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The equation to estimate the mean wind speed at height (meters) above the ground is: = [ + (,,)] where is the friction velocity (m s −1), is the Von Kármán constant (~0.41), is the zero plane displacement (in metres), is the surface roughness (in meters), and is a stability term where is the Obukhov length from Monin-Obukhov similarity theory.
The power law is often used in wind power assessments [4] [5] where wind speeds at the height of a turbine ( 50 metres) must be estimated from near surface wind observations (~10 metres), or where wind speed data at various heights must be adjusted to a standard height [6] prior to use.
Meteorological data includes wind speeds which may be expressed as statute miles per hour, knots, or meters per second. Here are the conversion factors for those various expressions of wind speed: 1 m/s = 2.237 statute mile/h = 1.944 knots 1 knot = 1.151 statute mile/h = 0.514 m/s 1 statute mile/h = 0.869 knots = 0.447 m/s. Note:
Wind speed on the Beaufort scale is based on the empirical relationship: [6] v = 0.836 B 3/2 m/s; v = 1.625 B 3/2 knots (=) where v is the equivalent wind speed at 10 metres above the sea surface and B is Beaufort scale number.
In common usage, wind gradient, more specifically wind speed gradient [1] or wind velocity gradient, [2] or alternatively shear wind, [3] is the vertical component of the gradient of the mean horizontal wind speed in the lower atmosphere. [4] It is the rate of increase of wind strength with unit increase in height above ground level.
The reason to vary the rotor speed is to capture the maximum aerodynamic power in the wind, as the wind speed varies. The aerodynamic efficiency, or coefficient of power, C p {\displaystyle C_{p}} for a fixed blade pitch angle is obtained by operating the wind turbine at the optimal tip-speed ratio as shown in the following graph.
Modern large wind turbines achieve peak values for C P in the range of 0.45 to 0.50, [2] [full citation needed] about 75–85% of the theoretically possible maximum. In high wind speed, where the turbine is operating at its rated power, the turbine rotates (pitches) its blades to lower C P to protect itself from damage.
Graphs by hour of California's total electric load, the total load less solar and wind power (known as the duck curve) and solar power output. Data is for October 22, 2016, a day when the wind power output was low and steady throughout the day. In electrical engineering, a load profile is a graph of the variation in the electrical load versus ...