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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.

Pre-Qualify the 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 covenants
• 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 Solar Site 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 energy.

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 path. 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 significantly. 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 rates.  

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.