wind energy. how wind turbines work wind is a form of solar energy. winds are caused by the uneven...

WIND ENERGY

How Wind Turbines Work

• Wind is a form of solar energy. Winds are caused by the uneven heating of the atmosphere by the sun, the irregularities of the earth's surface, and rotation of the earth.

• Wind flow patterns are modified by the earth's terrain, bodies of water, and vegetation. Humans use this wind flow, or motion energy, for many purposes: sailing, flying a kite, and even generating electricity.

• So how do wind turbines make electricity? Simply stated, a wind turbine works the opposite of a fan. Instead of using electricity to make wind, like a fan, wind turbines use wind to make electricity.

• The wind turns the blades, which spin a shaft, which connects to a generator and makes electricity. Take a look inside a wind turbine to see the various parts. View the wind turbine animation to see how a wind turbine works.

Types of Wind Turbines

• Modern wind turbines fall into two basic groups: the horizontal-axis variety, as shown in the photo, and the vertical-axis design, like the eggbeater-style Darrieus model, named after its French inventor.

• Horizontal-axis wind turbines typically either have two or three blades. These three-bladed wind turbines are operated "upwind," with the blades facing into the wind.

Sizes of Wind Turbines

• Utility-scale turbines range in size from 100 kilowatts to as large as several megawatts. Larger turbines are grouped together into wind farms, which provide bulk power to the electrical grid.

• Single small turbines, below 100 kilowatts, are used for homes, telecommunications dishes, or water pumping.

Inside the Wind Turbine

• Anemometer: Measures the wind speed and transmits wind speed data to the controller.

• Blades: Most turbines have either two or three blades. Wind blowing over the blades causes the blades to "lift" and rotate.

• Brake: A disc brake, which can be applied mechanically, electrically, or hydraulically to stop the rotor in emergencies.

• Controller: The controller starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and shuts off the machine at about 55 mph. Turbines do not operate at wind speeds above about 55 mph because they might be damaged by the high winds.

• Gear box: Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speeds from about 30 to 60 rotations per minute (rpm) to about 1000 to 1800 rpm, the rotational speed required by most generators to produce electricity. The gear box is a costly (and heavy) part of the wind turbine and engineers are exploring "direct-drive" generators that operate at lower rotational speeds and don't need gear boxes.

• Generator: Usually an off-the-shelf induction generator that produces 60-cycle AC electricity.

• High-speed shaft: Drives the generator.

• Low-speed shaft: The rotor turns the low-speed shaft at about 30 to 60 rotations per minute.

• Nacelle: The nacelle sits atop the tower and contains the gear box, low- and high-speed shafts, generator, controller, and brake. Some nacelles are large enough for a helicopter to land on.

• Pitch: Blades are turned, or pitched, out of the wind to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity.

• Rotor: The blades and the hub together are called the rotor.

• Tower: Towers are made from tubular steel (shown here), concrete, or steel lattice. Because wind speed increases with height, taller towers enable turbines to capture more energy and generate more electricity.

• Wind direction: This is an "upwind" turbine, so-called because it operates facing into the wind. Other turbines are designed to run "downwind," facing away from the wind.

• Wind vane: Measures wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind.

• Yaw drive: Upwind turbines face into the wind; the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. Downwind turbines don't require a yaw drive, the wind blows the rotor downwind.

• Yaw motor: Powers the yaw drive.

THE WINDMILL

• HISTORICALLY USED FOR MILLING GRAIN, PUMPING WATER, MECHANICAL POWER APPLICATIONS

• WIND TURBINES– WECS – Wind Energy Conversion Systems– WIND GENERATORS, IF THE PRODUCE

ELECTRICITY– AEROGENERATORS – BECAUSE OF

AERODYNAMICS OF BLADES

WIND TURBINES

• USED FOR ELECTRICAL GENERATION SINCE END OF 19TH CENTURY

• NOW ONE OF THE MOST COST-EFFECTIVE GENERATION TECHNIQUES

• ONE OF FASTEST GROWING SOURCES OF ELECTRICITY – 40% PER YEAR GROWTH RATE

WIND ENERGY

• MOST DEVELOPED ON LAND– CHEAPEST TO DEVELOP– FEWER TECHNICAL PROBLEMS– EASIER TO MAINTAIN

• OFFSHORE WIND GENERATORS– MORE EXPENSIVE TO CONSTRUCT– MORE DIFFICULT TO MAINTAIN– HARSHER ENVIRONMNET

THE WIND

• CAUSED BY VARIATIONS IN ATMOSPHERIC PRESSURE• DIFFERENTIAL WARMING OF ATMOSPHERE• TROPICS ARE WARMER THAN HIGHER LATITUTDES• ATMOSHPERIC PRESSURE CAUSED BY WEIGHT OF

COLUMN OF AIR ABOVE EARTH AT A SPECIFIC LOCATION

• MEASURED WITH A BAROMETER• UNIT OF MEASURE IS “BAR”

THE BAROMETER• CALIBRATED IN MILLIBARS

(mbar)• SEALEVEL ATMOSPHERIC

PRESSURE IS ~ 1013.2 mbar• ALSO CALIBRATED IN

KILLOPASCALS(95-105 kPa)• USA – GIVEN IN INCHES OF

MERCURY (28-31” NORMAL RANGE

TYPICAL HOME BAROMETER

• METAL CHAMBER WITH A CALIBRATED AIR PRESSURE

• ADJUST TO PRESSURE BASED ON ALTITUDE• USUALLY USED TO FORCAST WEATHER• ALSO CAN PREDICT WIND

GLOBAL WIND CIRCULATION

• FIG. 7.2 TEXT• Coriolis Effect – Due to Earth’s rotation– Trade winds in Northern Hemisphere veer to the right

(clockwise)– Trade winds in Southern Hemisphere veer to the left

(counter-clockwise)

• Read over Box 7.1• ISOBARS – LINES OF EQUAL PRESSURE • SEE FIG. 7.4

LOCAL EFFECTS

• COASTAL REGIONS– GENERATED BECAUSE OF DIFFERENT HEAT

CAPACITIES OF SEA AND LAND – LAND HAS LOWER HEAT CAPACITY THAN WATER– LAND HEATS UP QUICKER DURING THE DAY AND

COOLS QUICKER AT NIGHT– DAYTIME – COOL AIR FLOWS ON SHORE– NIGHTIME WARM AIR FLOWS OFFSHORE

SHORELINE DAYTIME WIND CIRCULATION

NIGHTIME SHORELINE WIND CIRCULATION

NIGHTIME SHORELINE WIND CIRCULATION

EFFECTS DUE TO MOUNTAINS

• Cool mountain air warms and rises quickly in the morning

• Cool valley air moves up• At night the cool mountain

air sinks to the valley.• Are these breezes large

enough to power a small wind turbine?

ENERGY AND POWER IN THE WIND

• REMEMBER – ENERGY CONTAINED IN THE WIND IS KINETIC ENERGY

• KE = ½ mass x velocity squared = ½ mV2

• m (kg) and V (ms-1)• MASS OF AIR = VOLUME x DENSITY• mair = vair

• Where kg m-3 at sea level

CALCULATING ENERGY DRIVING A TURBINE

• V = VELOCITY OF AIR (m s-1)• IN ONE SECOND A VOLUME OF AIR (MASS)

DETERMINED BY THE AREA OF THE BLADE CIRCLE (A) WILL FLOW PAST THE TURBINE m = AV

• WE HAVE THE ENERGY IN THE MASS OF AIR IN A CYLINDER OF AREA (A) TIMES THE LENGTH (VELOCITY x 1 SECOND)

• KINETIC ENERGY PER SECOND = ½ AV3

EXAMPLE

• SUPPOSE WE HAVE A WIND TURBINE WITH 30 m LONG BLADES AND THE WIND IS BLOWING A 5 m s-1

• AREA WOULD BE = R2 • A = m)2 = 2828.57 m2

• KE per second = ½ AV3(joules per second) = 0.5 x 1.2256 x 2828.57 x(5)3 = 216,668 joules per second = P (Watts)

IMPORTANT POINTS• KENITIC ENERGY IN WIND PER SECOND = P(W) = ½ AV3

• NOTE: THIS DOES NOT MEAN YOU CAN EXTRACT ALL OF THIS POWER!

• POWER IS PROPORTIONAL TO THE CUBE OF THE WIND VELOCITY

• POWER IS PROPORTIONAL TO THE SWEPT AREA OF THE WIND TURBINE

• DENSITY OF THE AIR IS LOWER AT HIGHER ELEVATIONS• DENSITY OF AIR IN COLD CLIMATES IS HIGHER (10%)

WIND TURBINES

• HISTORY ~4000 YEARS• NOTE FIG. 7.8– SCREENED WINDMILLS– CLAPPER WINDMILLS

• CUP ANENOMETER– USED TO MEASURE WIND VELOCITY– MEASUREMENT USUALLY TAKEN AT 30m OR

MORE

MAJOR TYPES OF WIND TURBINES

• VERTICAL AXIS WIND TURBINE (VAWT)• HORIZONTAL AXIS WIND TURBINE (HAWT)• BOTH ARE COMMON TODAY– VERTICAL AXIS IS MOST PREVALENT– HAWT SEEMS TO BE LOWER TO THE GROUND

BECAUSE OF STABILITY

Renewable Energy Problems• Solar based, dominated by Photovoltaic (PV)• Wind focused on large propeller based systems for

the ‘Farm & Barn’ community.• Both approaches are Expensive• Tall propellers are unsightly and noisy• Wind systems have not historically delivered their

rated power.• Energy is currently produced away from

consumption requiring expensive transmission and creating bottleneck problems.

WIND TURBINE TYPES

• VAWT – GENERALLY CROSS FLOW TYPE• HAWT – GENERALLY AXIAL FLOW TYPE– HIGH-SOLIDITY DEVICES – COMMON WINDMILL USED FOR

PUMPING WATER – FIG 7/11– LOW-SOLIDITY DEVICES – MOST WIND TURBINES FOR

ELECTRICAL GENERATION – PROPELLER TYPE BLADES FIG 7.12 – 7.14

EFFECT OF NUMBER OF BLADES ON HAWT

• DEFINITIONS– ROTATION SPEED • REVOLUTIONS PER MINUTE (rpm) = N• RADIANS PER SEC (radians per sec) = or• 1 rpm = 2/60 rad s-1 = 0.10472 rad s-1 • 2 radians = 360 degrees• 1 radian = 57o 17’ 44”

BLADE TIP SPEED

• TIP SPEED (U) = TANGENTIAL VELOCITY OF THE ROTOR AT THE TIP OF THE BLADE

• R = RADIUS OF ROTOR• U = R =2RN/60• EXAMPLE: N = 10 rpm, R = 30 m• U = 31.4 m/s = 70.24 mph

TIP SPEED RATIO

• IF WE DIVIDE THE TIP SPEED (U) BY THE UNDISTURBED WIND VELOCITY (VO) UPSTREAM OF THE TURBINE WE OBTAIN THE TIP SPEED RATIO ()

• USEFUL MEASURE TO COMPARE DIFFERENT TURBINES

• U/VO = R/VO

EXAMPLE

• IF THE TIP SPEED ON A HAWT IS 120 mph AND THE WIND SPEED IS 10 m/s WHAT IS THE TIP SPEED RATIO

• 120mph x 0.447ms-1/10ms-1 = 5.364

TIP SPEED RATIO

• WIND TURBINE OF A PARTICULAR DESIGN WILL OPERATE OVER A RANGE OF TIP SPEED RATIOS BUT WILL BE MOST EFFICIENT AT A CERTAIN TIP SPEED RATIO.

• OPTIMUM TIP SPEED RATIO FOR A TURBINE DEPENDS ON– NUMBER OF BLADES– WIDTH OF BLADES

SOLIDITY

• SOLIDITY = FRACTION OF THE SWEPT AREA THAT IS SOLID

• LOW-SOLIDITY – WIND MAY PASS THROUGH WITHOUT INTERACTING WITH THE BLADES

• HIGH NUMBER OF BLADES = HIGHLY SOLID SWEPT AREA = HIGH SOLIDITY

EXTRACTION EFFICIENCY

• TO EXTRACT AS MUCH OF THE KE AS POSSIBLE, BLADES HAVE TO INTERACT WITH AS MUCH OF THE WIND PASSING THROUGH THE SWEPT AREA AS POSSIBLE.

NUMBER OF BLADES

• LOW –SOLIDITY TURBINE WILL HAVE TO TRAVEL MUCH FASTER TO FILL UP THE SWEPT AREA THAN A HIGH-SOLIDITY TURBINE WILL

• TWO BLADED WIND TURBINE WILL HAVE AN OPTIMUM TIP SPEED RATIO 1/3 HIGHER THAN THAT OF A 3 BLADE WIND TURBINE.

OPTIMUM NUMBER OF BLADES

• MORE BLADES WILL INTERACT WITH WIND MORE, BUT TOO MANY BLADES INTERFER WITH EACH OTHER

• HIGH-SOLIDITY TURBINES TEND TO BE LESS EFFICIENT

• THREE BLADED TURBINES TEND TO BE MOST EFFICIENT