Library of Congress Cataloging-in-Publication Data Manwell, J. F. Wind energy explained: theory, design, and application / James Manwell, Jon McGowan. Wind energy explained: Theory, Design, and application [Book Review]. Article ( PDF Available) in IEEE Power and Energy Magazine 1(6) 51 · December with 4, Reads by J.F. Manwell, J.G. McGowan, and. Download as PDF, TXT or read online from Scribd Wind energy explained: theory, design, and application / James Manwell, Jon McGowan, Anthony Rogers .
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WIND ENERGY. EXPLAINED. Theory, Design and Application. Second Edition. J. F. Manwell and J. G. McGowan. Department of Mechanical and Industrial. Wind energy's bestselling textbook- fully revised. This must-have second edition includes up-to-date data, diagrams, illustrations and thorough. View Table of Contents for Wind Energy Explained. Wind Energy Explained: Theory, Design and Application. Author(s). J.F. Manwell · J.G.
Design and Application Table 2. Texas Rohatgi and Nelson. Wind Characteristics and Resources 41 14 13 Wind speed. In addition to a summary of the historic uses of wind power. The reason that wind energy began to disappear is primarily attributable to its non-dispatchability and its non- transportability. The propeller is usually made of polystyrene foam or polypropylene.
He holds an M.
His research and graduate student supervision at UMass has produced approximately technical papers in a wide range of energy conversion applications. His recent research interests in wind engineering have been concentrated in the areas of wind system siting, hybrid systems modeling, economics, and offshore wind engineering. He lives in Northfield, Massachusetts. Anthony Rogers holds both and M. He has had a long career in the wind energy field, and has been involved with a wide range of topics.
These have included wind turbine monitoring and control and the application of remote sensing devices. He lives in Amherst, Massachusetts. Please check your email for instructions on resetting your password.
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Wind Energy Explained: Theory, Design and Application Author s: Conference Series. Design and Application Spera. Fundamental Concepts of Wind Turbine Engineering. The section after that reviews wind resource measurement techniques and instrumentation. The chapter concludes with a summary of a number of advanced topics in the area of wind resource characterization. Wind Energy Explained: This information is used in the design and selection of a wind turbine intended for a particular site.
Operations — operation requirements include the need for wind resource information that can be used for load management. Siting — siting requirements can include the assessment or prediction of the relative desirability of candidate sites for one or more wind turbines.
Performance evaluation — performance evaluation requires determining the expected energy productivity and cost effectiveness of a particular wind energy system based on the wind resource.
The chapter starts with a general discussion of wind resource characteristics. Systems design — system design requires knowledge of representative average wind conditions. The material covered in this chapter can be of direct use to other aspects of wind energy which are discussed in the other sections of this book. The next two sections present a number of topics that enable one to analyze wind data.
Global Origins 2. The influence of each of these forces on atmospheric wind systems differs depending on the scale of motion considered. There is a pressure gradient force in the vertical direction. These include the classic references Putnam as well as books by Eldridge In addition to the pressure gradient and gravitational forces. Spera and Burton et al.
The circulation of the atmosphere that results from uneven heating is greatly influenced by the effects of the rotation of the earth at a speed of about kilometers per hour at the equator.
The variation in incoming energy sets up convective cells in the lower layers of the atmosphere the troposphere. In a simple flow model. These affect prevailing near surface winds. As shown in Figure 2. Design and Application There are a number of other sources of information on wind characteristics as related to wind energy. At the same time. It should be noted that this model is an oversimplification because it does not reflect the effect that land masses have on the wind distribution.
These include pressure forces. These include the work of Justus Hiester and Pennell Johnson The Coriolis force per unit mass. The direction of the Coriolis force is perpendicular to the direction of motion of the air. Coriolis force. The resultant of these two forces. Above the boundary layer. The resulting wind. This force decreases as the height above the ground increases and becomes negligible above the boundary layer defined as the near earth region of the atmosphere where viscous forces are important.
This imposes a further force on the wind. In reality. The gradient wind is also parallel to the isobars and is the result of the balance of the forces: Friction at the surface causes the wind to be diverted more toward the low-pressure region.
Smaller scale atmospheric circulation can be divided into secondary and tertiary circulation see Rohatgi and Nelson. Examples of tertiary circulation. During the day. Tertiary circulations are small-scale. These different surfaces can affect the flow of air due to variations in pressure fields. Secondary circulations include the following: The oceans act as a large sink for energy. All these effects lead to differential pressures which affect the global winds and many of the persistent regional winds.
Reproduced by permission of Alternative Energy Institute. An understanding of these wind patterns. These include sea breezes and mountain winds. The direction reverses at night. Figure 2. Secondary circulation occurs if the centers of high or low pressure are caused by heating or cooling of the lower atmosphere.
Wind Characteristics and Resources 27 land masses. These include the following: A review of each of these categories as well as comments on wind speed variation due to location and wind direction follows. Meteorologists generally conclude that it takes 30 years of data to determine long-term values of weather or climate and that it takes at least five years to arrive at a reliable average annual wind speed at a given location.
The ability to estimate the inter- annual variability at a given site is almost as important as estimating the long-term mean wind at a site. They can have a large effect on long-term wind turbine production. Time scale Figure 2. Inter-annual Inter-annual variations in wind speed occur over time scales greater than one year.
As will be discussed in later sections. Reproduced by permission of ASME 2. Design and Application Spring and summer maxima occur in the wind corridors of Oregon. Wind Characteristics and Resources 29 Figure 2. A typical diurnal variation is an increase in wind speed during the day with the wind speeds lowest during the hours from midnight to sunrise.
Researchers are still looking for reliable prediction models for long-term mean wind speed. Annual Significant variations in seasonal or monthly averaged wind speeds are common over most of the world. Winter maxima occur over all US mountainous regions. The largest diurnal changes generally occur in spring and summer.
It is interesting to note that this figure clearly shows that the typical behavior of monthly variation is not defined by a single year of data. Diurnal Time of Day In both tropical and temperate latitudes. Spring maxima occur over the Great Plains. Daily variations in solar radiation are responsible for diurnal wind variations in temperate latitudes over relatively flat land areas. The complexities of the interactions of the meteorological and topographical factors that cause its variation make the task difficult.
This variation can be explained by mixing or transfer of momentum from the upper air to the lower air. It is generally accepted that variations in wind speed with periods from less than a second to ten minutes and that have a stochastic character are considered to represent turbulence.
Design and Application Figure 2. For wind energy applications. Turbulence and its effects will be discussed in later sections of this chapter. As illustrated in Figure 2. More details on these factors as related to turbine design are discussed in Chapters 6 and 7 of this text. Reproduced by permission of Alternative Energy Institute vary with location and altitude above sea level.
Turbulence can be thought of as random wind speed fluctuations imposed on the mean wind speed. Texas Rohatgi and Nelson. These fluctuations occur in all three directions: Ten-minute averages are typically determined using a sampling rate of about 1 second. Short-term variations usually mean variations over time intervals of ten minutes or less.
Short-term Short-term wind speed variations of interest include turbulence and gusts. Although gross features of the diurnal cycle can be established with a single year of data.. Wind turbine structural loads caused by gusts are affected by these four factors.
Wyoming Hiester and Pennell. Wind Characteristics and Resources 31 10 January Wind speed. The graph shows monthly and five-year mean wind speeds for two sites 21 km apart. Short-term direction variations are the result of the turbulent nature of the wind. Seasonal variations may be small. Variations in Wind Direction Wind direction also varies over the same time scales over which wind speeds vary. These short-term variations in wind direction need to be Figure 2. Power from the wind is proportional to the area swept by the rotor or the rotor diameter squared for a conventional horizontal axis wind machine.
Crosswinds due to changes in wind direction affect blade loads. Wind Characteristics and Resources 33 considered in wind turbine design and siting. From the continuity equation of fluid mechanics.
Yawing causes gyroscopic loads throughout the turbine structure and exercises any mechanism involved in the yawing motion.
The wind power density is proportional to the density of the air. For standard conditions sea- level. The actual power production potential of a wind turbine must take into account the fluid mechanics of the flow passing through a power-producing rotor.
Horizontal axis wind turbines must rotate yaw with changes in wind direction. The wind power density is proportional to the cube of the wind velocity. The average wind power density. Design and Application Table 2. In practice. It is important to distinguish between the different types.
Table 2. If annual average wind speeds are known for certain regions. Using estimates for regional wind resources. Some sample qualitative magnitude evaluations of the wind resource are: The energy pattern factor is calculated from: More accurate estimates can be made if hourly averages. The economic potential is the technical potential that can be realized economically. The World Energy Council One such estimate World Energy Council.
On a global basis. Implementation potential takes into account constraints and incentives to assess the wind turbine capacity that can be implemented within a certain time frame. The majority of this land was in the West. Elliot et al. For worldwide wind resource assessments. These authors conclude that. This is based on the meteorological potential. More recent work on the world technical and economic potential of onshore wind energy is summarized in the paper of Hoogwijk et al. In comparison.
For offshore wind farms. Numerous wind resource estimates have been made for the potential of wind energy in the United States. In order to provide this fraction of the US electrical demand about billion kWh per year. In this study Gustavson based his resource estimate on the input of the solar energy reaching the earth and how much of this energy was transformed into useful wind energy.
As noted in Volume 1 of Wind Energy: The technical potential is calculated from the site potential. Wind Characteristics and Resources 35 of wind energy potential that can be estimated. They also feature expanded data collection methods and improved analysis techniques. The estimates published in the s are much more realistic than earlier ones. This is equivalent to the available wind resource.
In addition to variations due to the atmospheric stability. This variation of wind speed with elevation is called the vertical profile of the wind speed or vertical wind shear.
There are at least two basic problems of interest with the determination of vertical wind profiles for wind energy applications: Rotor blade fatigue life is influenced by the cyclic loads resulting from rotation through a wind field that varies in the vertical direction. It should be noted that these are separate and distinct problems.
For example.. Air density. Design and Application 0. The density of dry air can be determined by applying the ideal gas law. On the other hand. In wind energy engineering the determination of vertical wind shear is an important design parameter since: Instantaneous variation in wind speeds as a function of height e.
These factors will be discussed in the next sections. Seasonal variation in average wind speeds as a function of height e. Air density as a function of moisture content can be found in numerous books on thermodynamics such as Balmer Atmospheric stability is usually classified as stable. The stability of the atmospheric boundary layer is a determining factor for the wind speed gradients e.
Air pressure decreases with elevation above sea level. The international standard atmosphere assumes that the sea-level temperature and pressure are The pressure in the international standard atmosphere up to an elevation of m is very closely approximated by: The negative sign results from the convention that height.
As will be shown in the following analysis. Of course. Moist air is slightly less dense than dry air. If the atmosphere is approximated as a dry no water vapor in the mixture ideal gas. The first law of thermodynamics for an ideal gas closed system of unit mass undergoing a quasi-static change of state is given by: A summary of how the atmospheric temperature changes with elevation assuming an adiabatic expansion follows.
Above zi the temperature profile reverses. Assume the standard rate of 0.
For comparative purposes. Using conventional nomenclature. The air is heated near the ground. The surface layer of air extending to zi is called the convective or mixing layer. The concept of atmospheric stability is illustrated by considering the upward displacement of a small element of air to an altitude with a lower ambient pressure. The small element of air being lifted in this example will cool at the dry. The temperature profile before sunrise the solid line decreases with increasing height near the ground and reverses after sunrise dashed line.
Different temperature gradients create different stability states in the atmosphere. When one does exist. Each component is frequently conceived of as consisting of a short-term mean wind. The lateral component perpendicular to U is v z.
A summary and examples of these properties follows. Turbulent wind consists of longitudinal. More details concerning them are given in Appendix C and in the texts of Rohatgi and Nelson and Bendat and Piersol The sample would be denser and would tend to return to its original level.
This atmospheric state is called stable. If the test element of air had the same temperature as the surrounding air at the start. The longitudinal component. This explains the need for the daily balloon soundings taken at major airports worldwide to determine the actual lapse rate. Turbulent wind may have a relatively constant mean over time periods of an hour or more.
These features are characterized by a number of statistical properties: To generalize. One should note that the standard international lapse rate seldom occurs in nature. It is defined by the ratio of the standard deviation of the wind speed to the mean wind speed. This time period is usually taken to be ten minutes. Assuming that the sample interval is dt. In this calculation both the mean and standard deviation are calculated over a time period longer than that of the turbulent fluctuations.
In equation form: The sample rate is normally at least once per second 1 Hz. The short-term mean wind speed can then be expressed in sampled form as: For the sake of clarity. The turbulence intensity. The data. The lateral and vertical components can be decomposed into a mean and a fluctuating component in a similar manner. The length of this time period is normally no more than an hour.
In general. Note that the short-term mean wind speed.
Design and Application superimposed fluctuating wind of zero mean. Wind Characteristics and Resources 41 14 13 Wind speed. The Gaussian probability density function that represents the 0. The normal probability density function for continuous data in terms of the variables used here is given by: The probability density function that best describes this type of behavior for turbulence is the Gaussian.
Experience has shown that the wind speed is more likely to be close to the mean value than far from it. A measure of the average time over which wind speed fluctuations are correlated with each other is found by integrating the autocorre- lation from zero lag to the first zero crossing. Gusts are relatively coherent well correlated rises and falls in the wind. The autocorrelation function can be used to determine the integral time scale of turbulence as described below.
The single resulting value is known as the integral time scale of the turbulence. As described in Appendix C. While typical values are less than ten seconds.
The graph also includes the von Karman power spectral density function described above for comparison. Wind Characteristics and Resources 43 Multiplying the integral time scale by the mean wind velocity gives the integral length scale. Of most importance here. L is the integral length scale.
This is referred to as the von Karman psd elsewhere in this text. The mean wind velocity is A suitable model that is similar to the one developed by von Karman for turbulence in wind tunnels Freris. These sinusoidal variations will have a variety of frequencies. The power spectral density of the sample wind data above is illustrated in Figure 2. Appendix C gives details of how to determine psds.
Based on the autocorrelation function illustrated above. Other psds are also used in wind engineering applications see the discussion of standards in Chapter 7. A number of power spectral density functions are used as models in wind energy engineering when representative turbulence power spectral densities are unavailable for a given site. This type of analysis originated in electrical power applications. The integral length scale tends to be more constant over a range of wind speeds than is the integral time scale.
Since the average value of any sinusoid is zero. The first is that the average power in the turbulence over a range of frequencies may be found by integrating the psd between the two frequencies. The actual wind speed.
There are two points of particular importance to note regarding power spectral densities psds. Power spectral densities are often used in dynamic analyses. In wind energy studies. The first approach. It is based on a combination of theoretical and Figure The summary that follows will present some of the current models that are used to predict the variation in wind speed with elevation above ground.
The integration is from the lower limit of z0 instead of 0 because natural surfaces are never uniform and smooth. If one assumes a smooth surface. U the horizontal component of velocity. Wind Characteristics and Resources 45 empirical research. The second approach. This yields: In this region the pressure is independent of z and integration yields: Note that U is used here.
Near the surface of the earth the momentum equation reduces to: Combining Equations 2. A summary of each of these laws and their general application follows.
Both approaches are subject to uncertainty caused by the variable. Near the surface the pressure gradient is small. When this is used. Its basic form is: The log law is often used to extrapolate wind speed from a reference height.
Uref see Spera. Correlations Based on Both Surface Roughness z0 and Velocity Wind researchers at NASA proposed equations for a based on both surface roughness and the wind speed at the reference elevation. His expression has the form: It has been found that a varies with such parameters as elevation. Some researchers have developed methods for calculating a from the parameters in the log law.
A review of a few of the more popular empirical methods for determining representative power law exponents follows. Many researchers. Correlation Dependent on Surface Roughness The following form for this type of correlation was proposed by Counihan The wind velocity at 30 m is set out in Table 2.
Wind Characteristics and Resources 47 Table 2. Note that at 10 m.
Many authors define non-flat terrain as complex terrain this is defined as an area where terrain effects are significant on the flow over the land area being considered.
Design and Application wind power developers to accurately know the wind speed characteristics at turbine hub height generally between 60 to m — and across the rotor. Some of the effects of terrain include velocity deficits.
These were developed for flat and homogenous terrain. In the previous section. This conclusion was based on the use of experimental data sets from: The influence of terrain features on the energy output from a turbine may be so great that the economics of the whole project may depend on the proper selection of the site. Non-flat terrain has large-scale elevations or depressions such as hills.. Elevation differences between the wind turbine site and the surrounding terrain are not greater than about 60 m anywhere in an Flat terrain is terrain with small irregularities such as forest.
For these three types of terrain.
Note that some of these rules include wind turbine geometry: To qualify as flat terrain. Hiester and Pennell.
This section presents a qualitative discussion of a few of the more important areas of interest on the subject of terrain effects.
Recent work on this subject e. Flow in such terrain is divided into two classifications: Non-flat or complex terrain.
For man-made obstacles. This type of flow. Man-made obstacles are defined as buildings. Natural obstacles include rows of trees. Wind Characteristics and Resources 49 Figure 2. Flow conditions in mountainous terrain are complex because the elevations and depressions occur in a random fashion.
The elevation difference between the lower end of the rotor disc and the lowest elevation on the terrain is greater than three times the maximum elevation difference h within 4 km upstream see Figure 2.
An important point to be made here is that information on wind direction should be considered when defining the terrain classification. The distinction between the two is made with comparison to the planetary boundary layer.
This affects the local wind profile. For small-scale flows this classification is further divided into elevations and depressions. Note that the estimates in the figure apply at a level equal to one building height. Small-scale Features Researchers Hiester and Pennell. A summary of each follows. The results of an attempt to quantify data from man-made obstacles are shown in Figure 2. When the prevailing wind is not perpendicular. Depressions Depressions are characterized by a terrain feature lower than the surroundings.
In addition to diurnal flow variations in certain depressions. The ratio of length to height should be at least Ridges are elongated hills that are less than or equal to m above the surrounding terrain and have little or no flat area on the summit. Examples of the results for ridges follow. This classification includes features such as valleys. Characterization studies of this type of flow in water and wind tunnels.
Steeper slopes give rise to stronger wind flow. The slope of a ridge is also an important parameter. The change in speed of the wind is greatly increased if depressions can effectively channel the wind.
Wind Characteristics and Resources 51 Figure 2. The following types of large depressions have been studied under this terrain classification: Design and Application depressions.
Here the mountains can effectively channel and accelerate the flow. They include mountains. The flow over these features is the most complex.
An example of a large depression with the prevailing winds in alignment is shown in Figure 2. Large-scale Features Large-scale features are ones for which the vertical dimension is significant in relation to the planetary boundary layer. The large number of parameters affecting the wind characteristics in a valley. This occurs when moderate to strong prevailing winds are parallel to or in alignment within about 35 degrees with the valley or canyon.
These include orientation of the wind in relation to the depression. There are a number of ways to summarize the data in a compact form so that one may evaluate the wind resource or wind power production potential of a particular site. Later sections of this text will describe how such curves can be estimated from analytical models of the wind turbine system. This section will review the following topics: Normally these curves are based on test data.