RENEWABLE ENERGY

SYSTEMS
BY MORRIS MUGYEMA
 WIND ENERGY

• Wind power is the conversion of wind energy into

useful form of energy, such as using turbines to
make electricity, windmills for mechanical power,
wind pumps for water pumping or drainage, or sails
to propel ships.
• Wind is simple air in motion.
• It is caused by the uneven heating of the earth’s
surface by radiant energy from the sun.
 CONT’D
• Since the earth’s is made of very different types of land
and water, it absorbs the sun’s energy at different rates.
• A weather vane, or wind vane is used to show the
direction of the wind. A wind vane points toward the
source of the wind.
• Wind direction is reported as the direction from which
the wind blows, not the direction toward which the wind
moves. A north wind blows from north towards the
south.
 CONT’D
• The wind is one of the oldest sources of energy
harnessed by humankind.
• Windmills have been used for centuries to pump water
and to mill grain (that’s where the “mill” in “windmill”
comes from).
• Nowadays, wind power is making a comeback as
problems with conventional energy sources increase.
WIND SPEED OPERATING RANGE AND POWER
• Wind speed can be specified in meters per second (m/s),
km/hr, miles per hour (mi/hr) or nautical miles per hour
(nmi/hr) also know as knots (kt).
• Manufacturers of wind turbines, especially in Europe,
quantify wind speed in meters per second.
 CONT’D

• Most large wind turbines are designed to operate at wind

speeds ranging from 3 to 4 m/s up to 20 to 25 m/s.

• That corresponds to a minimum or cut in wind speed of

10.8km/hr to 14.4km/hr and a maximum or cut out
wind speed of 72 km/hr to 90km/hr against which its
difficult to walk.
 CONT’D
• Within this operating range, a large wind turbine can
generate anywhere from a few hundred kilowatts (KW) up
to several megawatts (MW) of usable electric power,
depending on the blade length, the wind speed , and the
size of the generator.
 CONT’D

• The capacity factor of wind energy. This is the proportion

of the time the resource can be exploited to produce
usable output.

• It is approximately 25 to 40 percent , depending on the

geographic location and on the design of the turbine.
The best sites for wind turbines are often far from
population centers
 DESIGN CONSIDERATIONS
• Large wind turbines require tall, strong towers for
support. A typical wind turbine tower measures between
50m to 80m high and is anchored in a mass of concrete.
Some towers are guyed for enhanced high wind survival.
The tower of the wind turbine carries the nacelle and the
rotor
• Most wind turbines have three blades. Some designs
have 2 blades, a few variants have four or more. The
rotational diameter is twice the blade length.
 CONT’D
• The rotor. The rotor provides the blades to rotate.

• The blade bearing, electric generator and generator

cooling apparatus are contained in a housing called a
nacelle.

• The system is designed to spin at a constant angular

speed of approximately 20 rpm for the entire workable
range of wind speed
 CONT’D
• A gear box translates the angular speed of the blades
into the proper angular shaft speed for a generator to
produce ac at a constant frequency of 50Hz

• In order to function properly, a large wind turbine must

be oriented so its blades rotate on an axis that points
into the wind.
 CONT’D

• This means that the plane defined by the rotating blades

must be perpendicular to the wind direction.

• To orient itself , the entire assembly can swivel through

360 degrees horizontal circle on a turntable
 CONT’D
• The wind direction and the wind speed are detected by a
wind vane and an anemometer similar to the instruments
used by meteorologists for the same purposes.

• If the wind becomes too strong, a fail safe braking

system stops the blades and locks them in place , and
the turntable (Yaw drive) rotates approximately 90
degrees so the assembly experiences the smallest wind
load
 CONT’D

• In most wind turbines, the rotor blades are on the

windward side of the nacelle, as shown in the figure a
below. This is called upwind design

• In some wind turbines, the rotor blades are on the

leeward side of the nacelle. This is known as the
downwind design
 CONT’D
• a) upwind design b) downwind design
WIND TURBINE
 WIND TURBINE
• Most wind turbines have the same basic parts; blades,
shafts, gears, a generator, and a cable (some turbines do
not have gear boxes). These parts work together to
convert the wind’s energy into electricity
1. The wind blows and pushes against the blades on top
of the tower, making them spin
 CONT’D

3. The turbine blades are connected to a low speed drive

shaft. When the blade spins the shaft turns. The shaft is
connected to a gearbox. The gears in the gearbox
increase the speed of the spinning motion on a high
speed drive shaft
4. The high speed drive shaft is connected to a generator.
As the shaft turns inside the generator , it produces
electricity
 CONT’D

5. To ensure the wind turbine is producing the maximal

amount of electric energy at all times, the yaw drive is
used to keep the rotor facing into the wind as the wind
direction changes.
6. The electricity is sent through a cable down the turbine
tower to a transmission line.
• The amount of electricity that a turbine produces
depends on its size and the speed of the wind
 HORIZONTAL AXIS WIND TURBINE ( HAWT )

• Horizontal axis wind turbines are shortened to HAWT,

are the common style that most of us think of when we
think of a wind turbine. A HAWT has a similar design to a
windmill, it has blades that look like a propeller that spin
on the horizontal axis
• Horizontal axis wind turbines have the main rotor shaft
and electrical generator at the top of the tower , and they
must be pointed into the wind.
 CONT’D
• Since a tower produces turbulence behind it, the turbine
is usually pointed upwind of the tower.
• Wind turbine blades are made stiff to prevent the blades
from being pushed into the tower by high winds.
• Additionally, the blades ae placed a considerable
distance in front of the tower and are sometimes tilted
up a small amount.
 CONT’D

• Downwind machines have been built , despite the

problem of turbulence, because they don’t need an
additional mechanism for keeping them in line with the
wind, and because in high winds, the blades can be
allowed to bend which reduces their swept area and thus
their wind resistance.

• Since turbulence leads to fatigue failures and reliability is

so important most HAWTS are upwind machines
 CONT’D

1. Small turbines are pointed by a simple wind vane

placed square with the rotor (blades), while large
turbines generally use a wind sensor coupled with a
servo motor.

2. Most large wind turbines have a gearbox, which turns

the slow rotation into a faster rotation that is more
suitable to drive an electrical generator
CONT’D
 HAWTS Advantages

1. The tall tower base allows access to stronger wind in

sites with wind shear.
• In some wind shear sites every 10 meters up the wind
speed can increase by 20% and the power output by 34%
 CONT’D

2. High efficiency, since the blades always move

perpendicularly to the wind, receiving power through
the whole rotation.
• In contrast, all vertical axis wind turbines and most
proposed airborne wind turbine design, involve various
types of reciprocating actions, requiring airfoil surfaces
to backtrack against the wind for part of the cycle.
• Backtracking against the wind leads to inherently lower
efficiency
 HAWTS Disadvantages
• Massive tower construction is required to support the
heavy blades, gearbox, and generator.
• Components of a horizontal axis wind turbine (gearbox,
rotor shaft and brake assembly) being lifted into position
is difficult.
• Their height makes them obtrusively visible across large
areas, disrupting the appearance of the landscape and
sometimes creating local opposition.
 CONT’D
• Downwind variants suffer from fatigue and structural
failure caused by turbulence when a blade passes
through the tower’s wind shadow (for this reason, the
majority of HAWTs use an upwind design, with the rotor
facing the wind in front of the tower).

• HAWTs require an additional yaw control mechanism to

turn the blades toward the wind.
 CONT’D
• HAWTs generally require a braking or yawing device in
high winds to stop the turbine from spinning and
destroying or damaging itself.
 CONT’D

• Cyclic stresses & vibration – when the turbine turns to

face the wind, the rotating blades act like a gyroscope.
As it pivots, gyroscopic precession tries to twist the
turbine into a forward or backward somersault. For each
blade on a wind generator’s turbine, force is at a
minimum when the blade is horizontal and at a
maximum when the blade is vertical. This cyclic twisting
can quickly fatigue and crack the blade roots, hub and
axle of the turbines.
 VERTICAL AXIS WIND TURBINES (VAWT)

• Vertical axis wind turbines, as shortened to VAWTs, have

the main rotor shaft arranged vertically.
• The main advantage of this arrangement is that the wind
turbine does not need to be pointed into the wind.
• This is an advantage on sites where the wind direction is
highly variable or has turbulent winds.
 CONT’D

• With a vertical axis, the generator and other primary

components can be placed near the ground, so the tower
does not need to support it, also makes maintenance
easier.

• The main drawback of a VAWT is it generally creates drag

when rotating into the wind.
 CONT’D

• It is difficult to mount vertical-axis turbines on towers,

meaning they are often installed nearer to the base on
which they rest, such as the ground or a building
rooftop.

• The wind speed is slower at a lower altitude, so less wind

energy is available for a given size turbine.
 CONT’D

• Air flow near the ground and other objects can create
turbulent flow, which can introduce issues of vibration,
including noise and bearing wear which may increase the
maintenance or shorten its service life.
 CONT’D

• However, when a turbine is mounted on a rooftop, the

building generally redirects wind over the roof, thus
doubling the wind speed at the turbine.
• If the height of the rooftop mounted turbine tower is
approximately 50% of the building height, this is near
the optimum for maximum wind energy and minimum
wind turbulence.
CONT’D
 VAWT ADVANTAGES

• They can produce electricity in any wind direction.

• Strong supporting tower is not needed because
generator, gearbox and other components are placed on
the ground.
• Low production cost as compared to horizontal axis wind
turbines.
 CONT’D
• As there is no need of pointing turbine in wind direction
to be efficient so yaw drive and pitch mechanism is not
needed.
• Easy installation as compared to other wind turbine.
• Easy to transport from one place to other.
 CONT’D

• Low maintenance costs.

• They can be installed in urban areas.
• Low risk for human and birds because blades moves
at relatively low speeds.
• They are particularly suitable for areas with extreme
weather conditions, like in the mountains where they
can supply electricity to mountain huts.
 VAWT DISADVANTAGES

• As only one blade of the wind turbine works at a time,

efficiency is very low compared to HAWTs.
• They need an initial push to start; This initial push that is
needed to make the blades continue spinning must be
provided by a small motor.
 CONT’D

• when compared to horizontal axis wind turbines they are

very less efficient because of the additional drag created
when their blades rotate.
• They have relative high vibration because the air flow
near the ground creates turbulent flow.
 CONT’D

• Because of vibration, bearing wear increases which

results in the increase of maintenance costs.
• They can create noise pollution.
• VAWTs may need guy wires to hold it up (guy wires are
impractical and heavy in farm areas).
 ADVANTAGES OF WIND POWER

• Wind is a renewable energy source and the supply is

practically unlimited.

• Wind power plants do not produce greenhouse gases, CO,

NOx, Sox, particulate pollutants, or waste products.
 CONT’D

• Once installed, wind turbine is comparatively easy and

inexpensive to maintain

• Wind power plants can reduce dependency on fossil

fuels, hydropower, and nuclear fission reactors for
electric generation
 CONT’D

• Large wind turbines can be dispersed over wide regions.

This distributes the energy source and should help in the
quest for a fault-tolerant utility grid ( a system not
vulnerable to catastrophic failure or sabotage).

• Wind power can be used to supplement other modes of

electric power generation. This increases diversity of a
nation’s electrical system.
 CONT’D

• Wind turbines can be placed offshore over large lakes or

over the ocean, as well as on land

• Even the largest wind turbines have small footprints and

can thus share land resources with other operations such
as farming and cattle ranching.
CONT’D
 LIMITATIONS
• The wind is an intermittent source of energy. The
capacity factor is lower than that of most other energy
sources
• A wind turbine can be damaged or destroyed by a severe
thunderstorm, hurricane, or ice storm
• Some people do not like the physical appearance of large
wind turbines.
 CONT’D

• Wind turbines make some noise. However at a

reasonable distance from the tower, blade and turbine
noise is rarely much louder than the wind itself.

• Wind turbines may occasionally injure or kill birds. This

problem can be mitigated by choice of location and by
not placing multiple turbines in close proximity .
 CONT’D
• Wind power cannot by itself totally satisfy the electrical
needs of a city or town. It is best a supplemental source,
used in conjunction with fossil fuels and hydro power

• Location with consistent usable winds are often far from

population centers, requiring the use of long
transmission lines.
 WIND POWER CALCULATIONS

• Wind turbine energy capture • Rotor power

P  ρAV C p
1 3
V2 2 1

where :
Cp  rotor power coefficient
  air density
V1
A  rotor swept area
• V = wind speed
 CONT’D
• A German physicist Albert Betz concluded in 1919 that
no wind turbine can convert more than 16/27 (59.3%) of
the kinetic energy of the wind into mechanical energy
turning a rotor.
• To this day, this is known as the Betz limit or Betz‘ law.
the theoretical maximum power efficiency of any design
of wind turbine is 0.593 (i.e. no more than 59.3% of the
energy carried by the wind can be extracted by a wind
turbine). This is called the “power coefficient” and is
defined as:
 CONT’D

Cp  0.593
Cp vs. PU Exit Velocity

0.7

0.6

where : 0.5

V2  1 V
3 1
0.4

Cp
0.3

(wind velocity slows by 2/3)

0.2

0.1

0
0 0.2 0.4 0.6 0.8 1 1
PU Exit Velocity

Cp vs. PU Exit Velocity

 CONT’D

• The swept area of the turbine can be calculated from the

length of the turbine blades using the equation for the
area of a circle:

• 𝐴 = 𝜋𝑟 2
CONT’D
 CONT’D

• We are given the following data:

• Blade length, l = 52 m
• Wind speed, v = 12 m/sec
• Air density, ρ = 1.23 kg/m3
• Power coefficient, Cp = 0.4
• Inserting the value for blade length as the radius of the
swept area into area equation we have:
 CONT’D

l = r= 52m
𝐴 = 𝜋𝑟 2
A = 𝜋 × 522
= 8495𝑚2
 CONT’D

• We can then calculate the power converted from the wind

into rotational energy in the turbine using the power
equation
 CONT’D

• The power coefficient is not a static value as defined; it

varies with the tip speed ratio of the turbine. Tip speed
ratio is
• Defined as:
 CONT’D

• The blade tip speed can be calculated from the rotational

speed of the turbine and the length of the blades used in
the turbine using the following equation:

• where D is the diameter of the turbine.

 CONT’D

• The tip speed ratio (often known as TSR) is of vital

importance in the design of wind turbine generators.

• If the rotor of the wind turbine turns too slowly, most of

the wind will pass undisturbed through the gap between
the rotor blades.
 CONT’D

• Alternatively if the rotor turns too quickly, the blurring

blades will appear like a solid wall to the wind.

• Therefore, wind turbines are designed with optimal tip

speed ratios to extract as much power out of the wind as
possible.
 CONT’D

Tip Speed Ratio Calculations

• If a 20mph wind is blowing on a wind turbine and the
tips of its blades are rotating at 80mph, then the tip
speed ratio is

80
• =4
20
 CONT’D
• Optimum Tip Speed Ratio
• The optimum tip speed ratio depends on the number of
blades in the wind turbine rotor. The fewer the number
of blades, the faster the wind turbine rotor needs to turn
to extract maximum power from the wind.
• A two bladed rotor has an optimum tip speed ratio of
around 6, a three bladed rotor around 5, a four bladed
rotor around 3.
 EFFECTS OF ROTOR SPEED RATIO

• The choice of the tip speed ratio of a particular wind

turbine design depends on several factors.
• In general , a high tip speed ratio is a desirable feature
since it results in a high shaft rotational speed that is
needed for the efficient operation of an electrical
generator.
• A high tip speed ratio however entails several possible
disadvantages.
 CONT’D
• Rotor blade tips rotating at a speed higher than 80m/sec
will be subject to erosion of the leading edges from their
impact with dust or sand particles in the air and will
require use of special erosion resistant coatings much
like in the design of helicopter blades
• Noise generation in the audible and non audible ranges
• Vibration particularly in case of two or single bladed
rotors
 CONT’D

• Starting difficulties if the shaft is stiff to start rotation

• Reduced rotor efficiency due to drag and tip losses
• Excessive rotor speeds would lead to run away turbine,
leading to its failure and disintegration
 QUESTION

• Why are 3-blade horizontal axis turbines optimal?