Transcript Document

PhD Candidate: Maarten Holtslag
Department: AWEP
Section: Wind Energy
Supervisor: W.A.A.M. Bierbooms
Promoter: G.J.W. van Bussel
Start date: 01-10-2011
Funding: FLOW
Cooperations: ECN-Ecofys-Eneco
Far Shore Wind Climate
Modelling
Background
As shown in previous figures, shear is highest for stable
conditions while turbulence is (generally) strongest for
unstable conditions. Despite the fact that the guidelines do
not overestimate either of these processes, the combined
level of moderately strong shear and turbulence never
occurs in reality. Wind shear and turbulence are thus not
independent, but are both coupled and dependent on
atmospheric stability. This fundamental coupling of
processes in the atmosphere is lacking in guidelines and,
as shown, results in an overestimation of wind turbine
fatigue loads of 10%.
As a final analyses, we assess if there is a significant
difference in loads caused by shear and loads caused by
turbulence by the guidelines. In this scope similar load
simulations where carried out for an atmosphere with only
turbulence present and for an atmosphere with only shear
present. The relative ratio of these simulations is plotted in
figure 4. It can be seen here that in fact for all wind
speeds the balance between shear and turbulence is
incorrect in the guidelines. This can be adjusted by
lowering wind shear, which similarly also reduced simulated
lifetime equivalent loads.
3
Very Unstable
2.5
Unstable
Equivalent turbulence [m/s]
Far shore wind conditions are expected to be favorable for
wind energy power production due to increased mean wind
speeds and reduced turbulence levels. Exact far shore wind
conditions are not known however due to absence of
intensive off shore measurement campaigns. This research
aims not only to define far shore wind conditions, but also
assess how general wind conditions should be transferred
to wind turbine design parameters. Besides, it is aimed to
define a methodology how one can determine wind
conditions for a far shore site, either with or without local
observation data. It is expected that this research also
contributes to fundamental insight in turbulence, wind
shear and atmospheric stability for offshore conditions.
It is recognized that atmospheric stability is a fundamental
parameter in meteorology, used to describe the general
state of the atmosphere. Due to its importance, it is
assessed if for a far shore site atmospheric stability does
influence key wind turbine design parameters, and what
the impact of atmospheric stability is on wind turbine
fatigue loads compared to using reference guidelines.
Coupling of atmospheric conditions
σEQ as a function of U and L
Neutral
Stable
2
Very Stable
Guidelines
1.5
1
0.5
0
0
5
10
15
20
25
30
Hub height wind speed [m/s]
Figure 2: Equivalent turbulence as a function of
atmospheric stability
Wind shear according to observations and shear models
2
Observations
Bin-ave Observations
Power Law
Free Convection
Holtslag
The validated wind profile and turbulence characteristics
can be used to assess the overall impact of stability on
wind turbine fatigue loads. This is done for the blade root
bending moment. The cumulative lifetime equivalent loads
are calculated as (Sathe et al., 2012)
u90/u27 [-]
1.6
1.4
25
FEQ  C 
d
U  4 L VS
1
0.8
-5
-4
-3
-2
-1
0
1
2
3
4
5
100/L [m-1]
Figure 1: Wind shear as a function of stability
Stability and Wind Shear
In meteorology Monin-Obukhov theory has been used
extensively in the past decades to describe the lowest
parts of the atmosphere. MO-Theory states that any nondimensional parameter is a function of the non-dimensional
stability parameter ζ (Obukhov, 1971). Assuming validity of
MO-Theory, wind profiles can be described by the stability
corrected logarithmic wind profile (Businger et al., 1971)
U  z 

u*   z 
ln



 
  
   z0 

Analyses of observation data taken from the far shore
meteomast IJmuiden shows this wind profile performs well
offshore. Results are shown in figure 1, where it can be
seen that for stable conditions wind shear increases.
Stability and Turbulence
Since turbulence (in terms of the standard deviation of the
horizontal wind speed) is a stochastic process, and one
cannot simulate every possible TI, one has to consider a
characteristic TI in wind turbine design. It can be shown
that if TI is log-linear distributed, the characteristic
turbulence level depends on wind turbine characteristics as
well as on distribution parameters of the log-linear
distribution of turbulence (Veldkamp, 2006). The impact of
atmospheric stability on the equivalent turbulence can be
seen in figure 2, where it is clear that turbulence increases
for unstable conditions and there is a non-linear
dependence of turbulence on mean wind speed. This nonlinear dependence is not included in the guidelines.
EQ
P  L | U  P U 
The last two terms in the summation are taken from the
same observation dataset and represent respectively the
chance that for a hub height wind speed U the stability
class L occurs, and the chance that the hub height wind
speed U occurs. The first term in the summation
represents the equivalent damage that occurs for given
(hub height) atmospheric conditions, and is calculated
based on simulations carried out with the design software
Bladed. For these simulations the 5MW NREL reference
turbine is used as reference wind turbine. The cumulative
loads before summation over all wind speeds are shown in
figure 3. The difference in lifetime cumulative equivalent
loads when using either guidelines or the stability
dependant results shown in figure 1 and 2 is 10%. These
difference are mainly found for moderate wind speeds,
ranging from 8 to 14 m/s. Missing values prevented
calculations for wind speeds below 6 m/s and above 21
m/s for stability dependant results. Since neither wind
shear (figure 1), nor turbulence (figure 2) is by definition
overestimated, it is questioned what causes the difference
in calculated lifetime equivalent loads.
Shear-loads / turbulence-loads
80
70
60
50
40
30
IEC-guidelines
Stability-Obs.
20
10
0
0
5
10
15
20
25
Figure 4: Relative ratio of wind turbine loads for
idealized atmospheres (shear only / turbulence only)
Outlook
The research presented so far is based on only 6 months
of observation data, which is questionable based on the
seasonal pattern of temperature (and thus stability).
Besides, so far fatigue loads of only one component of the
wind turbine are investigated. After proper extension of
this research it is expected that two journal papers will be
written, one dealing with far shore atmospheric conditions,
and one dealing with the impact of atmospheric stability on
wind turbine fatigue loads. Besides, results obtained so far
will be presented at the 2nd International conference on
Energy & Meteorology in Toulouse, June 2013.
100
IEC-total
80
Obs-total
60
40
20
0
0
5
10
15
20
25
30
Hub height wind speed U [m/s]
Figure 3: Cumulative loads as a function of wind speed
Publications
-
30
Hub height wind speed [m/s]
FEQ per wind speed
120
Equivalent load [kNm]
Aerospace Engineering
1.2
VU
Shear-loads / turbulence-loads [%]
1.8
Stability and Fatigue Loads
Obukhov, A. M. (1971), “Turbulence in an atmosphere with non-uniform temperature”, Boundary Layer Meteorology 2, pp 7-29
Businger, J. A., Wyngaard, J. C., Izumi, Y. & Bradley, E. F. (1971), “Flux-Profile relationships in the atmospheric boundary layer”, Journal of the Atmospheric Sciences 28 pp181-189
Veldkamp, H. F. (2006), “Chances in Wind Energy: A probabilistic approach to wind turbine fatigue design”, PhD-Thesis, Technical University Delft”
Sathe, A., Mann, J., Barlas, T., Bierbooms, W. A. A. M. & van Bussel, G. J. W. (2012), “Influence of atmospheric stability on wind turbine loads”, Wind Energy pp 49-61