Green Collar Careers - Wind
Turbine Site Survey
Wind energy is the invisible force that people take for granted.
Where the sun visible lights the sky even on cloudy days the wind may
not announce is presence as often even when there is more then ample
wind to drive a wind turbine. In most places wind energy is far
more variable then solar energy. The sun rises and falls at
times we can rely on; the wind blows on its own schedule giving
a relatively unreliable level of predictability. For that reason
the perception of wind energy suffers. The task of wind energy
site survey differs from a solar site survey.
Site for a Wind Turbine
Wind Turbines are structures that generally require a property size of
1 acre or larger. In worst case conditions if a storm knocks the
wind turbine down the base should be located 10 feet further from the
perimeter of the property then the height of the tower; that way
in case if does come down it shouldn't endanger neighboring
properties. Some wind turbines emit noise; generally the
smaller wind turbines may be relatively quiet. Wind Turbines,
particularly tall towers with guy wires may be considered an eyesore.
Check local code wind turbines restrictions:
- Property placement
- Noise emissions
- Other limitations and restriction, including any neighborhood
- Tower Foundations and Soil Requirements (see below)
Average wind speed is the first element to determine for a site
survey. Look up the local Regional Wind Speed worksheet in the
Survey workbook to identify the suitability of wind energy
available by location. This table comes from over 20 years of
measured wind speeds and provides only baseline values for wind
Wind Turbine not recommended
Wind Turbine will have limited production; operational less
the 30% of the time
Wind Turbine operational at least 30% of the time
Wind Turbine operational at least 40% of the time
Wind Turbine operational at least 75% of the time
Locations in red will not have enough wind energy for a viable wind
turbine installation. Locations in yellow are on the edge of
viability; generally a solar electric system will be a much
better option in yellow zones.
Green may be acceptable for a site
survey, but remains cautiously dependant on the local terrain.
Low lying valley or wind flow obstructions can quickly reduce the
suitability of the site. Look at the relative elevation to the
surrounding 5 mile radius; are there any terrain features that
would work for or against this site? Generally if there are
steep grades nearby and the site is on a hilltop wind speeds will be
amplified; likewise if the site is in a valley adjacent to a
creek the wind speed is nullified. Use your best judgment.
In sites rated Good and Excellent wind turbines are likely to be
considered a much better alternative then solar electricity.
Obstacles and Terrain Features
|Natural obstacles to
wind flow may be trees or rolling terrains that create
turbulence in the wind flow. Turbulence reduces the
effective energy that drives the wind turbine. A general
rule is to place the wind turbine a horizontal distance at least
ten times the vertical height of an obstructing object when that
obstructing object is as tall as the bottom point of the blade
If the obstruction is below the
blade path it may actually amplify the the wind energy to
channel the wind into the blade path although flexible objects
such as tree limbs will increase some of the turbulence.
A single tree below the blade path will generally increase
turbulence up to distances of about 5h horizontal. That takes an
evaluation of predominant wind direction; if the predominant
wind direction is in line with a row of trees or other obstacles that
will channel the wind there stands a good chance that feature can be
used to position the wind turbine tower. Again you will have to
use your best judgment with this unpredictable source of energy.
|Structural shape of
the obstacle also affects the turbulence zone. The typical
home with a sloped roof will direct the cross flow of air
upwards as the roof acts as a ramp. The diagram gives a
general view of where the turbulent zone will be.
The simple solution is to locate the wind
turbine as high as possible to avoid the turbulent zones.
The taller the tower the more it costs, and that cost factor
increases with height. Generally as heights exceed 60 feet
the cost factor begins to go up in a hurry.
Local Soil Conditions
|As the wind turbine
is perched on top of a tower the soil conditions will determine
the structure of the footing. Guy wire supported towers
distribute the load much more uniformly then a free standing
tower although they also have the least aesthetic appeal and the
most lifetime maintenance requirements.
Footing requirements will range widely both from soil conditions
and local code requirements. As free standing towers begin
to approach 60 feet the footing requirements begin to increase
Loose soft soils such as sand or sandy loam
generally require deeper footings up to 20% of the tower height,
with diameters about 2% of the tower height. These are
only approximations as every local code will vary.
You will need to understand the local codes
before doing the site survey. In some cases this may
require taking some soil samples.
Building Mounted Wind Turbines
Where this may be an acceptable practice on a steel building structure
is is not recommended for wood or concrete block structures.
Wind turbines will apply varying stresses to a building particularly
during high winds. While the structure may be "within limits"
for the structural rating there will be flexure of the structure
resulting in very undesirable effects; nails will come loose,
drywall and concrete joints will form cracks at greatly accelerated
Solar Site Survey Notes
Most of the data on the Site Survey Data worksheet should be
self explanatory although the number in cell B88 may provide a little
confusion. To derive this number number note any potential
obstacles and apply these rules:
- Take the approximate height of the obstacle and multiply by 10
- If the wind turbine tower is located in a horizontal location
of less then 10 times the height of the obstacle, then subtract
the tower height from the obstacle height.
For example; a 39 foot tall tree is 300 feet from the tower.
This is within the 10X rule. The wind turbine tower is 50 feet
tall. Therefore the Tower Obstruction Clearance is 11 feet.
Note that this a roughcut adjustment factor intended to derate annual
production based on near obstacles only.