GUIDELINES FOR WIND POWER APPLICATIONS
By Emil Bedi, CANCEEand Hakan Falk, "Energy Saving Now".
Wind turbines have to compete
with many other energy sources. It is therefore important that they be cost
effective. They need to meet any load requirements and produce energy at a
minimum cost . When you have decided that it is time to consider buying and
installing a wind turbine you have to examine first two things: how much energy
you require, and what is the average wind speed at the height of the wind
turbine. Sometimes, it sure seems windy in your area, at least part of the time
any way. But how can you tell if a wind turbine generator will really be
optimised in term of power output versus wind speed. The common response is that
you must monitor the wind speed at your site for at least one year and compare
the results with historical data that had been recorded for some years. Or,
contract a professional who will do a ‘feasibility study’ to estimate the yearly
average wind speed and the estimated annual energy that would be captured by the
wind turbine. Usually, which way to choose depends on the amount of investment
you are willing to pay for having the wind turbine. For small applications when
the amount of investment is relatively small, it is unrealistic to pay more than
the cost of the wind turbine for obtaining the yearly average wind speed.
Wind systems are at the mercy of their site survey. Without an extended
site survey or real wind data for a specific location, it is really impossible
to specify a wind turbine for the system. While PV and microhydro systems are
often effectively designed by their users, wind systems should seek help from
someone who really knows wind power. Here are some guidelines for siting and
sizing small wind turbines.
SITING A TURBINE
A common way of siting wind turbines is to place them on hills or ridges
overlooking the surrounding landscape. In particular, it is always an advantage
to have as wide a view as possible in the prevailing wind direction in the area.
On hills, one may also experience that wind speeds are higher than in the
surrounding area. You may notice that the wind can bend some time before it
reaches the hill, because the high pressure area actually extends quite some
distance out in front of the hill. Also, you may notice that the wind
becomes very irregular, once it passes through the wind turbine rotor. As
before, if the hill is steep or has an uneven surface, one may get significant
amounts of turbulence, which may negate the advantage of higher wind speeds.
DISTANCE BETWEEN OBSTACLE AND TURBINE
The distance between the obstacle and the
turbine is very important for the shelter effect. In general, the shelter effect
will decrease as you move away from the obstacle, just like a smoke plume
becomes diluted as you move away from a smokestack. In terrain with very low
roughness (e.g. water surfaces) the effect of obstacles (e.g. an island) may be
measurable up to 20 km away from the obstacle. If the turbine is closer to the
obstacle than five times the obstacle height, the results will be more
uncertain, because they will depend on the exact geometry of the obstacle.
ROUGHNESS
The roughness of the terrain between
the obstacle and the wind turbine has an important influence on how much the
shelter effect is felt. Terrain with low roughness will allow the wind passing
outside the obstacle to mix more easily in the wake behind the obstacle, so
that it makes the wind shade relatively less important. A good rule of
thumb is that we deal with individual obstacles which are closer than about 1000
metres from the wind turbine in the prevailing wind directions. The rest we deal
with as changes in roughness classes.
OBSTACLE HEIGHT
The taller the obstacle, the
larger the wind shade. If the turbine is closer to the obstacle than five times
the obstacle height, or if the obstacle is taller than half the hub height, the
results will be more uncertain, because they will depend on the exact geometry
of the obstacle. In that case the programme will put a warning in the text box
below the results.
WAKE EFFECT FROM WIND TURBINE
Since a wind
turbine generates electricity from the energy in the wind, the wind leaving the
turbine must have a lower energy content than the wind arriving in front of the
turbine. This follows directly from the fact that energy can neither be created
nor consumed. A wind turbine will always cast a wind shade in the downwind
direction. In fact, there will be a wake behind the turbine, i.e. a long trail
of wind which is quite turbulent and slowed down, when compared to the wind
arriving in front of the turbine. Wind turbines in parks are usually spaced at
least three rotor diameters from one another in order to avoid too much
turbulence around the turbines downstream. In the prevailing wind direction
turbines are usually spaced even farther apart.
TURBULENCE
Turbulence decreases the
possibility of using the energy in the wind effectively for a wind turbine. It
also imposes more tear and wear on the wind turbine, as explained in the section
on fatigue loads. Towers for wind turbines are usually made tall enough to avoid
turbulence from the wind close to ground level.
AVERAGE WIND SPEED
To correctly site and size a wind turbine, it is helpful to have the
information about average wind speed for the location. The annual average wind
speed is used to describe the general windiness of a place. Shorter-term
averages (monthly, hourly) are used in more precise analyses where the time
relation between wind energy availability and energy demand is particularly
important. The time variation of wind speed at a given site is described by the
relative probability of the wind speed at any moment being greater or less than
the average wind speed. A typical distribution of wind speed (called the
Rayleigh Distribution, special case of Weibull Distribution) usually means that
there is little probability of absolutely no wind; the most frequent wind speed
is about 75% of the average wind speed; and wind speeds above twice the average
wind speed do occur, but not often.
Wind Speed Measurement
Don’t consider wind power without a thorough measurement of the wind
speed at your specific location. In most cases, four months should be the
minimum recording interval and one year is preferred. If you are going to spend
a lot of money on a wind system, this extra eight months could mean the
difference between a good investment and a bad one.
 The measurement of wind
speeds is usually done using a cup anemometer. The cup anemometer has a vertical
axis and three cups which capture the wind. The number of revolutions per minute
is registered electronically. Normally, the anemometer is fitted with a wind
vane to detect the wind direction. Other anemometer types include ultrasonic or
laser anemometers which detect the phase shifting of sound or coherent light
reflected from the air molecules. Hot wire anemometers detect the wind speed
through minute temperature differences between wires placed in the wind and in
the wind shade (the lee side). The advantage of the non-mechanical
anemometers may be that they are less sensitive to icing. In practice, however,
cup anemometers tend to be used everywhere, and special models with electrically
heated shafts and cups may be used in arctic areas.
Determining the exact average
annual wind speed is not an easy task and it is an expensive process. After all
it might be unnecessary. For small wind turbines applications what we need to do
is get some idea of the average annual wind speed for the area, and that can be
available by observing few physical phenomena around the site. Start by your
feeling, while they are hardly scientific, then try to check the airport and
weather station data for your area. Use these data as a raw baseline, which you
have to tune to make them represent your area.
Meteorologists already collect wind data for weather forecasts and
aviation, and that information is often used to assess the general wind
conditions for wind energy in an area. Precision measurement of wind speeds, and
thus wind energy is not nearly as important for weather forecasting as it is for
wind energy planning, however. Wind speeds are heavily influenced by the surface
roughness of the surrounding area, of nearby obstacles (such as trees,
lighthouses or other buildings), and by the contours of the local terrain.
Unless you make calculations which compensate for the local conditions under
which the meteorology measurements were made, it is difficult to estimate wind
conditions at a nearby site. In most cases using meteorology data directly will
underestimate the true wind energy potential in an area.
It is
because weather stations monitor wind speeds at or slightly above street level,
where people live. They don’t monitor wind speeds at 20 - 30 meters, where the
wind turbine is usually located. Similarly, airports data has limited value.
Because airplanes traditionally had problems taking off and landing in windy
locations, airports were sited in rather sheltered locations. Virtually all
airports are sheltered. After having the raw data from nearby airport or
weather station, you need to extrapolate these numbers to your location using a
concept know as shear ‘factor’. Based on these numbers and the topographical
difference or similarity between your site and theirs (weather station and
airport), you can theoretically estimate your average wind speed at any proposed
height.
Very simple anemometer can be build by yourself. Here is the way how to
construct it. Materials needed : five paper Dixie cups, two straight plastic
soda straws, a pin scissors, paper punch, small stapler, sharp pencil with an
eraser.
Procedure: Take four of the Dixie cups. Using the paper
punch, punch one hole in each, about a half inch below the rim. Take the fifth
cup. Punch four equally spaced holes about a quarter inch below the rim. Then
punch a hole in the centre of the bottom of the cup. Take one of the four cups
and push a soda straw through the hole. Fold the end of the straw, and staple it
to the side of the cup across from the hole. Repeat this procedure for another
one-hole cup and the second straw. Now slide one cup and straw assembly through
two opposite holes in the cup with four holes. Push another one-hole cup onto
the end of the straw just pushed through the four-hole cup. Bend the straw and
staple it to the one-hole cup, making certain that the cup faces in the opposite
direction from the first cup. Repeat this procedure using the other cup and
straw assembly and the remaining one-hole cup. Align the four cups so that their
open ends face in the same direction (clockwise or counter clockwise) around the
centre cup. Push the straight pin through the two straws where they intersect.
Push the eraser end of the pencil through the bottom hole in the centre cup.
Push the pin into the end of the pencil eraser as far as it will go. Your
anemometer is ready to use. Your anemometer is useful because it rotates at the
same speed as the wind. This instrument is quite helpful in accurately
determining wind speeds because it gives a direct measure of the speed of the
wind. To find the wind speed, determine the number of revolutions per minute.
Next calculate the circumference of the circle (in feet) made by the rotating
paper cups. Multiply the revolutions per minute by the circumference of the
circle (in feet per revolution), and you will have the velocity of the wind in
feet per minute. The anemometer is an example of a vertical-axis wind collector.
It need not be pointed into the wind to spin.
FLAGGING
Another useful tool to help determine the
potential of a wind site is to observe the area’s vegetation. Trees, especially
conifers or evergreens, are often influenced by winds. Strong winds can
permanently deform the trees. This deformity in trees is known as flagging.
Flagging is usually more pronounced for single, isolated trees with some height.
On the upwind side of the tree, the branches are noticeably stunted. On the
downwind side, they’re long and horizontal. The flagging was caused by
persistent winds from, more or less, one direction. Look around especially for
single trees, or trees on the outskirts of a grove. Unless they have grown
considerably above the common tree line, trees in a forest will not show
flagging because the collective body of trees tends to reduce the wind speed
over the area. While the presence of flagging positively indicates a wind
resource, you should not conclude that the absence of flagging in your area
precludes any suitable average wind speeds. Other factors that you are not aware
of may be affecting the interaction of the wind with the trees.
For very rough estimate of the average wind speed Griggs-Putman
Index of Deformity can be used. 
VARIATION OF WIND SPEED
While average wind speed is meaningful, there are other wind parameters
that are just as meaningful. Other wind parameters worth knowing are maximum
wind speed, number of days (hours) between winds of greater than 5m/s. Number of
consecutive days (hours) where the wind is in excess of 5 m/s, and the times of
year where the either wind or not wind periods occur. The wind speed is always
fluctuating, and thus the energy content of the wind is always changing. Exactly
how large the variation is depends both on the weather and on local surface
conditions and obstacles. Energy output from a wind turbine will vary as the
wind varies, although the most rapid variations will to some extent be
compensated for by the inertia of the wind turbine rotor.
All important data is not available from garden variety recording
anemometers. A recording anemometer that will take all the data mentioned above
will cost much. Such anemometers are more computer than wind sensor and cost
between USD 2,000 and USD 4,000.
SIZING A SMALL TURBINE
This is a job for someone with experience with all types of wind
turbines. Not only must the wind turbine be well made, but it also must fit the
wind conditions at your particular site and must produce the power that the
system requires. Modern turbines usually produce some specie of low voltage and
only the very large units make 60 cycle, 120/240 VAC directly.
When
choosing a turbine the rated power for a wind turbine is not a good basis for
comparing one product to the next. This is because manufacturers are free to
pick the wind speed at which they rate their turbines. If the rated wind speeds
are not the same then comparing the two products is very misleading. Usually
manufacturers will give information on the annual energy output at various
annual average wind speeds. These figures allow you to compare products fairly,
but they don’t tell you just what your actual performance will be.
TOWER
The power in the wind is a function of
(among other things) the cube of the wind speed. Therefore, the easiest way to
increase the power available to a wind generator is to increase the wind speed.
We can increase wind speed by either installing a taller tower or by moving to a
windier location. Note that as a percentage, wind speed increases much faster
over terrain cluttered with trees and buildings than over flat open ground. With
the exception of the middle of a lake or desert, wind speed increases
significantly with height. For example, power available at 30 meters can be up
to 100% higher than power available at 10 meters. Said another way, two wind
generators on two 10 meters towers will produce as much power as one wind
generator on a 30 meter tower. And the system with the 30 meters tower will be
cheaper to install than the “twin” systems at 10 meters. The rule of thumb for
siting is that the wind generator must be at least 10 meters above any obstacle
within 100 meters. Consider 15 meters to be a realistic minimum and after that,
go as high as you can. Smaller turbines typically go on shorter towers than
larger turbines. A 250 watt turbine is often, for example, installed on a 15-20
meter tower, while a 10 kW turbine will usually need a tower of 20-30 meter. A
wind turbine must have a solid tower to perform efficiently. Turbulence, which
is highest close to the ground and diminishes with height, reduces the
performance of the turbine.
For small wind mills the least expensive
tower type is the guyed-lattice tower, such as those commonly used for ham radio
antennas. Smaller guyed towers are sometimes constructed with tubular sections
or pipe. Self-supporting towers, either lattice or tubular in construction, take
up less room and are more attractive but they are also more expensive. Telephone
poles can be used for smaller wind turbines. Towers, particularly guyed towers,
can be hinged at their base and suitably equipped to allow them to be tilted up
or down using a winch or vehicle. This allows all work to be done at ground
level. Some towers and turbines can be easily erected by the purchaser, while
others are best left to trained professionals. Anti-fall devices, consisting of
a wire with a latching runner, are available and are highly recommended for any
tower that will be climbed. Aluminium towers should be avoided because they are
prone to developing cracks. Towers are usually offered by wind turbine
manufacturers and purchasing one from them is the best way to ensure proper
compatibility. Be sure that the tower is strong and well installed. Sloppy tower
installation can bring the whole system crashing down. Guyed towers are more
secure and less expensive than unguided towers.
Choosing a wind controller
In almost every case, the manufacturer of
the wind machine also makes a regulator for that specific model. So, the user
doesn‘t have to select a regulator because it is bundled in with the wind
machine. These controls are shunt types that divert the turbine‘s output to
maintain control of the system‘s voltage. Diversion regulator schemes are really
the only type used, because unloading the wind machine will cause overspeeding
and damage to the turbine.
Sizing the Wind system‘s battery
The size of a wind system battery storage is
determined by the longest period of windless weather. This can be very difficult
to determine in advance. For this reason wind systems usually have more days of
battery storage than do PV systems. Shoot for a minimum of seven days of storage
and extend this to fourteen days if you can afford it. Wind power comes in gusts
and spurts, having a large battery makes more effective use of nature‘s least
consistent power source.
© Copyright energysavingnow.com 2000.
© Copyrights to Software @ this site
|
