Volume 2 wind energy 2 04 – wind energy potential

Volume 2 wind energy 2 04 – wind energy potential Volume 2 wind energy 2 04 – wind energy potential Volume 2 wind energy 2 04 – wind energy potential Volume 2 wind energy 2 04 – wind energy potential Volume 2 wind energy 2 04 – wind energy potential Volume 2 wind energy 2 04 – wind energy potential

2.04.1 Introduction

2.04.3.6.4 Landscape without neutral stability

2.04.3.6.7 Power spectrum of wind speed: Averaging periods

2.04.4.1.1 Instantaneous phenomenon: Wind gust

2.04.4.1.2 Diurnal breezes: Land–sea and valleys breezes

2.04.4.2.2 Weibull hybrid distribution

2.04.5.1 Stochastic Models for Simulating and Forecasting Wind Speed Time Series

2.04.5.2 Bayesian Adaptive Combination of Wind Speed Forecasts from Neural Network Models

2.04.5.2.1 Artificial neural network models

2.04.6.2 Wind Energy Potential 90

2.04.1 Introduction

The winds, in the macro-meteorological sense, are movements of air masses in the atmosphere These large scale-movements are generated primarily by temperature differences within the air layer due to differential solar heating Because the energy per unit surface area received from the sun depends on geographical latitude, temperature differences arise and hence pressure gradients which, together with the centripetal force and the Coriolis force associated with the rotation of the Earth, induce the movements of the air masses known as the gradient wind

Solar radiation, evaporation of water, cloud cover, and surface roughness all play important roles in determining the conditions

of the atmosphere The study of the interactions between these effects is a complex subject called meteorology, which is covered by

concerning the flow of wind will be considered in this chapter

2.04.2 Wind Characteristics

2.04.2.1 Origin of Wind

Wind is caused by differences in pressure When a difference in pressure exists, the air is accelerated from higher to lower pressure

On a rotating planet the air will be deflected by the Coriolis effects, except exactly on the equator Globally, the two major driving

the tropics and upward from frictional effects of the surface, the large-scale winds tend to approach geostrophic balance Near the

inward into low pressure areas

2.04.2.2 Meteorology of Wind

The basic driving force of air movement is a difference in air pressure between two regions This air pressure is described by several physical laws One of these is the ideal gas law:

where p is pressure in Pascal (N m−2), V is volume (m3), n is the number of moles, R is the universal gas constant (8.3144 J K−1 mol−1), and T is absolute temperature (°K)

2.04.2.3 Wind Direction and Wind Velocity

As the warm air expands and the cold air condenses, this process creates zones of relative high or low pressure Afterwards, the wind

is fundamentally the movement of air created by these differences in pressure

Theoretically, at the Earth’s surface, the wind blows from high-pressure areas toward low-pressure areas However, at the

of being perpendicular to them In the northern hemisphere, the wind rotates counter-clockwise round cyclonic areas and clockwise round anticyclonic areas In the southern hemisphere, these wind directions are reversed The wind direction is determined by the direction from which it blows For example, it is a westerly wind if the air blows from the West

2.04.2.4 Fundamental Causes of Wind

radiation received at different latitudes and the rotation of the earth Superimposed upon the general world wind circulation, due to these two factors, modifications arising from local disturbances, such as the tropical cyclone

We have on a terrestrial scale regular pressure systems that produce important winds, called dominant winds or general

most important global wind characteristics

Polar circle

Subtropical high pressure belt

Equator (low pressure)

Subtropical high pressure belt

Polar circle

South pole

7500 m

Polar

Polar easterlies

Westerlies

Westerlies

Front Storm

Trade Winds N.E

S.E

66°

30°

30°

66°

0°

Doldrums Trade Winds

Polar Front Storm Polar easterlies

Meridian cross section of the atmosphere showing the meridian circulation

Figure 1 General world wind circulation [2]

In each hemisphere, we can discern three more or less individualized cells: a tropical cell, a temperate cell, and a polar cell, which turn one against the other like cogs in a gear box The north and south tropical cells are separated from one another by the equatorial calm which is a low-pressure area and from the temperate cells by the subtropical high-pressure zones

Actually, the sketch is not perfect The unequal heating of oceans and continents surface, relief, vegetation, and seasonal variations deform and modify the high- and low-pressure zones There are also atmospheric disturbances created by masses of cold air that move, from time to time, from the poles towards the equator Thus, the state of the atmosphere is continually evolving

As a result, the most favorable area for wind energy production is situated on the continents near the seashores or in the sea

2.04.3 Wind Measurements

2.04.3.1 Wind Speed

An anemometer is a device for measuring wind speed and is a common weather station instrument The term is derived from the

2.04.3.2 Cup Anemometers

The cup anemometer consists of three or four hemispherical cups each mounted on one end of four horizontal arms, which in turn are mounted at equal angles to each other on a vertical shaft The airflow that pass the cups in any horizontal direction turns the cups

in a manner that is proportional to the wind speed

The three-cup anemometer was developed in 1926 It had a more constant torque and responded more quickly to gusts than the

2.04.3.3 Other Anemometers

Anemometer types include the propeller, cup, pressure plate, pressure tube, hot wire, Doppler acoustic radar, sonic, and so on The propeller and cup anemometers depend on rotation of a small turbine for their output, while the others basically have no moving parts

2.04.3.3.1 Pressure plate anemometer

Another type of anemometer is known as a pressure plate or normal plate anemometer This is the oldest anemometer known having been developed in 1667 It uses the principle that the force of moving air on a plate being normal to wind is proportional to the area of the plate and to the square of wind speed The main application of this type of anemometer has been in gust studies because of its very short response time [3, 5]

2.04.3.3.2 Pressure tube anemometer

Yet another type of anemometer is the pressure tube anemometer It is not used much in the field because of difficulties with ice, snow, rain, and the sealing of rotating joints However, it is often used as the standard in a wind tunnel when these difficulties are not present It has been known for almost two centuries that the wind blowing into the mouth of a tube causes an increase of

Figure 2 Cup anemometer [4]

2.04.3.3.3 Hot-wire anemometer

The hot-wire anemometer uses a very fine wire electrically heated up to some temperature above the ambient Air flowing via the wire has a cooling effect on the wire As the electrical resistance of most metals is dependent upon the temperature of the metal, a relationship can be obtained between the resistance of the wire and the flow speed

The hot-wire anemometer, while extremely delicate, has extremely high-frequency response and fine spatial resolution com­ pared to other measurement methods, and as such is almost universally employed for the detailed study of turbulent flows, or any flow in which rapid velocity fluctuations are of interest [3, 5]

2.04.3.3.4 Laser Doppler anemometer

The laser Doppler anemometer uses a beam of light from a laser that is divided into two beams, with one propagated out of the anemometer Particulates flowing along with air molecules near where the beam exits reflect, or backscatter, the light back into a detector, where it is compared to the original laser beam When t he particles are in great motion, they produce a Doppler shift f or measuring wind speed in the laser light, which is used to calculate the speed of the particles, and therefore the air around the anemometer [3, 5]

2.04.3.3.5 Sonic anemometer

The sonic anemometer was developed in the 1970s It uses ultrasonic sound waves to determine instantaneous wind speed by measuring how much sound waves traveling between a pair of transducers are sped up or slowed down by the effect of the wind Sonic anemometers can take measurements with very fine temporal resolution, 20 Hz or better, which makes them well suited for turbulence measurements The lack of moving parts makes it appropriate for long-term use in exposed automated weather stations and weather buoys where the accuracy and reliability of traditional cup-and-vane anemometers is adversely affected by salty air or large amounts of dust [3, 5] Two-dimensional (wind speed and wind direction) sonic anemometers are used in applications such as weather stations, ship navigation, wind turbines, aviation, and weather buoys

2.04.3.4 Wind Direction

The wind vane, used for indicating wind direction, is one of the oldest meteorological instruments When mounted on an elevated shaft or spire, the vane rotates under the influence of the wind such that its center of pressure rotates to leeward and the vane points

current blows from the West

Modern aerovanes combine the directional vane with anemometer Colocating both instruments allows them to use the same axis and provides a coordinated read out

2.04.3.5 Wind Rose

A wind rose is a graphic tool used by meteorologists to give a succinct view of how wind speed and direction are typically distributed at a particular location It summarizes the occurrence of winds at a location, showing their strength, direction, and frequency (Figure 4(a))

N

E

S Percentage of calms within circle arcs represent 5% intervals

Resultant vector

189° – 18%

Wind speed (knots)

NORTH

> = 22 17–21 11–17 7–11 4–7 1–4 Calms: 2.59%

20%

16%

12%

4%

8%

SOUTH Figure 3 Wind direction: vane and transmitter [5]

Figure 4 (a) Wind rose [6] (b) Wind rose [3]

Historically, the wind rose was a predecessor of the compass rose, as there was no differentiation between a cardinal direction and the wind which blew from such a direction It was included on maps in order to let the reader know the characteristics of the eight major winds Using a polar coordinate system of gridding, the frequency of winds over a time period could be plotted by wind direction, with color bands showing wind ranges Presented in a circular format, the wind rose shows the frequency of winds

blows from a particular direction Each concentric circle represents a different frequency, emanating from zero at the center to increasing frequencies at the outer circles A wind rose plot may contain additional information, in that each spoke is broken down into color-coded bands that show wind speed ranges The wind rose typically uses 16 cardinal directions, such as North (N), NNE,

NE, and so on

There are a number of different formats that can be used to display wind roses A particular method for describing wind speed

2.04.3.6 Wind Speed Profiles

2.04.3.6.1 Roughness classes and lengths

The shape of the terrain over which the wind is flowing will have a frictional effect upon the wind speed near the surface Both the height and the spacing of the roughness elements on the surface will influence the frictional effect on the wind A single parameter, the surface roughness length, z0, is used to express this effect

Roughness classes (RCs) and roughness lengths are characteristics of the landscape used to evaluate wind conditions at a

1 2 3 4 5 6

60 Height (m)

α= 0,1

α= 0,4

50 40

30 20

10

0

1

2

V

V10

V = V10 h

10

α

h

10

Figure 5 Wind shear related to a reference height of 10 m, for various roughness heights, α [6]

ln ðz0Þ

ln ðz0Þ

ln ð3:3333Þ

2.04.3.6.2 Wind shear

The wind speed profile tends to a lower speed as it moves closer to the ground level This is designated as wind shear It is found that

roughness of the terrain (Figure 5)

1 α

where V1 is the wind velocity at some reference height h1 and V2 is the wind velocity at height h2

follows [8]:

The variation of wind speed with height depends on the surface roughness and the atmospheric stability, and we could assume that wind speed grows logarithmically with height The wind speed at a certain height above ground can be also estimated as a function

ð

ln z

z0

The reference speed Vref is a known wind speed at a reference height zref

2.04.3.6.3 Turbulence

Wind flowing around buildings or over very rough surface locations exhibits rapid changes in speed and/or direction, called turbulence This turbulence decreases, for example, the power output of the windmill and can also lead to unwanted vibration of the

Hills can sometimes be used to give an enhancement of wind velocity over that on the surrounding plain The best hills are those

previously quoted relationship cannot be used and it is necessary to measure wind speed on the considered site

2.04.3.6.4 Landscape without neutral stability

The previous analysis assumed neutral atmospheric stability conditions meaning that a parcel of air is adiabatically balanced from a thermodynamic perspective Neutral stability is a reasonable assumption in high wind when shearing forces rather than buoyancy

2 H

H

20 H

≈

2 H

≈

Figure 6 Zone of turbulence over a small building [8]

High wind at low altitude

High wind

High wind

Inverse flow perturbations Inverse flow

1

2

3

Figure 7 Acceleration of wind over hills [6]

forces are dominant However, the atmosphere is rarely neutral and the buoyancy forces usually predominate over the shear forces

as noticed from the scattering of the measurements at neighboring data collection points

2.04.3.6.5 Modified log law

The logarithmic wind profile equation in the lowest 100 m may still be used under nonequilibrium conditions with some appropriate modifications [7]:

Wind speed (m/s)

20

15

10

5

150 m

50 m

10 m

Time (min)



½7

where Vfr is friction velocity, k is von Karman constant (k = 0.4), L is Monin–Obukhov length, Ψ is correction term, and z0 is roughness length

given by [7]

0cp

8

where T0 is ground surface absolute temperature (°K), q0 is ground surface heat flux, cp is heat capacity of the air at constant pressure, and g is gravity’s acceleration

2.04.3.6.6 Nature of atmospheric winds

To be able to understand and predict the performance, for example, of wind turbines, it is essential that the designer has knowledge

of the behavior and structure of the wind itself that will vary from time to time and from site to site dependent on the climate and topography of the region, the surface condition of the terrain around the site, and various other factors Since the early 1970s, significant progress has been made in our understanding of the structure of the wind, in our ability to predict the conditions likely to

be experienced at a site and to assess the suitability of a site for generating power from wind

conditions These records demonstrate the main characteristics of the flow in the region near the ground The turbulent fluctuations are random in character and they are not always governed by deterministic equations, but we will need to use statistical techniques This short-spanned change in wind speed is primarily due to the local geographic and weather effects

2.04.3.6.7 Power spectrum of wind speed: Averaging periods

In order to separate the short-period fluctuations of wind speed due to mixing from the long-term changes associated with

T

to þ

T

to −

2 where V(t) is the instantaneous wind speed component along the average wind direction at time, t, and we must define the time interval, T, over which the average is taken

is the gap between periods of about 10 min and 2 h where the spectrum contains very little energy The significance of the spectral gap is that, if the averaging period for the mean wind speed is chosen to lie within this range, the synoptic variations can be separated from those due to turbulence It has been suggested that a good averaging period for defining mean wind speeds lies between 20 min and 1 h

Figure 8 Simultaneous recordings of wind speed at three heights [9]

4

2

0

Spectral

gap

Figure 9 Spectrum of horizontal wind speed at Brookhaven National Laboratory [9]

2.04.4 Analysis of Wind Regimes

Here we will discuss the wind pattern as such and its characterization by numbers and graphs We assume that a set of short-time measurements from a meteorological station is available The reliability of the data is not questioned here

2.04.4.1 Wind Speed Variation with Time

2.04.4.1.1 Instantaneous phenomenon: Wind gust

The lower region of the atmosphere is known as the planetary boundary layer and the movement of the air is retarded by frictional forces and large obstructions on the surface of the Earth, as well as by the Reynolds stresses produced by the vertical exchange of momentum due to turbulence The turbulence, which may be mechanical and/or thermal in origin, also causes rapid fluctuations in

It is often useful to know the maximum wind gust that can be expected to occur in any given time interval This is usually represented by a gust factor G, which is the ratio of a wind gust to the hourly mean wind speed G is obviously a function of the turbulence intensity, and it also clearly depends on the duration of the gust Thus, the gust factor for a 1 s gust will be larger than for a

3 s gust, since every 3 s gust has within it a higher 1 s gust [10]

Time-varying wind speeds and, more specifically, the uncertainty over what wind speed will be during the next hour or by hour

necessitates a control system that adjusts the rotor speed and thus optimizes the turbine’s power output for slow wind speed variation and attenuates high-frequency wind gust effects to reduce the resulting fatigue

V in m/s

40

30

20

10

Time in sec

Figure 10 Wind gusts from Orkney (UK) [2]

day (a)

Figure 11 Diurnal wind

2.04.4.1.2 Diurnal breezes: Land–sea and valleys breezes

At the sea shore the General Circulation Patterns have superimposed on them an airflow from the sea to the land at night, and in the

These sea/land breezes can be quite strong and can dominate the wind pattern A rather similar situation to the sea breeze can arise between valleys and mountains

2.04.4.1.3 Seasonal phenomenon

The monthly variation of wind speed depends essentially on geographical location and only meteorological measurements can give information about these variations But, generally, the yearly variations are repetitive with a good precision

2.04.4.1.4 Time distribution

Plotting the monthly average of each hour of the day shows the diurnal fluctuations of the wind speed in that particular month

should be noted that in this specific case, the diurnal variation can be advantageous for wind energy generation as we may need more power during the daytime hours than at night

In a similar manner, the monthly average can be plotted to show the monthly fluctuations of the wind speed, compared with the

availability of power matches the demand

2.04.4.1.5 Calm wind spells

According to the Beaufort wind scale at a standard altitude of 10 m above an open, even surface, the action of wind is calm when the wind speed is less than 0.3 m s−1 and smoke rises vertically [1, 4]

2.04.4.1.6 Frequency distribution

Another type of information that can be extracted from the time distribution of the data is the distribution of periods with low wind

days This type of information is valuable for the calculation of the size of storage devices

Apart from the distribution of the wind speeds over a day or a year, it is important to know the number of hours per month or per year during which the given wind speeds occurred, that is, the frequency distribution of the wind speed (Figure 14) The maximal value of this histogram corresponds to the most frequent wind speed

It is often important to know the number of hours that a windmill, for example, will not run or the time fraction that a windmill produces more than a given power In this case, it is necessary to add the number of hours in all intervals above the given wind