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Windenergietechnik > The Wind > Flashcards

The Wind Flashcards

1
Q

Challenge in designing a wind turbine

A

Find the balance between high degree in efficiency and a low degree on mechanical loading

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2
Q

Definition Wind

A

Wind is the motion of air due to a potential gradient (thermal differences). Air particles move from a point with higher potential to a point with lower potential
- dynamic phenomenon

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3
Q

What are Potential differences caused by?

A
  • By solar radiation (changing in time)
  • more radiation at the equator lower potential at the poles
  • potential gradient results in passat winds
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4
Q

What is the Potential

A
  • Potential is a combination of pressure and temprature
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5
Q

What are the passat winds?

A
  • Northwinds in northern hemisphere

- Southwinds in southern hemisphere

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6
Q

Cyclonic western winds

A
  • Closer to the poles
  • Caused by earth rotation and coriolis forces
  • western winds on northern and southern hemisphere
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7
Q

Geostrophic winds

A
  • in a height where there is no influence of earth surface
  • undistributed winds (no boundary layer effects)
  • counter-clockwise in nothern and clockwise in southern hemisphere
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8
Q

Local wind systems

A
  • caused by sudden changes in landscape
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9
Q

Sea-land circulation

A
  • land heats faster than sea
  • daytime sea-land winds
  • land cools faster than sea
  • nighttime: land-sea winds
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10
Q

Mountain-valley circulation

A
  • daytime: uphill winds

- nighttime: downhill winds

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11
Q

Parameters for the description of the wind

A
  • Wind velocity
  • Wind direction
  • Frequency of occurence distributions
  • Wind shear
  • Turbulence intensity
  • Coherence in wind field
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12
Q

Wind velocity

A
  • Velocity of air particles in motion

- not constant in time and space and height

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13
Q

Planetary boundary layer

A
  • height range in which the wind velocity is gradient
  • layer around surface of the eartth where surface influences wind
  • thickness is weather dependent (100m at night to 2km at warm days)
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14
Q

Wind shear

A
  • gradient in wind velocity along the height depending on surface roughness
  • Surface friction is much larger for cities, thickness of layer is much bigger
  • the lower the roughness the thinner the PBL
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15
Q

Atmosphere Stability

A
  • stabil (lower temperatures at the surface than above, strong vertical massflow and turbulence)
  • neutral (temperatur at the surface is similar to higher hights, occurs with high velocities, mixing of temperature)
  • conncetive (higher temperatures at the surface than above, no vertical massflow)
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16
Q

Exponential model

A
  • commonly used in load analysis

- parameter alpha is given in the respective design standard

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17
Q

Logarithmic model

A
  • commonly used in energy assesments

- benefit: physical interpretation of the free parameter z0 (roughness length, height where windflow = 0)

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18
Q

Wind shear models

A
  • Exponential model

- Logarithmic model

19
Q

roughness length

A

the smaller the roughness length, the higher the wind velocity at the same height (Offshore: roughness length is smaller)
- more energy at lower hubheights

20
Q

Roughness classes

A 
0 = Offshore z0 = 5mm
3 = Strongly structured  landscape z0 = 1m
21
Q

Flow around obstacles

A
  • effects strongly depending on shape / slope etc.
  • disturbance increases with increasing distance and height
  • flow situation is non-linear (can have severe effects)
22
Q

Turbulence

A
  • Wind = Mean wind velocity u + turbulence intensity I
  • short term change in time and space of the wind velocity (deviation)
  • timescale btw. 10s - 10min
  • spacescale: smallscale 1m - 100m
  • the closer to earth the more deviations from the mean velocity curve
23
Q

Standard deviation

A

Square root of variance

24
Q

Turbulence intensity

A

Ratio between standard deviation and the mean wind velocity
- 5 - 40 %
-

25
Q

Differen offshore and onshore environments

A

If wind velocity increase, the roughness of the seasurface increases which leads to a increase in turbulence

26
Q

Coherence in the wind field

A
  • Captures the spatial distribution of turbulence in the frequency domain across the rotor area
27
Q

Frequency of occurence distribution based on a measurement

A
  • 10 min average measurements over one year
  • extrakt histogram:
  • 1m/s bins
  • count no. of events in each windspeed bin (histogram im absolut numbers)
  • devide the absolut numbers by the overall number of events (histogram in relative values)
  • percenctage of time in the year where there is a certain wind velocity
28
Q

Frequency of occurence distribution function

A
  • use of Weibull distribution function
  • A average windspeed at side
  • k: shape factor (1-4)
  • increase of k, more narrow Weibull function,
  • smaller standard deviation the more Weibull function approaches Gaussian function
29
Q

Weibull with k = 2

A
  • Central Europe shape factor
  • Rayleigh distribution
  • the larger the mean velocity, the smaller the peak in the rayleigh distribution and the wider the distribution function
30
Q

Windrose

A
  • plots the frequency of occurence distribution for each wind direction
  • can be transfered in a Energie rose (with mean wind velocity)
31
Q

Pressure sensors

A
  • Prandtl tubes
  • can measure static pressure and total pressure
  • you can calculate dynamic pressure (difference)
  • calculate velocity
  • frequent maintenance not usable for wind measurement campaign
32
Q

Cup anemometer

A
  • Standard measurement device
  • derive linear relation btw. rotation speed and wind velocity
  • calibrate devite in wind tunnel
  • issue: overspeeding effect
  • may freeze
33
Q

Overspeeding effect

A
  • rotor of cub anemometer has inertia does not follow all high frequency changes in wind velocity
  • overspeeding is increasing with increasing turbulence
34
Q

Inclined inflow

A

Cup anemometer is not influenced by wind direction (pro) but strong inclination angle might be problematic
- strong vertical component leads to wrong results

35
Q

Ultrasonic anemometer

A

+ accurate technique in 2D and 3D

  • supports creates wakes which disturb the measurement
  • expensive
36
Q

LiDAT measurement

A
  • light detection and ranging
  • laser based
  • velocity of particles in direction of laser beam
    + 2D or 3D
  • weaknesses for clear atmospheres
  • very expensive
    + usefull for valiadation
37
Q

Sphere anemometer

A
  • drag of sphere will result in a deflection of the beam
  • with turbulences the anemometer will start to vibrate
    + very accurate
    + low cost
    no disadvantages
38
Q

Wind resource database

A
  • European windatlas
  • local windmaps
    + cheap
  • only mean velocity
39
Q

Wind measurement

A
  • at least for 1 year and for full multiples only
  • highest height of measurement should at least be at 2/3 of expected hub height
  • via cup anemometer (wind speed), wind vane (direction) and thermometer (mass density) on met mast
40
Q

Assumptions Energy assessment

A
  • no dependency on wind direction
  • single wind turbine located at met mast position
  • no wake loss

(all not correct)

41
Q

Power curve

A
  • plots the power produced by the wind turbine against the wind speed
42
Q

AEP

A

Annual Energy Production (for each windspeed)
Product of the power produced by the turbine for the given wind velocity at the time period of the year where that velocity occurs (Product of frequency of occurce of wind speed and 1 year)

  • Overall AEP, sum of AEPs for all windspeeds (ur to uout)
43
Q

Wind Atlas Method

A
  • transfering data do different locations
  • correction of obstacles roughness and orology
    + high accuracy for less complex sites
  • spatial transfer based on laminar fluid flow theory (not valid)
  • least inaccurate model (not accurate)
  • alternative: CFD

Windenergietechnik (6 decks)

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