Wind Energy Outline
General Introduction
The Wind Resource
Wind Energy Systems Design
Disadvantages
Economics
The wind
The wind is solar power in mechanical form.
Wind energy is created by the uneven heating of the
earth by the sun;
A small part (around 2 %) of the energy of solar
radiation on Earth is converted into kinetic energy of
flowing air – the wind.
Wind’s velocity and direction depend on the imposed
pressure gradients, plus certain other forces, plus the
local geography.
Con’t…
The wind is a free-flowing fluid stream.
Energy conversion from free-flowing fluid streams is
limited because energy extraction implies decrease of
fluid velocity (decrease of kinetic energy of the
stream), which cannot fall down to zero, the stream
should continue traveling and cannot stop entirely.
The kinetic energy of a unit mass of flowing fluid is
So the power per unit mass flow would be:
Con’t…
Using the air density , the flow velocity , and the
area perpendicular to the flow, the mass flow
becomes: . dm dv dh
m A AV
dt dt dt
Then the total available power in the air flow is:
Con’t…
Hence, the energy transmitted by the wind to the rotor
blades is the difference between the upwind and the
downwind.
The kinetic is the difference between the upwind and
the downwind kinetic energies:
Power coefficient Cp
We can now define CP as the relation between converted
power to available power in the fluid flow:
The power coefficient CP is a function of the axial induction
factor. The optimum of this function (which is a maximum value
for CP) can be found from its first and second derivatives. The
optimum of CP is
Classification of Systems
Useful Output Type
– Mechanical Output (Wind Mill)
– Electrical Output (Wind Generators)
Orientation Type
– Horizontal Axis Wind Turbine (HAWT)
– Vertical Axis Wind Turbine (VAWT)
Number of Blades
– Single Blade
– Multi Blade
Number of Units
– Single Unit
– Multi Unit (Array or Farm)
HAWT …
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VAWT …
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HAWT
The most common type of lift-force wind turbines is the
horizontal axis wind turbine - HAWT. The rotor axis lies
horizontally, parallel to the air flow.
The blades sweep a circular (or slightly conical) plane normal to
the air flow, situated upwind (in front of the tower) or downwind
(behind the tower).
The main advantage of HAWTs is the good aerodynamic
efficiency (if blades are properly designed) and versatility of
applications.
Their main disadvantage is that the tower must support the rotor
and all gearing and electrical generator standing on top of it, plus
the necessity of yawing to face the wind.
VAWT
Another type of lift-force wind turbines is the vertical axis wind
turbine - VAWT.
The rotor axis is perpendicular to the air flow (usually vertical). The
blades sweep a cylindrical, conical or elliptical plane, perpendicular
to the air flow and parallel to the rotor axis.
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Starting Torque of a VAWT
The lift-force VAWT does not experience any starting
torque!!! This may be a critical issue for certain
applications.
Turbines connected to the electricity grid can use the
electric generator as a starting motor. In stand-alone
configurations, either electricity storage devices (again
using the generator as a starting motor) or integrated
drag-force turbines (as start turbines) can be applied in
order for the VAWT to spin up to a point where the lift
12 force can take over.
Advantages of VAWTs
All main power train components (gearbox, generator, brakes
and main bearing) are placed on the ground, allowing for easy
access for maintenance and lower stress on the tower.
Yaw mechanism for facing the wind is not needed – the
turbine accepts wind from any direction.
The blades are easier to manufacture (symmetrical airfoils
without any twist or taper).
All these features result in a simple machine, easily scalable
to large dimensions, at lower costs than a horizontal axis one.
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Advantages of VAWTs
The main advantage of the VAWT is believed to be the easier
geometrical scalability to large dimensions.
HAWTs are limited in future scale expansion for single turbines
due to quickly increasing masses of blades and fatigue loads
with scale.
Besides, the longer the blade for a HAWT, the lower the
aerodynamic efficiency of the part close to its root would be.
VAWTs would not face such problems.
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Ideal power output Curve
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Important Parameters …
Cut-In Speed
The wind speed at which a wind turbine begins to produce
power
Rated Speed
The "rated wind speed" is the wind speed at which the "rated
power" is achieved and generally corresponds to the point at
which the conversion efficiency is near its maximum .
In most cases, the power output above the rated wind speed is
maintained at a constant level.
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Important Parameters …
Rated (Power) Output
The power output at, or above, the rated speed.
Cut-Out Speed
The cut-out speed is the wind speed at which the turbine
may be shut down to protect the rotor and drive train
machinery from damage, or high wind stalling
characteristics.
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Output Power Regulations
Power output from the rotor can be decreased at high winds
by decreasing the power coefficient through aggravated
aerodynamic conditions, implying decreased lift forces.
There are two main types of power control for wind turbines:
Stall regulation, and
Pitch regulation
Reading assignment
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Comparison: Pitch - Stall
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Wind Potential Assessment - Ethiopia
Although the potential for wind energy as indicated by
varies studies is claimed to be vast and promising, so
far very little or no exploited wind energy has been
reported or seen to be used owing to its infancy even in
the global market.
The Ethiopian Rural energy development bureau is
making efforts, in aiding private investors, to invest in
renewable energy resources at different times.
Con’t…
Ethiopia plans 800 MW of wind power.
Adama I wind farm-51MW in 2011
The 120 MW Ashegoda Wind Farm opened in
October 2013 and was the largest wind farm in Africa
at that time.
The larger 153 MW Adama II wind farm went online
in May 2015, bringing Ethiopia's installed wind
capacity to 324MW total.
From Africa in this time the largest wind power is
found in Morocco which is more 300 MW. Check it?
Components of wind
Con’t…
The mechanical brake is used as a backup system for
the aerodynamic braking system.
The gearbox increases the slower speed of the wind
turbine to the higher speed required by the generator.
Yaw System wind turbine yaw mechanism is used to
turn the wind turbine rotor against the wind.
The hydraulics system might be used to reset the
aerodynamic brakes of the wind turbine.
The cooling unit contains an electric fan which is
used to cool the electrical generator.
The stator winding is connected directly to the grid
and the rotor driven by the wind turbine.
Con’t…
The power captured by the wind turbine is converted
into electrical power by the induction generator and
is transmitted to the grid by the stator winding.
The pitch angle is controlled in order to limit the
generator output power to its nominal value for high
wind speeds.
In order to generate power the induction speed must
be slightly above the synchronous speed but the
speed variation is typically so small that the WTIG is
considered to be at fixed speed wind generator.
Advantages and Dis advantages
Advantages
Plentiful, abundant
Inexhaustible
Non - polluting
No heat burden on atmosphere –release no heat like
thermal (no green effect)
Small project size
Short/flexible development time
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Con’t…
Dis advantages
Non steady – may be there now and you may not found
later/Intermittent output
Undependable –you can’t predict
The minimum speed required to generate is >30km/hr
The sites are isolated (not everywhere)
Always Backup is required for continuity
Generally remote location
Grid connectivity -- lack of transmission capability
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