What is the Wind?
Movement of air caused by the circulation of changing temperatures.
The uneven heating of the earths surface. Factors that Affect the
Shape and Direction of Mountains
Water / Land Connections
Time of day
Time of Year
physical features / terrain of the surface of a wind site.
α = Exponent that varies with surface roughness.
• .14 smooth terrain
• .20 small trees, buildings, corn fields
• .25 or higher with taller trees and/or buildings
• You can determine this for each particular site.
Shape and Direction of
Mountain –valley winds (page 24)
Water and Land Connections
Time of Day and Year
Higher winds found late in the day and evening Higher winds seen during the winter
Annual Average Wind Speed can fluctuate up to 25% from year to year.
Wind Direction and Wind Rose
Annual Frequency and Average
Annual Wind Speed in
Sixteen Compass Directions
Potato Hill, Watauga County, NC
Page 48 in text
North Carolina Wind
South Dakota Wind Resource Map
LiDAR and Sodar
Uses laser pulses for assessing wind speed, direction, and turbulence characteristics SODAR
Uses sounds pulses for assessing wind speed, direction, and turbulence characteristics
Anemometers & wind vanes
20 – 80 meter towers
Data collected and stored every
wind speed averages and wind directions
4 Standard Techniques
Wind Assessment Techniques
• Visual Assessment – Flagging
• Wind Maps
• Anemometer / Meteorological (Met.) tower
• LIDAR/SODAR (only for large wind installations)
Wind Speed, Power and Height
Use Metric!, Use Metric!, Use Metric!
Act like any intelligent industry and use a combination of both metric and standard • Often you must convert units
1 meter = 3.28 feet
1 meter/second = 2.24 mph
Power in the Wind
= rate @ which energy is available
is air(fluid) in motion (we want wind speed!!!)
has mass - 1.225 Kg/m3 at sea level & 59F.
of moving air contains kinetic energy
amount of power in the wind is a function of
speed & mass
is a cubic function of wind speed
• Double the speed and power increases 8 X
Wind Speed is Key!
Probably need a site with at least 10 mph average @ 30 meters for small wind turbines
& 15 mph for large
Wind resource assessment is essential !!!!!!!!!
Importance of Wind Resource Assessment
Marginal site Vs. Good site
Example: Bergey XL.1 $6500 system
4 m/s* avg. wind site. • AEO=1920
7 m/s avg. wind site. • AEO=4800
kWh/year kWh/year • $/kWh/20yr = $.17
• $/kwh/20yrs = $.07
• Lower output
• 2.5x higher output
• 40% in cost/kWh at good site vs.
cost cost/kWh marginal site.
* 1 m/s = 2.24 mph
(P/A in watts/m2) = ½ Density x V3
Power Density expressed in watts/m2
At sea level with a temperature of 15 C
(59 F) where air density is 1.225 kg/m3
P/A = ½ Air Density x V³
Air Density = 1.225 kg/m³
Velocity = 8m/s
P/A = (.5) (1.225 kg/m³) (8m/s)³
P/A = (.6125 kg/m³) (512m³/s³)
P/A = 313.6 W/m²
Air Density ( )
Inversely related to Temperature & Elevation
Decreases with increasing temperature & elevation
Cold and low places have higher air densities
Temperature is typically less significant and sometimes ignored (10 – 15 % yearly variation)
Elevation can be significant and a constant
(density @ 5,000’ is 15 % lower than sea level)
Air Density Changes with Elevation