Fluid Dynamics and Wind Energy

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Fluid Dynamics and Wind Energy
Peter Stuart
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Why is Fluid Dynamics Important in Wind Energy?

• Wind Turbine Aerodynamics
• Modern turbines are lift based devices.
  Understanding the aerodynamics of wind turbines
  allows turbine designers to maximise energy
  capture while keeping loads to a minimum.
• Wind Resource Assessment
• Wind turbines are sited in the atmospheric
  boundary layer (lowest 1km of atmosphere). Wind
  speeds and turbulence levels within the
  atmospheric boundary layer are strongly
  influenced by the terrain (hills, mountains,
  forests etc). A good understanding of the flow of
  the atmospheric boundary layer is essential to
  site wind turbines effectively.
• Wake Modelling
• Downwind of turbines a region of reduced mean
  wind speed and increased turbulence is produced.
  Wakes must be well understood to site wind
  turbines effectively.

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Wind Turbine Design (1) Energy in the Wind

• The amount of energy available in the wind is
  given by

• U Wind Speed
• ? Air Density
• A Rotor Swept Area

• Although the energy in the wind continues to
  increase with wind speed, wind turbines regulate
  their power output to their rated capacity above
  a certain wind speed.

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Wind Turbine Design (2)

• Number of Blades
• Wind turbines can have one, two, or three (or
  more) blades. Reducing the number of blades
  offers the potential to reduce the cost of the
  machine. However the less blades the faster a
  turbine needs to rotate to achieve optimum energy
  capture. Faster rotating machines produce more
  noise and are believed to have a greater visual
  impact. The popularity of the three bladed
  concept is largely due to it achieving optimum
  energy capture while minimising noise and visual
  impact.
• Wind Turbine Size
• We saw previously that the power produced by a
  wind turbine is directly proportional to its
  swept area. Historically wind turbines have got
  bigger and bigger in order to achieve greater and
  greater energy capture. Large modern turbines
  have a rated power of several Megawatts.
• Vertical / Horizontal
• A wind turbine rotor may rotate on either a
  horizontal or a vertical axis. Both design
  concepts are valid, however historically the
  horizontal axis design concept has achieved far
  greater popularity. Today most (and certainty all
  large scale) wind turbines are horizontal axis
  machines.

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Wind Turbine Design (3) Power Regulation

• The lift on turbine blades depends on the wind
  speed and the angle of attack.

• The angle of attack is determined by the wind
  speed and the rotational speed of the turbine.

• For a given pitch angle the angle of attack will
  increase with wind speed.

• Three power regulation philosophies
• (i) Passive Stall Regulation
• (ii) Active Stall regulation
• (iii) Pitch Regulation

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Resource Modelling (1) Turbulent Flow
Instantaneous wind speed is the sum of a mean
value and a fluctuation.
Aim to site turbines in areas of high mean wind
speed and low turbulence, maximising both annual
revenue and turbine lifetime.
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Resource Modelling (2) Why not just measure?

• Onsite measurements take time and cost money.
  Typically the wind speed and turbulence is
  measured at only one, perhaps two locations
  onsite

but wind speeds and turbulence levels can vary
significantly across a site.

• A flow model is required to extend the measured
  wind climate at the reference location across a
  site.

• Combining measurements at one location with a
  perfect wind flow model would be equivalent to
  placing measurement masts all over a site.

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Resource Modelling (3) Different types of Models

• Wind energy industry has traditionally relied
  upon simplified flow solvers which linearise the
  governing fluid flow equations.

Non-linear term makes solving flow equations more
difficult.

• Under the assumption of shallow gradients the
  simpler models replace the non-linear term with a
  linear term. Linearisation allows the equations
  to be solved but the assumption precludes
  application of models to complex/steeper terrain.

• CFD models do not perform this linearisation and
  therefore are not subject to the assumption of
  shallow gradients. CFD models can be applied in
  cases of complex/steeper terrain.

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Resource Modelling (4) Terrain complexity
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Resource Modelling (5) Example Hill (Max Slope
8º)
Assumptions of linear model are completely valid,
CFD and MS3DJH agree well.
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Resource Modelling Example Hill (Max Slope 22º)
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Resource Modelling Real Terrain

• Simulate the atmospheric boundary layer using
  real terrain data as an input.

• Essential to include forests as their influence
  is often as important as that of hills.

• Determine areas of highest wind speed (and lowest
  turbulences). These are good places to site wind
  turbines.

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Wake Modelling

• Wake models predict velocity deficit and
  increased turbulence in the lee of wind turbines

• Wake losses reduce the energy output of a wind
  farm.

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Layout Design and Optimisation

• The optimum layout is one which has maximum
  possible topographical gains while keeping wake
  losses to a minimum.
• The complexity of the optimisation process is
  determined by the wind rose
• (wind direction frequency distribution).
• For a unidirectional wind rose the turbines will
  be positioned in rows within the rows turbines
  are positioned reasonably close (2-3 rotor
  diameters) while the rows are spaced farther
  apart (gt5 rotor diameters).
• For multi-directional winds layout optimisation
  algorithms will be required.

Optimum layout for a site with a multidirectional
wind rose.
Optimum layout for a site with a unidirectional
wind rose.
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More Info on Wind Energy

• Very good introduction http//www.windpower.org/e
  n/tour/

• Wind Energy Handbook (Wiley)
• Aerodynamics of Wind Turbine Blades (James
  James)