules for the design of wind turbines began in the 1980s. The first publication was a set of regulations for certification drawn up by Germanischer Lloyd in 1986. These initial rules were subsequently considerably refined as the state of knowledge grew, leading to the publication by Germanischer Lloyd of the Regulation for the Certification of Wind Energy Conversion Systems in 1993. This was further amended by supplements issued in 1994 and 1998. Meanwhile national standards were published in The Netherlands (NEN 6096, Dutch Standard, 1988) and Denmark (DS 472, Danish Standard, 1992). The International Electrotechnical Commission (IEC) began work on the first international standard in 1988, leading to the publication of IEC 1400-1 Wind turbine generator systems - Part 1 Safety Requirements in 1994 (Second Edition IEC, 1997). A revised edition containing some significant changes appeared in 1999, bearing the new number...

411 The CP performance curve

The theory described in Chapter 3 gives the wind turbine designer a means of examining how the power developed by a turbine is governed by the various design parameters. The usual method of presenting power performance is the non-dimensional CP - curve and the curve for a typical, modern, three-blade turbine is shown in Figure 4.1.

1063 Interface protection

Section 10.6.1 considered the protection of the wind turbines from the effects of insulation failure and subsequent high fault currents, which were predominantly supplied by the distribution network. However, protection is also required to ensure that the wind farm does not feed into faults on the distribution network or attempt to supply an isolated section of network. The problem is illustrated in Figure 10.24. For faults on the network, the difficulty is that wind turbines are not a reliable source of fault current and so circuit breaker B cannot be operated by over-current protection. Thus, for the fault shown, the current-operated protection on the network is used to open circuit breaker A. This then isolates the wind turbine which begins to speed up as the wind input remains but it is no longer possible to export power to the network. In fact, this acceleration begins as soon as the fault occurs, and before circuit breaker A trips, as the fault depresses the network voltage and...

69 Fixed Speed Twospeed or Variablespeed Operation

Wind turbine rotors develop their peak efficiency at one particular tip speed ratio (see Figure 3.15), so fixed speed machines operate sub-optimally, except at the wind speed corresponding to this tip speed ratio. Energy capture can clearly be increased by varying the rotational speed in proportion to the wind speed so that the turbine is always running at optimum tip speed ratio, or alternatively a slightly reduced Noise considerations are often of greater significance than energy capture in the decision to opt for non-fixed speed operation. As noted in Section 6.4.4, the aerodynamic noise generated by a wind turbine is approximately proportional to the fifth power of the tip speed. Both variable speed and two-speed operation allow the rotational speed to be substantially reduced in low winds, thus reducing turbine aerodynamic noise dramatically when it could otherwise be objectionable because of low ambient noise.

911 Initial site selection

Initially a desk-based study is carried out to locate a suitable site and to confirm it as a potential candidate for the location of a wind farm. It may be recalled (see section 4.5) that the mean power production for a wind turbine (assuming 100 percent availability) is given by where P(U) is the power curve of the wind turbine, f(U) is the probability density function (PDF) of the wind speed, and T is the time period. The power curve is available from the potential turbine supplier(s) while an initial estimate of the PDF of the wind speed may be obtained from a wind atlas (European Wind Atlas, 1989). The PDF is generally based on a Weibull distribution and takes account of regional climatology, roughness of the surrounding terrain, local obstacles and topology. PDFs are calculated for 12 30 sectors and integrated with the power curve usually using numerical evaluation techniques. At this stage of the project development only an approximate indication of the wind-farm output is...

931 Terminology and basic concepts

Two distinctly different measures are used to describe wind turbine noise. These are the sound power level LW of the source (i.e., the wind turbine) and the sound pressure level LP at a location. Because of the response of the human ear, a logarithmic scale is used based on reference levels that correspond to the limit of hearing. The units of both LP and LW are the decibel (dB).

525 Functions of the control and safety systems

A primary function of the control system is to maintain the machine operating parameters within their normal limits. The purpose of the safety system (referred to as 'protection system' in IEC 61400-1) is to ensure that, should a critical operating parameter exceed its normal limit as a result of a fault or failure in the wind turbine or the control system, the machine is maintained in a safe condition. Normally the critical operating parameters are

562 Critical configuration for different machine types

It was shown in the preceding section that the maximum lift coefficient is likely to exceed the maximum drag coefficient for a wind turbine blade, so consequently the maximum loading on a stationary blade will occur when the air flow is in a plane perpendicular to the blade axis and the angle of attack is such as to produce maximum lift. For a stall-regulated machine, this will be the case when the blade is vertical and the wind direction is 70 -80 to the nacelle axis, whereas for a pitch-regulated machine the blade only needs to be approximately vertical with a wind direction at 10 -20 to the nacelle axis.

46 Windturbine Field Testing 461 Introduction

Wind-turbine field testing is undertaken in the main for two different reasons. First, as part of the development of new designs, manufacturers and researchers undertake a wide range of measurements to check on the operation of a given machine and in some instances to validate wind turbine models used in the design process. The second, and perhaps the most common, reason for testing is to establish the performance of a given wind turbine for commercial reasons. Because in the second case the objectives are better defined and since many of the problems are common to both sorts of tests, there will be a concentration on performance measurement. Performance measurement is also the area best covered by agreed standards and recommendations, although some difficulties and inconsistencies still remain. It should be mentioned that testing is also increasingly undertaken, in a commercial context by the operators, to verify the manufacturer's performance warranty. Such tests will tend to follow...

751 Induction generators

The induction generators commonly used on fixed-speed wind turbines are very similar to conventional industrial induction motors. In principle the only differences between an induction machine operating as a generator and as a motor are the direction of power flow in the connecting wires, whether torque is applied to or taken from the shaft and if the rotor speed is slightly above or below synchronous. The size of the market for induction motors is very large and so, in many cases, an induction generator design will be based on the same stator and rotor laminations as a range of induction motors in order to take advantage of high manufacturing volumes. Some detailed design modifications, e.g., changes in rotor bar material, may be made by the machine manufacturers to reflect the different operating regime of wind turbine generator, particularly the need for high efficiency at part load, but the principles of operation are those of conventional induction motors. The synchronous speed,...

845Optimal feedback methods

There is, however, a huge body of theory (and practice, although to a lesser extent) relating to more advanced controller design methods, some of which have been investigated to some extent in the context of wind-turbine control, for example If the system dynamics are known, then some very similar mathematical theory can be used, but applied in a different way. Rather than fitting an empirical model, a linearized physical model is used to predict sensor outputs, and the prediction errors are used to update estimates of the system state variables. These variables may include rotational speeds, torques, deflections, etc. as well as the actual wind speed, and so their values can be used to calculate appropriate control actions even though those particular variables are not actually measured. A subset of the known dynamics may be used to make estimates of a particular variable for example, some controllers use a wind-speed observer to estimate the wind speed seen by the rotor from the...

91 Project Development

The development of a wind farm follows a broadly similar process to that of any other power generation project, but with the particular requirements that the wind Table 9.1 Typical Breakdown of Costs for a 10 MW Wind Farm Element of wind farm of total cost Wind turbines 65 Wind farm electrical infrastructure 8 turbines must be located in high wind speed sites to maximize energy production and their size makes visual effects a particularly important aspect of the environmental impact. Guidance on the development of wind energy schemes has been issued by the British Wind Energy Association (BWEA, 1994) in their Best Practice Guidelines for Wind Energy Development. A similar document is published by the European Wind Energy Association (EWEA). The three main elements of the development of a wind-farm project are identified as (1) technical and commercial issues, (2) environmental considerations, and (3) dialogue and consultation. Perhaps surprisingly to many engineers and technologists...

1000 900 800 700 600 500 400 300 200 100

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Wind speed (m s) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Wind speed (m s) Wind speed Power Curve Figure 9.1 Annual Energy Calculation of a Wind Turbine 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Wind speed (m s) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Wind speed (m s) Wind speed Power Curve Figure 9.1 Annual Energy Calculation of a Wind Turbine amenity value and ensuring that no turbine is located so close to domestic dwellings that a nuisance will be caused (e.g., by noise, visual domination or light shadow flicker). A preliminary assessment of visual effects is also required considering the visibility of the wind farm particularly from important public viewpoints. If, within the wind-farm perimeter, there are areas of particular ecological value, due to flora or fauna, then these need to be avoided as well as any locations of particular archaeological or historical interest. Communication...

101 Powercollection Systems

Figure 10.1 shows a schematic of a typical fixed-speed wind turbine electrical system. The main power circuit is from the generator, via three flexible pendant cables to a moulded case circuit breaker (MCCB). The MCCB is fitted with an overcurrent protection trip with an instantaneous setting against faults and a delayed (thermal) function which operates at a lower current level and is intended to detect overload of the generator. This arrangement follows normal practice for the control of large motors. An anti-parallel thyristor soft-start unit, normally with a bypass contactor, is used to reduce the current inrush when the generator is energized. There are a number of additional circuits including those for the power factor correction capacitors (PFC) and the auxiliary supplies. The power factor correction capacitors are switched in stages to provide a greater degree of control of reactive power and also to limit capacitive switching currents. Small reactors may be connected in...

610 Type of Generator

Fixed-speed wind turbines differ from almost all conventional generating plant by using induction rather than synchronous generators. This choice is driven by the requirement for significant damping in the drive train due to the cyclic variations in the torque developed by the aerodynamic rotor. Therefore, to a first approximation, the behaviour of a synchronous machine may be considered to be analogous to a torsional spring. The torque is proportional to the angle between the rotor and the stator field. This angle is known as the load or power angle. In contrast, an induction generator can be thought of as a torsional damper where the torque is proportional to the difference in speed between the rotor and the stator field (the slip speed). This is illustrated in simple schematic form in Figure 6.14. It may be seen that if the simple model of a fixed-speed wind turbine, equipped with a synchronous generator, is excited by the cyclic torque from the wind-turbine rotor then there is no...

A39 Aerofoil Drag Characteristics

The definition of the drag coefficient for an aircraft wing or a wind-turbine blade is based not on the frontal area but on the plan area, for reasons that will become clear later. The flow past a body which has a large span normal to the flow direction is basically two-dimensional and in such cases the drag coefficient can be based upon the drag force per unit span using the stream-wise chord length for the definition

53 Turbulence and Wakes

Fluctuation of the wind speed about the short-term mean, or turbulence, naturally has a major impact on the design loadings, as it is the source of both the extreme The nature of free stream turbulence, and its mathematical descriptions in statistical terms, form the subject of Section 2.6. Within a wind farm, turbines operating in the wakes of other turbines experience increased turbulence and reduced mean velocities. In general, a downwind turbine will lie off-centre with respect to the wake of the turbine immediately upwind, leading to horizontal wind shear. Models describing velocity deficits and the increase in turbulence intensity due to turbine wakes are reported in Section 2.10 but, as is pointed out, no consensus has yet emerged as regards their use for wind turbine design calculations.

45 Estimation of Energy Capture

The quantity of energy that can be captured by a wind turbine depends upon the power versus wind speed characteristic of the turbine and the wind-speed distribution at the turbine site. The wind-speed distribution at a site can be represented by the Weibull function the probability that the wind speed will exceed a value U is where c, called the scale factor, is a characteristic speed related to the average wind speed at the site by (r being the gamma function and k is a shape parameter, see also Section 2.4). Let U U u a normalized wind speed. The wind speed distribution density is then the modulus of the derivative of Equation (4.2) with respect to u i.e., the probability that the wind speed lies between u and u + du is f(u)du. Alternatively, Equation (4.4) gives the proportion of time for which the wind speed u will occur. The performance curve shown in Figure 4.14 is for a turbine designed with an optimum tip speed ratio of 7. As an example, assume that the turbine is...

410 Aerodynamic Performance Assessment

There is often a need for assessing the aerodynamic or instantaneous performance of a wind turbine. Detailed features, such as the stall characteristic, will tend to be smoothed out by the 10 min averaging employed in power performance assessment. Consequently only short averaging periods can be used, but this introduces a further limitation of the method of bins. It has been shown that poor correlation between power and wind speed results in a systematic distortion of the binned relationship (Christensen and Dragt, 1986 Dragt, 1983) and shorter averaging times result in poorer correlation. The effect is to rotate the power curve about the point where the wind speed probability is highest as shown in Figure 4.24. This can be understood as follows. Consider a short gust of high wind at the anemometer. The likelihood is that at this instant, the wind speed at the turbine will be lower (i.e. nearer the mean), and consequently the power output measured will be less than would have been...

Extreme loading during operation stallregulated machines

As described in Chapter 5, wind turbine design codes specify a number of load cases consisting of various combinations of defined wind speed and direction changes - some of them involving external or machine faults - which are an attempt to define an envelope of the worst loadings to be expected in practice. It is instructive to take one such code, IEC 61400-1, and compare the blade loadings arising from the different load cases for a particular design. The WTG chosen is a 40 m diameter, 500 kW stall-regulated Class II machine fitted with TR blades (see Figure 5.2(a)) and operating at a single rotational speed of 30 rpm. The rated wind speed, Ur, and cut out speed, Uo, are 16 m s and 25 m s respectively. The shaft tilt with respect to the horizontal is taken as 5 so, allowing for a 8 inclination of the flow to the horizontal as specified in the code, the maximum shaft tilt with respect to the flow is 13 . Category A turbulence characteristics are assumed. Hub-height wind speed (m s)...

International Electrotechnical Commission IEC

The IEC is currently developing a range of standards specifically applicable to wind turbines. The work is being undertaken in the main by its TC88 Technical Committee, and covers power performance, acoustics, blade testing, mechanical loads and power quality. Power performance testing is covered by the published IEC standard 6140012 (1998). The IEC standard also exists as a British and European Standard BS EN 61400-12 1998. All national standards bodies have a responsibility which extends to any significant technology. Accordingly, national standards committees have been formed in most countries to cover wind energy technology. In many instances these merely formalize the national participation in international standards formation, and tend to adopt acceptable international standards as national standards.

1062 Islanding and selfexcitation of induction generators

Fixed-speed wind turbines use induction generators to provide damping in the drive train and, as there is no direct access to the field of an induction generator, the magnetizing current drawn from the stator leads to a requirement for reactive power. In order to reduce the reactive power supplied from the network it is conventional to fit fixed-speed wind turbines with local power factor correction (PFC) capacitors. As long as the induction machine is connected to a distribution network its terminal voltage is fixed and so the PFC capacitors merely reduce the reactive power drawn from the network. However, once the induction generator is isolated from the network then there is the possibility of a resonant condition, known as self-excitation, leading to large over-voltages (Allan, 1959, Hindmarsh, 1970). When the generator is isolated from the network it will tend to accelerate as its load is removed. This increase in rotational speed, and consequently of frequency, adds to the...

933 Measurement prediction and assessment of windfarm noise

The sound power level of a wind turbine is normally determined by field experiments. These were originally specified in the IEA Recommended Practice (International Energy Agency, 1994) but are now described in an international standard (BS, 1999, IEC, 1998). Outdoor experiments are necessary because of the large size of modern wind turbines and the necessity to determine their noise performance during operation. The sound power level cannot be measured directly but is found from a series of measurements of sound pressure levels made around the turbine at various wind speeds from which the background sound pressure levels have been deducted. The method provides the apparent A-weighted sound power level at a wind speed of 8 m s, its relationship with wind speed and the directivity of the noise source for a single wind turbine. It does not distinguish between aerodynamic and mechanical noise although there is a method proposed to determine if tones are 'prominent'. In addition to...

311 The Method of Acceleration Potential 3111 Introduction

An aerodynamic model that is applied to the flight performance of helicopter rotors, and which can also be applied to wind turbine rotors that are lightly loaded, is that based upon the idea of acceleration potential. The method allows distributions of the pressure drop across an actuator disc that are more general than the, strictly, uniform pressure distribution of the momentum theory. The model has been expounded by Kinner (1937), inspired by Prandtl, who has developed expressions for the pressure field in the vicinity of an actuator disc, treating it as a circular wing. To regard a rotor as a circular wing requires an infinity of very slender blades so that the solidity remains small. which is the Laplace equation governing the pressure field on and surrounding the actuator disc. Given the boundary conditions at the actuator disc Equation (3.146) can be solved for the pressure field and, in particular, the pressure distribution at the disc. The pressure is continuous everywhere...

475 Power measurement

As it is the net power which is of interest the power transducer should be located downstream of any auxiliary loads. It is generally assumed that the wind turbine will be operating at a nominally fixed speed. For variable speed operation, the IEA indicated that the rotor speed must also be measured to enable changes in kinetic energy to be calculated and compensated for, and this should be done to within 1 percent of the nominal rotor speed. No such prescription is included in the international standard.

6132 Downwind configuration

The wind velocity deficit behind a wind-turbine tower is much greater than that in front of it, to the extent that Powles (1983) has reported a turbulent region with essentially no forward velocity extending up to four tower diameters downstream of an octagonal tower. Beyond this distance, recovery is relatively rapid, with the deficit reduced to about 25 percent at seven tower diameters downstream. In addition to the mean wind-speed velocity deficit behind the tower, vortex shedding results in additional wind-speed fluctuations over and above those already present due to turbulence. The two effects combine to present a harsh environment to the blades immediately behind the tower. The blades are subjected to a large negative impulsive load each time they pass the tower, which contributes significantly to blade fatigue damage, and the audible tower 'thump' that results is liable to be unwelcome. Designers usually mitigate both effects by positioning the rotor plane well clear of the...

681 Independent braking systemsrequirements of standards

DS 472 (Section 5.1.4) and the GL rules (Section 5.1.3) both require that a wind turbine shall have two independent braking systems. On the other hand, IEC 614001 (Section 5.1.2) does not explicitly require the provision of two braking systems (stating that the protection system shall include one or more systems capable of bringing the rotor to rest or to an idling state), but it does require the protection system to remain effective even after the failure of any non-safe-life protection system component.

74 Gearbox 741 Introduction

The design of industrial fixed ratio gearboxes is a large subject in itself and well beyond the scope of the present work. However, it is important to recognize that the use of such gearboxes in wind turbines is a special application, because of the unusual environment and load characteristics, and the sections which follow focus on these aspects. Sections 7.4.2 to 7.4.6 consider variable loading, including drive train dynamics and the impact of emergency braking loads, and examine how gear fatigue design is adapted to take account of it. The relative benefits of parallel and epicyclic shaft arrangements are discussed in Section 7.4.7, while subsequent sections deal with noise reduction measures, and lubrication and cooling. A useful reference is the American Gear Manufacturers Association Information Sheet (1996) which covers the special requirements of wind turbine gearboxes in some detail.

813 The safety system

The normal wind-turbine supervisory controller should be capable of starting and stopping the turbine safely in all foreseeable 'normal' conditions, including extreme winds, loss of the electrical network, and most fault conditions which are detected by the controller. The safety system acts as a back-up to the main control system, and takes over if the main system appears to be failing to do this. It may also be activated by an operator-controlled emergency stop button.

752 Variablespeed generators

There are two fundamental approaches to electrical variable-speed operation. Either all the output power of the wind turbine may be passed through the frequency converter to give a broad range of variable speed operation or a restricted speed range may be achieved by converting only a fraction of the output power. Figure 7.32 shows in schematic form how a broad range, variable-speed generation system may be configured. Early broad range variable-speed wind turbines used a diode rectifier bridge in the generator converter and a naturally commu-tated, thyristor, current source, converter on the network side (Freris, 1990). However, naturally commutated thyristor converters consume reactive power and generate considerable characteristic harmonic currents. On weak distribution systems it is difficult to provide suitable filtering and power factor correction for this type of equipment. Hence modern practice is to use two voltage source converters (Heier, 1998) with either a synchronous or...

683Mechanical brake options

A wind turbine brake typically consists of a steel brake disc acted on by one or more brake callipers. The disc can be mounted on either the rotor shaft (known as the low-speed shaft) or on the shaft between the gearbox and the generator (known as the high-speed shaft). The latter option is much the more common because the braking torque is reduced in inverse proportion to the shaft speeds, but it carries with it the significant disadvantage that the braking torques are experienced by the gear train. This can increase the gearbox torque rating required by as much as ca 50 percent, depending on the frequency of brake application (see Section 7.4.5). Another consideration is that the material quality of brake discs mounted on the high-speed shaft is more critical, because of the magnitude of the centrifugal stresses developed.

742 Variable loads during operation

The torque level in a wind turbine gearbox will vary between zero and rated torque according to the wind speed, with excursions above rated on fixed-speed pitch-regulated machines due to slow pitch response. The short-term torque fluctuations will be subject to dynamic magnification to the extent that they excite drive train resonances (see Section 7.4.3 below). In addition there will be occasional much larger torques of short duration due to braking events, unless the brake is fitted to the low-speed shaft. Figure 7.24 shows example load-duration curves (excluding dynamic effects and braking) for two 500 kW, fixed-speed machines - one stall-and the other pitch-regulated. The curve for the former is calculated by simply combining the power curve with the distribution of instantaneous wind speeds, which is obtained by superposing the turbulent variations about each mean wind speed on the Weibull distribution of hourly means. Excursions above rated power are not included. In the case of...

100 1000 10000 Voltage changes per min

If a number of wind turbines are subject to uncorrelated variations in torque then their power outputs and effect on network flicker will reduce as where n is the number of generators, P and p are the rated power of the wind farm and wind turbine respectively and AP and Ap are the magnitude of their power fluctuation. There is some evidence (Santjer and Gerdes, 1994) that on some sites wind turbines can fall into synchronized operation and in this case the voltage variations become cumulative and so increase the flicker in a linear manner. The cause of this synchronous operation is not completely clear but it is thought to be due to interactions on the electrical system caused by variations in network voltage. However, experience on most UK sites, where the winds are rather turbulent, has been that any synchronized operation lasts only a short while before being disrupted by wind-speed changes. This may not be the case offshore when the turbulence of the wind is likely to be lower.

513 Germanischer Lloyd rules for certification

Germanischer Lloyd's Regulation for the Certification of Wind Energy Conversion Systems, commonly referred to as the GL rules, adopts the same classification of wind turbines as IEC 61400-1, but specifies a single value of hub-height turbulence intensity of 20 percent. A larger number of load cases are specified, but many of them parallel cases in IEC 61400-1. However, the GL rules also provide a simplified fatigue spectrum for aerodynamic loading and simplified design loads for turbines with three non-pitching blades.

922 Design and mitigation

Wind turbine designers have recognized for some years that the overall form of the structure of a large wind turbine has to be pleasing and this aspect of industrial design is considered early in the development of a new machine. It is now generally recognized that for aesthetic reasons three-bladed turbines are preferred (see Section 6.5.6). Two-bladed rotors sometimes give the illusion of varying speed of rotation, which can be disconcerting. In addition, for a similar swept area, a two-bladed rotor will operate faster than one with three blades. There is considerable evidence (Taylor and Rand, 1991, Gipe, 1995) that a slower speed of rotation is more relaxing to the eye. This effect works to the advantage of the large modern wind turbines, which operate typically at 30-35 r.p.m. There is some speculation that, in the future, very large wind turbines designed specifically for remote offshore installation may revert to two-bladed rotors to realize the engineering benefits of this...

106 Electrical Protection

The electrical protection of wind turbines and wind farms follows the same general principles which are applied to any electrical plant (ALSTOM, 1987) but there are two significant differences. Because wind farms are frequently connected to the periphery of the power system it is common to find that the fault currents which will flow in the event of insulation failure are rather small. Although this is desirable from the point of view of reducing hazards, this lack of current can pose significant difficulties for the rapid and reliable detection of faults. In particular, some designs of high-voltage fuses rely on the energy within the arc for their correct operation. Hence they cannot be relied on to interrupt small fault currents when the arc energy is low. Secondly, fixed-speed wind turbines use induction machines and variable-speed wind turbines are interfaced to the network through voltage source converters. Neither induction generators or voltage source converters are a reliable...

1047 Power system studies

Power system studies are often required in order to assess the impact of embedded wind generators on the network and to ensure that the network conditions are such as to allow the wind generators to operate effectively. In the past simple manual calculations were used but sophisticated computer programs are now available at relatively low cost. However, considerable care is necessary when using these programs as a number of them were designed primarily to investigate systems with conventional generation using synchronous machines. Load flow (or power flow) programs take as input the network topology and parameters of the lines, cables and transformers. The customers' load and generator outputs are then used to calculate the steady-state performance of the network in terms of voltages, real and reactive power flows and losses. Generally a balanced three-phase system is assumed. For wind-energy applications it is important that the load flow package includes a good model of the tap...

135 Utility Resource Planning Tool

The wind and photovoltaic power, in spite of their environmental, financial, and fuel diversity benefits, are not presently included in the utility resource planning analysis because of the lack of the familiarity and analytical tools for nondispatchable sources of power. The wind and pv powers are treated as nondispatchable for not being available on demand. The Massachusetts Institute of Technology's Energy Laboratory has developed an analytical tool to analyze the impact of nondispatchable renewables on the New England's power systems operation. Cardell and Connors3 have applied this tool for analyzing two hypothetical wind farms totaling 1,500 MW capacity for two sites, one in Maine and the other in Massachusetts. The average capacity factor at these two sites is estimated to be 0.25. This is good, although some sites in California have achieved the capacity factor of 0.33 or higher. The MIT study shows that the wind energy resource in New England is comparable to that in...

1075 Impact on the generation system

The connection of wind generation acts to displace conventional, often fossil fuelled, plant with the highly desirable consequence of reducing gaseous emissions. However, significant penetrations of wind energy will alter the operation of the conventional generators and their costs. Wind farms will respond to the wind resource and so the rest of the generation system needs to be able to accommodate the variations in their output. At present in the UK this is not considered to be a major issue as on a typical winter day conventional generation is scheduled to meet a load increase of up to 12 GW over the morning load pick-up period which lasts some 2 h. This need for flexible operation is much greater than any immediate requirement due to wind farms. Operational experience from Denmark (Falck Christensen et al., 1997) is that the worst-case change in total wind power over 15 min is of the order of 10-15 percent of wind output at that time. This includes shut-downs of entire wind farms...

148 Renewable Capacity Limit

A recent survey made by Gardner5 and Ris0 National Laboratory6 in the European renewable power industry indicates that the grid interface issue is one of the economic factors limiting full exploitation of the available wind resources. The regions of high wind power potentials have weak existing electrical grids. In many developing countries such as India, China, and (a) Before connecting the wind farm (a) Before connecting the wind farm (b) After connecting the wind farm

465 Energy Distribution

Then, it would look like that in Figure 4-12, which is for the Rayleigh speed distribution. The wind speed curve has the mode at 5.5 m s and the mean at 6.35 m s. However, because of the cubic relation with the speed, the maximum energy contribution comes from the wind speed at 9.45 m s. Above this speed, although V3 continues to increase in cubic manner, the number of hours at those speeds decreases faster than V3. The result is an overall decrease in the yearly energy contribution. For this reason, it is advantageous to design the wind power to operate at variable speeds in order to capture the maximum energy available during high wind periods.

48 Analysis of Test Data

Both the IEA and the IEC standard use a 10 min averaging time. This corresponds approximately to the 'spectral gap' (Section 2.1) and means that wind distributions of either 10 min or 1 h means can be used with reasonable confidence to estimate annual energy production. Once erroneous data have been eliminated and any corrections applied, 10 min averages of wind speed and wind power should be calculated. Scatter plots should be presented as shown in Figure 4.22. The data are then analysed using the 'method of bins' (Akins, 1978). According to this procedure the wind speed range is divided into a series of intervals (known as bins). The IEC standard requires 0.5 m s bins throughout the range. Data sets are distributed into the bins according to wind speed and the ensemble average of the data sets in each bin calculated as follows

743Drivetrain dynamics

All wind turbines experience aerodynamic torque fluctuations at blade-passing frequency and multiples thereof because of the 'gust slicing' phenomenon, and these fluctuations will inevitably interact with the dynamics of the drive train, modifying the torques transmitted. The resulting drive train torque fluctuations can be assessed by dynamic analysis of a drive train model consisting of the following elements connected in series

84 Closedloop Control Analytical Design Methods

Clearly the choice of controller gains is crucial to the performance of the controller. With too little overall gain, the turbine will wander around the set point, while too much gain can make the system completely unstable. Inappropriate combinations of gains can cause structural responses to become excited. This section outlines some of the techniques which have been found to be useful in designing closed-loop control algorithms for wind turbines, such as the gains of a PI or PID controller for example. Clearly it is only appropriate here to give some useful hints and pointers. There are many standard texts on control theory and controller design methods, to which the reader should refer for more detailed information, for example D'Azzo and Houpis (1981), Anderson and Moore (1979), and Astrom and Wittenmark (1990).

925 Sociological aspects

There are a number of computer-based tools available for quantifying visual effects and landscape architects and planners have developed techniques to place quantitative measures on visual impact using professional judgement. However, public attitudes, which ultimately determine whether a wind farm may be constructed, are influenced by many more complex factors. Public attitudes to wind farms have been studied on a number of occasions (e.g., ETSU, 1993, 1994) and Gipe (1995) discusses this subject in considerable detail. In general, the large majority of people approve of wind farms after they have been constructed although a significant minority remains opposed to them. In particular, there is the difficult issue that some local residents consider they are paying a high cost for a benefit, either financial or environmental, which accrues to others. The financial benefits may be shared with the community in a number of ways including by the development of co-operative or...

839 Individual pitch control

Although individual or cyclic pitch control has been proposed many times, it has yet to find a place in commercial wind turbines. Asymmetrical loadings across the rotor are responsible for a significant contribution to fatigue loads, and in principle it should be possible to reduce these loads by controlling the pitch of each blade separately according to the conditions experienced by each blade. This may become particularly important for large wind turbines. However, in order to achieve any useful benefit, there must be some measurement available which can distinguish between the different blades, so that the controller can generate appropriate pitch demand signals for each. The simplest measure which could be used is simply the rotor azimuth angle. Although in a turbulent wind the wind speed variations across the rotor are not particularly dependent on azimuth, there are some effects (wind shear, tower shadow, upflow and shaft tilt) which cause a systematic azimuth-dependent...

673Passive pitch control

An attractive alternative to active control of blade pitch to limit power is to design the blade and or its hub mounting to twist under the action of loads on the blades in order to achieve the desired pitch changes at higher wind speeds. Unfortunately, although the principle is easy to state, it is difficult to achieve it in practice, because the required variation in blade twist with wind speed generally does not match the corresponding variation in blade load. In the case of stand-alone wind turbines, the optimization of energy yield is not the key objective, so passive pitch control is sometimes adopted, but the concept has not been utilized as yet for many grid-connected machines.

104 Embedded Dispersed Wind Generation

The wind is a diffuse source of energy with wind farms and individual turbines often distributed over wide geographical areas, and so the public electricity distribution networks, which were originally constructed to supply customer loads, are usually used to collect the electrical energy. Thus wind generation is said to be embedded in the distribution network or the generation is described as being dispersed. The terms embedded generation and dispersed generation can be considered to be synonymous. Conventional distribution systems were designed for a unidirectional flow of power from the high-voltage transmission network to the customers. Significant generation was not considered in the initial design of the distribution networks and alters the way they operate. Connection of embedded generation to distribution networks has important consequences both for the windfarm developer and the operator of the distribution network and is the subject of continuing interest (CIRED, 1999,...

1041 The electric power system

The bulk supply transformers are used to extract power from the transmission network and to provide it to the distribution networks at lower voltages. Practice varies from country to country but primary distribution voltages can be as high as 150 kV. Distribution networks are normally operated radially with a single path between the bulk supply transformers and the loads. In urban areas with high loads the distribution networks use large cables and transformers and so have a high capacity. However, in rural areas the customer load is often small and so the distribution circuits may have only a limited capability to transport power while maintaining the voltage within the required limits. Most wind farms are connected to rural, overhead distribution lines. The design of these circuits tends to be limited by consideration of voltage drop rather than thermal constraints and this severely limits their ability to accept wind generation.

96 Finance 961 Project appraisal

When evaluating power projects it has been conventional for many years to use techniques based on discounted cash flow (DCF) analysis (Khatib, 1997). This is based on the recognition that most people and organizations have a time preference for money and that they would rather receive money today not next year and would rather pay out money next year rather than today. The use of DCF analysis, with a high discount rate, tends to favour projects with short construction times, low capital costs and high operating costs. Thus, it makes renewable energy schemes, particularly offshore wind farms, rather difficult to justify, as a large initial capital investment is required with an income stream stretching into the future. In the UK at present, discount rates of 8 -12 percent would not be uncommon for commercial power projects reflecting the value placed on capital and the perceived level of risks. Some years ago (when the electricity supply industry was state-owned) the British Government...

916 Public consultation

Prior to the erection of the site anemometer masts the wind farm developer may wish to initiate some form of informal public consultation. This is likely to involve local community organizations, environmental societies and wildlife trusts. It may also be appropriate to keep local politicians informed. Of course, the erection of meteorological masts does not necessarily imply that the wind farm will be constructed but, as they are highly visible structures, careful consultation is required to ensure that unnecessary public concern is allayed.

915 Site investigations

At the same time as wind speed data are being collected more detailed investigations of the proposed site may also be undertaken. These include a careful assessment of existing land use and how best the wind farm may be integrated with, for example, agricultural operations. The ground conditions at the site also need to be investigated to ensure that the turbine foundations, access roads and construction areas can be provided at reasonable cost. Local ground conditions may influence the position of turbines in order to reduce foundation costs. It may also be important to undertake a hydrological study to determine whether spring water supplies are taken from the wind farm site and if the proposed foundations or cable trenches will cause disruption of the ground-water flow. More detailed investigations of the site access requirements will include assessment of bend radii, width, gradient and any weight restrictions on approach roads. Discussions are also likely to continue with the...

912Project feasibility assessment

Once a potential site has been identified then more detailed, and expensive, investigations are required in order to confirm the feasibility of the project. The wind farm energy output, and hence the financial viability of the scheme, will be very sensitive to the wind speed seen by the turbines over the life of the project. Hence it is not generally considered acceptable in complex terrain to rely on the estimates of wind speed made during the initial site selection but to use the measure-correlate-predict (MCP) technique to establish a prediction of the long-term wind resource (Derrick, 1993, Mortimer, 1994).

Example 101 Calculation of Voltage Rise in a Radial Circuit Figure 1010

Consider a 5 MW wind farm operating at a leading power factor of 0.98. The network voltage (V0) is (1 + j0) per unit and the circuit impedance (Z) is (0.05 + j0.1) per unit on a 10 MVA base. A power factor of 0.98 leading implies a reactive power draw of 1.01 MVAr. Thus, following Equation (10.7), the calculation becomes

151 Energy Delivery Factor

The key economic performance measure of a power plant is the electrical energy it delivers over the year. Not all power produced is delivered to the paying customers. A fraction of it is used internally to power the control equipment, meeting the power equipment losses and for the housekeeping functions such as lighting. In a typical wind farm or pv park, about 90 percent of the power produced is delivered to the customers, and the remaining is self-consumed for the plant operation. The wind farm energy delivery factor varies with season and that must be taken into account. For example, the quarterly average EDF in England and Wales from the beginning of 1992 to the end of 1996 are shown in Figure 15-1. The operating data show rather wide variations, ranging from 15 to 45 percent. The EDF is high in the first quarter and low in the last quarter of every year. Even with the same kWh produced per year, the average price per kWh the plant may fetch could be lower if the seasonal...

107 Economic Aspects of Embedded Wind Generation

In a deregulated power supply system it is important that all those involved in competitive activities (i.e., generators and suppliers of electrical energy) have equal access to the transmission and distribution monopoly networks in an open and non-discriminatory manner. This concept of 'open-access' is potentially very important for embedded wind generation as it guarantees that a wind farm can be connected to the network. A corollary of this free-market approach is that any change in the costs of the network caused by the wind farm need to be recognized and suitable payments made.

Behaviour of pitchregulated machines in fatigue

For pitch-regulated machines, the highest flapwise bending moment ranges occur at high wind speeds and yaw angles, but the largest mean values occur around Mean wind speed (m s) Figure 7.13 Relative Contribution to Life-time Fatigue Damage for Different Wind Speeds for a 1.5 MW Stall-regulated Machine, Including Effect of Mean Load, after Thomsen (1998) Mean wind speed (m s) Figure 7.13 Relative Contribution to Life-time Fatigue Damage for Different Wind Speeds for a 1.5 MW Stall-regulated Machine, Including Effect of Mean Load, after Thomsen (1998) rated wind speed. Moreover, blade pitching results in a rapid fall-off in bending moment with short-term mean wind speed just above rated. This behaviour is illustrated in Figure 7.10, which shows the variation in flapwise moment with short-term mean wind speed and yaw angle at 60 percent radius for a 40 m diameter machine. It transpires that the combination of the steep bending moment short-term wind speed characteristic, high mean...

54 Variable Speed Operation

At a given site, the wind speed can vary from zero to high gust. As discussed in Chapter 4, the Rayleigh statistical distribution is found to be the best approximation to represent the wind speed at most sites. It varies over a wide range. Earlier in Chapter 4, we defined the tip-speed ratio as follows Free upstream wind velocity V For a given wind speed, the rotor efficiency Cp varies with TSR as shown in Figure 5-11. The maximum value of Cp occurs approximately at the same wind speed that gives peak power in the power distribution curve of Figure 5-10. To capture the high power at high wind, the rotor must also turn at high speed, keeping the TSR constant at the optimum level. The centrifugal mechanical stress in the blade material is proportional to the TSR. The machine working at a higher TSR is stressed more. Therefore, if designed for the same power in the same wind speed, the machine operating at a higher TSR would have slimmer rotor blades. The ability of a wind turbine to...

43 Comparison of Measured with Theoretical Performance

The turbine considered in this section is run at constant rotational speed, the most common mode of operation, because this allows electricity to be generated at constant frequency. More detail about this method of operation will be discussed in the next section but the main feature is that there is, theoretically, a unique power output for a given wind speed. When the turbine was under test the chosen rotational speed was 44 r.p.m. Energy output and wind speed were measured over 1 min time intervals and the average power and wind speed determined. The test was continued until a sufficient range of wind speeds had been covered. The results were then sorted in 'bins' 0.5 m s of wind speed wide and a fairly smooth power versus wind-speed curve was obtained as shown in Figure 4.11. The turbine has a diameter of 17 m and would be expected to produce rather more power than shown above if operated at a higher rotational speed.

67 Power Control 671 Passive stall control

The simplest form of power control is passive stall control, which makes use of the post-stall reduction in lift coefficient and associated increase in drag coefficient to place a ceiling on output power as wind speed increases, without the need for any changes in blade geometry. The fixed-blade pitch is chosen so that the turbine reaches its maximum or rated power at the desired wind speed. Stall-regulated machines suffer from the disadvantage of uncertainties in aerodynamic behaviour post-stall which can result in inaccurate prediction of power levels and blade loadings at rated wind speed and above. These aspects are considered in greater detail in Section 4.2.2.

Behaviour of stallregulated machines in fatigue

For stall-regulated machines, the highest out-of-plane bending moment ranges and means normally occur at high wind speeds and yaw angles. This is illustrated in Figure 7.9, which shows the variation in this moment with wind speed and yaw angle at 60 percent radius for a 40 metre diameter machine, based on the three-dimensional data referred to at the start of Section 7.1.8 above. Note that above rated wind speed, the bending moment plots level off, so that a given departure of the lateral wind component from the zero mean, sustained over half a revolution, results in a larger bending moment fluctuation than a change in the longitudinal component of twice this magnitude. For example, if the mean wind speed is 24 m s, a lateral component of 6 m s (corresponding to a yaw angle of 14 ) causes a bending moment variation of 20 kNm when the blade rotates from 0 to 180 azimuth, compared to a variation of 17 kNm as a result of a 6 m s fluctuation in longitudinal wind speed (which, in any case,...

545 Operational load casesmachine fault states

Load case 2.1 Control system fault, with steady hub-height wind speed equal to Ur or Uo and normal wind shear. Partial safety factor normal. Load case 2.2 Protection system fault or preceding internal electrical fault, with steady hub-height wind speed equal to Ur or Uo and normal wind shear. Partial safety factor abnormal.

478 Data acquisition rate

For the purpose of power performance estimation the collected data are averaged to increase the correlation between wind speed and power. Consequently high rates of data sampling are not required. Where pulse generating instruments are used the logging interval should be chosen long enough to provide an acceptable resolution. For example, an anemometer might give 20 pulses m of wind run. If this is sampled at 0.5 Hz at a wind speed of 5 m s the resolution error will be 1 in 200 or 0.5 percent which is adequate. Analogue measurements are more likely and the international standard specifies a minimum sampling rate of 0.5 Hz.

4112 Sensitivity factors

The sensitivity factors indicate how changes in a particular measured parameter affect the relevant measurand. For example, temperature measurements are used to calculate the air density used in the power curve calculation through correction of the wind speed or power. We are interested in the rate of change of power (the measurand) with temperature, i.e. the gradient dP dK. From the correction formula, this factor is Pi 288.15 (kW K). Similarly the sensitivity factor for air pressure measurement is P 1013 (kW hPa). The sensitivity factor for the impact of wind-speed error on the power curve is given directly by the local gradient of the power curve, calculated from Wind speed The same sensitivity factor applies to all other influences on wind-speed measurement such as flow distortion and anemometer mounting effects.

423 Effect of rotational speed change

The power output of a turbine running at constant speed is strongly governed by the chosen, operational rotational speed. If a low rotation speed is used the power reaches a maximum at a low wind speed and consequently it is very low. To extract energy at wind speeds higher than the stall peak the turbine must operate in a stalled condition and so is very inefficient. Conversely, a turbine operating at a high speed will extract a great deal of power at high wind speeds but at moderate wind speeds it will be operating inefficiently because of the high drag losses. Figure 4.7 demonstrates the sensitivity to rotation speed of the power output - a 33 percent increase in r.p.m. from 45 to 60 results in a 150 percent increase in peak power, reflecting the increased wind speed at which peak power occurs at 60 r.p.m. At low wind speeds, on the other hand there is a marked fall in power with increasing rotational speed as shown in Figure 4.8. In fact, the higher power available at low wind...

472 Windspeed measurement

Wind speed is the most critical parameter to be measured so considerable emphasis should be placed on its accuracy. According to the IEA (1982) the anemometer should have an accuracy of 5 percent or better over the range of relevant wind speeds and according to the revised IEA recommendation (1990) it should be accurate to 0.1 m s or less for wind speeds between 4 and 25 m s. Finally the IEC have opted to eschew a stated precision, and require instead calibration against a traceable instrument. The instrument should be calibrated, before and after the test, so as to establish that its accuracy has been maintained throughout the test (MEASNET have documented a specified calibration procedure). To avoid problems, it is advisable to run in a new anemometer for a period of about 2 months before use, to allow the bearings to ease. Another characteristic of an anemometer is its distance constant, which the IEC states, should be 5 m or less. The distance constant is an indication of the...

831 Control of fixedspeed pitchregulated turbines

A fixed-speed pitch-regulated turbine usually means a turbine that has an induction generator connected directly to the AC network, and which therefore rotates at a nearly constant speed. As the wind speed varies, the power produced will vary roughly as the cube of the wind speed. At rated wind speed, the electrical power generated becomes equal to the rating of the turbine, and the blades are then pitched in order to reduce the aerodynamic efficiency of the rotor and limit the power to the rated value. The usual strategy is to pitch the blades in response to the power error, defined as the difference between the rated power and the actual power being generated, as measured by a power transducer. The primary objective is then to devise a dynamic pitch control algorithm that minimizes the power error, although as explained above, this may not be the only objective. When the power falls below rated, the pitch demand saturates at the fine pitch limit, maximizing the aerodynamic...

674Active stall control

Active stall control achieves power limitation above rated wind speed by pitching the blades initially into stall, i.e., in the opposite direction to that employed for active pitch control, and is thus sometimes known as negative pitch control. At higher wind speeds, however, it is usually necessary to pitch the blades back towards feather in order to maintain power output at rated. stalled above the rated wind speed, so that gust slicing (see Section 6.7.2) results in much smaller cyclic fluctuations in blade loads and power output. It is found that only small changes of pitch angle are required to maintain the power output at rated, so pitch rates do not need to be as large as for positive pitch control. Moreover, full aerodynamic braking requires pitch angles of only about 20 , so the travel of the pitch mechanism is very much reduced compared with positive pitch control. Figure 6.11 compares schedules of pitch angle against wind speed for active stall control and active pitch...

832 Control of variablespeed pitchregulated turbines

With a variable-speed generator, it becomes possible to control the load torque at the generator directly, so that the speed of the turbine rotor can be allowed to vary between certain limits. An often-quoted advantage of variable-speed operation is that below rated wind speed, the rotor speed can be adjusted in proportion to the wind speed so that the optimum tip speed ratio is maintained. At this tip speed ratio the power coefficient, CP, is a maximum, which means that the aerodynamic power captured by the rotor is maximized. This is often used to suggest that a variable-speed turbine can capture much more energy than a fixed-speed turbine of the same diameter. In practice, however, it may not be possible to realize as much gain as this simple argument would suggest. Maximum aerodynamic efficiency is achieved at the optimum tip speed ratio opt, at which the power coefficient CP has its maximum value CP(max). Since the rotor speed Q is then proportional to wind speed U, the power...

543Operational load casesnormal machine state

Several load cases have to be investigated in this category, so that the effects of extremes of gust loading, wind direction change and wind shear can be evaluated in turn. Two types of deterministic discrete gust models are used for the gust loading the 'rising gust' and the 'rising and falling gust'. In the former case, the wind speed rises sinusoidally over the time interval t 0to t T 2toa new value, according to the formula U(t) U + aU(1 cos 2 t T), and remains there. In the simple version of the 'rising and falling gust', the sinusoidal wind speed variation continues over the full cycle until the wind speed has returned to its original value. However, IEC 61400-1 defines a more sophisticated version, incorporating a brief dip in the wind speed before and after the rising and falling gust, the complete cycle being defined by where au is the standard deviation of the turbulent wind speed fluctuations, and the factor P takes the values 4.8 and 6.4 for gusts with recurrence periods...

82 Closedloop Control Issues and Objectives 821 Pitch control see also Sections 424 and 673

Below rated wind speed, the turbine should simply be trying to produce as much power as possible, so there is generally no need to vary the pitch angle. The aerodynamic loads below rated wind speed are generally lower than above rated, so again there is no need to modulate these using pitch control. However, for fixed-speed turbines, the optimum pitch angle for aerodynamic efficiency varies slightly with wind speed. Therefore, on some turbines, the pitch angle is varied slowly by a few degrees below rated in response to a heavily averaged anemometer or power output signal. Above rated wind speed, pitch control provides a very effective means of regulating the aerodynamic power and loads produced by the rotor so that design limits are not exceeded. In order to achieve good regulation, however, the pitch control needs to respond very rapidly to changing conditions. This highly active control action needs very careful design as it interacts strongly with the turbine dynamics. One of the...

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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