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Mini Project Harout Charoian
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Transcript of Mini Project Harout Charoian
WIND FARM DESIGN AND
OPTIMIZATION
Harout Charoian Mini-Project at National Technical University of Athens
Date: 05/13/2016
1
Table of Contents Abstract: ........................................................................................................................................................ 2
Wakes: ........................................................................................................................................................... 3
Wake Models: ............................................................................................................................................... 4
Wake Effects: ................................................................................................................................................ 5
Wake effects in wind farms: ..................................................................................................................... 5
Wind Farm Shadowing: ............................................................................................................................. 6
Optimization: ................................................................................................................................................ 7
Wind Farm Layout Optimization: .............................................................................................................. 7
Pitch Based Active Wake Control: ............................................................................................................. 8
Yaw Based Active Wake Control: .............................................................................................................. 9
Loads due to Yaw Misalignment: ............................................................................................................ 10
Conclusion: .................................................................................................................................................. 12
Bibliography: ............................................................................................................................................... 13
2
Abstract:
The following report has been prepared for the 4th module of European Master in Renewable
Energy (2015/16) at the National Technical University of Athens specializing in Wind Energy.
The purpose of this project is to identify the importance of wake effects and ways of decreasing
wake effect losses.
Wind energy is one of the most mature technology in renewable energy industry. Due to its
success, many wind farms are being built with bigger sizes and number of turbines. Although the
technology itself is mature, there are still many studies especially on wakes to make wind farms
more efficient.
The wakes result of energy loss in wind farms, these wakes have to be minimized in order to make
wind energy more competitive. The energy market is being more and more competitive and the
additional losses through wakes are important to be analyzed and studied in order to have lower
cost of energy production and compete even better in the market.
This report provides an overview of different wake models and an explanation on how wakes are
created, its effects on a wind farm and also effects on neighboring wind farms. The report will also
discuss some optimization techniques that are used or under study to minimize wake effect losses.
The optimization that is recently introduced now is by de-rating the upstream wind turbine to
minimize the wakes for the downstream wind turbine. Another optimization which is still under
study is by yaw misalignment to deflect the wakes away from the downstream wind turbines. The
optimization techniques was concluded that it will increase the production of energy in wind farms,
but in the case of yaw misalignment technique more research has to be done on the fatigue loads
due to the misalignment.
3
Wakes:
Wakes are created by the wind turbines due to the extraction of kinetic energy from the air. The
wakes happen downstream of a wind turbine, which illustrates that it does not create problems if
there was only one wind turbine, but many. In the case of many wind turbines, which is called a
wind farm, it is very difficult to avoid wakes from other turbines unless if you have one row of
wind turbines and a prevailing wind direction.
The Levelized Cost of Energy (LCOE) in a wind farm is reduced or increased significantly by the
change in velocity and the turbulence. The target of wind farm design is to maximize the energy
yield as a whole wind farm and minimizing the fatigue loading on wind turbines. The success of
every investment is measured by the profit and designing a good wind farm will play a major
contribution to the successfulness.
Wakes are important to be taken into account while doing a wind farm due to two main reasons,
one reason is that wakes are the effect of velocity deficit and this deficit is directly effecting the
production of the turbine by the third degree shown in the equation𝑃 =1
2∗ 𝜌 ∗ 𝐶𝑃 ∗ 𝑈
3 ∗ 𝐴. The
second reason to understand the significance of wakes in wind farms is that they have much higher
turbulences, these turbulences will have negative effect on the lifetime of the wind turbines due to
the increase of fatigue loads.
Figure 1: Velocity Deficit Model
In the figure above, it is shown how a wind turbine results in a velocity deficit with distance and
it is seen that the wake expands and gets larger with distance. It also suggests that the further away
you are from the turbine, the velocity deficit will recover and eventually turn to zero. This is the
reason why we don’t see turbines positioned right after each other.
In order to do these studies, it is important first to make some models which are very accurate to
real testing conditions. These models will then be used in trying to find the best wind farm lay-out
as well as the best way of using wind turbine control to maximize the wind farm.
4
Wake Models: In order to prevent cases of very harmful wakes, some studies and researches were needed to be
done. Figure 1 looks simple to simulate but that’s only because it is a very simplified figure. In
reality, there are many conditions that need to be taken into account in order to have correlation
with real life scenarios. These conditions must be taken into account: the effect from the towers,
shear effect, rotation effect from the spinning blades, ground effects on the boundary layer and
turbulence from wind turbines and atmosphere. It is important to note that all these factors make
it very hard and complex to simulate perfect results. To simulate wake effects, there are two main
ways: the simple engineering models and the 3D models.
In the simple engineering models, it uses simplified momentum equations and mass conservation
equations to predict wakes, these are simple models which do not take into account the terrain
(ground effect) and the atmospheric boundary layer. Due to the simplicity of these concepts
calibrations is needed through experimental data to have close results. These simple engineering
models are based Prandtl, GCL semi-empirical and Abramovich’s theory for jets which could be
corresponded to wakes. The simple engineering models are not valid for near wakes and are only
valid for flat terrain and they also need calibration with experimental data.
There are more complicated but more accurate models which are the 3D models being used in the
industry. They are very difficult because it is hard to simulate turbulence and you have the
interaction of two different turbulences, the atmospheric turbulence and the turbulence that is
caused by the wind turbines. Due to the development of super computers it is now possible to run
full 3D Navier-Stokes equation simulations of wind farms, Reynolds Averaged Navier-Stokes
(RANS) have been used for the last 2 decades and having enough experience in applications. There
are two models in defining the Reynolds stresses for the eddy viscosity; the k-ε and the k-ω
turbulence models. Both of which need to be calibrated because the closure coefficients in them
are the numbers that are obtained through wind tunnel testing, these numbers change sometimes
in real conditions and that is the reason for the calibration. These models are now models that
could be used for the site to approximate wind and wake effect. But, there should be other models
to model the wind turbines that will be placed in the sites. The wind turbine modeling has been
done using two main techniques, the actuator disk concept and the actuator line concept.
There are two different ways of applying the actuator disk concept. The first way is that the loading
is considered to be uniform over the rotor disk, it is used widely due to its easy application and the
fact that only the thrust coefficient curve is needed. While the second way is that the loading is
variably distributed over the rotor disk, the thrust coefficient is also needed but this time along the
span to make the variable loading and it could be also done by using the Blade Element Theory.
For the both concepts there’s also the need of reference velocity; it is the velocity of the wind
before the velocity deficit happens due to the extraction of energy. To get the reference velocity,
the induction factor concept could be used. The second way of modeling a wind turbine is the
actuator line approach, Blade Element Theory is used to calculate the aerodynamic forces and
distribution the Gaussian distribution of forces are calculated by radial distances away. By having
these models all working together, it is possible to do many tests in improving the lay-out of the
wind farm and using wind turbine control techniques.
5
Wake Effects: Wake effects in wind farms:
Now that there are the tools to model simulations with good precision, the effect of the wakes
could be simulated. The wakes depend on many factors such as: the stability of the atmosphere,
stability of wakes (meandering, turbulence), the turbulence intensity and distances between wind
turbines. If the site has a high turbulence intensity then the recovery of the velocity deficit will be
in less distances due to more “turbulence mixing” happening with the atmospheric boundary layer.
Usually, the turbulence intensity of offshore is much lower than the turbulence intensity of onshore
sites. Thus the distances between wind turbines are greater on offshore.
The figure below has been has been simulated by Torbon J. Larsen (Riso-DTU) and it shows how
the intensity of ambient turbulence has effect on wake effects and production of the wind farm.
Figure 2: Shows a wind farm distribution with a 5x5 grid and a spacing of 8D
In figure 2 it is shown the wake effect of different turbulence intensities on the left side shows the
ambient turbulence intensity of 1% and on the right side with a turbulence intensity of 10%. The
total power output is shown for wind speeds of 8m/s and all directions, and the decrease by just
having the turbulence intensity difference is huge. The wind farm with 1% turbulence intensity is
producing 20MW and 60MW was produced if the turbulence intensity was 9% instead. The losses
were three times more with the same wind farm layout!
Figure 3: Is taken by measurements from a Danish offshore wind farms.
6
Figure 3 represents an unstable atmosphere on the left side and a stable atmosphere on the right
side in a Danish offshore wind farm which revealed a significant dependence of the atmospheric
stability on the wake losses. As seen in the figure there is the double amount of wake losses if the
atmosphere is stable rather than being unstable.
The stability of the atmosphere and the turbulence intensity of the wind has a very huge impact on
the energy production, but these effects are something that we cannot control and if there is
something that is not controllable then it cannot be improved as well. Two things could be done in
order to improve the output of the wind farm. Either by trying different lay-out arrays or use wind
turbine control option to maximize the output.
Wind Farm Shadowing:
Big wind farms cause velocity deficit downwind the farm, this is due to the wakes that we have
discussed before. Studies have shown now that due to increasing number of wind farms some wind
farms are being developed either upstream or downstream the wind farms causing them to be in
the wakes of the other wind farm. This effect is called also wind farm shadowing.
Figure 4: Wind speed effect due to neighboring wind farms (RisoDTU)
Figure 4 represents the velocity deficit due to a neighboring wind farm, it is seen that the deficit
starts as soon as the wind enters the first wind farm and recovery of the wind starts as soon as it
leaves the wind farm. But the downstream wind farm sees lower wind speed because it has not
fully recovered from the wakes of the neighboring wind turbines. This is a major challenge to
advance in which raises questions to the policies of offshore wind farms especially. Offshore wind
has usually a low turbulence intensity and also its flat terrain makes a slow recovery of wakes
between wind turbines. Thus, larger area is need for the wakes to recover and the velocity deficit
to recover as well. The challenge in the policy is how far should a new wind farm be located away
from the older wind farm for them not to have wind farm shadowing, according to (RisoDTU,
2007) the recovery of the wind deficit between wind farms are between 30-60km. The distance of
the recovery depends on the ambient turbulence, the stability of the atmosphere and the number of
turbines in the wind farm. The smaller the wind turbine groups then the reduction of the wind
speed is less.
7
Optimization: Wind Farm Layout Optimization:
The effects of wind turbine wakes sometimes even decrease the energy production by 50%, and
this is not a small number, this is the difference between making your project liable or a huge
failure. To optimize the design of a wind farm there are some software already available in the
market such as WindFarmer. The WindFarmer optimizes the wind farm while taking into account
restrictions such as the reachability, slopes, cabling and much more if needed. It takes into account
the wind directions and with the restrictions given, it does iterations with random places for each
wind turbine within the boundaries and gives us the best locations for the wind turbines.
Figure 5: Represents the difference between not optimized and optimized wind farms (TUDelft)
Figure 5 shows us the difference between a not optimized and optimized wind farm. On the right
is the optimized wind farm and on the left is a regular wind farm with no optimization at all. The
optimization was done without the restriction of cable costs, and it has increased the production of
energy but due to the cabling costs and longer routs it was not a liable investment thus another
restriction was added to have more real solution.
In figure 6, below it is seen that there has been put some restrictions that the wind turbines cannot
move further than a certain location from its original space, this is to minimize the cabling costs
and at the same time optimizing the wind farm.
Figure 6: Optimized wind farm lay-out (TUDelft)
8
The different colors in figure 6 show us two iterations of optimization and the green color is the
original place of the wind turbines without any optimization. These optimizations have been put
to practice now in a more serious matter in the offshore wind farms due to the flat terrains and less
ambient turbulence which wakes are penetrating more in wind farms than on complex terrains
onshore.
Pitch Based Active Wake Control:
Other ways to increase the energy output of the wind farm is by wind turbine control techniques.
There are two different kinds of wind turbine control. These wind farm optimization techniques
are regularly used to put wind turbines closer together thus decreasing the electrical infrastructure
costs or if the wind farm already exists but optimization was needed. The first method is by de-
rating the upstream wind turbine by using the pitch control of the system and making the wind
turbine more transparent for the wind turbine.
Figure 7: The velocity deficit by two systems (ECN)
In figure 7, it shows on the left hand side that the operation of the upstream wind turbine is on full
production and the velocity deficit downstream causes much less wind speed to reach the second
turbine due to the wakes. On the right hand, it is shown that by de-rating the upstream wind turbine
the velocity deficit is much less for the second turbine since there would be less wake effects. It
might seem that both wind farms would produce the same amount of energy due to the same
positioning, wind speed and direction. But, in practice the main idea is to compensate the losses
of the de-rated wind turbine by the downstream wind turbine and in most cases it the de-rating of
the wind turbine by just a small amount could up-rate the downstream wind turbine by more. De-
rating the wind turbine upstream will also have positive effect on the downstream wind turbines
since it will decrease the turbulence intensity thus less fatigue loads for the wind turbines. ECN
has done some research on Nordsee Ost wind farm layout, by using FarmFlow software, on the
maximization of the wind farm power production as well as measuring the ‘damage equivalent
loads’. The experiment was concluded with a 3.3% more power production and in some cases
reduction of damage equivalent loads of 40%, (Brand, Bot, Kanev, Savenije, & Ozdemir, 2014).
The increase of AEP 3.3% might not seem much, but these kind of increases add up to make the
industry more competitive. Also looking at the reduction of the fatigue loads has direct effects by
making the system needing less O&M.
9
As a summary de-rating wind turbines upstream will increase the power production of the wind
farm and in the meantime decrease the loading.
Yaw Based Active Wake Control:
The second way to use wind turbine control in optimizing the wind farm is by setting the turbine
misaligned with the wind direction, which is called yaw misalignment. It is misaligned mainly to
divert the wakes away from the downstream wind turbines. Unlike the de-rating of the wind
turbines the yaw misalignment will increase the loads on the system thus very careful operation
has to be done in order the wind turbine not to be performed more than its designed limit.
Figure 8: Diverging wakes by the method of Yaw-Misalignment
Figure 8 shows a simple visual example on how yaw misalignment could diverge the wakes for
the downstream wind turbine. The yaw misalignment technique has higher potential than the de-
rating of the wind turbine method because the wakes are being directed away rather than being
reduced. Diverging the wake would mean that the downstream wind turbine would get most of its
wind from the undisturbed wind flow rather than having the wake of the one upstream. The
disadvantage of the yaw misalignment method compared to the de-rated power is that it is only
beneficial for only two wind turbines in a row compared to the de-rated power technique it is
possible to do it for more than three turbines in a row.
The same simulation has been done by ECN with the same Nordsee Ost wind farm layout and it
is interesting to see that the production has of the top row has increased by 17.4%. Since the wakes
could only be diverted for the two turbines in a row that is the reason why the top row as been
measured. 17.4% increase by yaw misalignment compared to 3.3% due to de-rating is a huge
difference; however, the big increase will also have big impacts on the fatigue loading of the
turbine. The effect of yaw misalignment in downstream wind turbines should also be studied. I
have stated that it will decrease the fatigue loading of the downstream wind turbines since they
operate outside the wakes. But what if the downstream wind turbines operate in half-wake
operations? The fatigue loading on the downstream wind turbine increase due two reasons, one
reason is that the wind speed is faster than being in the wakes and the second reason is that
sometimes operation in half-wake makes asymmetry in loading which causes tremendous amount
10
of fatigue loading on the turbines. This topic is yet to be studied more in depth, but the results of
researches that are already available show positive signs that using yaw-misalignment technique
will cause considerable amount of power production increase in the expense of fatigue loads on
some turbines.
Loads due to Yaw Misalignment:
In order to prove if yaw misalignment has any effects on loading, using hGAST a script provided
by National Technical University of Athens, I have experimented two cases of loading. The first
case that was experimented is the loads during normal operation and the second case with 20
degree yaw misalignment. In both cases the simulations were identical, the wind turbine was a
5MW, 126 meters diameter, 89.5 meters tower height and under perfect wind conditions (no
turbulence or shear) with 9m/s wind speed. By applying these in the hGAST script, I have looked
at the forces on the tower and blades. The graphs are shown below:
Figure 9: The graph above show ‘pitching’ moment on the root of the tower
In figure 9, the ‘tilt’ moment causes the tower to move forward and backward oscillation due to
the thrust. The purple line is the loads with perfect alignment with the wind and it is seen that the
oscillations compared to the one with misalignment has higher amplitude. The high amplitude
directly effects the fatigue loading thus proving us that misalignment has more fatigue loading
than the one with perfect alignment. Another explanation could be seen through the two plots, is
that the misalignment has less average force compared to the normal operating turbine. This could
11
be explained that the thrust of the normal operating is more and it is extracting more kinetic energy
from the wind.
Figure 10: The graph represent the ‘roll’ moment on the tower
Figure 10 shows the two case of turbines, the graph represents the ‘roll’ moment which causes the
tower to oscillate sideways. In the chart it is seen that the average for the misaligned is also less
compared to the normal operating one, this load is mainly due to the generator torque. The torque
for the misaligned is less because it does not capture as much energy as the normal operating wind
turbine thus having lower average loads. But looking at the amplitudes of the two cases, the
misaligned wind turbine has higher amplitudes and this causes more fatigue loads.
The main factors of fatigue loading is the number of cycles and its amplitudes. In both cases the
number of cycles were the same for both tested turbines. But, the amplitudes of the loads were
higher when the turbines were misaligned compared to the normal operating turbines. This
suggests that misalignment technique does put extra fatigue loadings on the towers especially
during low operational wind speeds. This leads us to a conclusion that the fatigue loads are effected
by the yaw misalignment and at higher wind speeds there will also be bigger impacts since the
amplitudes will be much more.
12
Conclusion:
The importance of wakes to make the wind turbines more competitive in the market is undeniable.
Due to the wakes and bad positioning of wind turbines it is possible for some businesses to fail if
the wakes are not considered or taken seriously. Due to the effects on production it is very difficult
to find wind farms that agree to do experiments on them and there are only few wind farms built
to study wake effects because of its high investment costs. Instead, many models have been built
to simulate the wakes instead of using real wind farms. The models were discussed which were
from simple engineering models to complex models such as Navier Stokes models.
The effects of wakes depend on many factors, such as the ambient turbulence, the stability of the
atmosphere, the turbulences created due to upstream wind turbines and the distances between wind
turbines. The ambient turbulence and the stability of the atmosphere are natural phenomenon
which engineers can’t do anything about it. But, in order to have the most output out of the natural
conditions there are many ways to optimize the wind farms.
The first optimization that was discussed was having a better lay-out configuration depending on
the wind direction and minimizing the wake losses with some optimization programs such as
WindFarmer. Other ways optimizing wind farms were discussed as well, which is by the use of
the control systems on the wind turbine. There are two types of wind turbine control techniques:
the pitch based active wake control and yaw based active wake control. The pitch based active
wake control is already used commercially with positive results which decreases the fatigue loads
for the downstream wind turbines as well as increasing their production by de-rating the upstream
wind turbine and results in producing less wakes. The second technique is still not used
commercially and is under study, it was resulted that there is huge potential in the increase of
power production in this technique but there are also some concerns about the fatigue loadings and
the probability of having loads more than its designed capability. A simple simulated model was
run to see the difference on the loads of the tower if it has been misaligned by 20 degrees. In both
graphs in figure 9 & 10, suggest higher amplitudes of moment forces will happen which will result
in more vibrations and more fatigue loads. More research needs to be done also to see if these
fatigue loads are compensated by the production boost and to make this technique also
commercially applied.
13
Bibliography:
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