The Hula Hoop. Used for Fun and Exercise Since 500 B.C.
Helps to Explain Wind Power Concepts
41 in.
The Cross Sectional Area of a Hula Hoop is a Little Less Than 1 Square Meter
SUN
Hula
Hoop
1 square
meter
• Face the Hula Hoop toward the
bright sun, and the solar power
passing through it is about 1000
Watts), or
• 1 kilowatt per square meter.
• 1 kilowatt per square meter is
the standard sun intensity by
which solar panels are tested.
Your electrical harvest is 14% of
this.
1
Now, Face the Hula Hoop Toward the Wind
Hula
Hoop
1 square
meter
The wind power flowing through the Hula Hoop increases dramatically
with wind speed (varies by the cube)
“Cut-in” speed
Generator is at
rated output
power
Wind Speed
Power Through
Hula Hoop
8 mph
29 Watts
10 mph
55 Watts
20 mph
428 Watts
28 mph
Approx. 1000
Watts
Same 1000 Watts
as solar, but you
harvest 40%
instead of 14%
2
Wind Speed is Higher Above Ground
Even a modest increase in wind speed produces
an impressive increase in wind power
Up here at 213 feet
will be 13.2 mph
Example.
Measured at 30
feet,10 mph
126 Watts per
square meter
At top, 15.1 mph, 153
Watts per square meter
331 feet
213
feet
55 Watts per
square meter
Ground
3
At Rated Wind Speed and Wind Power (1000 Watts per square meter),
How Much of the Wind Power Can be Harvested?
Betz Law Says the Theoretical Limit is 60%. Realistic Value is 40%.
Upstream wind
speed V
331 feet
Wind speed is slowed by blades
to one-third of upstream V
The max harvest is when blade tip
speed is about 6 times the
upstream wind speed.
28 mph * 6 = 168 mph tip speed,
and 20 revolutions per minute.
4
The First Principle of Wind Turbine Operation, Discovered by Experiments
More than 200 Years Ago Is:
“To harvest the maximum power out of the wind, the turbine blade speed
(RPM) should be proportional to wind speed”
So, if rated wind speed = 28 mph corresponds to 20 RPM, then
● When wind speed = 20 mph, turbine speed should be 14.3 RPM
● When wind speed = 10 mph, turbine speed should be 7.1 RPM
● When wind speed = 8 mph, turbine speed should be 5.7 RPM
Note: If wind speed is higher than 28 mph, the turbine blades are pitched
back to hold generator power at rated 1.5 megawatts, so that turbine speed
never exceeds 20 RPM.
5
Turbine Placement on West Texas Mesa Top
The best performers are on the opposite edge of the mesa where turbulence is lowest
Prevailing wind
direction
6
Wind Farms Usually Have 100 to 150 Wind Turbines
• Each turbine occupies a rectangle of about 50-60 acres
• A 300 foot work area “pad” (about 2 acres) is needed for the cranes to
maneuver
• Electric power cables are underground
• Wind farms do not interfere with the ranchers or cattle, and the income from
royalties is welcome
2500 ft
• Wind farms are NOT noisy. When standing at the base, they are awe inspiring.
300 ft
pad
800 ft
Prevailing wind direction
7
Mesa Top, King Mountain Wind Farm, McCamey, TX
8
9
Big Wind
Load
Centers
Some
Wind
10