Section II Wind Resource Assessment

1. Section IIWind Resource Assessment

2. Learning Objectives • Understand wind resources • Understand use of Wind Resource maps • Learn how to interpret wind resource data • Learn how to present wind data • Understand the Power available in the wind

3. Wind • Wind Patterns from pressure differentials. • Uneven heating by solar radiation • Rotation of Earth • Seasonal variations of solar flux • Vertical pressure gradients are mostly balanced by gravity so most wind patterns are horizontal.

4. Wind Resource Facts • Double the wind speed results in 8 times the power! • Wind speed is larger as you move higher above the ground. • Doubling the area swept by the blades doubles the power captured from the wind.

5. Wind Resource Maps • The US Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) in Golden, Colorado is responsible for production of the wind resource maps. • NREL contracted with a company to create these maps. • Available wind speed and direction data from the US Weather service, airports, etc. were used as input data.

6. Wind Map Creation • Topographic maps and sophisticated air flow modeling was done to connect the specific data sites together integrating local topographic features and to normalize data at specific heights above the ground: 50 m and 70 m typically. • The results are continental US and state-by-state maps.

9. Warning! • Due to the inherent inaccuracies in the modeling and the scarcity of data at the proper elevation and locations, the wind maps should only be used as an indicator of wind resource. The only reliable and accurate data at a specific site is to measure the wind there. Proper data requires measurement of wind speed and direction for at least 1 year!

10. Wind Physics • Power available in the wind is proportional to the • Air Density, r (kg/m3) (kilograms per cubic meter) • Air density depends on altitude, air temperature, humidity, and local weather conditions (barometric pressure) • The typical value used for wind resource estimates is at sea level at a temperature of 59 oF (15 oC) and is 1.225 kg/m3 The air temperature decreases at about 3.57 oF/1000 ft. (6.5 oC/1000 ft.) between sea level and 10,000 ft. of altitude. The air density decreases as temperature increases.

11. Air Density • Even though the temperature drops as we increase elevation (hence the air density should be higher), gravity causes the air to be concentrated closer to the ground (higher pressure). • Gravity beats temperature so the net effect is for the air density to be lower as we increase our height above the ground.

12. A graph of the drop in air density as a function of elevation above sea level is given. The equations below are to estimate the air density at a specific elevation of the turbine hub where the elevation, h, is given in feet or meters. The equations can be used to extend the air density calculation beyond the elevation values shown in the graph. • = 1.225 - 8.5662 x 10-6 h (h in feet) • = 1.225 - 2.8097 x 10-5 h (h in meters) Note that the air density is approximately linear with elevation up to about 10,000 m

13. Wind Physics • Power available in the wind is proportional to the • Air Density, r • Swept Area of the Rotor, A (m2) • We will discuss what “swept area” means later

14. Wind Physics • Power available in the wind is proportional to the • Air Density, r • Swept Area of the Rotor, A • Cube of the Air Velocity, (v x v x v , each in units of m/s) (Pw = ½ rAv3) W = kg·m2/s3

15. Wind-Turbine Physics • Power available in the wind is proportional to the • Air Density, r • Swept Area of the Rotor, A • Cube of the Air Velocity, (v x v x v) (Pw = ½ rAv3) • Betz Limit – theoretical maximum power that can be extracted from the wind is 0.59 or 59%

16. Betz Limit • As we extract energy from the moving air it slows down on the back-side of the turbine rotor. If we tried to extract more kinetic energy from the moving air, it would pile up on the rotor’s back-side, inhibiting more air to move past and through the rotor – The big traffic jam and less power into the turbine! • Therefore, it turns out that we can extract only a maximum of about 59% (the exact number is 16/27) of the wind’s power.

17. Betz Limit • This theoretical limit is based on the physics of air flow NOT a limit of technology or being able to design a better turbine. • Modern good turbines are able to extract about 40% of the power available in the wind. Engineering work is continual in pushing the performance toward the Betz limit of 59%. Note: there are many turbine designs, particularly small turbines, that extract a much lower percentage of the wind’s power! Beware!

18. Swept Area • Swept area is the area that the blades of a wind turbine intersect with the wind. NOTE: Swept Area is NOT the surface area of the blades! • A HAWT with one or more blades has them moving in a circular motion so the swept area, A, is a circle with a radius, R, equal to the blade length, L, plus the rotor hub radius. Since the blades are typically much longer than the hub, the blade length is often used to estimate the swept area radius. • Therefore, the swept area for this type of blade arrangement is A = pR2 ≈ pL2

19. Swept Area W • Swept area for most vertical axis wind turbines (VAWT) is a rectangle. • Therefore, the swept area for this type of blade arrangement is L x WA = L x W L

20. Example • Given an average annual wind velocity of 8 m/s (17.9 mph) at a site, the expected Wind Power Density available for capture is Pw = ½ rAv3 OR Pw /A = ½ rv3 (power density or power per area) Pw /A = ½ x 1.225 kg/m3 x (8 m/s)3 = 0.5 x 1.225 x 8 x 8 x 8 = 313.6 W/m2

21. Example (cont.) • Further, if we are using a turbine that has a circular area that is swept out by the blades of about 7,854 m2 (corresponds to 50 m length blades). • This would give us an available wind power of 313.6 W/m2 x 7,854 m2 = 2,463,014 W = 2.46 MW

22. Assume dry air

23. Wind Regimes Let’s discuss this box on the wind map

24. Look at the first line where the corresponding Wind Power Density is about 300 to 400 W/m2. • By definition the lower end of the range determines the numerical Class number. This lower range is then a Class 3 wind regime (300 W/m2) and is considered a “Fair” wind resource area. • The next-to-last line has a wind power density of 600 to 800 W/m2. The associated regime is Class 6 (for the 600 W/m2 at the lower end of this range) and is considered “Outstanding.”

25. The Wind Power Density is calculated using a standard air density value of 1.225 kg/m3 • Wind Power Density, Pw/A = 0.5rv3 • The distribution of wind velocities (generally measured over a year) at any given site tends to look like the graph below. The vertical axis on the graph is the relative probability of the wind velocity being a particular value. Note that the wind speed at maximum probability is NOT the average wind speed. • The wind velocity ranges that are given in the map’s table are associated with the distribution defined below. This wind velocity “probability” curve can be described mathematically by something known as a Weibull function. One of the fitting parameters used to specifically quantify the function’s shape and fit is “k”. This Weibull function uses a k = 2 value. Any more detail is beyond the scope of this course. Suffice it to say that the Weibull k value of 2.0 describes a specific form of the Weibull function used to estimate wind velocity probability distributions across large geographic regions. v

26. Exercise 3 1). A GE model 1.5s turbine (with blades type GE 34a) has a rotor diameter of 70.5 m. What is the swept area of the rotor? • 4,970 m2 • 15,615 m2 • 3904 m2 • 141 m2

27. Exercise 3 2). Scotsbluff, NE has an average wind speed at 70 m of what?

28. Exercise 3 3). The power available in the wind depends on the • cube of the wind velocity, v3 • wind velocity, v • square of the air density, r2 • tower diameter, D

29. Exercise 3 4). Given the wind velocity of 15.3 mph, and a HAWT turbine with a rotor diameter of 42 m, the power available in the wind is • About 12.16 MW • About 1.088 MW • About 3.04 MW • About 271.9 kW

30. Exercise 3 5). Given that we want an average wind power availability of 160 kW and using a blade with a length of 25 m on a HAWT, what is the average wind velocity required at our site? • 6.3 m/s • 5.1 m/s • 8.2 m/s • None of the above

31. Exercise 3 6). Given that we want an average wind power availability of 160 kW and our site has an average wind velocity of 7.2 m/s, what is the approximate blade length we need on a HAWT? • About 25 m • About 15 m • About 6 m • About 22.2 m

32. Wind Velocity and Height

33. Wind and Height • Generally, the wind velocity is on average a larger value as you move above the ground. • This should be intuitive as the ground interferes less with the wind so the air is undisturbed and free to flow better as we get higher. • Therefore, placing a turbine as high as possible above the ground will provide better performance on average.

34. Wind and Height • The variation of wind velocity with respect to height is termed Shear. • Empirical* relationships for the wind shear are typically made using ratios of heights and associated velocities. * Empirical means an equation based on measurements, not an equation derived from fundamental physics.

35. Wind Shear • Why are wind shear calculations important? • Answer: We often have wind data at a height lower than we would like to place a turbine. Therefore we need to estimate the wind speeds at the expected height of our turbine.

36. Annual average shear exponents can vary from 1/7 to 0.25, causing considerable uncertainty in vertical extrapolations of wind resource.

37. Example Given a wind velocity of 8 m/s at a height of 10 m above the ground, we can estimate the wind velocity at 70 m using the 1/7thshear exponent (1/7 = 0.14286): m/s

38. Example (cont.) • What’s the big deal about a difference in wind velocity of 10.56 m/s as compared to 8 m/s? • Remember the wind power depends on the wind velocity cubed. • 10.56 m/s wind has more than twice (2.3 times) the available power than 8 m/s wind!! Lesson: Knowing the wind velocity at the turbine height is IMPORTANT!!

39. Exercise 4 1). The measured wind velocity at 15 m is 6.2 m/s. What is the estimated velocity at 30 m? (Hint: use a shear exponent of 1/7th) • About 12.40 m/s • About 5.615 m/s • About 6.845 m/s • About 20.65 m/s

40. Exercise 4 2). The measured wind velocity at 10 m is 5.4 m/s. What is the estimated velocity at 50 m using the two shear exponents of 1/7th and 0.2 ? • 6.8 and 7.45 m/s • 5.1 and 5.3 m/s • 5.8 and 6.1 m/s • None of the above

41. Wind Data Collection • Data is most often collected using Meteorological Towers (called MET towers) that have: • Anemometers to measure wind speed, • Wind Vanes to measure wind direction and often have • Thermometers to measure air temperature • Barometers to measure barometric pressure to refine air density calculations

42. Wind Data Collection • MET towers are either tubular steel 6-8 inches in diameter or for taller towers, are a steel lattice structure (similar to radio towers). Guy wires secure these towers at connections from the tower at several heights to anchors in the ground. • The anemometers and wind vanes are usually positioned at two or three heights on the tower, with two anemometers and one vane at each height. Three six-foot long booms, one for each anemometer and wind vane, are attached to the tower extending out horizontally. Each sensor is mounted at the end of the boom to minimize the effects of the tower’s wind wake. The booms for the anemometers are positioned in opposite directions. • The sensors produce data that give the wind speed and direction averaged over 10-minute intervals. This data is recorded and stored by a data logging system at the bottom of the tower. The sensors on the tower are connected to the logger by cable running down the tower. • Wind data is collected at several heights to determine the local wind shear.

43. Wind Data Collection • The alternate equipment to a MET tower is the SOnic Detection And Ranging system (SODAR). • An ultrasonic signal is directed upwards into the air. The signal bounces back and is recorded. • The signal’s transit time gives the height. • The signal’s frequency shift (Doppler shift) gives the wind speed at height.

44. Wind Rose • The wind data gathered over a year can be graphically plotted to give a quick picture of the relative wind velocity and prevailing direction during the year. • The rose is a circular plot (polar) where the 16 main compass directions are identified. The percentage time that the wind blows in a particular direction is plotted as a wedge. Each wedge is divided into color-coded segments. The width of the segments corresponds to the percentage time over the year that the wind has a velocity in the range defined by the color used for the wedge segment.

45. Wind Rose • A look at the wind rose is extremely useful for siting wind turbines. If a large share of the energy in the wind comes from a particular direction, then you will want to have as few obstacles as possible, and as smooth a terrain as possible in that direction, when you place wind turbines in the landscape.

46. Each wedge shows the percentage of time that the wind is from that direction.(usually along one of 16 compass directions)The radial length of the wedge indicates the total percentage of time for wind from that direction.The relative radial lengths of each color indicate the percentage time in a particular wind speed range. Example:In slightly North of NW direction the wind blew 1.5 % of the time (21.5% - 20%) at speeds of 25-30 knots (red). A turbine sited at this location would need unobstructed terrain in the prevailing wind direction of North to NW.

47. Examples of other Wind Rose Plots • The station identification, month, and period of record (1984-1992) are located at the top left of each wind rose. • The percent frequency of calm winds is located in the lower left side of the wind rose plot. • The frequency of occurrence of winds is plotted in the sixteen points of the compass (north, north-northeast, northeast, etc.) with a maximum of six wind speed classes for each direction. • The legend for the wind speed colors is at the bottom left of each wind rose.

48. Exercise 5 1). Using the left-most wind rose from the previous slide, what is the direction of the predominant wind? 2). In the Westerly direction, what is the percentage of time that the wind blows in the range of 7 to 10 knots?