:::renewable
energy education:::solar energy education:::environmental
education:::
SolarSteve's
School
of Terrestrial Wind Energy
invites you to
consider issues concerning
:::
Wind System Siting and Placement :::
The first step
in the wise selection of a site is to understand
the general topography of the land you have to work with.
1. The
best site in general is one completely unobstructed for at least
300 yards in all directions.
2. The generator ought to be up high enough so as to clear all
obstacles
around by at least 15-20 feet, preferably more.
3. Locate wind generators between the direction of prevailing winds and
the object(s). Any object higher than the wind generator will cause a
disturbance
in air flow for approximately 300 yards upstream of the generator and
50-100
yards downstream.
Flat
Expanses
The
windiest areas are wide open spaces because there is less frictional
drag than over rugged terrain. One of the windiest spots in the country
is Fargo, North Dakota (with an average ground wind velocity of 14.4).
Fargo
is located on one of the flattest large expanses of land in the
continental
United States.
Proximity to Water
Sites
near a body of water generally experience higher wind velocity,
caused by the temperature differences between the air over the water
and
that of the air over the land. Count on a daily shift of wind direction
both in the morning and in the evening. Temperatures of land masses
rise
and fall more quickly than water surfaces, i.e., land is warmer than
water
during the day and cooler at night. During the day the air moves from
the
cooler water towards the warmer land and at night it shifts direction
and
moves from the warmer water to the cooler land mass.
Slopes
Mountainous
areas have local wind patterns similar to a site near a
body of water. In the daytime, air next to a mountain slope is heated
by
contact with the ground from solar. This air usually becomes warmer
than
air at the same altitude, but farther from the slope. Surrounding
colder,
dense air settles downward and forces the warmer air, near the ground,
up
the mountain, producing the "valley wind," called thus because
the air seems to be flowing up and out of the valley.
Conversely, at night the air in contact with the mountain is cooler
than
farther from the land mass and so the cooler air sinks along the slope
producing
the mountain breeze.
These constant temperature differences produce consistently higher wind
velocities than in other locales. Hills and ground wells cause an
unpredictable
variety of wind patterns. At some locations the wind is obstructed
completely,
at others a venturrie effect is caused by converging winds, thereby
creating
a very favorable wind generator site.
Barriers
Barriers
produce disturbed areas of airflow downwind, called wakes.
In barrier wakes, wind speed is reduced and rapid changes in wind speed
and direction, called turbulence, are increased. Because most wind
generators
have relatively thin blades that rotate at high speeds, barrier wakes
should
be avoided whenever possible, not only to maximize power, but also to
minimize
turbulence. Exposure to turbulence may greatly shorten the lifespan of
small
wind systems.
Buildings
Since it
is likely that buildings will be located near a wind system
site, it is important to know how barriers affect airflow and available
power. Building wakes increase in height immediately downstream. The
wind
flows around the building forming a horseshoe-shaped wake, beginning
just
upstream of the building and extending some distance downstream.
A general rule of thumb for avoiding most of the adverse effects of
building
wakes is to site the wind system:
* upwind a distance of more than two times the height of the building.
Upwind
and downwind indicate directions along the principal power direction.
* Downwind a minimum distance of ten (preferably 20) times the building
height, or
* At least the building height above ground if the wind system is
immediately
downwind of the building.
Ridges
Ridges
are defined as elongated hills that reach less than or equal
to 2,000 ft. above surrounding terrain and have little or no flat area
on
the summit. There are three advantages to locating a wind system on a
ridge:
1. The ridge acts as a tower.
2. The undesirable effects of cooling near the ground are partially
avoided.
The ridge may accelerate the airflow over it, thereby increasing the
available
power. The first two advantages are not unique to ridges but apply to
all
topographical features having high relief (hills, mountains, etc.)
The most important considerations in siting wind systems on or near
ridges
are summarized thus:
1. The best ridges or sections of a single ridge are those most nearly
perpendicular
to the prevailing wind. (however, a ridge several hundred feet higher
than
another and only slightly less perpendicular to the wind is preferable.)
2. Ridges or sections of a single ridge having the most ideal slopes
within
several hundred yards of the crest should be selected. Ridge sites
meriting
special consideration are those with features such as gaps, passes, or
saddles.
3. Sites where turbulence or excessive wind shear cannot be avoided
should
not be considered.
4. Roughness and barriers must be considered.
5. If siting on the ridge crest is not possible, the site should be
either
on the ends or as high as possible on the windward slope of the ridge.
The
foot of the ridge and the leeward side should be avoided.
6. Vegetation may indicate the ridge section having the strongest winds.
Flat-topped ridges present special problems because they can actually
create
hazardous wind shear at low levels. Siting a wind system at these low
levels
will cause unequal loads on the blade as it rotates through areas of
different
wind speeds, will create rapid fluctuations in direction, and could
decrease
performance and the life of the blades. These problems can be avoided
by
increasing the tower height to allow the blade to clear the shear zone.
"The road to Carbon Emission Reduction is always under
construction"
©S.K.Lowe1995-2008