Lesson 19: Wind Energy PDF

Lesson 19: Wind Energy Overview When the term renewable energy is mentioned, two technologies immediately come to mind: wind and solar. This lesson focuses on wind energy, which has been taken advantage of by humans at least as far back as 5000 B.C. Wind mills, the precursors to modern wind turbine...

Lesson 19: Wind Energy Overview When the term renewable energy is mentioned, two technologies immediately come to mind: wind and solar. This lesson focuses on wind energy, which has been taken advantage of by humans at least as far back as 5000 B.C. Wind mills, the precursors to modern wind turbines, were invented in Persia (southwest Asia, now modern day Iran). While modern wind turbines are considerably different in design, they still operate on the same basic principal; extracting power from the wind to turn a turbine to do work. In modern wind turbines, that work is generating electricity. This lesson begins with an overview of how much power there is in the wind, and how much of that power can in fact be extracted. The mechanics and power output of wind turbines are then delved into, followed by a discussion of wind resources in the U.S. and their impact, along with expansion of wind use plus government incentives, on the cost of wind. Lecture Outline 1. Wind energy over time a. Wind energy is one of the oldest forms of energy used by mankind i. Sailing ii. Grain milling iii. Water pumping iv. Milling wood b. Up to the present, two basic designs have emerged i. Vertical axis turbines 1. Advantages a. Omnidirectional 전방향성 b. Can be installed in small spaces 2. Disadvantages a. Height and thus power output limited  ii. Horizontal axis turbines 1. Builds off class wind mill design 2. Advantages a. Omnidirectional b. Can be built tall with big turbine blades i. Results in much higher power output 2. Wind power a. Power in the wind passing through the circular area swept by a wind turbine is given by the following equation i. Power = ½ x Air Density x Circular Area Radius x Wind velocity cubed 3. Betz limit a. Only a fraction of the power of the wind passing through the circular area swept by the wind turbine can be extracted to produce electricity i. If all the power were extracted, there be no air movement on the other side of the turbine ii. Instead, the wind is slowed and the area it occupies expands behind the turbine iii. The maximum power that can be extracted from the wind because of this is k.a. as the Betz Limit and it is 59% 4. Blade lift vs. drag a. Wind blades have a tear-drop shape to promote lift b. The lift is created when front of the blade is slightly angled or pitched upward relative to the oncoming wind c. The pitched blade’s shape causes wind passing over the top of the blade to move faster than the air underneath i. This creates a pressure drop over the top of the blade relative to the bottom that acts to push or lift the blade upwards d. If the pitch is too steep (too angled), the wind passing over the top of the  blade separates from into an eddy that exerts a drag on the blade that counteracts the lift i. In an airplane, when the wings are pitched too much, the plane stalls and begins to fall out of the sky e. On a wind turbine, the direction of the air passing over the blade changes with increasing distance from the turbine’s rotation axis i. This is because all parts of the blade must complete a 360° rotation in the same time, so the tip of the blade is moving fastest over the course of this rotation, while the base of the blade is moving the slowest f. The direction of air flow over the blade is a combination of the direction of the oncoming wind and the rate that the blade is turning i. Near the base, the dominant direction is that of the oncoming wind ii. At the tip, the dominant direction is opposite to that that the blade is turning g. As a consequence of the change in flow direction across the blade with increasing distance from the axis of rotation, the blade is twisted i. It is pitched just above the direction of the oncoming wind near the base of the blade ii. And it is pitched more in the direction of rotation at the tip of the blade. 5. Practical limits on blade number a. Blade number and power extraction efficiency i. One blade = 51% 1. Greater vibration and thus requires stronger, more costly tower ii. Three blades = 55% iii. Betz Limit = 59% 1. So increasing the number of blades leads to diminishing returns wrt extracting greater power from the wind 2. Three blades also dampen tower vibration reducing fortification needs 완화하다  and thus providing best return on cost 6. Wind turbine mechanics a. Main parts i. Blades with adjustable pitch ii. Rotor iii. Brake iv. Transmission 1. Drive shaft 2. Gears v. Electric generator 1. Stator 2. Rotor vi. Gears and motor to turn the rotor into the wind (yaw) vii. Nacelle: housing which contains parts (iii-vi) viii. Wind vane and anemometer for measuring wind direction and speed ix. Tower b. Note that many of these parts require electric energy to run (e.g., yaw motor) 7. Wind capacity vs. output a. Power output from a wind turbine given by its power curve i. X-axis = wind speed ii. Y-axis = power output iii. Minimum wind speed that produces net power output = cut-in speed 1. Speeds less than this do not produce enough power to power the wind turbine iv. Above cut-in speed, power output rises until reaching a maximum  v. Maximum maintained at higher wind speeds by raising the pitch of the blades to increase drag and slow rotation vi. Above a maximum wind speed, the turbine is shut down to prevent mechanical/electrical damage 1. Done in part by rotating the rotor so that it is parallel rather than facing the wind 8. Wind farm layout a. Wind farms made up of multiple wind turbines b. Turbines placed to minimize disturbing wind passing by other turbines, i.e., minimize wind wake effects i. General rule of thumb for turbine spacing is ~7 rotor diameters c. Wind farms require less equipment and less personnel to run i. There is no fuel needed and no high-pressure system to be maintained ii. Entire site typically manage by small onsite building iii. Can also be monitored and even managed using laptop with appropriate SCADA software d. Power output from each turbine can vary with wind speed, so each turbine often contains an AC->DC->AC converter, with the latter AC synced to the grid e. The synced current is collected at medium voltage and then stepped up to high voltage by a step-up transformer and put onto high voltage transmission lines 9. Wind resource vs. wind farm output a. The best wind resources in the U.S. i. U.S. interior ii. U.S. mountain ridges iii. U.S. offshore 1. These resources are often far away from population centers and so require new, expensive high-voltage transmission lines to be built to tap into them  b. Wind energy highly variable i. In some places, is most powerful at night when grid demand is low ii. One way to deal with this is to separate wind farms far enough apart so that the variability of power output from each farm sums to a relatively consistent base power level iii. Where wind power is least variable is offshore 1. Large fetch area for strong consistent winds to form 2. Also, much lower surface roughness, so wind speeds stay strong nearer the surface 10. Wind economics a. Main costs are capital and fixed O&M i. Fuel is free and it takes far less to run a wind farm so variable O&M is negligible b. Other significant costs i. Land lease ii. Permitting iii. Installation c. Total costs have fallen as wind energy has been built out and economies of scale have been achieved d. Important driver for the latter here in the U.S. has been the Federal Gov’t’s incentive k.a. the production tax credit (PTC) i. At present, the PTC = $0.015/kWh 1. Was $0.023/kWh ii. Like the solar investment tax credit (ITC), this incentive would be available for two years and then lapse 1. As a result, wind development would wax and wane with the coming and going of the PTC iii. Since the Great Recession, the PTC has in place, but has recently been  stepped down iv. Again, it will be interesting to see if and how the incentive may be retained under the Infrastructure Bill