Wind Energy (2).pdf

Energy Systems Engineering Technology ESET 222 Wind & Solar Energy Winter 2024 Professor: Arun Hor. Wind Energy ESET 222: Wind & Solar Energy ( Winter 2023 ) Wind Turbine TURBULENCE: Turbulence (also referred to as stall) prevents the lifting force of the rotor blade from acting on the rotor when th...

Energy Systems Engineering Technology ESET 222 Wind & Solar Energy Winter 2024 Professor: Arun Hor. Wind Energy ESET 222: Wind & Solar Energy ( Winter 2023 ) Wind Turbine TURBULENCE: Turbulence (also referred to as stall) prevents the lifting force of the rotor blade from acting on the rotor when the wind is too high. This occurs because as the angle of the wind increases, the lift also increases until the wind can no longer smoothly travel over the upper surface of the blade. The wind begins to tumble over the top of the rotor which destroys the low pressure area created by the smooth flowing air over the top of the rotor blade. This causes lift to no longer be affective and therefore the wind turbine does not create energy. ESET 222: Wind & Solar Energy ( Winter 2023 ) Wind Turbine BLADES - AEROFOILS Apparent wind – resulting wind between blade speed and wind speed. Angle of Attack – angle between apparent wind and chord line Tip speed ratio – ratio of tangential velocity at the tip of the blade to the undisturbed upwind velocity Each turbine has a maximum operating efficiency defined by a particular tip speed ratio Tip speed ratios typically between 4 – 8 for low solidity turbines ESET 222: Wind & Solar Energy ( Winter 2023 ) Wind Turbine TIP SPEED: The tip of the turbine blade travels at the highest speed of any part of the turbine blade when it is rotating. Conversely, the part of the turbine blade that is connected to the hub, near the shaft, is traveling at the slowest speed when the blade is rotating. Tip speed is defined as the measured speed at the blade tip as it rotates through the air. Because the tip is traveling at the highest speed, it comes under considerable stress caused by centrifugal force when it is rotating. TIP SPEED RATIO: By definition, TSR is the speed of the blade at its tip divided by the speed of the wind. For example, if the tip of a blade is travelling at a speed of 100 mph and the wind speed is 20 mph, then the TSR is 5 which means the tip of the blade is travelling five times faster than the speed of the wind. ESET 222: Wind & Solar Energy ( Winter 2023 ) Power in the Wind Power Extracted from the Wind SPEED CONTROL: The speed control methods fall into the following categories: no speed control: In this method, the turbine, the electrical generator, and the entire system is designed to withstand the extreme speed under gusty wind. yaw and tilt control: Here the rotor axis is shifted out of the wind direction when the wind speed exceeds the design limit. pitch control: This changes the pitch of the blade with the changing wind speed to regulate the rotor speed. stall control: In this method of speed control, when the wind speed exceeds the safe limit on the system, the blades are shifted into a position such that they stall. The turbine has to be restarted after the gust has gone. TIP SPEED RATIO: video Tip speed = π D N / 60 where N is the rotor speed (rpm) , D is the rotor diameter (m); and v is the wind speed (m/s) upwind of the turbine. ESET 222: Wind & Solar Energy ( Winter 2023 ) Power in the Wind Power Extracted from the Wind ESET 222: Wind & Solar Energy ( Winter 2023 ) Power in the Wind Power Extracted from the Wind Spacing of the Towers: The spacing depends on the terrain, the wind direction, the speed, and the turbine size. The optimum spacing is found in rows 5 to 9 rotor diameters apart in the wind direction, and 3 to 5 rotor diameters apart in the crosswind direction. Effect of Height: The wind shear at ground surface causes the the wind speed increase with height in accordance with the expression where V1 = wind speed measured at the reference height h1 V2 = wind speed estimated at height h2, and α = ground surface friction coefficient. ESET 222: Wind & Solar Energy ( Winter 2023 ) Power in the Wind Given the nonlinear relationship between power and wind, we can’t just use average wind speed to predict total energy available, as the following example illustrates. Example: Don’t Use Average Wind speed. Compare the energy at 150C, 1 atm pressure, contained in 1 m2 of the following wind regimes: a. 100 hours of 6 m/s winds (13.4 mph), b. 50 hours at 3 m/s plus 50 hours at 9 m/s (i.e., an average wind speed of 6 m/s) Solution a. With steady 6 m/s, Energy (6 m/s) = 1/2 ρAv3t = 12 · 1.225 kg/m3 ·1m2 · (6 m/s)3 · 100 h = 13,230 Wh b. With 50 h at 3 m/s, Energy (3 m/s) = 1/2 · 1.225 kg/m3 ·1 m2 · (3 m/s)3 · 50 h = 827 Wh With 50 h at 9 m/s, Energy (9 m/s) = 1/2 · 1.225 kg/m3 ·1 m2 · (9 m/s)3 · 50 h = 22,326 Wh for a total of 827 + 22,326 = 23,152 Wh The above example dramatically illustrates the inaccuracy associated with using average wind speeds. While both of the wind regimes had the same average wind speed, the combination of 9-m/s and 3-m/s winds (average 6 m/s) produces 75% more energy than winds blowing a steady 6 m/s. Later we will see that, under certain common assumptions about wind speed probability distributions, energy in the wind is typically almost twice the amount that would be found by using the average wind speed. ESET 222: Wind & Solar Energy ( Winter 2023 ) Power in the Wind IMPACT OF TOWER HEIGHT: Wind speed is greatly affected by the friction that the air experiences as it moves across the earth’s surface. Surface winds are slowed considerably by high irregularities such as forests and buildings. the impact of the roughness of the earth’s surface on wind speed is the following: Where, v is the wind speed at height H, vo is the wind speed at height Ho (often a reference height of 10 m), α is the friction coefficient, and z is the roughness length(m). For neutral stability conditions, α is approximately 1/7, or 0.143. (1/7th power law) ESET 222: Wind & Solar Energy ( Winter 2023 ) Power in the Wind Example: An anemometer mounted at a height of 10 m above a surface with crops, hedges, and shrubs shows a wind speed of 5 m/s. Estimate the wind speed and the power density in the wind at a height of 50 m. Assume 150C and 1 atm of pressure. Solution: The friction coefficient α for ground with hedges, and so on, is estimated to be 0.20. From the 150C, 1-atm conditions, the air density is ρ = 1.225 kg/m3. Wind speed at 50 m will be Power density will be, That turns out to be more than two and one-half times as much power as the 76.5 W/m2 available at 10 m. We can now say that, As (v/vo)3 = (H/Ho)α.3 = (H/Ho)3α ESET 222: Wind & Solar Energy ( Winter 2023 ) Power in the Wind Example: A wind turbine with a 30-m rotor diameter is mounted with its hub at 50 m above a ground surface that is characterized by shrubs and hedges. Estimate the ratio of specific power in the wind at the highest point that a rotor blade tip reaches to the lowest point that it falls to. Solution: The friction coefficient α for ground with hedges and shrubs is estimated to be 0.20. The ratio of power at the top of the blade swing (65 m) to that at the bottom of its swing (35 m) will be So, the power in the wind at the top tip of the rotor is 45% higher than it is when the tip reaches its lowest point. This illustrates an important point about the variation in wind speed and power across the face of a spinning rotor. For large machines, when a blade is at its high point, it can be exposed to much higher wind forces than when it is at the bottom of its arc. This variation in stress as the blade moves through a complete revolution is compounded by the impact of the tower itself on wind speed, especially for downwind machines, which have a significant amount of wind “shadowing” as the blades pass behind the tower. The resulting flexing of a blade can increase the noise generated by the wind turbine and may contribute to blade fatigue, which can ultimately cause blade failure. ESET 222: Wind & Solar Energy ( Winter 2023 ) Power in the Wind Our fundamental relationship for the power delivered by the rotor is: Where, theoretical maximum rotor efficiency Cp is 59.3%. This is also called Betz efficiency or Betz’s law. It is to note that 100% energy can never be extracted from the wind. In fact, under the best operating conditions, about 35 to 45 percent efficiency can be achieved in converting the power in the wind into the power of a rotating generator shaft. Tip Speed ratio: If the rotor turns too slowly, the efficiency drops off since the blades are letting too much wind pass by unaffected. If the rotor turns too fast, efficiency is reduced as the turbulence caused by one blade increasingly affects the blade that follows. The usual way to illustrate rotor efficiency is to present it as a function of its tip-speed ratio (TSR). The tip-speed-ratio is the speed at which the outer tip of the blade is moving divided by the wind speed: where rpm is the rotor speed , D is the rotor diameter (m); and v is the wind speed (m/s) upwind of the turbine. ESET 222: Wind & Solar Energy ( Winter 2023 ) Power in the Wind Example: A 40-m, three bladed wind turbine produces 600 kW at a wind speed of 14 m/s. Air density is the standard 1.225 kg/m3. Under these conditions, a. At what rpm does the rotor turn when it operates with a TSR of 4.0? b. What is the tip speed of the rotor? c. If the generator needs to turn at 1800 rpm, what gear ratio is needed to match the rotor speed to the generator speed? d. What is the efficiency of the complete wind turbine (blades, gear box,generator) under these conditions? ESET 222: Wind & Solar Energy ( Winter 2023 ) Power in the Wind Solution: ESET 222: Wind & Solar Energy ( Winter 2023 ) Power in the Wind Q: Does Temperature Influence Wind Power Generation? A: The turbine will produce about 16 per cent more on a cold day. ESET 222: Wind & Solar Energy ( Winter 2023 ) Power in the Wind Problem 1: Using the data of a Vesta V90 – 3.0 MW turbine (diameter = 90 m), find the turbine’s efficiency for a) just above the cut-in speed (5m/s), b) the average speed (15 m/s). c) Why does the power output level off at 3,000 kW? Solution: a) We know: Wind speed = 5 m/s Diametre of the turbine = 90 m Actual power output = 250 kW (see p.curve) Input power, P= ½ ρAv3 = ½ x 1.2 x 3.14x 45x45 x 5 x 5 x 5 = 477,125 W = 480 kW Efficiency = Output Power / Input Power = 250kW / 480kW = 52% ESET 222: Wind & Solar Energy ( Winter 2023 ) Power in the Wind b) Wind speed = 15 m/s Diametre of the turbine = 90 m Actual power output = 3,000 kW Input power = P = 13,000 kW Efficiency = 23.2% c) Why does the power output level off at 3,000 kW? We have to limit the turbine from rotating faster so that there is not too much force on it, leading to damage. ESET 222: Wind & Solar Energy ( Winter 2023 ) Wind Energy Quiz World's Biggest Wind Turbine: The Vestas V164 has a rated capacity of 8 MW, later upgraded to 9.5 MW. The wind turbine has an overall height of 220 m (722 ft), a diameter of 164 m (538 ft), is for offshore use, and is the world's largest-capacity wind turbine since its introduction in 2014. ESET 222: Wind & Solar Energy ( Winter 2023 ) Power in the Wind